Hirayama disease is a distinct type of cervical myelopathy characterized by juvenile onset of unilateral muscular atrophy of a distal upper extremity. We report herein a case with Hirayama disease-like juvenile muscular atrophy involving proximal muscles in the upper extremities. In this case, in the flexion position of the neck, cervical magnetic resonance imaging revealed that the spinal cord was compressed by expansion of the posterior extradural space with forward displacement of the dura matter. These neuroimaging results are identical to those of Hirayama disease. However, the involved muscles in this case were the proximal muscles, unlike Hirayama disease. Five previous cases have displayed this rare subtype of Hirayama disease. The cause of the unique phenotype may be abnormal cervical column alignment, with upper cervical kyphosis producing a higher apex of the vertebral level in a cervical flexion position, resulting in mid-cervical segmental myelopathy.