Microscopic polyangiitis

1. High-Yield Summary

• Definition & Core Pathology: Microscopic polyangiitis (MPA) is an idiopathic, systemic, necrotizing small-vessel vasculitis belonging to the antineutrophil cytoplasmic antibody (ANCA)-associated vasculitides (AAV). Unlike granulomatosis with polyangiitis (GPA), MPA characteristically lacks granulomatous inflammation and typically spares the upper respiratory tract.
• Immunodiagnostic Benchmark: Circulating perinuclear ANCA (p-ANCA) directed against myeloperoxidase (anti-MPO) is identified in 70% to 85% of affected individuals; a smaller subset (10% to 15%) demonstrates cytoplasmic ANCA (c-ANCA) targeting proteinase 3 (PR3).
• Musculoskeletal Cardinal Manifestations: Prominent early features include symmetric, non-erosive polyarthralgias and migratory arthritis (50% to 70%), profound myalgias, cutaneous palpable purpura, and ischemic neuropathy manifesting as asymmetric mononeuritis multiplex (e.g., acute peroneal nerve foot drop or radial nerve wrist drop).
• Life-Threatening Hallmarks: Rapidly progressive glomerulonephritis (RPGN) characterized by pauci-immune necrotizing crescentic glomerulonephritis, and alveolar hemorrhage secondary to necrotizing pulmonary capillaritis.
• Orthopaedic & Surgical Relevance: Primary orthopaedic considerations involve differentiating vasculitic arthropathy from septic arthritis; safe biopsy techniques (sural nerve and muscle); management of mononeuritis multiplex and foot drop with bracing (AFO) or delayed tendon transfer; conservative demarcation of ischemic digital gangrene; and rigorous surveillance for secondary corticosteroid-induced osteonecrosis (avascular necrosis of the femoral and humeral heads).
• First-Line Therapeutic Regimens: Remission induction requires pulsed high-dose intravenous methylprednisolone combined with either rituximab (anti-CD20) or pulsed intravenous cyclophosphamide; adjunctive avacopan (oral C5a receptor antagonist) enables rapid glucocorticoid withdrawal while preserving renal recovery.

2. Anatomy

Microscopic polyangiitis preferentially targets the microvasculature, defined anatomically as vessels with internal diameters under 300 micrometers. This encompasses postcapillary venules, capillaries, terminal arterioles, and small-caliber distributing arteries. Understanding the vascular distribution is essential for diagnostic tissue harvesting, soft-tissue debridement, and preventing catastrophic wound complications.

Neurovascular Microanatomy & Danger Zones

• Vasa Nervorum & Epineurial Architecture: Peripheral nerve trunks derive their blood supply from extrinsic longitudinal nutrient vessels that pierce the epineurium to form an anastomotic microvascular network. In MPA, necrotizing vasculitis of these 50-to-150-micrometer epineurial arterioles induces severe endoneurial hypoxemia and ischemic axonal degeneration, producing classic mononeuritis multiplex. The common peroneal nerve (fibular head) and tibial nerve (tarsal tunnel) represent critical anatomical danger zones where baseline physiological tethering compounds vasculitic ischemia.
• Microvascular Synovium: Synovial subintimal beds possess an extensive capillary network that becomes engorged with neutrophils during active vasculitis, producing inflammatory effusions without pannus formation or enzymatic cartilage breakdown.
• Renal & Alveolar Microcirculatory Beds: The glomerular tuft (fenestrated capillaries) and alveolar capillary bed (thin gas-exchange barrier) possess continuous high blood flow and high shear stress, rendering them vulnerable to neutrophil adhesion, transmural fibrinoid necrosis, and extravasation of erythrocytes into Bowman space or alveolar sacs.

Surgical Exposure & Biopsy Corridors

• Sural Nerve Biopsy Corridor: Located posterior and superior to the lateral malleolus. Dissection must identify and preserve the lesser saphenous vein while harvesting a 2 to 3 cm segment of the sural nerve. Complete transection of the nerve induces permanent numbness over the lateral border of the foot; patients must be counseled preoperatively.
• Combined Muscle Biopsy (Gastrocnemius / Peroneus Brevis): Harvesting a strip of adjacent muscle along the epimysium significantly increases diagnostic yield by capturing intermediate-sized feeding arterioles. Care must be taken to avoid violating the superficial peroneal nerve along the lateral compartment.

