Rheumatology

The civil war of the immune system is hard to read because the enemy is the self. A twenty-four-year-old female student comes in with a butterfly rash and joint pain; the resident checks an antinuclear antibody, finds it positive, and relaxes, ready to write the diagnosis — but the attending shakes his head and says that is only the ticket, not admission yet. The whole set of rheumatology and immunology antibodies looks like rote pairing, but every antibody has something real that it is attacking, and once you understand that target, the pairing forms in your mind on its own, with no need to force-memorize it. The double-stranded DNA antibody deposits with immune complexes into the glomerulus, so in lupus it rises and falls together with disease activity and nephritis; the Smith antibody attacks the small nuclear ribonucleoproteins inside the spliceosome — highest specificity, unfortunately low sensitivity, but once positive it is almost a signature of lupus; the histone antibody arises because certain drugs remodel the antigenicity of histones, and the immune system treats those remodeled histones as strangers, so stopping the drug resolves it.

The mechanism of lupus can be broken into five steps: genetic susceptibility comes first, then failure of self-tolerance, then T-cell and B-cell dysregulation combined with poor clearance of apoptotic cells, which leads to a set of antibodies against nuclear proteins and chromatin, and finally immune complexes deposit into the glomerulus, skin, serosa, and CNS, causing multisystem damage. The diagnostic logic is therefore likewise three-staged: first screen with the sensitive antinuclear antibody, whose sensitivity exceeds ninety-five percent so that almost every lupus patient is positive — its value lies in a negative result excluding the diagnosis, but its specificity is low, since healthy people and other autoimmune diseases can also be positive, which is why treating it as the answer leads you astray. The next step adds double-stranded DNA and Smith antibodies; the 2019 joint European-American criteria fixed this chain into a rule, with the entry criterion being an antinuclear antibody of 1:80 or higher, followed by weighted scoring across seven clinical domains and three immunologic domains, and only a total score of ten or more with at least one clinical criterion classifies the patient as lupus. Complement is consumed and falls during active disease as immune complexes use it up, which is why it serves as a marker of activity; but C-reactive protein is often not elevated in uncomplicated lupus, so if it rises markedly, one should first suspect concurrent infection or serositis rather than assume the disease itself has worsened. The cornerstone of treatment is the quinine-class drug hydroxychloroquine, which is not a symptom-relief agent but the backbone that changes the disease course — it lowers relapse, reduces organ damage, and improves survival, though long-term high-dose use causes retinopathy, so periodic fundoscopy is required; acute flares are controlled with corticosteroids, and for severe organ involvement such as proliferative lupus nephritis or CNS disease, induction requires cyclophosphamide or mycophenolate mofetil.

Drug-induced lupus is a clean branch off this main line. The usual culprit drugs form a mnemonic called HIP — hydralazine, isoniazid, procainamide — plus minocycline, anti-TNF agents, quinidine, and others. The mechanism is that in people with a slow-acetylator phenotype, these drugs remodel the antigenicity of histones, and the immune system treats these remodeled histones as foreign, producing anti-histone antibodies; but this reaction stays shallow and never reaches the double-stranded DNA and Smith layer, and complement is not consumed, so the kidneys and central nervous system are left alone. The differentiating table makes sense once you follow the mechanism: drug-induced disease shows a positive histone antibody while double-stranded DNA and Smith are negative, complement is normal, and the sex difference is not pronounced. The iron rule of management is to stop the drug — treating it like primary lupus with long-term immunosuppression is therapeutic violence; at most, severe symptoms are carried through with a short course of anti-inflammatories or low-dose steroids. Each connective tissue disease is triaged by a different antibody, because what the antibody attacks determines what the disease looks like. Mixed connective tissue disease is the nothing-quite-fits overlap of lupus, sclerosis, and myositis, with Raynaud phenomenon and puffy hands as its signature, and the hard evidence is a high titer of the U1 ribonucleoprotein antibody. Systemic sclerosis splits into two paths by centromere and topoisomerase antibodies: the centromere antibody tracks limited sclerosis, namely CREST, where sclerosis stays confined to the fingers and face, with the five-piece set being calcinosis, Raynaud phenomenon, esophageal dysmotility, sclerodactyly, and telangiectasia; the topoisomerase type I antibody tracks diffuse disease, with widespread skin sclerosis plus pulmonary fibrosis; the RNA polymerase type III antibody likewise maps to diffuse disease, but should raise alertness for renal crisis and malignancy. Sjögren's syndrome attacks the exocrine glands, hence the dry eyes and dry mouth, with Ro and La as its antibodies; the Ro antibody is small enough to cross the placenta, so the mother may have nothing worse than dryness while the fetus can develop congenital heart block because its conduction system is struck — a detail the exam loves to bury; Sjögren's extraglandular manifestations, such as vasculitis and interstitial nephritis, require corticosteroids plus immunosuppression, not simply artificial tears. Behind the Jo-1 antibody of polymyositis and dermatomyositis lies antisynthetase syndrome, in which myositis, interstitial lung disease, mechanic's hands, arthritis, and Raynaud phenomenon all arrive together as a set.

