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Musculoskeletal & Rheumatology

Vasculitis Syndromes

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Vasculitis is a diverse group of disorders characterized by inflammation of blood vessel walls, leading to vessel damage, narrowing, occlusion, or rupture with resulting tissue ischemia and infarction. These disorders affect vessels of all sizes (large, medium, small) and represent a spectrum of pathologic mechanisms ranging from immune-complex deposition to antineutrophil cytoplasmic antibody (ANCA)-mediated injury to direct endothelial injury. Vasculitis may present as a primary systemic disease or secondary to underlying malignancy, infection, or medication exposure, with annual incidence varying from 1-4 per 100,000 population depending on the specific subtype. Understanding vasculitis classification, pathogenic mechanisms, and diagnostic criteria is essential for early recognition and treatment, as many forms carry significant morbidity and mortality if untreated, making this a high-yield topic for both board examinations and clinical practice. The Chapel Hill Consensus Conference classification system provides the contemporary framework for categorizing vasculitis by vessel size and pathologic mechanism, fundamentally guiding diagnostic and therapeutic approaches.

Vasculitis results from breakdown of vascular homeostasis through multiple interconnected immunologic and inflammatory mechanisms. The fundamental pathologic process involves leukocyte infiltration of vessel walls with destruction of the normal architecture, though the specific triggers, vessel targets, and molecular drivers vary substantially among different vasculitis syndromes.

- Immune-Complex-Mediated Vasculitis (Type III Hypersensitivity)

Circulating immune complexes composed of antigen-antibody pairs deposit in vessel walls, particularly in small vessels of skin, joints, kidneys, and gastrointestinal tract. These complexes activate the classical complement cascade through C1q binding, generating potent chemotactic fragments (C3a, C5a) that recruit neutrophils and macrophages. Complement activation also generates the membrane attack complex (C5b-9), which directly damages endothelial cells and increases vascular permeability. Deposition of IgA-dominant complexes in IgA vasculitis (formerly Henoch-Schönlein purpura) occurs through mesangial binding in the kidney and mucosal sites, explaining the characteristic clinical distribution. The size and charge of immune complexes determine vessel tropism: smaller complexes penetrate into small vessels, while larger complexes lodge in larger vessel walls. Tissue damage results from both direct complement-mediated injury and secondary recruitment of inflammatory cells releasing proteolytic enzymes, reactive oxygen species, and additional pro-inflammatory cytokines.

- ANCA-Associated Vasculitis (AAV): Antineutrophil Cytoplasmic Antibody-Mediated Mechanisms

Autoantibodies directed against neutrophil cytoplasmic antigens (primarily myeloperoxidase [MPO] and proteinase 3 [PR3]) drive a distinct pathogenic pathway. These antibodies bind to their respective antigens on the surface of activated neutrophils and monocytes, causing antibody-dependent cellular cytotoxicity and direct neutrophil activation through Fc receptor engagement. Activated neutrophils undergo NET formation (NETosis), whereby activated neutrophils undergo programmed death with extracellular release of chromatin decorated with antimicrobial and autoantigenic proteins, including MPO and PR3. These NETs trap bacteria but also provide a scaffold for amplifying autoimmunity and recruiting additional inflammatory cells to vessel walls. The ANCA-antigen complex also triggers complement activation via the alternative pathway and promotes endothelial cell dysfunction through direct ANCA-mediated signaling, leading to upregulation of adhesion molecules and cytokine production. Molecular mimicry with infectious agents (particularly respiratory pathogens) is hypothesized to trigger initial autoimmunity in genetically predisposed individuals, with subsequent bystander activation of autoreactive T and B cells perpetuating disease. MPO-ANCA typically associates with microscopic polyangiitis and vasculitis with pulmonary-renal syndrome, while PR3-ANCA associates with granulomatosis with polyangiitis (GPA, formerly Wegener's granulomatosis), reflecting different patterns of antigen accessibility and tissue localization.