3. Pathogenesis

The pathogenesis of microscopic polyangiitis is an immune-mediated cascade centered on dysregulated neutrophil activation, alternative complement pathway amplification, and endothelial necrosis without immune complex deposition (pauci-immune).

• Neutrophil Priming & Antigen Translocation: Circulating quiescent neutrophils encounter low-grade inflammatory cytokines, specifically tumor necrosis factor-alpha (TNF-α) and interleukin-1-beta (IL-1β). This interaction triggers the translocation of intracellular primary granule proteins, predominantly myeloperoxidase (MPO), to the external cell membrane surface.
• Pathogenic Autoantibody Binding (Anti-MPO): Immunoglobulin G (IgG) autoantibodies directed against MPO bind directly to the translocated antigens via Fab2 fragments while concurrently engaging neutrophil Fc-gamma receptors (FcγRIIa). This dual engagement initiates hyperactivation of the neutrophil.
• Respiratory Burst & NETosis: Activated neutrophils undergo violent degranulation, releasing toxic oxygen radicals, proteolytic enzymes (lysozyme, elastase, myeloperoxidase), and extruding web-like structures composed of chromatin and cytotoxic histones known as neutrophil extracellular traps (NETs). These NETs directly damage the underlying vascular endothelium and sustain autoantigen presentation.
• Alternative Complement Pathway Amplification: Neutrophil activation activates the alternative complement pathway, generating anaphylatoxin C5a. C5a binds to its cognate G-protein-coupled receptor (C5aR1 / CD88) on neutrophils, creating an intense self-amplifying chemotactic and inflammatory feedback loop.
• Fibrinoid Necrosis & Pauci-Immune Destruction: As the microvascular wall degrades, plasma proteins including fibrinogen infiltrate the necrotic media and intima, coagulating into amorphous eosinophilic “fibrinoid necrosis.” Direct immunofluorescence microscopy confirms the absence or striking paucity of immunoglobulin and complement deposition within the vascular lesions, cleanly differentiating MPA from immune-complex vasculitides (such as IgA vasculitis, lupus vasculitis, and cryoglobulinemic vasculitis).

4. Natural History

Epidemiology & Demographics

Microscopic polyangiitis has an estimated annual incidence of 2.5 to 10 cases per million individuals, with an overall prevalence ranging from 20 to 50 cases per million. While GPA is more common in high-latitude northern European populations, MPA exhibits a relatively higher frequency in southern European populations, Japan, and East Asia. The disease typically manifests in older adults, with peak onset between 55 and 75 years of age. A slight male predilection exists (male-to-female ratio approximately 1.2:1 to 1.5:1).

Untreated Natural History vs. Modern Immunosuppressive Course

In the historical pre-immunosuppressive era, MPA carried a catastrophic prognosis, with 1-year and 2-year mortality rates approaching 80% to 90%, driven primarily by fulminant uremia from rapidly progressive crescentic glomerulonephritis or asphyxiation from diffuse alveolar hemorrhage. The introduction of systemic corticosteroids and pulsed cyclophosphamide in the late 20th century transformed MPA into a manageable chronic disease, achieving remission rates of 80% to 90%.

With contemporary protocols utilizing rituximab, reduced-dose glucocorticoids, and targeted C5a-receptor blockade, the 5-year overall survival rate exceeds 75% to 85%. However, relapses occur in approximately 25% to 35% of MPA patients within 5 years (a lower relapse rate than PR3-ANCA positive GPA, which exceeds 50%). Progressive chronic kidney disease (CKD) develops in over 30% of patients, with 15% to 25% progressing to end-stage renal disease (ESRD) requiring chronic hemodialysis or renal transplantation. The leading causes of mortality in modern cohorts are opportunistic infections secondary to profound immunosuppression within the first 12 months, followed by cardiovascular events and end-stage organ damage in long-term survivors.

