Polyarteritis nodosa

Key points

• Medium-vessel necrotising vasculitis - spares small vessels (no glomerulonephritis, no pulmonary involvement)
• ANCA-negative - key differentiator from microscopic polyangiitis (MPA)
• Classic angiographic finding: multiple small microaneurysms at renal, hepatic, and mesenteric artery branch points
• Strong association with hepatitis B (up to 30% of cases)
• Does not affect the lungs - if lung involvement is present, consider MPA, GPA, or EGPA instead

Epidemiology & risk factors

• Peak incidence: 40-60 years, M > F
• Declining incidence due to HBV vaccination
• Associations: HBV (classic), HCV, hairy cell leukaemia
• Rare in children (consider if Kawasaki-like illness with visceral involvement)

Pathophysiology

• Segmental, transmural necrotising inflammation of medium-sized muscular arteries
• Fibrinoid necrosis of vessel wall - weakening leads to aneurysm formation
• Lesions at different stages simultaneously (acute + healed) - unlike hypersensitivity vasculitis
• Healed segments show fibrosis and luminal narrowing - causes downstream ischaemia/infarction
• Immune complex-mediated (especially HBV-associated cases)

Clinical features

• Constitutional: fever, weight loss, malaise, myalgia
• Renal: hypertension (renovascular), renal infarcts, haematuria (but no glomerulonephritis)
• GI: mesenteric ischaemia, post-prandial pain, bowel perforation/haemorrhage
• Neurological: mononeuritis multiplex (classic), peripheral neuropathy
• Skin: livedo reticularis, subcutaneous nodules, ulcers
• MSK: arthralgia, myalgia
• Testicular: orchitis (relatively specific for PAN among vasculitides)

Imaging / investigations

Conventional angiography (gold standard for vascular findings)

• Multiple small microaneurysms (1-5 mm) at branch points of renal, hepatic, mesenteric arteries
• Irregular stenoses and occlusions alternating with normal segments ("string of beads" or "rosary bead" appearance)
• Aneurysms may thrombose or rupture - can cause retroperitoneal haemorrhage

CT

• Renal infarcts (wedge-shaped cortical non-enhancement)
• Hepatic and splenic infarcts
• Bowel wall thickening/ischaemia (mesenteric involvement)
• Retroperitoneal or intraperitoneal haemorrhage (ruptured aneurysm)
• CTA: can demonstrate microaneurysms if large enough, but conventional angiography remains more sensitive for small aneurysms

MRI

• Renal infarcts on post-contrast sequences
• Vessel wall oedema/enhancement on MRA (active inflammation)
• Less sensitive than conventional angiography for microaneurysms

Ultrasound

• Renal: small kidneys with cortical thinning (chronic), wedge-shaped hypoechoic infarcts (acute)
• Testicular: hypoechoic lesion mimicking tumour - can be a diagnostic pitfall
• Hepatic/splenic infarcts

Laboratory

• Raised ESR/CRP, neutrophilia
• ANCA-negative (p-ANCA/c-ANCA)
• HBsAg positive in HBV-associated cases
• Biopsy of affected medium vessel: fibrinoid necrosis, transmural inflammation

Differentials

• Microscopic polyangiitis (MPA): small vessel, p-ANCA positive, glomerulonephritis and pulmonary haemorrhage present, no microaneurysms
• GPA (Wegener's): small vessel, c-ANCA, upper/lower respiratory tract + renal involvement
• Fibromuscular dysplasia: string of beads on angiography but no systemic inflammation, affects young women, renal arteries
• Segmental arterial mediolysis: non-inflammatory, can mimic PAN angiographically, tends to affect coeliac/mesenteric arteries
• Mycotic aneurysms: infectious aetiology, often fewer in number, clinical context differs
• SLE vasculitis: ANA/dsDNA positive, multi-system but different angiographic pattern

Management

• Non-HBV PAN: corticosteroids + cyclophosphamide (induction), then azathioprine/methotrexate (maintenance)
• HBV-associated PAN: short course steroids + antiviral therapy (entecavir/tenofovir) + plasma exchange; prolonged immunosuppression avoided
• Interventional radiology: coil embolisation of ruptured aneurysms (acute haemorrhage)
• Monitoring: serial inflammatory markers, renal function, repeat imaging for aneurysm surveillance