What are the key clinical features that suggest a diagnosis of Microscopic Polyangiitis (MPA) in a patient presenting with systemic symptoms?

Microscopic Polyangiitis (MPA) is a necrotizing small-vessel vasculitis that typically presents with systemic symptoms and often affects multiple organ systems (Lhote et al., 1996; Chevet et al., 2023). It is a form of ANCA-associated vasculitis (AAV) and is commonly associated with myeloperoxidase (MPO)-ANCA, also known as p-ANCA (Chevet et al., 2023; Sharma et al., 2024).

Key clinical features suggesting MPA include:

• Renal involvement: Rapidly progressive glomerulonephritis (RPGN) is a hallmark feature, often leading to renal failure (Lhote et al., 1996; Chevet et al., 2023).
• Pulmonary involvement: Pulmonary capillaritis, which can manifest as alveolar hemorrhage, is another common and serious presentation (Lhote et al., 1996; Chevet et al., 2023). Patients may present with hemoptysis or dyspnoea (Lhote et al., 1996).
• General systemic symptoms: Non-specific symptoms such as fever, weight loss, malaise, and fatigue are frequently reported (Lhote et al., 1996).
• Musculoskeletal symptoms: Myalgia and arthralgia are common (Lhote et al., 1996).
• Skin manifestations: Palpable purpura is a characteristic skin lesion that may be present (Lhote et al., 1996).
• Neurological involvement: Peripheral neuropathy can occur (Lhote et al., 1996).
• Gastrointestinal involvement: Abdominal pain, bleeding, or other gastrointestinal symptoms may be present (Lhote et al., 1996).

While the provided UK guidelines ¹,² do not specifically address Microscopic Polyangiitis, the diagnosis relies heavily on the combination of these clinical features, serological testing for ANCA, and often biopsy confirmation (Chevet et al., 2023; Sharma et al., 2024).