- Large Vessel Vasculitis: Endothelial Dysfunction and Adventitial Inflammation

Giant cell arteritis (GCA) and Takayasu arteritis (TAK) involve CD4+ and CD8+ T cell-mediated inflammation of large elastic arteries, particularly affecting the intima and media. Dendritic cells in the adventitia and vasa vasorum present antigens to T cells, initiating a Th1 and Th17-skewed response with production of interferon-gamma (IFN-γ), tumor necrosis factor-alpha (TNF-α), and interleukin-6 (IL-6). Activated macrophages (epithelioid and multinucleated giant cells) accumulate at the media-adventitia junction, and these cells produce transforming growth factor-beta (TGF-β) and platelet-derived growth factor (PDGF), triggering smooth muscle cell proliferation and intimal hyperplasia. This combination of inflammatory infiltration plus smooth muscle proliferation causes progressive vessel wall thickening, luminal narrowing, and eventual fibrosis. The chronic inflammatory state activates the coagulation cascade on inflamed endothelium, predisposing to in situ thrombosis. In GCA, the temporal artery is classically affected due to its unique muscular wall composition and potentially to recognition of sequestered antigens within the arterial wall. The pathophysiology explains both acute ischemic complications (from thrombosis or critical stenosis) and chronic sequelae (from progressive fibrosis and remodeling).

- Medium Vessel Vasculitis: Direct Endothelial Injury and Fibrinoid Necrosis

Polyarteritis nodosa (PAN) and other medium vessel vasculitides involve fibrinoid necrosis of the media of muscular arteries, characterized by transmural infiltration of neutrophils and mononuclear cells with deposition of fibrin and immune complexes within the vessel wall. The mechanism of initial endothelial injury remains incompletely understood but likely involves direct immune-complex deposition (in hepatitis B-associated PAN), direct viral infection of endothelial cells, or bystander inflammatory activation. The result is segmental inflammation creating characteristic "skip lesions" (patchy distribution with normal vessel segments between affected areas), which explains the aneurysmal dilations observed on angiography and the unpredictable distribution of organ involvement.

- Small Vessel Vasculitis: Endothelial Proliferation and Leukocytoclasia

Small vessel vasculitis (affecting capillaries and small arterioles/venules) classically presents with leukocytoclasia, the fragmentation of neutrophil nuclei in response to immune-complex deposition, creating a "nuclear dust" appearance on histology. In addition to immune-complex mechanisms, direct neutrophil infiltration causes release of neutrophil elastase and other serine proteases, amplifying endothelial damage. The small caliber of affected vessels makes them particularly vulnerable to complete occlusion from even modest inflammation or thrombosis, explaining the characteristic presentation with cutaneous palpable purpura (from red blood cell extravasation) and glomerulonephritis (from capillary wall damage in the glomerulus).

- Contribution of Genetic Factors and Environmental Triggers

Twin and family studies demonstrate genetic predisposition to vasculitis, with HLA associations (e.g., HLA-B51 in Behçet's disease, **HLA-DRB1*03** in ANCA-associated vasculitis) providing immunologic context. Environmental triggers including infections (particularly respiratory infections preceding AAV), medications, and occupational exposures (silica exposure in GPA) may elicit or perpetuate autoimmune responses through molecular mimicry or bystander activation mechanisms. Aging contributes to GCA through age-related immune senescence and reduced regulatory T cell function, explaining the dramatic increase in incidence after age 50.

- Primary Systemic Vasculitides

These idiopathic disorders represent the majority of vasculitis encountered in clinical practice and are classified by Chapel Hill Consensus Conference criteria based on predominant vessel size affected.

Large Vessel Vasculitis

  • Giant Cell Arteritis (GCA): Affects adults >50 years old (mean age 70), with female predominance (2-3:1). Annual incidence 15-25 per 100,000 in populations of Northern European descent; substantially lower in African and Asian populations. Associated with polymyalgia rheumatica (PMR) in 15-20% of cases. Strong HLA associations (HLA-DRB1*04).
  • Takayasu Arteritis (TAK): Primarily affects young women (female-to-male ratio 8-9:1), particularly in populations of Asian, African, and Latin American descent. Peak incidence in second and third decades; rare in populations of European descent. Associated with HLA-B52 in Japanese populations.

