Neurology

The alphabet of neurology begins with a single verb, and that verb is calcium coming in. Every muscle contraction, whether in skeletal or smooth muscle, is at its core nothing more than this one sentence. The only difference is whom the calcium grabs once it arrives, and who serves as the switch. In skeletal muscle, calcium bursts out of the sarcoplasmic reticulum through a gate called RyR1, but what opens that gate is not the electrical signal itself — it is DHPR on the T-tubule, a voltage sensor that, once it senses the cell membrane depolarize, mechanically tugs RyR1 to one side, and calcium comes gushing out. So DHPR is the button and RyR1 is the actual door; confusing these two identities is the most common way to bleed points on the exam. Once calcium bursts out, it goes and binds troponin C, moving tropomyosin off actin, and only then can the myofilaments slide. Smooth muscle follows a different script: calcium comes in from a dual source, the sarcoplasmic reticulum plus the extracellular space, and instead of binding troponin, it grabs a calcium-sensing protein called calmodulin, which then activates the enzyme MLCK; once MLCK phosphorylates the light chain of myosin, the muscle finally contracts, and to relax, MLCP strips the phosphate group back off. One more line to remember at the neuromuscular end plate: the receptor there is nicotinic, not muscarinic, and the antibodies discussed later under myasthenia gravis attack exactly this receptor — so this basic question already holds that disease's pathology in your hand.

Once you understand this passage on muscle, sensation falls into place easily on the return trip. Why cholecystitis hurts in the right shoulder and why myocardial infarction radiates to the left arm is no coincidence — behind both lies the same chain of anatomical causation. Visceral afferent fibers and somatic afferent fibers converge in the spinal cord onto the same second-order neuron; the brain only ever sees the signal that second-order neuron sends up and cannot tell which extension placed the call, so it defaults to the body surface, because skin pain is an everyday event and visceral pain is far rarer. So the gallbladder and the diaphragm share the phrenic nerve, C3 to C5, and the pain maps onto the right shoulder; the heart's visceral afferents share the T1-to-T4 pathway with the left arm and jaw, and the pain radiates there instead. Exam questions love to write this convergence site as the medulla — that is wrong; the convergence happens in the spinal cord. There is one more corner in the movement section that is easily misunderstood: the plan for a voluntary movement is not dreamed up by the motor cortex alone — the motor cortex is only responsible for executing the output, while the actual design is completed jointly by the basal ganglia and the cerebellum. The basal ganglia govern the initiation and selection of movement, the cerebellum governs timing and precision, and only once the cortex receives the design does it send the command down through the corticospinal tract. So the Parkinson's pattern of wanting to move but being unable to, and the cerebellar-lesion pattern of moving but crookedly, are both, at their core, failures at the design stage.

For special sensation, remember three lines: the auditory receptor is the organ of Corti in the inner ear, not the middle ear; taste is carried by cranial nerves seven, nine, and ten, while the trigeminal only touches and never tastes; the optic chiasm has only nasal fibers crossing while temporal fibers do not, so when a pituitary adenoma compresses its center, the result is bitemporal hemianopia. Arriving at the ophthalmology section, the story of that 32-year-old woman now reads easily. Her vision dropped to 0.3 over three days, with a dull ache on eye movement, worsening color vision, and a positive RAPD in the affected eye on the pupillary light reflex, yet her fundus was nearly normal. A normal fundus is exactly the point, because hers is retrobulbar optic neuritis, with the lesion sitting behind the globe — two-thirds of optic neuritis looks exactly like this, and it may be the first gunshot of multiple sclerosis. Treatment calls for a high-dose intravenous pulse of methylprednisolone, and here lies a very easy pit to fall into: according to the ONTT trial, using standard-dose oral prednisone alone actually increases the relapse rate, so it is forbidden, because a standard oral dose never reaches the high concentration needed within the optic nerve and instead leaves behind a half-baked immunosuppression that lets T cells reactivate. For contrast, two other causes of acute vision loss: in an elderly patient with sudden painless blindness, think vascular occlusion — the signature of CRAO is the cherry-red spot, because full-thickness inner retinal ischemia leaves the fovea, which has no ganglion cell layer, showing the red of the choroid through it, while CRVO gives a blood-and-thunder fundus because blocked venous outflow causes the entire retina to hemorrhage and swell; an elderly patient with temporal headache, jaw claudication, and a sky-high ESR should make you think of giant cell arteritis, an emergency that demands high-dose steroids immediately without even waiting for the biopsy, because the other eye could go blind at any moment.

The extraocular muscles and convergence hold two points that are easily flipped in memory. Pure abduction of the eyeball relies solely on the lateral rectus, innervated by the sixth cranial nerve; depression in the adducted position is the job of the superior oblique, the fourth cranial nerve, and this is the signature of a fourth-nerve palsy — the patient cannot see clearly going down stairs and must tilt the head to compensate. Convergence comes in three types: tonic convergence is the baseline tone maintained while awake to hold resting eye position, proximal convergence is triggered by perceiving an object as near, and accommodative convergence is dragged along by the accommodation used to look at something near — it is its own accommodative type, distinct from proximal convergence, and not the tonic one. The story of accommodative esotropia flows cleanly: a hyperopic child must accommodate desperately at any distance, and over-accommodation drags along excess convergence, forming esotropia, so the first-line treatment is neither surgery nor prisms but full hyperopic correction with glasses first, which suppresses the accommodative demand and lets convergence return on its own. By the same logic, hyperopia is more amblyogenic than an equivalent degree of myopia, because looking both near and far demands heavy accommodation, which easily triggers accommodative esotropia or unequal images between the two eyes, forcing the brain to suppress one eye, which then goes to waste as amblyopia. So the claim that children's weaker accommodative power makes them prone to amblyopia has the direction backward — a child's lens is soft and its accommodative power is in fact far stronger than an adult's, and it is precisely because they can accommodate so well that problems arise. One last, very important red flag: an abducens palsy in a child must never be observed for three months the way an elderly diabetic's would be, because the sixth cranial nerve runs an unusually long course hugging the clivus, and any rise in intracranial pressure or any posterior fossa tumor will crush it first — so the moment a child has a sixth-nerve palsy, imaging must be obtained to find the cause. The same logic, that a longer course means earlier damage, also explains why an early warning sign of idiopathic intracranial hypertension is likewise a bilateral sixth-nerve palsy. One small trap in passing: myasthenia gravis can present with nothing more than unilateral ptosis or diplopia and does not have to begin bilaterally, so unilateral ptosis cannot rule out myasthenia gravis — this thread will be picked back up in Chapter Six. The whole chapter is really saying just one thing: how calcium gets in, how signals converge, how the eyeball moves, and why the optic nerve hurts — this is the alphabet underlying every disease that follows.

The spinal cord is not a uniform wire but a cable of three long tracts arranged with precision, and which one crosses, and which does not, decides exactly what is lost when a given site is injured. The lateral corticospinal tract governs ipsilateral motor function because it already crossed back at the decussation of the pyramids in the medulla, so once inside the spinal cord it is, in fact, ipsilateral; the posterior columns govern ipsilateral proprioception and vibration and likewise cross the street at the medulla, so they too are ipsilateral; the lateral spinothalamic tract governs pain and temperature, and it crosses to the opposite side within one or two segments of entering the cord, so inside the cord it is contralateral. In one sentence: motor and proprioceptive fibers have not yet crossed the street while still inside the cord, so they are ipsilateral, while pain and temperature cross the street the instant they come in, so they are contralateral — and this is the anatomical reason the crossed pattern shows up in a Brown-Séquard hemisection. Beyond the crossing point itself, the lateral corticospinal tract holds one more small point of internal topography that determines central cord syndrome: the cervical fibers to the arms run most medially, and the sacral fibers to the legs run most laterally, so when the center of the cord is injured, the medial fibers fail first and the arms end up worse off than the legs. An anterior spinal artery infarction seizes both motor function and pain/temperature sensation together, but because the posterior columns are supplied by a separate posterior circulation, proprioception is preserved — and that is the signature of anterior cord syndrome. This same logic of a central crossing point also explains syringomyelia: the cavity forms right at the midline and cuts off the pain-and-temperature fibers at the waist just as they are about to cross the street, producing a symmetric, cape-distribution loss of pain and temperature while proprioception and touch are preserved — this is dissociated sensory loss.

