Polyarteritis Nodosa

Definition/diagnostic criteria Polyarteritis nodosa (PAN) is a rare, necrotising vasculitis affecting medium-sized arteries, typically characterised by segmental inflammation and damage to blood vessel walls. This can lead to aneurysm formation and organ damage due to compromised blood supply. The Chapel Hill Consensus Conference nomenclature provides classification criteria, which, while not diagnostic, assist in differentiating PAN from other vasculitides. Diagnosis is based on clinical features, imaging and histopathology, with the presence of microaneurysms on angiography or histological evidence of small and medium-sized artery necrotising inflammation being highly suggestive of PAN.

Epidemiology PAN is uncommon, with an estimated incidence of 0 to 1.6 cases per 100,000 per year. It usually affects adults in their 50s and 60s, with no clear gender predilection. The prevalence in the UK is not precisely known but reflects these low incidence rates.

Diagnosis
Clinical features: Symptoms of PAN can be nonspecific and include systemic features such as fever, malaise and weight loss. Organ-specific manifestations might include abdominal pain due to mesenteric artery involvement, hypertension from renal artery stenosis, cutaneous nodules, livedo reticularis, testicular pain and neurological symptoms such as mononeuritis multiplex. Musculoskeletal symptoms like arthralgia and myalgia are also common.

Investigations: Diagnosis involves a combination of laboratory and imaging studies. The laboratory tests may reveal raised inflammatory markers (ESR, CRP), anaemia and renal impairment. Hepatitis B and C serologies should be performed due to an association with PAN. Imaging with Doppler ultrasound, CT or MRI angiography can detect aneurysms or stenosis in medium-sized arteries. The definitive diagnosis often requires a biopsy of affected tissue demonstrating necrotising arteritis.

Typical abnormalities found: On angiography, characteristic findings include microaneurysms and abrupt cut-offs in affected arteries. Biopsy samples may show transmural inflammation, fibrinoid necrosis, and aneurysm formation in medium-sized arteries without involvement of smaller arterioles or venules.

Treatment The management of PAN depends on severity and the presence of hepatitis virus association. For non-hepatitis-associated PAN, treatment involves immunosuppression with high-dose corticosteroids (e.g., prednisolone) and cyclophosphamide for induction of remission, followed by maintenance therapy with less toxic agents such as azathioprine or methotrexate. Plasma exchange may be considered in severe cases. Antiviral therapy should be initiated in hepatitis-associated PAN. It is crucial to monitor treatment response and side-effects, including the risk of infection due to immunosuppression.

Prognosis Without treatment, PAN is often fatal, but with appropriate therapy, five-year survival exceeds 80%. Prognosis depends on the severity of organ involvement and response to treatment. Residual damage in affected organs may persist, and long-term monitoring for relapses and treatment complications is essential.