Microscopic polyangiitis (MPA)

Microscopic polyangiitis (MPA) is a primary necrotizing vasculitis of small vessels (arterioles, capillaries, venules) without granulomas. Although it is characterized by the presence of focal segmental glomerulonephritis in 80-100% of cases, MPA can affect all organs and, unlike polyarteritis nodosa (PAN), pulmonary involvement is common.

It is a rare to exceptionally rare disease, previously not differentiated from PAN.

Clinical Features

Similar to PAN. It should be noted that renal alterations are almost constant (80-100% according to series), and unlike PAN, pleuro-pulmonary manifestations are frequent (alveolar hemorrhages ++). The presence of a pulmonary-renal syndrome is highly suggestive. Other common involvements include: skin, muscle, joints, and gastrointestinal. Other signs are much less frequent than in PAN (e.g., only 20% have multiple neuropathies).

Diagnosis

Clinical Suspicion

Similar to PAN, except for the high frequency of renal insufficiency, which can be isolated, and a very suggestive pulmonary-renal syndrome (alveolar hemorrhages + renal insufficiency) (differential diagnosis: Goodpasture syndrome, Wegener’s granulomatosis, SLE, etc.).

Laboratory Tests

• Presence of p-ANCA (anti-MPO type) in 60%, +- anti-basement membrane / antinuclear antibodies
• Other findings similar to PAN

Angiography

Not very contributive (no microaneurysms)

Biopsy

Most contributive for diagnosis when positive. Renal/pulmonary biopsy should be performed urgently in the case of suggestive clinical/laboratory findings with rapidly progressing renal/pulmonary involvement.

Therapeutic Management - Treatments

Therapeutic management is similar to that of polyarteritis nodosa (see corresponding chapter). Relapses are more frequent in MPA associated with ANCA.

Bibliography

EMC, traité de médecine AKOS, Elsevier, 2018

Jameson JL et al., Harrison's Principles of Internal Medicine, 20th edition, McGraw Hill Higher Education, 2018