For cervical fractures, see the word and pick the diagnosis. A Jefferson fracture is the C1 ring shattered into three or more burst fragments under vertical compression to the top of the head; odontoid fractures come in three types, and the type II base fracture is the most common and hardest to heal because blood flow to the base is poor; a Hangman fracture is a bilateral C2 pedicle fracture plus anterior displacement of C2, named for judicial hanging but seen today more often from dashboard hyperextension in a car crash; the double-lumen sign does not represent two actual spinal canals but rather the image produced when the facet joints are completely dislocated and the vertebral bodies stack on top of each other, most often seen at C6 to C7, and it signals a highly unstable injury. In the acute phase, the two things that most need to be told apart are the two kinds of shock, which are entirely different concepts: neurogenic shock is a hemodynamic shock in which a high spinal cord injury, usually at or above T6, completely severs the descending sympathetic outflow, while the vagus nerve, traveling as a cranial nerve, is left intact, so the result is vasodilation without compensatory tachycardia — hypotension paired with a normal or slow heart rate and warm, flushed skin, treated with a vasopressor rather than fluids alone, because fluids alone will not bring the heart rate down. Spinal shock is a shock at the level of reflexes: after the injury, all reflexes below the level are lost and flaccid paralysis sets in, a state that usually lasts from hours to weeks, and its end is marked by the return of the bulbocavernosus reflex. Do not mix the two up — if the stem gives quadriplegia plus hypotension plus a heart rate of 60, pick neurogenic; if it gives complete loss of reflexes after injury, pick spinal. Acute management is always ABCs plus spinal immobilization first, CT first to look at bone when a fracture is suspected, and MRI to look at cord edema and hemorrhage. On high-dose steroids, the modern view is that they are no longer standard treatment, at most an option within eight hours of injury, not recommended beyond eight hours, and absolutely withheld in penetrating trauma — the answer "standard of care" is simply wrong today.

The most frequently tested topic in the chronic phase is autonomic dysreflexia. Why is T6 the dividing line? Because the bulk of the visceral sympathetic fibers run all the way from T6 down to L2, so an injury at or above T6 completely severs the descending inhibition from above, and the sympathetic outflow below loses its brake entirely. The trigger is a noxious stimulus below the level of injury, most commonly a distended bladder or urinary retention from a blocked catheter, followed by constipation or fecal impaction. Once the stimulus arrives, the sympathetic chain below the injury fires massively, the vessels constrict violently, and the blood pressure spikes. The brain detects the hypertension and tries to hit the brake, but the signal is blocked at the site of injury and cannot reach the lower body, so it has no choice but to let the upper body compensate, and the upper body flushes, sweats, and develops a headache while the vagus nerve steps in to slow the heart, producing reflex bradycardia. So the signature is paroxysmal severe hypertension plus bradycardia plus a pounding headache, with a flushed, sweating upper body and a pale lower body — this upper-lower contrast is its fingerprint, a farce in which the lower body is on fire while the upper body slams the brakes. The first step in management is not an antihypertensive but finding the source of the fire: sit the patient upright to let blood pressure drop somewhat from the postural change, immediately remove the trigger — catheterize first, then check for constipation and tight clothing — and only give a short-acting antihypertensive if the pressure remains high. Reaching for an antihypertensive alone without resolving the trigger is a major error, because once the drug wears off the pressure will simply spike back up, and this is an emergency that can end in intracranial hemorrhage. The ASIA scale is determined by two things: whether any sensation or motor function remains in the S4-to-S5 saddle region, and whether most key muscles reach a strength of three. Any voluntary anal contraction means it cannot be grade A and must be incomplete; strength then decides between C and D — most muscles below three is C, most at or above three is D. The complications of the chronic phase are easiest to remember along the timeline: a urinary tract infection is the most common complication, from impaired bladder emptying combined with chronic catheterization; heterotopic ossification grows abnormal bone around a joint, with ALP elevated and a three-phase bone scan turning positive early, and surgical resection is forbidden while the ossification is still immature, because cutting it out only provokes even more severe ossification — wait until ALP normalizes and the bone scan returns to baseline before resecting; a reflexogenic erection is triggered by peripheral stimulation through the S2-to-S4 parasympathetic outflow, while a psychogenic erection relies instead on the T11-to-L2 sympathetic outflow, and reversing these two is a common way to lose points on the exam; sleep-disordered breathing after cervical cord injury is predominantly obstructive, not central; neural tube defects occur most often at the lumbosacral spine, and myelomeningocele is frequently associated with a Chiari II malformation, with hydrocephalus present in roughly 80 to 90 percent of cases, linked to maternal folate deficiency.

Last, let's finish off the two peripheral branch lines. In an adult, the spinal cord ends at the inferior border of the first and second lumbar vertebral bodies, and the bundle of nerve roots below that is called the cauda equina, so even though "low back pain plus leg weakness" sounds the same, an injury above versus below L1 produces an entirely different clinical picture. Conus syndrome is a mixed upper- and lower-motor-neuron injury, more symmetric, with symmetric saddle sensation and early bowel and bladder involvement; cauda equina syndrome is pure lower-motor-neuron, often asymmetric, with both the ankle jerk and the bulbocavernosus reflex absent. Both are surgical emergencies, but because compression in cauda equina syndrome most often arises from a disc, a tumor, or a hematoma, decompression within twenty-four to forty-eight hours is critical for neurologic recovery — missing the red-flag combination of saddle numbness plus urinary difficulty and sending the patient home is the mistake every resident fears most. The brachial plexus is formed from five roots, C5 to T1, and two patterns are tested most often: Erb-Duchenne palsy injures the upper trunk, C5 to C6, from a mechanism of the shoulder being pulled downward, as in birth trauma or a motorcycle fall, with the signature waiter's-tip posture; Klumpke's palsy injures the lower trunk, C8 to T1, from a mechanism of the arm being pulled upward, as in an infant lifted by the arm or a fall while grabbing a branch, with the signature claw hand plus Horner syndrome, because T1 carries preganglionic sympathetic fibers, and severing it produces ptosis, miosis, and anhidrosis on that half of the face — this same shared pathway explains why a Pancoast tumor at the lung apex also produces Horner syndrome. The whole chapter strings together into a single timeline: at the moment of injury, look first to ABCs and spinal immobilization; in the acute phase, tell neurogenic and spinal shock apart, and use CT for bone and MRI for the cord; moving into the chronic phase, stay alert for this whole cluster of ghosts — dysreflexia, infection, ossification, sexual function, and breathing; and finally, do not miss the peripheral branch lines reaching out to the cauda equina and the brachial plexus.

Four in the morning, and three heart-stopping patients crowd into the emergency department: a grandmother who was playing chess just yesterday cannot recognize her own son tonight, her eyes drifting unsteadily under the lights, drowsy one moment and yanking at her IV in agitation the next; in another bed, a middle-aged man who, over the past three months, has gone from fixing plumbing and wiring to forgetting the way home, walking as if his shoes were glued to the floor, and now wetting himself; in the farthest bed, a young man who, moments ago in his office lobby, went rigid all over for a few seconds, then jerked rhythmically for two minutes, biting the side of his tongue, and is still groggy now. The investigation neurology must run is to ask first how fast the illness came on, then the state of attention and consciousness, then match it against the pattern of symptoms, and finally connect it to the deposits under the microscope and the signal on the MRI. When an elderly patient suddenly turns confused, the worst thing to do is slap a dementia label on it right away, because delirium is, at its core, acute and reversible, with attention failing first, consciousness flickering clear and then foggy like a television with poor reception, worsening after dark, and behind it there is almost always a findable trigger — infection, an electrolyte disturbance, hypoxia, a drug, alcohol withdrawal — and once the trigger is removed, the patient comes back. Dementia, by contrast, is a chronic, steadily progressive decline, with attention relatively preserved early on and consciousness fully alert — it is simply that what should be remembered cannot be remembered, and what should be planned cannot be planned. So when facing an elderly patient who has suddenly turned confused, the first step is always to rule out delirium rather than to slap on a dementia label — think of delirium as a fever of the brain and dementia as rust in the brain, one acute, one chronic, one an event, the other a process.