Rheumatic fever is another story of the self mistaken for the enemy, and its five-step causal chain unfolds like this: after group A β-hemolytic streptococcal pharyngitis, the bacterium's M protein happens to resemble antigens in human myocardium, valves, and joint synovium; the antibodies the immune system manufactures engage in molecular mimicry, and while attacking the bacteria they also strike the body's own valves — a textbook case of type II hypersensitivity. It therefore strikes two to four weeks after the pharyngitis, because time is needed to manufacture the antibodies. The Jones major criteria include carditis, migratory polyarthritis, Sydenham chorea, erythema marginatum, and subcutaneous nodules, and the most classic long-term sequela is mitral stenosis. The exam loves asking whether the mechanism is molecular mimicry or immune complex deposition — remember that rheumatic fever is molecular mimicry, while immune complex deposition belongs to the stories of IgA vasculitis, cryoglobulinemia, and lupus nephritis. Finally, two immune exceptions that keep sneaking in deserve special emphasis: leukocyte adhesion deficiency is an integrin defect in which neutrophils lose the ability to adhere to the vessel wall and extravasate, so the circulating neutrophil count actually rises, wounds fail to form pus, and the umbilical cord separates late; most autoimmune diseases favor women over men, but type 1 diabetes affects men and women similarly and ankylosing spondylitis is more common in men than women — these two are frequently tested reversal traps. Hold onto the core question of what the antibody is attacking throughout this chapter, and the pairings will grow on their own once you understand it.

A forty-two-year-old woman has had whole-body aching for more than six months, wakes up groggy no matter how much she sleeps, cannot concentrate, and tosses and turns at night; she has been to three hospitals where blood work, imaging, and antibodies all came back normal, the first two doctors suspected she was faking it, and the third gave her sleeping pills, resolving nothing. The group most often misunderstood in rheumatology and immunology is patients with fibromyalgia, and the exam's core question is always the same sentence: this is a disease in which the central nervous system amplifies the pain signal, not a disease in which tissue is inflamed. Once this axis is accepted, every seemingly contradictory clue becomes self-evident.

The five-step mechanism chain runs as follows: genetic susceptibility and a stress trigger come first, then insufficient serotonin- and norepinephrine-mediated inhibition along the descending inhibitory pathways, which leads to hyperexcitable dorsal horn neurons and elevated substance P, producing central sensitization and a lowered pain threshold, and finally presenting as widespread body-wide pain plus fatigue, poor sleep, and cognitive impairment. This path is therefore one of abnormal signal processing rather than tissue destruction. The joints are not being gnawed and the muscles are not under attack, so blood work naturally shows no evidence of inflammation whatsoever — C-reactive protein and ESR are normal, autoantibodies are negative, imaging shows no destruction; the pain in this disease is real, but the inflammation is not. Once this axis is established, the differentiating table is just the same sentence written a different way: when the question gives whole-body aching plus every marker normal plus negative antibodies plus normal imaging, the answer is fibromyalgia; the moment an inflammatory marker rises, think toward a genuine inflammatory rheumatic disease instead, since rheumatoid arthritis, lupus, ankylosing spondylitis, and polymyalgia rheumatica each have their own pattern of elevation and corresponding antibody or gene. Polymyalgia rheumatica deserves special note: age over fifty, shoulder-and-hip-girdle pain, ESR often spiking above fifty, and a dramatic response to low-dose steroids — a completely different scent from fibromyalgia's normal markers.

