Selective expression of MHC class I in the affected muscle of a patient with idiopathic inflammatory myopathy.

The dominant clinical feature of polymyositis/dermatomyositis is weakness in proximal, rather than distal, musculature. Although rare, cases of focal/localized myositis in which polymyositis-like muscle inflammation is present in only one muscle or extremity have also been reported. The underlying mechanisms dictating involvement of specific muscle groups in polymyositis/dermatomyositis and focal/localized myositis have not been identified. Here, we describe a rare case of dropped-head syndrome due to localized idiopathic inflammatory myopathy (IIM) in the splenius capitis (neck extensor) muscle where major histocompatibility complex (MHC) class I expression was up-regulated in involved muscle fibers. Interestingly, the adjacent trapezius muscle was not affected, corresponding to muscle biopsy findings that did not show any sign of inflammation or MHC class I expression. Our case report therefore suggests that selection of affected muscle in IIM might be influenced by the MHC class I expression of the muscle.