Once you have confirmed the chronic path, the next dividing line comes down to only one thing: whether activities of daily living can still be managed independently. The various types of dementia are not something to memorize as fixed pairings — they should be reasoned out along the line of which protein piles up where, and which pathway is destroyed. Alzheimer's disease damages recent memory first, with an insidious, gradual course, and its pathology is extracellular Aβ senile plaques plus intracellular hyperphosphorylated tau neurofibrillary tangles; its CSF changes are the thing most often tested in reverse, but they make sense the moment you think through one sentence — Aβ42 piles up in large amounts in the brain's plaques and naturally cannot make its way into the CSF, so Aβ42 is actually decreased, while once a neuron collapses tau spills out, so phosphorylated tau is increased in the CSF — the more that piles up in the brain, the less makes it into the CSF. The signature of dementia with Lewy bodies is fluctuating cognition plus visual hallucinations plus spontaneous parkinsonism plus REM sleep behavior disorder, with α-synuclein as the pathology, and its most lethal landmine is extreme sensitivity to both antipsychotics and dopamine agonists, because its nigrostriatal D2 receptors are already fragile from α-synuclein deposition and have upregulated in compensation, so giving a typical antipsychotic crushes what little transmission remains and triggers even worse hallucinations and a neuroleptic-malignant-like syndrome — so motor symptoms should be treated with low-dose levodopa and cognition with a cholinesterase inhibitor, because the cholinergic system in DLB degenerates even earlier than in AD. The triad of normal pressure hydrocephalus is gait disturbance first, then urinary incontinence, then dementia, and this order is the thing most often tested in reverse — gait disturbance appears earliest and is also the most likely to improve first after lumbar drainage, because when the ventricles enlarge, the first structure compressed is the corticospinal tract's leg fibers lateral to the lateral ventricle, so the diagnosis is confirmed by draining thirty to fifty milliliters in a tap test. Creutzfeldt-Jakob disease is rapidly progressive dementia on the scale of months, plus myoclonus, plus cerebellar dysfunction, and the culprit, the prion protein, cannot be killed by ordinary boiling — it takes extreme high pressure at 134 degrees for 18 minutes to destroy it. Transient global amnesia is sudden amnesia with the patient repeating the same question over and over, yet orientation is preserved, and it resolves on its own within hours — it is not a stroke, and it should not be over-treated.

The distinction between a seizure and syncope is actually very clean: syncope is always preceded by an autonomic prodrome of sweating, dizziness, and the vision going dark, the trigger is clear, and recovery is rapid and complete; a seizure has no such prodrome, the jerking is regular and often bites the side of the tongue, and afterward there is confusion or drowsiness — so "night sweats plus fatigue" as a prodrome leans toward syncope and argues against a seizure. Absence seizures occur in children, a sudden staring pause lasting a few seconds with 3 Hz spike-and-wave on EEG, and the patient is immediately alert once it ends with no postictal drowsiness; myoclonic seizures are extremely rapid, brief jerks with consciousness usually preserved, and they are the finding most easily misjudged as loss of consciousness. Temporal lobe epilepsy arises from the hippocampus and amygdala of the medial temporal lobe, so bilateral hippocampal sclerosis forbids bilateral resection, because the hippocampi on both sides are responsible for forming new memories. Status epilepticus is now defined as lasting more than five minutes, or continuous seizures without recovery of consciousness in between, and there is a clear logic behind the drug ladder: the first line, the benzodiazepines, increase how often the GABA-A channel opens and act fastest, directly amplifying the inhibitory signal that has already been overwhelmed, but the dose cannot be escalated indefinitely, because it will suppress respiration and the receptor will desensitize, diminishing the effect, so the second line must switch to a different mechanism — levetiracetam, valproate, or (fos)phenytoin — to achieve steady suppression, and the third line is an anesthetic agent plus intubation; every step switches to a different mechanism, and this is the causal logic behind the sequence, not something to memorize by rote. Pediatric mortality is roughly three to nine percent, so seeing a question claim more than fifty percent is an exaggeration and should be marked wrong immediately. Two direction-reversal questions come up often: the compensation for raised intracranial pressure is the Cushing reflex — hypertension plus bradycardia plus irregular breathing — because raised intracranial pressure compresses the brainstem, and the body raises blood pressure to maintain cerebral perfusion, and the moment the baroreceptors are pushed up they reflexively lower the heart rate, so it is pressure up, rate down, not pressure down, rate up; and the seizure incidence in Sturge-Weber syndrome is seventy to ninety percent, very high, so do not underestimate it. The narcolepsy tetrad is excessive daytime sleepiness, cataplexy, sleep paralysis, and hypnagogic hallucinations, with hypothalamic orexin deficiency as the mechanism, and confusional arousal does not count as part of that tetrad.

Turning to imaging, hyperacute ischemia lights up on the diffusion-weighted sequence within minutes, because within minutes of ischemia ATP is exhausted, the Na⁺/K⁺ pump fails first, and water floods into the cell, producing cytotoxic edema with restricted diffusion; FLAIR suppresses the CSF signal so that lesions adjacent to the ventricles and the cortex become visible, but the edema must accumulate to a certain degree before it alters the T2 signal, so it takes six to twelve hours. Gradient echo and susceptibility-weighted sequences have an extremely high sensitivity to the magnetic effect of blood, so they detect hemorrhagic transformation, microbleeds, and hemosiderin even more sharply than CT — which runs against intuition — with acute large hemorrhage best seen on CT and chronic microbleeds best seen on gradient echo. Venous sinus thrombosis tends to strike a young woman in the postpartum period, on oral contraceptives, and dehydrated, and after contrast CT the superior sagittal sinus shows a filling defect called the empty delta sign. Tuberous sclerosis is mTOR overactivation, producing cortical tubers, subependymal calcified nodules, and subependymal giant cell astrocytomas that favor the region beside the foramen of Monro and can cause obstructive hydrocephalus. A calcified intra-axial tumor in an adult is an oligodendroglioma, roughly ninety percent calcified and carrying 1p/19q co-deletion; the most common posterior fossa lesion in an adult is a metastasis, while in children it is dominated instead by pilocytic astrocytoma and medulloblastoma — the two patterns run in opposite directions, so do not mix them up.

Finally, arriving at the microscope: neurodegeneration is, at its core, misfolded proteins slowly filling up brain cells. Parkinson's disease and dementia with Lewy bodies involve α-synuclein forming Lewy bodies; Alzheimer's disease involves extracellular Aβ plaques plus intracellular hyperphosphorylated tau tangles; frontotemporal dementia and amyotrophic lateral sclerosis involve TDP-43 and SOD1, not tau; PSP and corticobasal degeneration involve tau; and Creutzfeldt-Jakob disease is caused by the prion protein producing spongiform degeneration — so the moment you see ALS paired with tau, mark it wrong. The location of an intracerebral hemorrhage is almost the same thing as its cause: chronic hypertension causes hyaline degeneration of the deep penetrating arteries, forming Charcot-Bouchard microaneurysms, hence hemorrhage in the basal ganglia; cerebral amyloid angiopathy, by contrast, is Aβ deposition in the meningeal and cortical arterioles, hence lobar hemorrhage, favoring the elderly, associated with Alzheimer's disease, and linked to ApoE polymorphisms; a berry aneurysm forms at the branch points of the circle of Willis, a congenital defect of the tunica media that ruptures to cause subarachnoid hemorrhage. The signature of HSV encephalitis is Cowdry A intranuclear inclusions with hemorrhagic necrosis of the temporal lobe, and caseating granulomas never occur in viral encephalitis — that is the marker of tuberculosis. HSV's fondness for the medial temporal lobe and orbitofrontal cortex arises because it normally lies latent in the trigeminal ganglion, and upon reactivation travels upward along the meningeal branch of the first division of the trigeminal nerve and along the olfactory bulb's olfactory tract — these two anatomical pathways deliver the virus precisely to those two regions, so this regional preference is no coincidence; it is the result of the virus hitching a ride. The prognosis of medulloblastoma molecular subtypes, from best to worst, runs WNT, then SHH and Group 4, with Group 3 the worst; craniopharyngioma arises from Rathke's pouch and appears as enamel-organ-like tissue plus wet keratin plus calcification. The finding least associated with the sequelae of traumatic brain injury is demyelinating disease, because it is immune-mediated and has no causal link to trauma. The whole chapter strings together into the same investigation chain: first sort by acute versus chronic, then look at attention and consciousness, then look at the pattern, and finally match it against the protein and the sequence.