Diagnosis is a scale-based clinical judgment, resting on neither blood work nor imaging. The 2010 and 2016 American College of Rheumatology criteria rely on two scales: the Widespread Pain Index scores the number of painful sites over the past week, and the Symptom Severity Scale scores fatigue, unrefreshing sleep, cognitive symptoms, and the severity of somatic symptoms, plus a symptom duration of at least three months with other explanatory diseases excluded. One conceptual update worth remembering: the old eighteen tender points have been replaced by the questionnaire-based scales and are no longer a rigid threshold; the 2016 version further requires that the pain be widely distributed, to avoid misclassifying localized pain as fibromyalgia. Every test in this disease plays only the role of ruling out something else, never of confirming it: C-reactive protein and ESR rule out inflammatory disease, rheumatoid factor and anti-cyclic citrullinated peptide antibody screen for rheumatoid arthritis, cervical and lumbar MRI rule out nerve root compression and myelopathy, thyroid-stimulating hormone rules out hypothyroidism, and hormone panels are unrelated to the diagnosis. In other words, fibromyalgia has no positive confirmatory test — every blood draw and image exists to rule out something else, not to detect it, and treating rheumatoid factor as the gold standard for fibromyalgia is using a screening tool in the wrong place.

Treatment also follows the mechanism. Because the core problem is central sensitization rather than inflammation, the main approach is non-pharmacologic therapy — exercise and patient education — as first line; the drugs chosen modulate central pain processing: the antidepressants duloxetine and amitriptyline, the anticonvulsants pregabalin and gabapentin — what they share is that they directly modulate the central pain-processing circuit rather than suppressing inflammation; conversely, anti-inflammatory analgesics and steroids are essentially useless, because they act on inflammation and the lesion here is never inflammation to begin with. This is a frequently tested reversal trap: the question describes fibromyalgia, and the options offer anti-inflammatory analgesics and low-dose steroids — that is the wrong direction. Strung together, this whole chapter is really just different faces of the same principle: an amplified signal is not the same as damaged tissue, so normal inflammatory markers are no contradiction, negative antibodies are no contradiction, and unremarkable imaging is no contradiction; diagnosis rests on scales, treatment targets the center — once this causal chain clicks, seeing whole-body aching plus normal markers tells you at once which direction to head.

The old man woken at midnight by pain is sixty-seven, never misses sorghum liquor and organ meats at a meal, and recently started a diuretic for hypertension; at four this morning the big toe on his left foot is red, swollen, and shiny, screaming at the lightest touch. Next door, the grandmother's knee suddenly swelled up, with a thin white line of cartilage calcification on X-ray. Two joints that are both acutely red, swollen, hot, and painful — one crystal a needle, the other a rhombus — and the moment the aspirated joint fluid is viewed under polarized light, the two stories fork apart. The gold standard for diagnosis is polarized-light microscopy of joint fluid showing crystals engulfed by white cells; relying on serum uric acid or X-ray alone leads you astray.

The mechanism of gout can be broken into five steps: hyperuricemia, usually the under-excretion type, comes first; monosodium urate crystals precipitate in the joint space; the crystals are engulfed by macrophages, activating the NLRP3 inflammasome; IL-1β is released; and finally massive neutrophil infiltration produces redness, swelling, heat, and pain — once this chain clicks, it is clear why colchicine and IL-1 inhibitors can put out the fire. The gout crystal is monosodium urate, needle-shaped, with strong negative birefringence: when the crystal's long axis lies parallel to the slow axis of the red compensator it appears yellow, and blue when perpendicular; the pseudogout crystal is calcium pyrophosphate, rhomboid, with weak positive birefringence — blue when parallel and yellow when perpendicular, exactly the reverse — yet downstream it runs through the same NLRP3 pathway, so colchicine is effective for both in treatment. The favored sites are also cleanly divided: gout favors the big toe, pseudogout favors the knee, and on X-ray pseudogout shows a thin white line of cartilage calcification. Memorize the eight words needle, negative, yellow, big toe against rhomboid, positive, blue, knee, and the polarized-light question becomes a coin flip you can't lose.