5. Clinical Presentation

Patient History & Review of Systems

• Constitutional Prodrome: Over 75% of patients present with an insidious, non-specific constitutional prodrome lasting weeks to months, including profound fatigue, low-grade fevers, drenching night sweats, anorexia, and involuntary weight loss (>10% body mass).
• Musculoskeletal Inquiries: Patients report diffuse, debilitating arthralgias and inflammatory morning stiffness affecting large and small peripheral joints (knees, ankles, wrists, metacarpophalangeal joints). Intense calf and thigh myalgias are common. Surgeons must ask specifically about the tempo of symptom onset to distinguish vasculitic arthritis from chronic degenerative conditions.
• Neurological Symptoms: Elicit sudden, burning paresthesias, sensory dysesthesias, or acute motor weakness in a discrete peripheral nerve distribution (e.g., foot slap when walking or inability to open jars due to wrist drop), signaling mononeuritis multiplex.
• Cardiopulmonary & Renal Warnings: Inquire about cough, dyspnea on exertion, episodic hemoptysis (indicating pulmonary capillaritis), frothy urine (proteinuria), and dark tea- or cola-colored urine (gross hematuria).

Physical Examination & Diagnostic Maneuvers

6. Imaging and Diagnostic Studies

Laboratory & Serologic Hierarchy

• ANCA Serology (Indirect Immunofluorescence & Antigen-Specific ELISA): Indirect immunofluorescence reveals a perinuclear staining pattern (p-ANCA) in 75% to 85% of cases. Quantitative enzyme-linked immunosorbent assay (ELISA) or chemiluminescence immunoassay confirms high-titer antibodies targeting myeloperoxidase (anti-MPO). A minority (10% to 15%) demonstrates c-ANCA / anti-PR3 positivity. Serum ANCA testing carries an overall sensitivity of 80% to 90% and specificity exceeding 95% in active systemic disease.
• Urinalysis & Urine Microscopy (Essential Bedside Screen): Demonstrates an active nephritic sediment: microscopic dysmorphic red blood cells (>80% acanthocytes), red blood cell casts, granular casts, and proteinuria. Proteinuria typically ranges from sub-nephrotic (1 to 3 g/day) to nephrotic levels.
• Renal Function Biomarkers: Rapidly escalating serum creatinine and precipitously declining estimated glomerular filtration rate (eGFR) confirm rapidly progressive glomerulonephritis.
• Systemic Inflammatory Markers: Markedly elevated erythrocyte sedimentation rate (ESR > 80 mm/hr) and C-reactive protein (CRP > 100 mg/L) reflect high systemic vascular inflammation. Complete blood count typically demonstrates a normocytic, normochromic anemia of chronic disease and reactive thrombocytosis.
• Complement Levels: Serum C3 and C4 concentrations are characteristically normal or elevated (acute-phase reactants), corroborating a pauci-immune mechanism and ruling out hypocomplementemic immune-complex vasculitides.

Diagnostic Imaging Modalities

• High-Resolution Chest Computed Tomography (HRCT): The gold-standard imaging modality for pulmonary assessment. Demonstrates diffuse, bilateral, patchy ground-glass opacities, consolidative alveolar infiltrates, and interlobular septal thickening indicative of diffuse alveolar hemorrhage (DAH). In chronic or smoldering MPA, HRCT frequently reveals dependent basilar subpleural reticulation, traction bronchiectasis, and honeycombing characteristic of fibrosing interstitial lung disease (usual interstitial pneumonia [UIP] pattern). Unlike GPA, thick-walled cavitating nodules and upper airway stenoses are absent.
• Musculoskeletal Plain Radiography: Standard anteroposterior and lateral radiographs of symptomatic joints (knees, ankles, wrists) reveal soft-tissue swelling and periarticular osteopenia without cortical erosions, joint space narrowing, or periosteal reaction.
• Magnetic Resonance Imaging (MRI): Non-contrast MRI of symptomatic joints confirms joint effusion and non-proliferative synovitis without cartilage loss. Bilateral hip and shoulder MRI plays a crucial surveillance role in identifying early, pre-collapse (Ficat stage I/II) osteonecrosis secondary to high-dose corticosteroid regimens.
• Electrodiagnostic Testing (EMG / Nerve Conduction Studies): Demonstrates an asymmetric, multifocal sensorimotor axonal neuropathy with marked reduction in sensory nerve action potential (SNAP) and compound muscle action potential (CMAP) amplitudes with relatively preserved conduction velocities, diagnostic of mononeuritis multiplex.