Five patients are wheeled into the emergency department at once on a Saturday afternoon: an elderly man whose left hand suddenly cannot lift with a drooping corner of the mouth; a hypertensive woman who has just collapsed in the bathroom, her putamen glowing a glaring white on CT; a young woman who swears this is the worst headache of her life, peaking within three seconds, her neck now stiff; a 12-year-old Taiwanese-American child who goes weak on one side for a few minutes every time she cries too hard or blows up a balloon, with progressive occlusion at the ends of the ACA and MCA and a smoke-like network of vessels growing beside them; and finally an elderly man, three months out from a stroke, still weak on the right side, whose left shoulder hurts too much to lift and whose whole hand is swollen like a sausage. On this afternoon, the vascular service is not just fighting the battle of acute stroke — hemorrhage, moyamoya, and rehabilitation are all on this same front line. The core question in stroke is for how long the oxygen supply to a given patch of brain tissue has been cut off; ischemia, hypoxia, ATP depletion, failure of the Na⁺/K⁺ pump, cytotoxic edema, cell death — once this chain is set in motion, roughly 1.9 million neurons die every minute, which is why it is called time is brain. The iron rule is to first look at a non-contrast CT to see whether it is white or black — white is blood, black is ischemia, and this decides whether tPA can be given — but an early infarct is often normal on CT, so do not rule out a stroke just because you cannot see it. Ischemic stroke subtypes sort into four buckets: large-artery atherosclerosis commonly shows cortical signs such as aphasia, neglect, and hemianopia; a cardioembolic stroke is sudden, with the maximal deficit present right at onset, and can involve multiple vascular territories; a small-vessel lacunar stroke is the product of hyaline degeneration of the deep penetrating arteries from hypertension or diabetes, producing five classic syndromes — pure motor, pure sensory, clumsy hand, and dysarthria — but because the lesion is small and deep, it absolutely cannot produce a cortical sign like aphasia, neglect, or hemianopia, and getting this direction backward is a guaranteed way to fail the question.

The real logic behind the time window is this: within the ischemic core, ATP is already exhausted and the cells are already dead and cannot be saved, but surrounding the core, in the ischemic penumbra where blood flow has fallen to roughly a quarter to half of normal, the neurons are electrically silent yet just barely maintain their membrane ion gradients and structure, and they can recover if blood flow is restored; this penumbra is exactly what reperfusion is trying to rescue, and as time passes it is swallowed into the core minute by minute, which is why roughly 1.9 million neurons die for every minute of delay, and the brain ages by 3.6 years for every hour of delay. Four and a half hours is the statistical turning point in the risk-benefit ratio of IV tPA — beyond this window the blood-brain barrier has already failed from the ischemia, and giving tPA instead ruptures the vessels within the already-dead core, causing symptomatic intracranial hemorrhage in about six percent of cases, a risk that far outweighs the shrinking penumbra left to save — so the answer to why it is four and a half hours is not an arbitrarily chosen cutoff, but the tipping point of how much penumbra remains weighed against how much hemorrhage risk that would cost. Mechanical thrombectomy for a large-vessel occlusion, by contrast, can be extended to six to twenty-four hours, because the DAWN and DEFUSE-3 trials used imaging mismatch — a small core, a large penumbra — to screen out patients who still have salvageable tissue, and for these patients thrombectomy remains effective even beyond the standard time window. One direction-reversal question that comes up often concerns acute blood pressure management: in ischemic stroke, if tPA is not being given, blood pressure is not urgently lowered, and a reading below 220/120 is generally treated as permissive hypertension to preserve cerebral perfusion; only when tPA is going to be given does the pressure need to be controlled below 185/110 — over-aggressive lowering will starve the penumbra right along with everything else and expand the infarct.

Wallenberg syndrome is a must-know classic because occlusion of a single PICA or the distal vertebral artery can somehow produce five seemingly unrelated groups of symptoms all at once, and behind this lies the anatomical overcrowding of an enormous number of nuclei and tracts packed into that one small patch of lateral medulla. Think of the lateral medulla as a hub station: the spinothalamic tract governs contralateral trunk pain and temperature, the spinal trigeminal nucleus governs ipsilateral facial pain and temperature, the nucleus ambiguus governs the motor components of the glossopharyngeal and vagus nerves, controlling swallowing and the vocal cords, the vestibular nuclei govern vertigo and nystagmus, the inferior cerebellar peduncle governs ipsilateral cerebellar signs, and the descending sympathetic fibers innervate the ipsilateral face's pupil, eyelid, and sweat glands — all six of these major structures are crammed into this one small region. A single PICA occlusion blows up this entire hub station, producing contralateral trunk loss of pain and temperature plus ipsilateral facial loss of pain and temperature, which together are the crossed loss of pain and temperature, plus dysphagia and hoarseness, plus vertigo and nystagmus, plus ipsilateral cerebellar dysfunction, plus ipsilateral Horner syndrome with ptosis, miosis, and anhidrosis on that half of the face — these five symptom clusters are really just five anatomical neighbors losing power together. On the hemorrhage side, the location of an intracerebral hemorrhage almost directly tells you the cause: chronic hypertension causes hyaline degeneration of the deep penetrating arteries, forming Charcot-Bouchard microaneurysms that rupture in deep structures such as the basal ganglia, putamen, thalamus, pons, and cerebellum; cerebral amyloid angiopathy is Aβ deposition in the meningeal and cortical arterioles, so it produces lobar hemorrhage, favoring the elderly, often coexisting with Alzheimer's disease, and linked to ApoE polymorphisms. The three must-know principles for ICH are that blood pressure should be moderately lowered to around one hundred forty, neither left completely alone nor slammed down; steroids are ineffective against the edema and are harmful, and should not be given; and a cerebellar hemorrhage over three centimeters, or one compressing the brainstem, calls for aggressive surgical decompression. Subarachnoid hemorrhage presents as a thunderclap headache plus neck stiffness plus a star-shaped pattern of white on CT, and if the CT is negative but suspicion remains high, a lumbar puncture should be done to look for xanthochromia; aneurysmal SAH also demands vigilance for rebleeding and for cerebral vasospasm peaking on days four through fourteen after the bleed, prevented with twenty-one days of nimodipine. An epidural hematoma is lens-shaped, from the middle meningeal artery, with a lucid interval and rapid progression; a subdural hematoma is crescent-shaped, from the bridging veins, seen in the elderly or in alcoholics, and can be subacute.