Medium Vessel Vasculitis

  • Polyarteritis Nodosa (PAN): Rare disorder (incidence 0.5-2 per 100,000) affecting middle-aged men slightly more than women. Hepatitis B virus (HBV) infection is the most important identifiable risk factor, present in 5-80% of cases depending on geographic region; hepatitis C less common. Previously associated with amphetamine abuse (particularly methamphetamine) and cocaine use, now uncommon in developed countries.
  • Kawasaki Disease: Medium vessel vasculitis predominantly affecting children <5 years old, with highest incidence in East Asian populations (particularly Japan) but increasing prevalence in other populations. Leading cause of acquired heart disease in children in developed nations. Likely infectious trigger but specific pathogen not definitively identified; seasonal and epidemic patterns suggest infectious etiology.

Small Vessel Vasculitis

  • ANCA-Associated Vasculitis (AAV): Encompasses granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA), and eosinophilic granulomatosis with polyangiitis (EGPA). Annual incidence 10-20 per million; higher in populations of Northern European descent. Peak incidence 40-60 years. PR3-ANCA associated with GPA; MPO-ANCA with MPA and EGPA. Environmental triggers (silica exposure) and respiratory infections documented.
  • IgA Vasculitis (IgAV, formerly Henoch-Schönlein Purpura): Most common systemic vasculitis in children (incidence 13-50 per 100,000 children), with peak age 4-6 years but occurs across all ages. Clear male predominance in childhood (1.5-2:1). Seasonal variation noted, suggesting environmental/infectious trigger. Higher prevalence in East Asian and Caucasian populations.
  • Immune-Complex Small Vessel Vasculitis: Associated with systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), Sjögren's syndrome, and chronic infections (endocarditis, hepatitis C).

- Secondary Vasculitides and Associated Conditions

Drug-Induced Vasculitis

  • Antithyroid medications (propylthiouracil [PTU]): Most common medication-induced AAV; causes MPO-ANCA-positive microscopic polyangiitis.
  • Antibiotics: Sulfonamides, fluoroquinolones, and other drugs associated with immune-complex and ANCA-mediated mechanisms.
  • Hydralazine, procainamide, minocycline: Associated with drug-induced lupus and vasculitis.
  • Cocaine (particularly when contaminated with levamisole): Causes cutaneous necrotizing vasculitis with characteristic dual positivity for ANCA and antihistone antibodies; associated with severe agranulocytosis.
  • NSAIDs, statins, checkpoint inhibitors: Less common associations.

Infection-Associated Vasculitis

  • Hepatitis B virus: Primary association with PAN; HBsAg found in circulating immune complexes in majority of HBV-PAN cases.
  • Hepatitis C virus: Associated with mixed cryoglobulinemia, small vessel vasculitis, and occasionally medium vessel disease.
  • HIV: Increases risk of PAN, GCA, and other vasculitides; associated vasculitis may improve with antiretroviral therapy.
  • Bacterial endocarditis: Immune-complex vasculitis from bacterial antigen-antibody deposition.
  • Streptococcal infection: Associated with IgA vasculitis (post-streptococcal exacerbations common).
  • Tuberculosis, syphilis: Historical associations; rare in modern developed countries.

Vasculitis Associated with Systemic Autoimmune Diseases

  • Systemic Lupus Erythematosus (SLE): Small, medium, and large vessel vasculitis; associated with circulating immune complexes and anti-C1q antibodies.
  • Rheumatoid Arthritis (RA): Rheumatoid vasculitis typically affects small/medium vessels in seropositive (RF+, anti-CCP+) disease; associated with worse prognosis.
  • Sjögren's Syndrome: Predominantly small vessel vasculitis; higher frequency in patients with cryoglobulinemia.
  • Antiphospholipid Syndrome: Thrombotic rather than truly inflammatory, but associated with vasculopathy.