Histopathological Evaluation (Definitive Standard)

Biopsy provides definitive histopathologic confirmation and must be obtained whenever clinically feasible without delaying life-saving therapy:

• Renal Biopsy: Light microscopy reveals focal segmental necrotizing crescentic glomerulonephritis. Immunofluorescence microscopy demonstrates an absence or minimal “pauci-immune” staining for immunoglobulins (IgG, IgA, IgM) and complement (C3, C1q).
• Combined Sural Nerve & Muscle Biopsy: Demonstrates transmural inflammatory infiltration of epineurial and perimysial arterioles by neutrophils and mononuclear cells, accompanied by fibrinoid necrosis of the arterial wall, fragmentation of internal elastic lamina, and secondary axonal degeneration. Granulomas are conspicuously absent.
• Skin Punch Biopsy: Demonstrates leukocytoclastic vasculitis of postcapillary venules characterized by neutrophilic infiltration, nuclear debris (leukocytoclasia), erythrocyte extravasation, and endothelial swelling.

7. Classification

Microscopic polyangiitis is classified using international consensus frameworks that define its nosological boundaries, diagnostic scoring, and systemic prognostic risk.

2012 Revised Chapel Hill Consensus Conference (CHCC) Nomenclature

The 2012 CHCC consensus defines MPA as a necrotizing vasculitis with few or no immune deposits, predominantly affecting small vessels (capillaries, venules, or arterioles). Necrotizing arteritis involving small- and medium-sized distributing arteries may be present. Necrotizing crescentic glomerulonephritis is very common, and pulmonary capillaritis often occurs. Granulomatous inflammation is absent.

2022 ACR / EULAR Classification Criteria for Microscopic Polyangiitis

Developed to standardize clinical research and distinguish MPA from GPA and eosinophilic granulomatosis with polyangiitis (EGPA). Classification requires a documented diagnosis of small- or medium-vessel vasculitis and a cumulative score of 5 or more points (sensitivity 91.3%, specificity 94.4%).

Prognostic Staging: Revised Five-Factor Score (FFS)

The revised Five-Factor Score (Guillevin et al.) stratifies 5-year mortality risk based on the presence of adverse prognostic factors (age > 65 years, renal insufficiency with serum creatinine ≥ 150 μmol/L, severe gastrointestinal involvement, cardiomyopathy, and absence of ear/nose/throat manifestations). An FFS of 0 correlates with a 5-year mortality of ~9%, FFS of 1 carries a mortality of ~21%, and FFS ≥ 2 carries a 5-year mortality of ~40%, dictating aggressive initial immunosuppression.

8. Treatment

The therapeutic management of microscopic polyangiitis is structured into distinct phases: rapid induction of remission to extinguish life- or organ-threatening inflammation, followed by sustained maintenance of remission to prevent relapse, paired with tailored orthopaedic and supportive interventions.

Medical Therapy: Remission Induction

• Glucocorticoid Foundation: Immediate administration of intravenous methylprednisolone pulses (500 to 1000 mg daily for 3 consecutive days), followed by oral prednisone initiated at 1 mg/kg/day (maximum 80 mg/day). Following the landmark PEXIVAS protocol, clinicians should execute an accelerated glucocorticoid taper, reducing the dose to approximately 5 to 7.5 mg/day by month 6, which halves severe infectious complications without compromising remission rates.
• Rituximab (First-Line Preferred Agent): Administered either as 375 mg/m² weekly for 4 doses or as two 1000 mg intravenous infusions separated by 14 days. Rituximab is the preferred first-line remission induction agent across contemporary EULAR and ACR guidelines, demonstrating non-inferiority to cyclophosphamide and superior tolerability, especially in relapsing disease, pediatric/young adults, or to preserve fertility.
• Cyclophosphamide (Alternative Induction Agent): Reserved for severe organ-threatening disease with profound renal failure (creatinine > 350 μmol/L) or when rituximab is unavailable or contraindicated. Administered as pulsed intravenous infusions (15 mg/kg every 2 to 3 weeks, adjusted for age and eGFR) for 3 to 6 months. Intravenous mesna and hydration are mandatory to minimize acrolein-induced hemorrhagic cystitis.
• Targeted Complement Inhibitor (Avacopan): Oral selective C5a receptor antagonist (30 mg twice daily) approved as an adjunctive agent for active, severe MPA in combination with rituximab or cyclophosphamide. Avacopan facilitates substantial steroid minimization, accelerates eGFR recovery, and reduces glucocorticoid-related toxicity.
• Plasma Exchange (PLEX): Routine therapeutic plasma exchange is not recommended based on PEXIVAS trial results, which demonstrated no significant reduction in end-stage renal disease or all-cause mortality. PLEX is strictly reserved for rare clinical scenarios involving concurrent anti-glomerular basement membrane (anti-GBM) autoantibodies or refractory, life-threatening alveolar hemorrhage failing pulsed corticosteroids and rituximab.