The core of moyamoya disease is progressive stenosis and occlusion at the terminal internal carotid artery plus the origins of the ACA and MCA, with the PCA relatively spared — this is the direction the exam most loves to flip into an error, so never write it as MCA plus PCA. As the anterior circulation grows increasingly narrowed, the brain tissue, fighting to survive, grows a compensatory network of small vessels from basal collaterals that looks exactly like a puff of smoke on DSA, but these compensatory vessels are fragile and prone to rupture, and combined with the ischemia of the anterior circulation itself, children present predominantly with the ischemic form — crying, blowing up a balloon, or hyperventilating triggers a TIA, because the resulting low CO₂ reflexively constricts the cerebral vessels and the compensatory network cannot hold up under the strain — while adults present predominantly with the hemorrhagic form, from rupture of the compensatory vessels. It is usually bilateral and favors East Asians, and treatment with medication is only adjunctive; the real fix is an STA-MCA bypass, letting the ECA come to the rescue of the anterior circulation. Surviving a stroke is only the first half; rehabilitation is the second half. Remember aphasia along three axes — fluency, comprehension, repetition: Broca's is nonfluent but comprehension is intact and repetition is poor, the expressive type; Wernicke's is fluent but comprehension fails and repetition is poor, the receptive type; conduction aphasia is fluent with intact comprehension but repetition alone is impaired, from the arcuate fasciculus; and the hallmark of transcortical aphasia is preserved repetition. The Barthel Index scores ten basic ADLs and does not include taking medication, cooking, shopping, or managing finances — these IADLs belong to the Lawton scale. The direction of stroke complications must be remembered correctly: CRPS type I, shoulder-hand syndrome, favors the shoulder, wrist, and fingers, with the elbow relatively spared; never forcefully pull on a subluxed shoulder; spasticity is treated with baclofen plus botulinum toxin injections; and DVT should not be managed with a week of bed rest — early mobilization is what is actually needed instead. The whole chapter strings acute hemorrhage, ischemia, moyamoya, and rehabilitation together into the same map, and there is only one core question: how long has the tissue been starved of oxygen, and can it still be won back?

The neurology consult desk receives three signals in one morning: a retired engineer whose right hand has trembled at rest for three years, whose gait grows increasingly shuffling, whose face has gone masklike — the signature of Parkinson disease; a young female office worker with unilateral throbbing headache plus nausea and photophobia — migraine; a middle-aged woman with two months of morning headache and vomiting, whose imaging shows a mass hugging the dura — meningioma. These three battles differ clinically, but all circle the same core question of which neural circuit has been pushed out of balance: movement disorder is an imbalance between the two opposing basal ganglia pathways, headache is the intracranial and extracranial neurovascular system triggered by some mechanism, and a mass lesion is the sheer bulk of tissue squeezing out three clusters of symptoms. The core of Parkinson disease is degeneration of the dopaminergic neurons of the substantia nigra pars compacta; the basal ganglia's two opposing pathways are the direct pathway, which promotes movement, and the indirect pathway, which suppresses it; dopamine normally stimulates D1 along the direct pathway and inhibits D2 along the indirect pathway, keeping movement fluid; once dopamine runs short, facilitation weakens and suppression strengthens, so movement becomes sparse, rigid, and tremulous. The signature TRAP is resting tremor, pill-rolling in character, cogwheel rigidity, bradykinesia — the most diagnostically significant feature — and postural instability in late disease; the pathology is Lewy bodies formed from α-synuclein; dysmetria is cerebellar and does not belong to PD. Receptor specialization dictates the side effects: D3 sits in the reward circuit, so dopamine agonists provoke impulse control disorders such as pathological gambling; levodopa has a short half-life, and prolonged pulsatile stimulation drives aberrant striatal plasticity, producing dyskinesia — so levodopa breeds dyskinesia, agonists breed gambling and hallucinations. Carbon monoxide poisoning causes globus pallidus necrosis and delayed-onset parkinsonism, at a different site from primary PD's substantia nigra — don't confuse them; Huntington disease is a CAG repeat expansion with degeneration of the caudate nucleus, chorea early on but actually diminishing in late disease as it shifts to dystonia; essential tremor is a postural/action tremor, the opposite of PD's resting tremor, transiently improved by alcohol, with propranolol or primidone as first-line; Wilson disease is an ATP7B mutation with copper deposition in the lenticular nucleus, onset before age forty, plus liver disease, plus psychiatric symptoms, plus a Kayser-Fleischer ring, plus low ceruloplasmin, plus high urinary copper, treated with chelation or zinc; valproate causes postural tremor. The four URGE criteria for restless legs syndrome are an urge to move, worse with rest, better with movement, worse in the evening, with pregnancy, iron deficiency, and uremia as high-risk groups; supplement iron once serum ferritin falls below 50; alcohol before bed does not relieve it and instead causes rebound worsening.

Headache is triaged for secondary causes before being split among the primary types. Every letter of the SNOOP red flags has a mechanistic reason: systemic features (S) point to secondary causes with systemic signs such as infection, tumor, or vasculitis; a neurologic deficit (N) points to a structural mass lesion or raised intracranial pressure, because a focal sign requires a localizing lesion; a thunderclap, sudden onset (O) should raise suspicion for SAH, because only a ruptured vessel produces pain that peaks instantly; new onset after age fifty (O) should raise suspicion for temporal arteritis, because GCA favors vessel-wall inflammation in the elderly; a change in pattern or positional quality (P) points to abnormal CSF dynamics, with cough-induced worsening suggesting pressure sensitivity. The mechanisms of the three primary headaches: migraine is activation of the trigeminovascular system plus massive CGRP release causing meningeal vasodilation and inflammation, with aura arising from cortical spreading depression; tension-type is myofascial tension, bilateral pressure, the most common type; cluster headache is dysregulation of the hypothalamic circadian nucleus plus the trigeminal-autonomic reflex, hence unilateral retro-orbital stabbing pain plus ipsilateral lacrimation, conjunctival injection, nasal congestion, and Horner syndrome, plus restless agitation, plus a fixed time of day. For acute moderate-to-severe migraine, first-line is a triptan, a 5-HT1B and 1D agonist that constricts intracranial vessels and inhibits CGRP release, contraindicated in coronary artery disease and uncontrolled hypertension; ergotamine is a potent vasoconstrictor combining α-adrenergic agonism and 5-HT agonism, contraindicated in peripheral vascular disease — note that it is not a β-agonist, since β stimulation instead dilates vessels, a direction the exam frequently reverses; the newer CGRP antagonists and lasmiditan cause no vasoconstriction and are usable in cardiovascular patients. Acute cluster headache is treated with high-flow 100% oxygen plus sumatriptan, oxygen being specifically effective for cluster headache. Among preventive agents, migraine prevention uses propranolol, topiramate, valproate, amitriptyline, and CGRP monoclonal antibodies; carbamazepine is first-line for trigeminal neuralgia and does not prevent migraine; first-line prevention for chronic tension-type headache is amitriptyline; cluster headache prevention is verapamil.

Temporal arteritis is a large- and medium-vessel vasculitis in patients over fifty, with a tender, palpable temporal artery and elevated ESR; give steroids immediately and do not wait for the biopsy, to prevent blindness. Spontaneous intracranial hypotension is a positional headache from a CSF leak, worse standing and relieved lying down, with contrast MRI showing diffuse dural enhancement plus brain sagging, managed with an epidural blood patch. Idiopathic intracranial hypertension is an obese young woman with headache, pulsatile tinnitus, and papilledema, with normal imaging but elevated LP opening pressure, treated with acetazolamide plus weight loss; left untreated it risks blindness, the opposite direction from intracranial hypotension. Medication overuse headache is pre-existing migraine or tension-type headache plus analgesic use more than ten to fifteen days per month; the management is simply to stop the drug. Finally, intracranial tumor symptoms arise from three things: the mass effect raising intracranial pressure — morning headache, vomiting, papilledema, because lying down slows CSF drainage, raises CO2, dilates cerebral vessels, and raises intracranial pressure further; focal destruction causing weakness and aphasia; and cortical irritation causing seizures — which one appears first depends on growth rate and location. Low-grade gliomas actually present with seizures more often than high-grade ones, because they grow slowly and continuously irritate the cortex, while high-grade tumors grow fast and present mainly with focal deficits and raised intracranial pressure, with ring enhancement, central necrosis, and a butterfly pattern crossing the corpus callosum. The most common adult primary tumor is meningioma, at about thirty-seven percent, extra-axial, with a dural tail sign, usually benign, able to enlarge with female sex and pregnancy; glioma is the most common malignant primary tumor; but the most common intracranial tumor overall is actually metastasis — don't conflate the two.