Malignancy-Associated Vasculitis

  • Paraneoplastic vasculitis: Rare but reported with lymphoproliferative malignancies (lymphoma, leukemia) and solid tumors.
  • Direct vasculitis from tumor infiltration: Distinguishing from true immune-mediated vasculitis crucial for management.

- Risk Factors for Disease Development and Severity

  • Genetic predisposition: HLA associations vary by vasculitis subtype; family studies show aggregation.
  • Environmental exposures: Silica (associated with GPA), smoking (risk factor for AAV), occupational exposures.
  • Infections: Preceding upper respiratory infection common in AAV; streptococcal infection in IgA vasculitis.
  • Age: Peak incidence varies dramatically by subtype (children for IgAV and Kawasaki; elderly for GCA).
  • Sex: Female predominance in most vasculitides except PAN and medium vessel diseases.
  • Ethnicity: Significant variation in incidence and disease phenotype across populations.

Vasculitis presentation varies dramatically based on vessel size affected, anatomic distribution, and disease severity. The cardinal principle is that symptoms result from tissue ischemia and infarction secondary to vascular inflammation and obstruction.

Cardinal Systemic Manifestations (Occur in Majority of Vasculitis Syndromes)

- Constitutional Symptoms: Fever, malaise, weight loss, and night sweats represent the acute phase inflammatory response with elevation of TNF-α, IL-6, and other pro-inflammatory cytokines. These nonspecific symptoms often precede organ-specific manifestations by weeks to months, creating diagnostic delay. Fever is present in 40-60% of AAV and medium vessel vasculitis but less common in IgA vasculitis. The mechanism involves hypothalamic stimulation by circulating pyrogens.

- Musculoskeletal Manifestations

  • Arthralgias and arthritis: Occur in 40-80% of vasculitis cases; typically non-erosive in small vessel vasculitis but may involve multiple large joints. IgA vasculitis characteristically causes arthritis of knees and ankles; GPA may cause polyarticular arthritis. Mechanism involves immune-complex deposition in synovial tissue and synovial inflammation.
  • Myalgias: Particularly prominent in large vessel vasculitis (TAK, GCA) due to vascular insufficiency of skeletal muscle; associated with elevated creatine kinase (CK) if significant myositis present.
  • Polymyalgia Rheumatica (PMR): Classic association with GCA in elderly patients, presenting with aching and stiffness in shoulders, hips, and neck; dramatically responsive to corticosteroids (classic board question).

Organ-Specific Manifestations by Vessel Size and Location

Small Vessel Vasculitis (Cutaneous and Systemic)

- Cutaneous Manifestations (Most Common Presentation)

  • Palpable purpura: Hallmark finding representing leukocytoclastic vasculitis with red blood cell extravasation through damaged capillaries. Characteristically appears on lower extremities and buttocks (gravity-dependent) in nonblanching, raised lesions that may progress to necrosis. Histology shows fibrinoid necrosis of capillaries and small arterioles with leukocyte infiltration. Palpable purpura present in >80% of IgA vas

Step 1 — establish that inflammation is present

  • Acute-phase reactants: ESR and CRP are elevated in most active vasculitis; an ESR >50 mm/hr is one of the 1990 ACR classification items for giant cell arteritis. Normal inflammatory markers make active large-vessel or ANCA-associated vasculitis unlikely but do not exclude it.
  • Baseline organ survey: CBC (normochromic anemia, thrombocytosis, eosinophilia in EGPA), creatinine, LFTs, and above all urinalysis with microscopy — dysmorphic RBCs and RBC casts signal glomerulonephritis and convert an outpatient workup into an urgent one.

Step 2 — serology to sort the mechanism

  • ANCA testing: antigen-specific immunoassay for PR3 and MPO is now the preferred primary screen (2017 international consensus on ANCA testing), with indirect immunofluorescence (c-ANCA, p-ANCA) as an adjunct. PR3 tracks with GPA, MPO with MPA and EGPA.
  • Complement and immune complexes: low C3/C4 suggests cryoglobulinemic or lupus-associated immune-complex vasculitis; complement is characteristically normal in pauci-immune AAV.
  • Infection/mimic screen: hepatitis B and C serologies (HBV–polyarteritis nodosa, HCV–cryoglobulinemia), HIV, blood cultures to exclude endocarditis, and cryoglobulins.

