, David G. I. Scott2 and Chetan Mukhtyar2
Department of Rheumatology, Ipswich Hospital NHS Trust, Ipswich, UK
Department of Rheumatology, Norfolk and Norwich University Hospital, Norwich, UK
Microscopic polyangiitis (MPA) is a medium, small-vessel vasculitis characterized by pauci immune glomerulonephritis, pulmonary involvement, and the presence of ANCA. It was first described by Davson in 1948 and called microscopic polyarteritis, it was more widely recognized following the discovery of the association with ANCA.
9.2 Definition and Classification
The Chapel Hill Consensus conference in 2012 defined MPA in the ANCA associated group as “Necrotizing vasculitis, with few or no immune deposits, predominantly affecting small vessels (i.e. capillaries, venules, or arterioles). Necrotizing arteritis involving small and medium-sized arteries may be present. Necrotizing glomerulonephritis is very common. Pulmonary capillaritis often occurs. Granulomatous inflammation is absent.”  The ACR did not consider MPA in their 1990 classification scheme. There are no validated diagnostic criteria.
The annual incidence of MPA in the UK is approximately 5/million with a prevalence of approximately 100/million. The peak age of onset is 65–75 years and it is more common in men than women. It is very rare in childhood. There is evidence for variation in incidence between populations. In Europe, MPA appears to be more common than GPA in the South, while the reverse is true in the North of Europe . In Japan, MPA is the predominant form of ANCA-associated vasculitis.
The etiology of MPA is unknown but like most autoimmune conditions is believed to result from the interaction of trigger factors with a genetically predisposed host. ANCA in animals have been shown to be pathogenic. ANCA can activate TNF-alpha primed neutrophils, inducing production of reactive oxygen species and release of proteolytic enzymes including ANCA-target antigens.
9.5 Clinical Features
Renal involvement is seen in most patients (up to 90 %) with MPA and is the most common organ involved. An active urinary sediment may be present, with red cells and red cell casts which are characteristic of glomerular involvement. Rapidly progressive glomerulonephritis is common leading to acute or subacute renal failure and up to 20 % require dialysis. Proteinuria is common and, rarely, a nephrotic syndrome may develop. The prognosis is determined by the extent of renal involvement.
Pulmonary involvement is common in MPA and may vary clinically from mild dyspnoea to life-threatening massive pulmonary hemorrhage. Pulmonary hemorrhage occurs in up to 30 % of patients with MPA and is associated with a worse prognosis. Pulmonary fibrosis may develop as a consequence of pulmonary hemorrhage. Patients present with dyspnoea and hemoptysis; the chest radiograph shows patchy alveolar shadowing (Figs. 9.1 and 9.2). Nodular or cavitating lesions on a chest radiograph suggest granulomatous lesions more typical of GPA. Pulmonary function tests undertaken during active pulmonary hemorrhage show a raised KCO, and blood- or hemosiderinladen macrophages are found at bronchoalveolar lavage.
CXR showing pulmonary hemorrhage in a patient with microscopic polyangiitis (MPA)
CT thorax from a patient with MPA showing extensive pulmonary hemorrhage
Skin involvement is frequent (36 % at diagnosis, 25–60 % overall), typically a purpuric rash. Nailbed infarcts, splinter hemorrhages, livedo, infarction, or ulceration can occur.
Neuropathy occurs in up to 30 % of patients. Peripheral neuropathy is more common than mononeuritis multiplex affecting either peripheral or cranial nerves. Cerebral vasculitis is rare but may manifest with cerebral hemorrhage infarction seizures or headache.
Cardiac involvement is rare (3 %) but pericarditis and cardiac failure have been reported. This is in contrast to CSS, which is also associated with MPO-ANCA, in which cardiac disease is one of the predominant features (occurring in 26 %).
Abdominal pain, diarrhea, and gastrointestinal bleeding occur in 30 % but are less common than in PAN.
Mild symptoms of rhinitis, epistaxis, and sinusitis are compatible with a diagnosis of MPA but severe destructive disease, infiltrative, or granulomatous disease is not seen and its occurrence is highly suggestive of GPA.
Florid uveitis, retinal vasculitis, optic neuropathy, and orbital granulomata occur more commonly in GPA rather than MPA, where a mild episcleritis may occur in 20 %.
9.5.10 Venous Thromboembolism
Venous thomboembolism is a common occurrence in MPA, occurring in around 7.6 % of patients within a median 5.8 months from diagnosis.
9.6 Laboratory Features
MPA is a systemic inflammatory illness and therefore there is often a marked acute phase response with a nonspecific elevation of ESR and CRP, a mild normochromic normocytic anemia, thrombocytosis and low serum albumin. Eosinophilia is not characteristic of MPA and if present is suggestive of EGPA.
Perinuclear ANCA with an anti-MPO specificity is characteristically found . A few patients are cANCA positive with PR3 specificity. ANA and RF may be present but are typically negative. Complement levels are normal. Anti-GBM antibodies are not present.