The trap cluster around vestibular schwannoma: its origin is the vestibular nerve, not the cochlear nerve, even though hearing loss is the dominant symptom; bilateral vestibular schwannoma equals NF2; the most common surgical complication is injury to the adjacent facial nerve, not the abducens nerve; small tumors can be treated with radiosurgery as one first-line option, not surgery alone. Pituitary physiology relies on descending dopamine to inhibit prolactin; a sellar tumor compressing the pituitary stalk cuts off that inhibition, so prolactin rises mildly — the stalk effect — while a prolactinoma itself raises it markedly; first-line treatment is the dopamine agonist cabergoline or bromocriptine, which directly shrinks the tumor and lowers prolactin, making it one of the few brain tumors where medication precedes surgery; so a pituitary tumor causing low prolactin is the wrong direction; a macroadenoma can present acutely as pituitary apoplexy — sudden severe headache, vision loss, acute hypopituitarism — an endocrine emergency. The most common source of leptomeningeal metastasis is breast cancer plus leukemia and lymphoma plus lung cancer; don't remember only lymphoma and miss breast cancer. Among the neurocutaneous syndromes, Sturge-Weber syndrome is a port-wine stain in the V1 distribution plus a leptomeningeal angioma plus tram-track cortical calcifications, with seizures in seventy to ninety percent — don't underestimate it; TSC is TSC1 and TSC2 mutations causing mTOR hyperactivation, facial angiofibromas, subependymal nodules, SEGA, and infantile spasms; NF1 is a mutation in the NF1 gene on 17q causing loss of neurofibromin and Ras hyperactivation, with café-au-lait macules, optic glioma, and Lisch nodules clinically; NF2 is loss of merlin on 22q, with bilateral vestibular schwannomas and meningiomas. Childhood and adult brain tumor locations are reversed: children favor the posterior fossa, with pilocytic astrocytoma the most common benign tumor and medulloblastoma the most common malignant tumor, while adults favor the supratentorial compartment. Reason each of the three battlefields through its own mechanistic chain, and differential diagnosis and drug choice will fall into place like dominoes.

The neurology consult service receives five people in one day: a homemaker with increasing difficulty keeping her eyelids open by evening and jaw muscle weakness after chewing, for the past two months — myasthenia gravis; a female student with a cold two weeks ago who today suddenly cannot stand from numb legs, with knee reflexes entirely absent and vital capacity falling — Guillain-Barré syndrome; a female teacher with recurrent episodes over three years — blurred vision in the right eye once, numbness in the left hand once, weakness in the right leg now — the dissemination in time and space of multiple sclerosis; a baby boy with a high fever and a five-minute seizure — a febrile seizure; a baby girl with recurrent head-nodding, hugging-type jerks since four months of age plus hypsarrhythmia — West syndrome. The five stories span five different systems, and immunology, infection, and development are all packed into this chapter. The shared line of inquiry is to ask first at which level the destruction occurs — the endplate, the presynaptic membrane, the muscle itself, the peripheral nerve myelin, the central myelin, the meninges, the brain parenchyma, a suppurating focus in the brain, or immature development — and the level decides the diagnosis, the management, and the prognosis. Normal transmission at the neuromuscular junction happens in two steps: the action potential reaches the presynaptic nerve terminal, opens P/Q-type calcium channels, lets calcium in, and releases ACh vesicles; then ACh binds the nicotinic receptor on the endplate, letting sodium in, depolarizing the muscle, and triggering contraction. So this chain has only two positions that can be attacked: the presynaptic calcium channel, whose attack is LEMS, and the postsynaptic receptor, whose attack is MG. Why does MG tire with use while LEMS strengthens with use? Because the MG problem is too few postsynaptic receptors — each burst of released ACh gets broken down before it can bind enough receptors, so it depletes faster the more it is used; the LEMS problem is a blocked presynaptic calcium channel with intrinsically insufficient ACh release, but repeated stimulation lets calcium accumulate presynaptically, so the more it is used, the more ACh is released and the stronger the muscle becomes — remember: postsynaptic fatigues, presynaptic warms up. MG paired with thymoma and LEMS paired with small cell lung cancer are both essential pairings to memorize; aminoglycosides are contraindicated in MG because they worsen neuromuscular blockade.

Failure of the muscle itself: Duchenne muscular dystrophy is X-linked recessive with complete absence of dystrophin; the boy's calf pseudohypertrophy occurs because dead muscle fibers are replaced by fat and fibrous connective tissue, so the whole calf looks bulky when it is actually filled with fat — that is the origin of the word "pseudo." Gower sign is the child, rising from the floor, pushing on the knees with the hands and climbing up the body as if climbing stairs, because the proximal gluteus maximus and thigh muscles are weak; markedly elevated CK reflects instability of the sarcolemma with massive leakage of intracellular muscle enzymes. Becker is the same gene but with dystrophin partially preserved, giving milder symptoms. DM1 is autosomal dominant with a CTG repeat, predominantly distal weakness, myotonia with difficulty releasing the grip, facial muscle involvement, plus cataracts, diabetes, arrhythmia, and testicular atrophy — "predominantly proximal weakness, commonly with diplopia" is a wrong description. CMT is autosomal dominant, most commonly CMT1, a demyelinating type, with distal muscle atrophy, pes cavus, foot drop, inverted champagne bottle legs, and both motor and sensory involvement; the reason for pes cavus is that the tibialis anterior and gastrocnemius weaken first while the small peroneal muscles retain their tone, and the resulting medial-lateral imbalance pulls the arch of the foot upward. GBS is an autoimmune demyelinating polyradiculoneuropathy whose core mechanism is molecular mimicry: one to three weeks after infection, the antibodies the immune system produces mistake the lipopolysaccharide on the outer membrane of Campylobacter for the glycolipid antigens on the patient's own peripheral nerve myelin, and so turn their weapons on the patient's own myelin; once the myelin strips away, conduction slows and blocks, producing symmetric weakness that ascends from distal to proximal, plus sensory disturbance, plus absent deep tendon reflexes. Why does it start at the feet? Because the peripheral nerve roots there are longest, so the effect shows earliest; why does it ascend? Because the demyelination gradually spreads proximally; the most dangerous feature is respiratory muscle weakness — when the weakness ascends to the phrenic nerve at C3 to C5, respiratory failure follows, the leading cause of death in GBS, so monitoring vital capacity and oxygen saturation is mandatory in the emergency department. The CSF shows albuminocytologic dissociation, with protein raised and cell count normal; treatment is IVIG or plasma exchange, and steroids alone are ineffective; a bone scan is entirely useless in GBS and is a frequent trap answer. The Miller Fisher variant is the triad of ophthalmoplegia plus ataxia plus areflexia, with anti-GQ1b antibody. Why are steroids alone ineffective in GBS but effective in MS? Because in GBS the damage has already occurred and the myelin has already been attacked, so what is needed now is to clear the circulating attacking antibodies with plasma exchange, or neutralize them with IVIG; steroids have no ability to clear antibodies that already exist. MS is a relapsing, acute-attack disease, and high-dose steroids can suppress activated T cells and shorten an attack.

MS is an immune-mediated demyelinating disease of the central nervous system, at its core T cells and B cells crossing the blood-brain barrier to attack oligodendrocytes and myelin; contributing factors include vitamin D deficiency, prior EBV infection, the HLA class II DRB1 allele, and smoking, which is why it favors young women in high-latitude regions. Once demyelination occurs, the signal along that segment is conducted slowly or not at all, so the symptoms are momentary malfunctions of mismatched function: optic neuritis can be the presenting feature; INO is MLF damage; Lhermitte sign is an electric-shock sensation down the spine on neck flexion; the Uhthoff phenomenon is symptoms worsening with a rise in body temperature, because demyelinated nerves are heat-sensitive and conduction failure worsens. The imaging signature is periventricular ovoid plaques seen on FLAIR and T2, called Dawson's fingers, following venous distribution and perpendicular to the lateral ventricles, plus lesions in the spinal cord, optic nerve, and brainstem; new plaques enhance while old plaques do not, and CSF oligoclonal bands are positive. Diagnosis relies on the McDonald criteria, at its core dissemination in time plus dissemination in space. Treatment operates on three levels: acute attacks use high-dose intravenous methylprednisolone pulse therapy to shorten the attack; disease-modifying therapies include interferon-β, glatiramer, teriflunomide, dimethyl fumarate, fingolimod, natalizumab — watch for PML — ocrelizumab, and others; symptomatic treatment uses baclofen for spasticity, amantadine for fatigue, and anticholinergics for the bladder. Cognitive impairment in MS is quite common, about forty to sixty-five percent, not very rare — this is a frequent trap.

