Background/Purpose: Inflammatory myopathies are a heterogeneous group of diseases. We investigated if the location and pattern of deposition of complement membrane attack complex (MAC) can be used in the differential diagnosis of inflammatory myopathies.

Methods: We reviewed histological sections of 227 cases of muscle biopsies in which MAC was requested for diagnosis and analysed 145 cases with clear clinical and pathological diagnosis; 88 inflammatory myopathies, 31 muscular dystrophies and 26 controls. We reviewed the immunohistichemical location of MAC, HLA Class I, utrophin, neo-myosin, together with the standard histological sections of the muscle biopsies.

Results: MAC deposition was demonstrated in 86.2% of dermatomyositis (DM), 86.5% of inclusion body myositis (IBM) and about 50% of patients with polymyositis (PM) and mixed connective tissue disease( MCTD). Most of the DM cases (55%) showed continuous solid and strong staining pattern in the capillary wall (pattern 1) with clear tendency of perifascicular deposition but with no sarcolemmal labelling. Samples from muscles with IBM, PM and MCD showed lighter granular segmental staining (pattern 2) in the capillaries, which is frequently associated with frequent granular staining in sarcolemma of muscle fibres across the fascicles. About 31% of DM biopsies showed a mixture of patterns 1 and 2.

There were infrequent granular and infrequent granular deposits in the capillaries in about 13% of muscular dystrophies and 11% in control cases.

Conclusion: Pattern 1 deposition of MAC in capillaries appears to be more frequent in DM and could contribute to the diagnosis, in addition to other criteria. This pattern of deposition may indicative of primary humoral mechanism involving capillaries in DM. Pattern 2 deposition of MAC in capillaries and sarcolemma of other inflammatory diseases, such as IBM and PM, could suggest complement system activation secondary to the inflammatory process and muscle necrosis.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/is-the-pattern-of-capillary-deposition-of-complement-membrane-attack-copmlex-useful-in-the-differential-diagnosis-of-inflammatory/