29th January 2024

Microscopic polyangiitis

Background

Epidemiology 

This disease may affect all ethnic backgrounds in any age group, with the typical patient being middle-aged males or females. 

Aetiology

The disease is mediated by the immune system, with no obvious cause of the immune hyperactivity identified.

Pathophysiology

Microscopy polyangiitis is an ANCA-associated vasculitis. ANCA resides in neutrophils and macrophages and leads to destruction of white blood cells at the site of disease with release of enzymes within blood vessel walls, causing damage to the blood vessels. 

Clinical features

Manifestations are similar to those with granulomatous polyangiitis; however, the nasopharynx is usually not affected. The majority of patients have renal involvement with pauci-immune glomerulonephritis with hypertension. The inflammation also commonly affects the peripheral nervous system causing mononeuritis multiplex with presentations such as foot drop, pins and needles or numbness. Approximately half of the patients have skin involvement, which is typically a dependent palpable purpuric rash. Often, patients will also have constitutional symptoms such as fever and weight loss. Lung involvement can be life-threatening with inflammatory infiltrates and alveolar haemorrhage presenting with haemoptysis. 

Diagnosis

Laboratory findings of anti-MPO, pANCA are present in around 70% of cases. ESR and CRP are typically raised to suggest an inflammatory process. CT scan can demonstrate lung involvement. Diagnosis of the condition involves biopsy of the involved organ, typically the kidney skin, nerve, muscle or lung. This will demonstrate fibrinoid necrosis with infiltration of neutrophils with no evidence of granulomas.

Treatment

Immunosuppression with corticosteroids and steroid-sparing agents such as cyclophosphamide is usually first-line. Following the control of disease, a course of maintenance therapy is required to maintain remission.

References

• About vasculitis, Vasculitis UK. Available at: https://www.vasculitis.org.uk/about-vasculitis [Accessed 28th January 2022].
• Vasculitides, Amboss. Available at: https://www.amboss.com/us/knowledge/Vasculitides/ [Accessed 17th January 2022].
• Wallace, Z.S. and Miloslavsky, E.M., 2020. Management of ANCA associated vasculitis. BMJ, 368.