Medical Therapy: Remission Maintenance

• Rituximab Maintenance (Gold Standard): Initiated 4 to 6 months after induction at a fixed dose of 500 mg to 1000 mg intravenously every 6 months for a minimum duration of 24 to 36 months (MAINRITSAN trial protocols).
• Oral Immunosuppressive Alternatives: In patients with contraindications to rituximab, oral azathioprine (2 mg/kg/day) or oral methotrexate (20 to 25 mg/week with folic acid supplementation) serves as maintenance therapy in combination with low-dose oral prednisone (5 mg/day).
• Antimicrobial Prophylaxis: All patients receiving induction immunosuppression must receive mandatory Pneumocystis jirovecii pneumonia (PJP) prophylaxis with trimethoprim-sulfamethoxazole (one single-strength tablet daily or one double-strength tablet three times weekly) throughout induction and for at least 6 months following rituximab infusion. In sulfa-allergic patients, atovaquone or monthly inhaled pentamidine is prescribed.

Orthopaedic & Surgical Management

• Diagnostic Biopsy Technique: Sural nerve and muscle biopsy should be performed under local anesthesia with minimal blunt traction to preserve vessel architecture. Electrocautery must be avoided near the nerve specimen to prevent thermal crush artifact.
• Management of Mononeuritis Multiplex & Foot Drop: Acute foot drop requires immediate non-operative bracing with a lightweight custom polypropylene or carbon-fiber ankle-foot orthosis (AFO) to maintain functional clearance during the swing phase of gait and prevent fixed equinovarus contracture. Active physical therapy with passive Achilles stretching is initiated immediately.
• Surgical Tendon Transfer for Irreversible Foot Drop: If severe motor deficit (tibialis anterior MRC grade ≤ 2/5) persists beyond 12 to 18 months despite complete systemic remission and electrodiagnostic studies demonstrate absent motor reinnervation, reconstructive tendon transfer is indicated. Transfer of the posterior tibial tendon through the interosseous membrane to the lateral cuneiform and cuboid restores active dorsiflexion and eliminates brace dependence. Surgery must be scheduled during stable systemic remission.
• Ischemic Ulceration & Digital Gangrene: Digital infarctions and dry acral gangrene must be managed non-operatively with dry protective dressings, avoidance of topical silver sulfadiazine on dry eschars, and aggressive systemic immunosuppression to restore microvascular perfusion. Premature operative amputation must be avoided; debridement or formal revision amputation is performed only after complete demarcation of dry gangrene or if secondary moist gangrene and deep infection develop.
• Corticosteroid-Induced Osteonecrosis (AVN): Patients presenting with groin or shoulder pain after high-dose pulsed glucocorticoid therapy must undergo early MRI. Pre-collapse lesions (Ficat stage I/II) of the femoral head may be treated with core decompression with or without bone marrow aspirate concentrate (BMAC). Post-collapse disease with articular incongruity (Ficat stage III/IV) requires total hip arthroplasty (THA) or total shoulder arthroplasty (TSA).

Perioperative Optimization & Infection Control

• Timing of Elective Surgery: Elective reconstructive procedures (arthroplasty, tendon transfers) should be postponed until systemic disease is quiescent (Birmingham Vasculitis Activity Score [BVAS] of 0 for at least 6 months). For patients on maintenance rituximab, elective procedures are ideally scheduled at the end of the 6-month dosing cycle (just prior to the next scheduled infusion) to minimize deep periprosthetic infection risk.
• Stress-Dose Glucocorticoids: Patients with hypothalamic-pituitary-adrenal (HPA) axis suppression from prolonged prednisone use (>5 mg/day for >3 weeks in the preceding 6 months) require perioperative intravenous hydrocortisone coverage (e.g., 50 to 100 mg IV prior to incision, followed by 50 mg IV every 8 hours for 24 hours, then rapid taper to baseline oral dosing).
• Wound Healing Protocols: Systemic vasculitis and chronic glucocorticoids impair capillary angiogenesis and collagen cross-linking. Surgical incisions must be closed without tension using non-absorbable monofilament sutures, which should remain in place for 21 to 28 days (prolonged compared to standard intervals) to prevent wound dehiscence.

