Muscle biopsy is at present the gold standard for the diagnosis of idiopathic inflammatory myopathies (IIMs), which include dermatomyositis, polymyositis, and inclusion body myositis. Currently, there is no definite diagnostic marker that helps in the discrimination of different subgroups of IIMs and the discrimination of IIMs from other clinical and morphologic mimics. Major histocompatibility complex (MHC) class I and II antigens are not expressed on normal muscle fibers.
To determine the diagnostic utility of MHC class I and II antigen expression on the muscle biopsies from patients with various neuromuscular disorders and to validate its addition in the existing diagnostic armamentarium.
Expression of MHC class I and II antigen was studied on 126 muscle biopsies, of which 62 were IIMs and 64 were controls (taken from patients with other neuromuscular disorders).
When compared with controls, the sensitivity of MHC class I expression for diagnosis of IIMs was 88.7% (100%, 81.6%, and 100% for dermatomyositis, polymyositis, and inclusion body myositis, respectively), with a specificity of 89.1%. The specificity of MHC class II expression was 100% for all IIMs.
Detection of MHC class I and II antigen expression is extremely helpful in the diagnosis of IIMs and has high sensitivity and specificity, especially in dermatomyositis. This expression can be used as a diagnostic tool in discriminating IIMs from other muscle diseases in which it is either absent or weakly expressed.