Timing logic is the heart of the question. Why can't a urate-lowering drug be started during an acute flare? Because the essence of acute inflammation is a fire lit by crystals being engulfed by white cells inside the joint space and detonating; if you give a xanthine oxidase inhibitor at this point and crash the serum uric acid, it will destabilize and dissolve the urate crystals already deposited in tissue, which paradoxically triggers or worsens the flare — whether uric acid rises or falls, any sharp swing in either direction lights the fire. The iron rule is therefore that the acute phase uses only anti-inflammatory analgesia, colchicine, or corticosteroids to put out the fire, and urate-lowering therapy begins only two to four weeks after remission, with colchicine co-prescribed at the start to prevent recurrence. A patient already on a urate-lowering drug who has an acute flare should not stop it, because stopping likewise causes a swing in uric acid. Two counterintuitive but frequently tested points deserve emphasis: serum uric acid can fall within the normal range during an acute flare, because uric acid excretion increases during inflammation and the crystals were already deposited in the joint, so a normal serum uric acid cannot exclude gout; and asymptomatic hyperuricemia is, as a rule, not routinely treated with urate-lowering therapy — if it is only elevated with no flare and no stones, there is no need to start medication right away. The common drugs that affect uric acid all work through excretion: diuretics reduce excretion and raise uric acid, low-dose aspirin also inhibits excretion and raises it, while losartan also promotes excretion and therefore lowers it slightly, making it a preferred choice for hypertension with comorbid gout.

The last piece of the puzzle is pharmacogenomics: allopurinol can trigger severe cutaneous adverse reactions such as Stevens-Johnson syndrome or toxic epidermal necrolysis, and the risk rises sharply in patients positive for HLA-B*58:01; the carrier rate is high in the Han Chinese Taiwanese population, so screening for HLA-B*58:01 before starting the drug is required, with another agent substituted if positive. A paired mnemonic: carbamazepine corresponds to HLA-B*15:02, and both lead to severe skin-necrosis reactions. Hold onto the main line of crystals activating NLRP3 and IL-1β throughout this chapter, together with the iron rule of leaving uric acid alone during the acute phase, and the question becomes a coin flip you can't lose.

The axis of spondyloarthritis is a story of the immune system attacking the wrong place in a different organ. Its primary lesion sits at the point where tendon and ligament attach to bone, not rheumatoid arthritis's synovium and not degenerative cartilage. The causal chain strings together in five steps: first the susceptible HLA-B27 constitution plus a trigger from gut flora or mechanical stress, then local immune activation at the enthesis, switching on the Th17 axis of IL-23 plus IL-17, producing enthesitis; after the inflammation, the body launches repair, and excessive repair means ossification, so bone spurs and bony bridges eventually grow and cause stiffness. Once this mechanism clicks, the bamboo spine, the enthesis pain, why treatment calls for continuous rather than as-needed anti-inflammatory analgesics, and why IL-17 inhibitors work all make sense. Grasp the family's shared features as a five-piece set: enthesitis at the Achilles tendon and plantar fascia; axial involvement whose inflammatory low back pain is worse with rest, better with activity, with morning stiffness over thirty minutes and pain that wakes the patient at night; an HLA-B27 association with a negative rheumatoid factor; dactylitis presenting as sausage digits; and extra-articular features of uveitis, psoriasis, and inflammatory bowel disease.

Ankylosing spondylitis favors men aged twenty to thirty, with a male-to-female ratio of roughly two or three to one; the B27 positivity rate is about ninety percent, but only a minority of carriers ever develop the disease, so B27 alone cannot make the diagnosis. The clinical reasoning is that the sacroiliac joints become inflamed first, followed by ascending spinal stiffness, ligamentous ossification growing bony bridges, and an X-ray that shows a bamboo spine. The imaging sequence is highly instructive: while X-ray still shows nothing, MRI can already reveal sacroiliac bone marrow edema, which is why the modern classification labels patients who already show structural damage on X-ray as radiographic axial disease, and those whose X-ray remains normal but whose MRI already shows inflammation as non-radiographic axial disease — the two are simply the early and late ends of the same disease spectrum, and MRI can catch early patients before X-ray ever can. The most common extra-articular finding is acute anterior uveitis, or iritis; don't forget aortic regurgitation and upper-lobe pulmonary fibrosis. The treatment ladder has two points worth memorizing cold: first line is anti-inflammatory analgesics plus exercise and physical therapy, and continuous use suppresses ossification better than as-needed use; only after failure do you switch to a biologic such as an anti-TNF agent or an IL-17 inhibitor such as secukinumab or ixekizumab, and only after that fails do you move to a JAK inhibitor. Sulfasalazine works only on peripheral joints and is useless for axial disease, conventional disease-modifying antirheumatic drugs are equally powerless against axial disease, and oral corticosteroids likewise perform poorly for axial disease — the complete opposite of rheumatoid arthritis, and a frequently tested reversal trap.