Step 3 — tissue or imaging confirmation (the gold standard)

  • Biopsy of an affected organ is definitive: pauci-immune crescentic glomerulonephritis on renal biopsy in AAV; leukocytoclastic vasculitis with fibrinoid necrosis on skin biopsy; granulomatous inflammation with giant cells and fragmented internal elastic lamina on temporal artery biopsy, where skip lesions mandate an adequate-length specimen. ACR/Vasculitis Foundation (2021) conditionally favors temporal artery biopsy over ultrasound in the US, and treatment should never be delayed for it.
  • Angiography/cross-sectional imaging substitutes for biopsy in large- and medium-vessel disease: CT/MR angiography or PET for aortic wall thickening in Takayasu arteritis; catheter angiography showing microaneurysms and beading of mesenteric or renal arteries in PAN.
  • Named schemes: Chapel Hill Consensus Conference nomenclature for classification, ACR/EULAR 2022 classification criteria for research entry (not diagnosis), the Birmingham Vasculitis Activity Score for activity, and the Five-Factor Score for prognosis in PAN/EGPA.

Immediate stabilization (do not wait for confirmatory testing)

  • Glucocorticoids: high-dose oral prednisone for suspected giant cell arteritis, and pulse IV methylprednisolone when there is vision loss or amaurosis fugax — ACR/Vasculitis Foundation (2021) explicitly endorses starting steroids before temporal artery biopsy, since biopsy yield persists for days to weeks.
  • Organ-threatening presentations — diffuse alveolar hemorrhage or rapidly progressive glomerulonephritis — require pulse steroids plus simultaneous induction immunosuppression and often ICU-level airway/dialysis support.

First-line remission induction

  • Severe ANCA-associated vasculitis: glucocorticoids plus an anti-CD20 monoclonal antibody, rituximab, which ACR/VF (2021) conditionally prefers over cyclophosphamide — equally effective, better for relapsing disease and fertility preservation. A rapid steroid taper is preferred over prolonged high-dose steroids.
  • Adjunctive C5a receptor antagonist: avacopan is FDA-approved as an adjunct in severe GPA/MPA and permits substantial steroid sparing.
  • Giant cell arteritis: steroids plus the IL-6 receptor antagonist tocilizumab as the recommended steroid-sparing agent (ACR/VF 2021); methotrexate is an alternative.
  • Takayasu arteritis: steroids plus a non-glucocorticoid immunosuppressant (methotrexate, or a TNF inhibitor such as infliximab, or tocilizumab).
  • Kawasaki disease: IVIG 2 g/kg as a single infusion plus aspirin, given within 10 days of fever onset to prevent coronary aneurysms (AHA 2017).
  • HBV-associated PAN: antiviral therapy with a short steroid course and plasma exchange — prolonged immunosuppression alone worsens viral replication.

Escalation, maintenance, and procedures

  • Plasma exchange is not routine after PEXIVAS; reserve for select severe renal disease or alveolar hemorrhage.
  • Maintenance: scheduled rituximab, or azathioprine/methotrexate, typically for years given high relapse risk in PR3-ANCA disease.
  • Revascularization/surgery in Takayasu arteritis and aortic aneurysm repair should be performed when inflammation is quiescent.

Avoid: cyclophosphamide, methotrexate, and mycophenolate in pregnancy; live vaccines during immunosuppression; and glucocorticoid monotherapy for organ-threatening AAV. Add Pneumocystis prophylaxis with trimethoprim-sulfamethoxazole.