Sorting CNS infection by CSF: bacterial meningitis shows sky-high WBC predominantly neutrophils, protein raised, glucose lowered, with first-line empiric therapy of vancomycin plus a third-generation cephalosporin, adding ampicillin over age fifty to cover Listeria; draw blood cultures first, and if there are no focal signs, give antibiotics immediately and follow with the LP — don't delay antibiotics for the sake of the LP. Viral meningitis shows WBC one hundred to one thousand, predominantly lymphocytes, protein mildly raised, glucose normal, managed mainly with supportive care. Herpes simplex virus type 1 encephalitis presents with fever plus altered consciousness plus seizures plus personality change, with the lesion in the medial temporal lobe; give intravenous acyclovir immediately without waiting for the PCR, because untreated mortality reaches seventy percent and remains twenty to thirty percent even after treatment, with every hour of delay worsening the prognosis, and the safety of acyclovir far outweighs the cost of delay. HSV favors the medial temporal lobe and orbitofrontal lobe because the virus normally lies latent in the trigeminal ganglion and, on reactivation, ascends along the ophthalmic division of the trigeminal nerve and the olfactory tract. Tuberculous meningitis is subacute with cranial nerve palsies, lymphocytes, markedly raised protein, markedly lowered glucose, and raised ADA, treated with four-drug anti-tuberculous therapy plus steroids. The most common sources of brain abscess are direct spread from sinusitis, otitis media, and dental abscess, or hematogenous spread from endocarditis and lung abscess; on MRI it shows ring enhancement with central DWI hyperintensity because the pus restricts diffusion, exactly the opposite of GBM's ring enhancement with central DWI hypointensity — a differentiation that is always tested. In pediatrics, cerebral palsy is a non-progressive motor and postural disorder from early brain injury; spastic diplegia comes from prematurity plus periventricular leukomalacia, with the legs worse than the arms, a scissoring gait, intelligence that may be normal, and hearing loss is not a feature; the athetoid type comes from kernicterus and is most commonly associated with hearing impairment, because bilirubin crosses the blood-brain barrier and deposits in the lipid-rich basal ganglia and the cochlear nuclei, both regions attacked together, so movement abnormality and hearing loss appear side by side. Febrile seizure is a seizure triggered by fever in a child aged six months to five years; the simple type, lasting under fifteen minutes, generalized, and not recurring within twenty-four hours, does not damage the brain, does not increase the risk of intellectual disability, and needs no long-term antiepileptic drug — educate the family to keep the child on their side to maintain the airway, not put anything in the mouth, and go to the emergency department if the seizure lasts over five minutes. West syndrome is head-nodding, hugging-type spasms beginning at four to eight months, plus developmental regression, plus EEG hypsarrhythmia — highly disorganized, high-amplitude, multifocal discharges — with diverse causes including TSC, HIE, and cortical dysplasia, treated with ACTH injection or vigabatrin, with vigabatrin first-line in patients with TSC; without timely treatment the prognosis is very poor and it can progress to Lennox-Gastaut syndrome. Among the congenital infections of TORCHeS, CMV is the most common, characterized by periventricular calcification plus hearing loss plus microcephaly plus hepatosplenomegaly plus the blueberry-muffin rash; Toxoplasma is diffuse intracranial calcification plus chorioretinitis plus hydrocephalus; rubella is cataracts plus hearing loss plus PDA. The core of the whole chapter in one line: immunology, infection, and development all come down to first asking the level, then matching it to the diagnosis and the treatment.

Neurology resident Little Du has his notes spread across the whole table in the final week before the exam: the histology of how neurons and glial cells divide labor, the embryology of how the three primary and five secondary brain vesicles correspond, the pharmacology of receptors and second messengers, and the anatomy of which cranial foramen each nerve exits through and which disc herniation compresses which root. A senior resident passes by and says these four dictionaries aren't meant to be crammed — they're meant to help build a map you can look up at any time; spread them out on the table, and every question that comes in, you'll be able to look it up — that is this chapter. Every day a neuron carries out two major projects: it synthesizes large quantities of neurotransmitter receptor and channel proteins, so its nucleolus is unusually large and its rough endoplasmic reticulum piles up into what is seen as the Nissl body; then it must ship these proteins out to the tip of an axon that can grow a meter long, so it needs axonal transport, shipping outward on the anterograde motor kinesin and recycling on the retrograde motor dynein. Three frequent traps collapse automatically: the nucleolus has no membrane, unlike a membrane-bound organelle; the Nissl body is confined to the cell body and dendrites and never enters the axon hillock or the axon; the central chromatolysis that appears after axotomy is the active face of repair being initiated, not a form of death; rabies virus and tetanus toxin travel into the central nervous system by riding retrograde dynein. Rather than memorizing the whole table of glial cells by rote, remember one general rule instead: how many axons one cell wraps decides who provides central myelin and who provides peripheral myelin; who is responsible for sweeping the debris decides who is the phagocyte and who is the one being destroyed. The central oligodendrocyte can wrap multiple axons with one cell, while the peripheral Schwann cell wraps only one segment of one axon with one cell, so the main factory of central myelin is the oligodendrocyte — but it has no cleanup ability of its own, and myelin debris is cleared by microglia together with macrophages, and microglia are the only glial cell derived from the mesodermal monocyte lineage. Astrocytes press their end-feet against the cerebral microvasculature, and that embrace is the third layer of the blood-brain barrier, joining the endothelium's tight junctions and basement membrane to form the three-piece assembly — but the blood-brain barrier is not watertight throughout the entire central nervous system: circumventricular organs, such as the area postrema (the chemoreceptor trigger zone of the vomiting center) and the posterior pituitary, deliberately lack a barrier, so that they can sense toxins in the blood in order to trigger vomiting, and can let hormones be secreted outward. Central injury is cleared by microglia and leaves a glial scar, with very poor axonal regeneration; peripheral injury is cleared by Schwann cells and macrophages, and Schwann cells even personally lay down a Büngner band for the axon to regenerate along — this is the anatomical reason a severed peripheral nerve still has a chance to come back, while a severed central pathway almost never does.

The embryo is a way of folding a sheet of paper: the neural plate folds upward along the midline into the neural fold, and the two sides meet to close into the neural tube; the front end swells into brain vesicles, and the back end elongates into the spinal cord. The three primary vesicles are the prosencephalon, mesencephalon, and rhombencephalon, which further differentiate into the five secondary vesicles: the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon. The two facts most often reversed on exams are these: the cerebellum arises from the rhombencephalon, not the prosencephalon; and the cerebral aqueduct develops from the cavity of the mesencephalic vesicle, so obstruction there is non-communicating hydrocephalus. The neural tube is sealed at both ends by the neuropores, the rostral one around day twenty-five and the caudal one around day twenty-seven to twenty-eight; failure of the rostral neuropore to close produces an anencephalic infant, and failure of the caudal neuropore to close produces spina bifida, mainly caused by folic acid deficiency. Malformations mapped onto the brain vesicles: holoprosencephaly is associated with trisomy 13, maternal diabetes, and the sonic hedgehog pathway; Dandy-Walker malformation is hypoplasia of the cerebellar vermis plus cystic dilation of the fourth ventricle bulging outward; Chiari II malformation is herniation of the cerebellar vermis and medulla through the foramen magnum, pushed downward, associated with myelomeningocele and hydrocephalus. The dorsal alar plate of the brainstem governs sensation and the ventral basal plate governs motor function, and the red nucleus, that midbrain motor-coordination nucleus, arises from the basal plate. The optic vesicle is neuroectoderm bulging outward from either side of the diencephalon; the optic nerve is a central extension, so its myelin is formed by oligodendrocytes and extends only as far as the optic disc. The posterior pituitary arises from a downgrowth of the diencephalon and stores antidiuretic hormone and oxytocin; the anterior pituitary arises from Rathke's pouch, and its residual overgrowth is a craniopharyngioma, hence its suprasellar cystic character, its lining of keratinized squamous epithelium with machinery-oil-like cyst fluid, and its frequent calcification.