Reactive arthritis is another family member likewise linked to B27; its mechanism is sterile arthritis triggered one to four weeks after a distant gastrointestinal or genitourinary infection, with no organism found in the joint space — it is a post-infectious immune reaction, not the infection itself. The classic triad is arthritis plus conjunctivitis plus urethritis; the triggering organisms include Chlamydia for the genitourinary tract and Shigella, Salmonella, Yersinia, and Campylobacter for the gut, with Clostridium not a trigger — a high-frequency trap. Chlamydia causes reactive arthritis rather than infective endocarditis, which is also a common decoy. The other HLA associations must be kept straight: psoriasis itself corresponds to Cw6, the spine corresponds to B27, and rheumatoid arthritis is the one tested with DR4. Psoriatic arthritis can involve the distal interphalangeal joints, sausage digits, a pencil-in-cup deformity, and nail pitting. Inflammatory bowel disease-associated arthritis, meanwhile, has peripheral joint activity that tracks bowel inflammation activity. Hold onto the core idea of the enthesis as the primary lesion throughout this chapter, and everything from the imaging sequence to the choice of biologic follows naturally from it.

A referral letter to the clinic describes a seventy-six-year-old woman with new-onset temporal headache, jaw pain that stops her chewing halfway through, ESR spiking to ninety-five, and a left eye that hasn't been seeing too well lately; the resident is about to schedule a temporal artery biopsy, but the attending simply writes immediate high-dose corticosteroids and adds that the biopsy can wait. Vasculitis is a disease in which the immune system attacks the vessel wall, and the exam's skeleton is very simple: first sort by vessel caliber into large, medium, and small, then use ANCA subtype to pin down the small-vessel vasculitides. The large vessels are represented by giant cell arteritis and Takayasu arteritis — giant cell arteritis is the story of an elderly patient, the temporal artery, and vision, while Takayasu arteritis is the story of a young Asian woman with absent pulses. The medium vessels are represented by polyarteritis nodosa and Kawasaki disease — polyarteritis nodosa is often associated with hepatitis B, spares the lung, and shows beaded microaneurysms on angiography, while Kawasaki disease causes coronary artery aneurysms in children. The small vessels split into two camps: the ANCA-associated group of granulomatosis with polyangiitis, microscopic polyangiitis, and eosinophilic granulomatosis with polyangiitis, and the immune-complex-driven group of IgA vasculitis and cryoglobulinemic vasculitis, the latter often associated with hepatitis C.

The five-step mechanism of ANCA-associated vasculitis runs as follows: first, autoantibodies form against the neutrophil cytoplasmic enzymes PR3 or MPO; infection or environmental stimuli make neutrophils express these antigens on their surface; the moment ANCA binds, the neutrophils become hyperactivated, degranulate, and release NETs, attacking the small-vessel endothelium, and finally producing necrotizing vasculitis and necrotizing glomerulonephritis. It is therefore autoimmune, a different group from the autoinflammatory diseases of innate immune or inflammasome dysregulation, such as familial Mediterranean fever; the exam loves labeling granulomatosis with polyangiitis as autoinflammatory as a decoy, and remembering that it is autoimmune keeps you from being fooled. The three small-vessel vasculitides are split apart by whether the antigen is PR3 or MPO and whether the ANCA is c or p. Granulomatosis with polyangiitis, formerly called Wegener's, has the highest c-ANCA positivity rate with PR3 as its target; its clinical iron triad is upper-airway involvement such as sinusitis and saddle nose, lower-airway pulmonary nodules and cavities, and renal necrotizing glomerulonephritis, with granulomas seen on histology. Microscopic polyangiitis is p-ANCA with MPO as its target, likewise a pulmonary-renal syndrome but without granulomas and without upper-airway destruction. Eosinophilic granulomatosis with polyangiitis, or Churg-Strauss, is p-ANCA with MPO as its target, but ANCA positivity is only about thirty to forty percent, and its clinical iron triad is asthma, eosinophilia above ten percent, and vasculitis. The classic question gives recurrent asthma, palpable purpura, and twenty-seven percent eosinophils; the most specific test is ANCA, not total IgE and not ANA — but remember one trap: because positivity is only thirty to forty percent, a negative ANCA cannot exclude this disease, and diagnosis then still rests on the asthma history, eosinophilia, and a tissue biopsy showing necrotizing vasculitis with eosinophilic infiltration. As a treatment principle, severe disease with organ threat — kidney, heart, nerve, or pulmonary hemorrhage — has its remission induced with high-dose corticosteroids plus cyclophosphamide or rituximab, with azathioprine, rituximab, or methotrexate for maintenance; non-severe eosinophilic granulomatosis can add mepolizumab, an anti-IL-5 agent, to control eosinophils and spare corticosteroids.