9. Outcome

Modern multimodal therapeutic regimens have substantially improved both short- and long-term outcomes for patients with microscopic polyangiitis:

• Remission Induction Rates: Combination therapy with glucocorticoids and either rituximab or cyclophosphamide achieves complete clinical remission in 85% to 90% of patients within 3 to 6 months of treatment initiation.
• Renal Recovery & Long-Term Dialysis Dependency: Renal recovery depends directly on the baseline serum creatinine and the extent of chronic glomerulosclerosis at biopsy presentation. In patients presenting with non-dialysis-dependent renal impairment, eGFR improves by an average of 10 to 15 mL/min/1.73 m² over the first 12 months. However, 15% to 25% of patients with severe crescentic glomerulonephritis progress to end-stage renal disease (ESRD) within 5 years. Renal transplantation in quiescent MPA demonstrates graft and patient survival rates comparable to non-vasculitic end-stage renal disease controls.
• Relapse Dynamics: Relapse occurs in 25% to 35% of MPA patients over a 5-year surveillance window. Patients with anti-MPO antibodies experience a lower cumulative relapse rate compared to anti-PR3 positive patients, but MPO-associated relapses are frequently insidious, manifesting as silent progressive renal insufficiency or worsening pulmonary fibrosis.
• Neurological Recovery: Recovery of motor and sensory function following mononeuritis multiplex is slow, reflecting the biological rate of peripheral axonal regeneration (approximately 1 mm/day). Approximately 50% to 65% of patients regain functional motor power (MRC grade ≥ 4/5) over 12 to 24 months, while 30% to 40% experience permanent residual paresis or persistent neuropathic dysesthesias requiring permanent orthotic support.
• Patient-Reported Outcomes (PROMs): Quality-of-life assessments using the SF-36, EQ-5D, and PROMIS instruments document substantial functional improvements following remission induction. However, persistent fatigue, fear of disease relapse, and chronic musculoskeletal pain remain prevalent in up to 40% of patients in long-term clinical remission.

10. Complications

Complications in microscopic polyangiitis stem from both primary disease-mediated vascular damage and the cumulative toxicities of intensive immunosuppressive therapy. They are categorized chronologically in the table below:

11. Pearls and Pitfalls

• Diagnostic Pearl: Suspect systemic vasculitis in any adult patient presenting with migratory, symmetric polyarthralgias accompanied by constitutional weight loss, an elevated inflammatory marker (ESR > 80 mm/hr, CRP > 100 mg/L), and an abnormal urine dipstick positive for microscopic hematuria and protein.
• Neurologic Pearl: An acute, painless foot drop in a middle-aged or elderly patient without back pain is mononeuritis multiplex until proven otherwise. Do not misattribute this to an L5 radiculopathy without ruling out systemic vasculitis via urgent ANCA serology and urine microscopy.
• Biopsy Pearl: When performing a diagnostic sural nerve biopsy, harvest an adjacent segment of peroneus brevis or gastrocnemius muscle through the same incision. The combined nerve-muscle biopsy increases diagnostic histological yield by over 30% compared to nerve biopsy alone.
• Surgical Pearl (Avascular Necrosis Vigilance): Maintain a low threshold for ordering bilateral non-contrast hip and shoulder MRIs in any MPA patient who develops joint aching after receiving pulsed methylprednisolone. Detecting osteonecrosis in Ficat stage I or II enables joint-preserving core decompression prior to subchondral collapse.
• Amputation Pitfall: Avoid emergency amputation of dry, necrotic digits in acute vasculitis. These lesions represent dry, ischemic necrosis from arteriole thrombosis; aggressive early surgical intervention into unperfused tissue leads to non-healing wounds and proximal stump necrosis. Allow the digits to autoamputate or fully demarcate under medical immunosuppressive therapy.
• Serologic Pitfall: Do not rely solely on declining MPO-ANCA titers to judge disease quiescence. Approximately 20% to 30% of clinical relapses occur in the setting of stable or undetectable ANCA titers, while persistent high titers can be observed in patients in long-term clinical remission. Treat the patient’s clinical activity, urine sediment, and histological markers rather than the serologic titer in isolation.
• Infection Pitfall: The leading cause of death during the first year of MPA treatment is not active vasculitis, but severe opportunistic infection. Never omit trimethoprim-sulfamethoxazole PJP prophylaxis during induction immunosuppression.

