[Anti-nuclear matrix protein 2 antibody-positive dermatomyositis with the preferential involvement of neck extensors: a case report].

A 68-year-old man with a 2-month history of progressive weakness and spontaneous pain in proximal limb muscles presented to our hospital with a dropped head. He started experiencing progressive dysphagia several days before admission. On admission, he had muscle weakness of the limbs and neck extensors with edema and induration in distal extremities. Laboratory tests showed elevation of muscle enzymes. FDG-PET/CT demonstrated multiple hypermetabolic lymph nodes, but the primary site was not identified; thus, metastatic carcinoma of unknown primary origin was considered. The patient was diagnosed with anti-nuclear matrix protein 2 antibody-positive paraneoplastic myopathy based on serum tests. Histological findings of the left biceps brachii muscle biopsy revealed severe variation in fiber size and perifascicular myofiber atrophy. Myofibers exhibited myxovirus resistance protein A expression predominantly in the perifascicular region. Following intravenous methylprednisolone pulse therapy and intravenous immunoglobulin, the patient's muscle strength improved with normalization of muscle enzyme levels. The dropped head was considered to have resulted from the preferential involvement of neck extensors based on the observed FDG-PET/CT uptake in neck extensors.