Giant cell arteritis is the representative large-vessel disease, favoring patients over fifty and often over seventy, frequently associated with polymyalgia rheumatica. Its four clinical hallmarks are new-onset temporal headache, scalp tenderness, jaw claudication, and vision loss, namely anterior ischemic optic neuropathy, with markedly elevated ESR. The iron rule of management is to give high-dose corticosteroids immediately on high suspicion, without waiting for the biopsy result; the logic behind this sequence is that once the optic nerve becomes necrotic from ischemia the damage is irreversible, and waiting a day or two for the biopsy could leave the patient blind, while the biopsy still retains diagnostic value for several days after corticosteroids are started, because the granulomatous changes in the vessel wall do not disappear right away — so the eye is saved first and the diagnosis sought second, rather than confirming first and treating second. The newer drug tocilizumab is an IL-6 receptor antagonist that can serve as a steroid-sparing agent, reducing relapse and cumulative steroid dose, a recently and frequently tested update. Two easily confused miscellaneous conditions deserve attention: IgG4-related disease involves infiltration by IgG4-positive plasma cells, storiform fibrosis, and obliterative phlebitis, and can involve the periorbital region such as the lacrimal gland and orbit, the salivary glands, the pancreas causing autoimmune pancreatitis, and the retroperitoneum, often associated with an atopic constitution, eosinophilia, and elevated serum IgG4. Hereditary angioedema arises from C1 inhibitor deficiency or dysfunction, leading to bradykinin accumulation, recurrent non-pruritic subcutaneous and mucosal edema, and potential laryngeal edema; the screening clue is a persistently low C4, and total IgE having no diagnostic value is a trap — antihistamines, corticosteroids, and epinephrine are all ineffective, requiring a C1 inhibitor concentrate or a bradykinin or kallikrein inhibitor instead. The contrast worth writing in red is that hereditary angioedema is bradykinin edema — it doesn't itch, doesn't depend on IgE, and doesn't respond to antihistamines; check C4 and C1 inhibitor — utterly unlike the histamine-driven, itchy edema of urticaria, from mechanism all the way to treatment.

Finally, polyarteritis nodosa deserves one frequently tested trap called out explicitly: it is a medium-vessel disease, often associated with hepatitis B, spares the lung, ANCA is usually negative, and angiography shows beaded microaneurysms; its renal lesion is renovascular disease of the renal artery branches, such as infarction, microaneurysms, and renovascular hypertension, and it typically does not cause glomerulonephritis — glomerulonephritis is a hallmark reserved for GPA and MPA among the ANCA-associated vasculitides, and mistaking polyarteritis nodosa for an ANCA-associated vasculitis is a high-frequency wrong answer. This whole chapter is really just one causal chain unfolding across different calibers and different antigens: the immune system manufactures a mistaken antibody, or a group of wrongly activated cells, that goes to war against the vessel wall, and the clinical picture then follows from the caliber and location of the vessel under attack. From the temporal artery to the sinuses, from the lung to the kidney, from skin purpura to laryngeal edema, every clinical clue traces back to some segment of vessel — hold the caliber-and-ANCA axis firmly in mind, and the questions sweep past with ease.