Pharmacology reduces every switch to three axes: channels, receptors, and reuptake; every drug acts on one of these three switches, and once the direction is right, the side effects follow automatically. The essence of antiepileptics is to suppress excitation and strengthen inhibition: topiramate works on multiple fronts at once, blocking sodium channels, strengthening GABA, and antagonizing AMPA and kainate receptors — the one thing it does not do is block the glutamate transporter; ethosuximide blocks the thalamic T-type calcium channel and is first-line for absence seizures; baclofen is a GABA-B agonist used to treat spasticity — don't confuse it with the A type. Among opioid analgesics, nalbuphine is a μ partial antagonist plus a κ agonist, and it still relieves pain, just with a ceiling; the true pure antagonists with no analgesic effect are naloxone and naltrexone. Olanzapine causes fewer extrapyramidal symptoms because of its low affinity for dopamine D2 receptors combined with strong serotonin 5-HT2A antagonism, at the cost of metabolic syndrome; risperidone is the exception among the atypical drugs, with D2 blockade remaining strong and a tendency toward hyperprolactinemia; clozapine requires monitoring for agranulocytosis. First-line for attention-deficit hyperactivity disorder is methylphenidate, which inhibits reuptake; pitolisant is a histamine H3 inverse agonist that releases the autoreceptor brake on histaminergic neurons, letting more histamine be released, and treats narcolepsy; ondansetron is a 5-HT3 antagonist used to treat chemotherapy-induced nausea; first-line for antipyresis in viral illness in children is acetaminophen, with aspirin contraindicated to avoid Reye syndrome. Autonomic drugs follow three paths: β receptors run through cAMP; α1 together with the cholinergic M1 and M3 receptors run through calcium; α2 and M2 inhibit cAMP; cGMP is not a signal that belongs to β. Bladder contraction and relaxation: the storage phase relies on β3 relaxing the detrusor plus α1 contracting the sphincter, the voiding phase relies on M3 contracting the detrusor; overactive bladder is treated with an M3 antagonist or a β3 agonist, and difficulty voiding from prostatic hyperplasia is treated with an α1 antagonist. In ophthalmology, bimatoprost lowers pressure most strongly, timolol is contraindicated in asthma, and mydriasis with cycloplegia uses an anticholinergic, not a sympathomimetic. In the respiratory tract, albuterol is for acute asthma rescue, tiotropium is a long-acting muscarinic antagonist for COPD maintenance, and cromolyn is for prevention, not rescue.

Anatomical localization by origin site: norepinephrine in the locus coeruleus, serotonin in the raphe nuclei, dopamine in the substantia nigra and ventral tegmental area, acetylcholine in the nucleus basalis of Meynert, histamine in the tuberomammillary nucleus; epinephrine itself is never the central nervous system's major source. The corpus striatum equals the caudate nucleus plus the putamen plus the globus pallidus, while the striatum proper is only the caudate plus the putamen; the substantia nigra and the subthalamic nucleus belong to the basal ganglia circuit but are not inside the striatum; the limbic system contains the amygdala, hippocampus, cingulate gyrus, hypothalamus, mammillary body, olfactory bulb, anterior thalamic nucleus, and fornix, and neither the dentate nucleus nor the substantia nigra belongs to it. The thalamus has two sensory relay stations: hearing runs through the medial geniculate body, vision through the lateral geniculate body, and both geniculate bodies belong to the thalamus, not the midbrain. Conjugate eye movement is a beautiful mirror-image question: a cortical lesion makes the eyes look toward the affected side, a brainstem lesion makes the eyes look toward the healthy side, because the job of the frontal eye field is to push the eyes to the opposite side, so destruction lets the opposite side push the eyes over, making them appear to deviate toward the lesion. The circuit of the corneal reflex runs in through the first division of the trigeminal nerve and out through the facial nerve; the trochlear nerve is the only one of the twelve cranial nerves that exits from the dorsal brainstem and decussates to the opposite side. The back has four layers from superficial to deep: trapezius plus latissimus dorsi, then the middle layer of the posterior serratus driving respiration, then the erector spinae, and deepest of all the suboccipital muscle group. Dermatomes to memorize: the nipple line at T4, the xiphoid process at T6 to 7, the umbilicus at T10, the groin at L1. A lumbar paracentral disc herniation compresses the traversing root of the level below, so an L4-to-L5 herniation compresses L5 and an L5-to-S1 herniation compresses S1; only a far lateral herniation compresses the exiting root of the level above. The anterior longitudinal ligament runs anterior to the vertebral bodies, the posterior longitudinal ligament runs inside the spinal canal and continues upward as the tectorial membrane; hypertrophy of the ligamentum flavum squeezes the spinal canal into stenosis. Dorsal rootlets are purely sensory and ventral rootlets are purely motor, while the anterior and posterior rami of the combined spinal nerve are both mixed. The atlanto-occipital joint performs nodding, the atlanto-axial joint performs head rotation, and the alar ligament connects the dens to the occipital condyles. A few main lines among the cranial exit foramina: the ophthalmic artery travels through the optic canal, the second division of the trigeminal nerve through the foramen rotundum, the third division through the foramen ovale, the auditory nerve through the internal acoustic meatus, and the facial nerve exits the skull through the stylomastoid foramen. Tongue muscles: protrusion relies on the genioglossus, and apart from palatoglossus, which runs on the vagus nerve, every other tongue muscle runs on the hypoglossal nerve (CN XII); with a unilateral hypoglossal palsy, the protruded tongue deviates toward the affected side. Sensation of the tongue: the anterior two-thirds carries general sensation through the third division of the trigeminal nerve and taste through the chorda tympani branch of the facial nerve, the posterior one-third carries both general sensation and taste through the glossopharyngeal nerve, and the epiglottic region runs through the vagus nerve. Pharyngeal muscles: among the longitudinal elevator muscles, stylopharyngeus is the only one innervated by the glossopharyngeal nerve; the constrictor muscles are responsible for pushing food downward and are entirely innervated by the vagus nerve; so vagus nerve injury commonly causes aspiration pneumonia, because once that one nerve fails, the soft palate, the vocal folds, and the pharyngeal constrictors all fail together. The infrahyoid strap muscles governed by the ansa cervicalis are the sternohyoid, sternothyroid, omohyoid, and thyrohyoid; among these, the geniohyoid carries C1 fibers riding on the hypoglossal nerve and does not belong to the ansa cervicalis, while stylohyoid is innervated by the facial nerve. The Vidian nerve equals the greater petrosal plus the deep petrosal; the greater petrosal comes from the facial nerve's parasympathetic fibers, the deep petrosal comes from the sympathetic plexus; the lesser petrosal nerve comes from the glossopharyngeal nerve and is not part of the Vidian nerve. The four parasympathetic ganglia: the ciliary ganglion runs on the oculomotor nerve, the pterygopalatine and submandibular ganglia run on the facial nerve, and the otic ganglion runs on the glossopharyngeal nerve. The dorsal scapular nerve arises from C5, pierces the middle scalene, and innervates the rhomboids and levator scapulae; injury to the long thoracic nerve causes winging of the scapula. Hold onto these four dictionaries through the whole chapter, and any question you meet can be looked up like turning the pages of a dictionary.