12. Controversy

1. Role of Plasma Exchange (PLEX) After the PEXIVAS Trial

For more than three decades, therapeutic plasma exchange was widely utilized as standard adjuvant therapy for severe ANCA-associated vasculitis presenting with advanced renal failure (creatinine > 500 μmol/L) or diffuse alveolar hemorrhage. The landmark PEXIVAS trial (the largest randomized controlled trial in AAV, involving 704 patients) demonstrated that the addition of plasma exchange to standard immunosuppression did not reduce the incidence of death or end-stage renal disease at a median follow-up of 2.9 years. Consequently, 2024 EULAR and 2021 ACR guidelines recommend against the routine use of plasma exchange in MPA. Current debate centers on whether a niche subgroup with profound, life-threatening hypoxemic alveolar hemorrhage or concurrent anti-GBM double-positivity derives a survival benefit from rescue plasma exchange.

2. Glucocorticoid Minimization vs. Replacement with Avacopan

Systemic glucocorticoids are responsible for the majority of early treatment-related morbidity, including life-threatening infections, osteonecrosis, diabetes mellitus, and accelerated osteoporosis. The phase 3 ADVOCATE trial demonstrated that avacopan (a selective oral C5a receptor antagonist) was non-inferior to a tapering prednisone protocol at 26 weeks and superior in sustaining clinical remission at 52 weeks, accompanied by significant recovery of renal function and marked reduction in glucocorticoid toxicity scores. However, widespread clinical adoption faces debate regarding drug acquisition costs, insurance pre-authorization barriers, and whether avacopan can completely supplant glucocorticoid pulses during the hyperacute phase of alveolar hemorrhage and rapidly progressive glomerulonephritis.

3. Optimal Duration of Remission Maintenance Therapy

While the MAINRITSAN trials established the superiority of maintenance rituximab over azathioprine, the optimal total duration of maintenance therapy remains contentious. Discontinuing maintenance therapy after 18 to 24 months is associated with late relapses in up to 30% of patients. Conversely, extended rituximab administration beyond 36 to 48 months carries cumulative risks of hypogammaglobulinemia, persistent late-onset neutropenia, and severe infections. Clinicians must weigh individualized biomarkers (such as CD19+ B-cell repopulation, persistent ANCA positivity, and organ reserve) against infection risk when deciding whether to extend maintenance therapy indefinitely.

13. Evidence-Based Synthesis

Modern clinical decision-making is grounded in high-quality randomized controlled trials and prospective registry analyses, corroborated by seminal biomechanical and anatomical descriptions.

14. References

• Hellmich B, Sanchez-Alamo B, Schirmer JH et al. EULAR recommendations for the management of ANCA-associated vasculitis: 2022 update. Ann Rheum Dis. 2024. doi:10.1136/ard-2022-223764 [Level I: Systematic Review / Meta-Analysis]
• Chung SA, Langford CA, Maz M et al. 2021 American College of Rheumatology/Vasculitis Foundation Guideline for the Management of Antineutrophil Cytoplasmic Antibody-Associated Vasculitis. Arthritis Rheumatol. 2021. doi:10.1002/art.41773 [Level I: Systematic Review / Meta-Analysis]
• Biddle K, Jade J, Wilson-Morkeh H et al. The 2025 British Society for Rheumatology management recommendations for ANCA-associated vasculitis. Rheumatology (Oxford). 2025. doi:10.1093/rheumatology/keaf240 [Level I: Systematic Review / Meta-Analysis]

15. Master Class Pro-Tip

When evaluating an adult patient with new-onset symmetric arthralgias and an acute unilateral foot drop, suspect vasculitic mononeuritis multiplex immediately and order emergency p-ANCA/anti-MPO serologies and urine microscopy. In patients undergoing high-dose glucocorticoid pulses for systemic vasculitis, establish a baseline bilateral hip and shoulder screening MRI at 6 to 12 months; diagnosing osteonecrosis in Ficat stage I or II preserves the option for joint-sparing core decompression before irreversible articular collapse mandates total joint replacement. For ischemic digital lesions, resist the surgical impulse to perform early amputation: maintain dry protective dressings and let intensive immunosuppression restore microvascular perfusion, reserving operative intervention strictly for defined dry demarcation or secondary infection.

Master Class Surgical Pearl: Musculoskeletal Management in ANCA-Associated Vasculitis