A forty-eight-year-old woman has symmetric swelling of the metacarpophalangeal joints of both index and middle fingers, morning stiffness lasting more than two hours, both ESR and C-reactive protein elevated, and a strongly positive anti-cyclic citrullinated peptide antibody; she says she is too tired even to carry her dinner plate, and lately she has also been short of breath, with the chest film showing a small pleural effusion. The core causal chain of rheumatoid arthritis strings together in five steps: first the genetic HLA-DR4 shared epitope plus environmental factors such as smoking or periodontal bacteria, then proteins are citrullinated, producing the anti-cyclic citrullinated peptide antibody and rheumatoid factor, the synovium is activated and infiltrated by CD4 T cells and B cells, synovial proliferation forms pannus that releases matrix metalloproteinases, RANKL, TNF, and IL-6, and finally bone erosion is gnawed out from the inside, starting at the joint margin. The key imaging finding is therefore marginal bone erosion rather than bone spurs; long-standing disease deforms into ulnar deviation, swan-neck deformity, and boutonnière deformity.

Serologically, rheumatoid arthritis requires distinguishing sensitive from specific: rheumatoid factor is sensitive but not specific, since Sjögren's and hepatitis C can also be positive, while the anti-cyclic citrullinated peptide antibody is the most specific antibody and can also predict erosion and prognosis; the strongest genetic association is the HLA-DR4 shared epitope, the signature gene of rheumatoid arthritis, not to be confused with ankylosing spondylitis's B27 or psoriasis's Cw6. In its course, rheumatoid arthritis is predominantly symmetric and polyarticular but can begin as a single joint early on, a frequently tested reversal trap — do not rigidly exclude a monoarticular onset. In terms of joint distribution, rheumatoid arthritis involves the metacarpophalangeal joints, proximal interphalangeal joints, and wrist in a symmetric polyarticular pattern, usually sparing the distal interphalangeal joints; involvement there should suggest concurrent degenerative disease. Extra-articular manifestations fall along two axes of frequency and specificity: the most common is osteoporosis, produced by the triple blow of inflammatory cytokines, corticosteroids, and reduced activity, the invisible killer underlying rheumatoid arthritis as a whole; the most specific are the rheumatoid nodule, found on extensor surfaces and associated with high-titer rheumatoid factor, and Felty syndrome, the three-piece set of rheumatoid arthritis plus splenomegaly plus neutropenia — forgetting the low blood count costs you points.

Pulmonary manifestations include interstitial lung disease and pleural effusion, with the pleural effusion carrying a signature feature of extremely low glucose; ocular involvement is secondary Sjögren's plus scleritis; Caplan syndrome is rheumatoid arthritis plus pneumoconiosis such as silicosis or coal-worker's disease, producing multiple pulmonary nodules; cardiovascular disease represents accelerated atherosclerosis driven by chronic inflammation and is rheumatoid arthritis's leading cause of death, not joint destruction itself. One high-frequency safety point is C1–C2 atlantoaxial subluxation: rheumatoid arthritis's chronic inflammation destroys the ligaments between C1 and C2, loosening this vertebral segment relative to the odontoid process, and if the neck is hyperextended during intubation it can compress the spinal cord or medulla with catastrophic results — so rheumatoid arthritis patients must have their cervical spine assessed before intubation for general anesthesia, a consensus shared by anesthesiology and rheumatology that is tested without fail every time.

The iron rule of treatment is early and aggressive management aiming for remission or low disease activity, since delaying treatment is the same as letting the pannus gnaw away at bone unchecked. Symptom bridging uses anti-inflammatory analgesics or a short course of low-dose corticosteroids, but this does not alter the disease course and must not be used alone long-term; the cornerstone first-line disease-modifying antirheumatic drug is methotrexate, co-administered with folate to reduce side effects, and contraindicated in pregnancy because it is teratogenic; when methotrexate fails, biologics such as an anti-TNF agent, an IL-6 inhibitor such as tocilizumab, abatacept, or rituximab are used, or else a JAK inhibitor. Medication in pregnancy calls for switching sides rather than stopping treatment altogether, since stopping everything would let the disease flare massively and harm the fetus and mother instead; the teratogenic drugs that must be stopped are methotrexate and leflunomide, with leflunomide additionally requiring a cholestyramine washout to be cleared out completely, switching instead to the pregnancy-safe quinine-class hydroxychloroquine and sulfasalazine with folate supplementation, adding the Fc-fragment-truncated certolizumab if needed, with low-dose corticosteroids also considered safe. Hold onto the main line of the synovium gnawing bone throughout this chapter, and everything from imaging, blood work, and extra-articular disease to medication in pregnancy follows naturally from it.