Disease-related — emergencies first

  • Permanent vision loss in giant cell arteritis (emergency): arteritic anterior ischemic optic neuropathy from posterior ciliary artery occlusion; heralded by amaurosis fugax or diplopia, and typically irreversible once established — treat on suspicion.
  • Diffuse alveolar hemorrhage (emergency): pulmonary capillaritis producing hemoptysis (which may be absent), falling hemoglobin, and diffuse ground-glass opacities; hallmark of the pulmonary–renal syndrome of MPA and GPA.
  • Rapidly progressive glomerulonephritis (emergency): crescent formation with a rising creatinine over days and an active urine sediment; delay costs nephrons and leads to dialysis dependence.
  • Aortic aneurysm, dissection, and large-artery stenosis: chronic transmural inflammation with medial destruction in GCA and Takayasu arteritis; signalled by limb claudication, blood pressure discrepancy between arms, bruits, or new aortic regurgitation.
  • Coronary artery aneurysm and thrombosis in Kawasaki disease: the leading cause of acquired pediatric heart disease; screen with serial echocardiography.
  • Bowel infarction, perforation, and intussusception: mesenteric medium-vessel ischemia in PAN; ileoileal intussusception in IgA vasculitis presenting as abdominal pain with a mass.
  • Mononeuritis multiplex: vasa nervorum infarction causing sequential asymmetric palsies (wrist/foot drop), classic for PAN and EGPA.
  • Subglottic stenosis and saddle-nose deformity in GPA from destructive granulomatous upper-airway inflammation; stridor is an airway emergency.

Treatment-related

  • Cyclophosphamide: hemorrhagic cystitis from the metabolite acrolein (prevented by mesna and hydration), later bladder carcinoma, myelosuppression, and premature ovarian failure/infertility.
  • Rituximab: hypogammaglobulinemia with recurrent sinopulmonary infection, hepatitis B reactivation (screen HBsAg and anti-HBc before dosing), and rarely progressive multifocal leukoencephalopathy.
  • Glucocorticoids: osteoporotic fractures, hyperglycemia, adrenal suppression, cataract, and opportunistic infection — notably Pneumocystis jirovecii pneumonia.
  • Tocilizumab: gastrointestinal perforation, and suppression of CRP so that infection is masked — fever may be the only clue.
  • IVIG: aseptic meningitis, volume overload, thrombosis; avacopan: hepatotoxicity requiring LFT monitoring.

  • Vessel size is the organizing principle: the Chapel Hill Consensus Conference sorts vasculitis by caliber, and vessel size predicts the syndrome — palpable purpura and glomerulonephritis mean small vessel, microaneurysms and mononeuritis multiplex mean medium vessel, pulse deficits and bruits mean large vessel.
  • Single best next step in suspected GCA: start high-dose glucocorticoids immediately, then obtain the temporal artery biopsy. The stem that offers "biopsy first, then steroids" is the trap; steroid delay costs the eye and does not erase histology for days to weeks.
  • Pauci-immune is the giveaway for AAV: renal biopsy shows crescents with absent or scant immunofluorescence staining, and serum complement is normal — versus bright IgA staining in IgA vasculitis and low C3/C4 in cryoglobulinemic and lupus vasculitis.
  • PAN is ANCA-negative and spares the lungs, associates with hepatitis B, and shows beads-on-a-string microaneurysms on mesenteric or renal angiography. Pulmonary involvement should push you toward GPA, MPA, or EGPA instead.
  • The ANCA–antigen pairing examiners repeat: PR3/c-ANCA with granulomatosis with polyangiitis (sinusitis, saddle-nose, cavitary lung nodules, RPGN); MPO/p-ANCA with microscopic polyangiitis and EGPA (asthma, eosinophilia, mononeuritis multiplex).
  • Kawasaki disease requires prolonged fever plus the mucocutaneous features in a child under 5; the answer is IVIG plus aspirin with echocardiography — this is the exception where aspirin is given to a child despite Reye syndrome concerns.
  • Induction for severe AAV is rituximab or cyclophosphamide plus steroids, never steroids alone (ACR/Vasculitis Foundation 2021); routine plasma exchange is not recommended after PEXIVAS.
  • Common distractor: normal ESR/CRP does not exclude vasculitis, and a positive ANCA alone does not diagnose it — drugs (propylthiouracil, hydralazine, levamisole-adulterated cocaine) and infective endocarditis produce ANCA positivity, so culture and drug history come before immunosuppression.

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