Next into the clinic comes a seventy-year-old woman with hard Heberden's nodes at her distal interphalangeal joints, knee pain when climbing stairs that worsens the longer she is active, and a C-reactive protein that comes back completely normal; she says she played volleyball for twenty years when she was young. The core sentence for osteoarthritis is that it is mechanical wear of cartilage plus failed repair, not an immune attack. The five-step mechanism strings together like this: mechanical loading and aging come first, chondrocytes release matrix metalloproteinases and ADAMTS to degrade collagen and proteoglycans, cartilage thins and fissures, the subchondral bone is forced to sclerose and grows osteophytes and cysts at its margins, and finally the disease presents as mechanical pain and joint deformity. Systemic inflammatory markers are therefore usually normal, which is exactly the root reason the two diseases sit at opposite poles on both imaging and blood work; trap questions often describe the mechanism as cartilage simply being worn away, but the correct answer is to remember that matrix metalloproteinases enzymatically degrade the cartilage, not pure physical abrasion.

Osteoarthritis's clinical features and risk factors tell the classic story of mechanical pain plus reparative bone overgrowth: pain with activity, relief with rest, morning stiffness under thirty minutes, favoring Heberden's nodes at the hand's distal interphalangeal joints, Bouchard's nodes at the proximal interphalangeal joints, the first carpometacarpal joint, the knee, the hip, and the spine, typically sparing the metacarpophalangeal and wrist joints; involvement of the metacarpophalangeal joints should suggest hemochromatosis or calcium pyrophosphate deposition arthropathy. Risk factors are advanced age, female sex, obesity especially of the knee, joint trauma or overuse, genetic susceptibility, and poor mechanical alignment.

The logic for reading joint fluid analysis fits in one sentence: look at clarity, viscosity, white cell count, and cell type, with the white cell count as the main axis for sorting inflammatory from non-inflammatory — the higher the white cell count and the more neutrophils present, the closer you are to the infection end of the spectrum. Osteoarthritic joint fluid, having no inflammation, retains the viscosity of its high hyaluronic acid content, with a white cell count usually under two thousand and monocytes predominating, so an eighty-year-old with knee pain whose joint fluid is viscous, with a white cell count of about a hundred and predominantly mononuclear, points to osteoarthritis. One frequently tested extension is a well-controlled rheumatoid arthritis patient who develops a single joint effusion with a white cell count of only about eight hundred, within the low-grade inflammatory range — this is most likely concurrent osteoarthritis, not rheumatoid arthritis worsening or infection.

The treatment ladder has two points that keep getting revised, so use the current version: first line is non-pharmacologic — weight loss, exercise, strength training, patient education, assistive devices — the intervention with the strongest evidence for long-term management of knee and hip osteoarthritis; for medication, the knee gets topical anti-inflammatory analgesics preferred, because elderly patients carry high GI/renal/cardiac risk, and an oral anti-inflammatory analgesic is not automatically the first choice over ninety percent of the time — risk must be assessed; acetaminophen has limited efficacy and is no longer a mainstay, reserved only as a fallback when contraindicated. Locally, intra-articular corticosteroids give short-term relief; evidence for intra-articular hyaluronic acid is weak, and the 2019 guideline conditionally recommends against it for the knee and hip — do not treat it as routine; joint replacement comes only at the end stage. One final differential trap deserves emphasis: an imaging abnormality confined to a single wrist bone is not osteoarthritis — lunate abnormality or collapse on X-ray is avascular necrosis, namely Kienböck disease; non-union after a scaphoid fracture is avascular necrosis of the scaphoid; and hand metacarpophalangeal involvement plus chondrocalcinosis plus iron overload should suggest hemochromatosis-associated arthropathy. Hold onto the main line of cartilage being enzymatically degraded by matrix metalloproteinases throughout this chapter, and everything from imaging, blood work, and joint fluid to the treatment ladder follows naturally from it, with the pairings growing on their own once you understand it.