Monomelic amyotrophy (hirayama disease) with upper motor neuron signs: a case report

Abstract

Monomelic amyotrophy (MMA), also known as Hirayama disease, is a sporadic juvenile muscular atrophy in the distal upper extremities. This disorder rarely involves proximal upper extremities and presents minimal sensory symptoms with no upper motor neuron (UMN) signs. It is caused by anterior displacement of the posterior dural sac and compression of the cervical cord during neck flexion. An 18-year-old boy visited our clinic with a 5-year history of left upper extremity pain and slowly progressive weakness affecting the left shoulder. Atrophy was present in the left supraspinatus and infraspinatus. On neurological examination, positive UMN signs were evident in both upper and lower extremities. Electrodiagnostic study showed root lesion involving the fifth to seventh cervical segment of the cord with chronic and ongoing denervation in the fifth and sixth cervical segment innervated muscles. Cervical magnetic resonance imaging (MRI) showed asymmetric cord atrophy apparent in the left side and intramedullary high signal intensity along the fourth to sixth cervical vertebral levels. With neck flexion, cervical MRI revealed anterior displacement of posterior dural sac, which results in the cord compression of those segments. The mechanisms of myelopathy in our patient seem to be same as that of MMA. We report a MMA patient involving proximal limb with UMN signs in biomechanical concerns and discuss clinical importance of cervical MRI with neck flexion. The case highlights that clinical variation might cause misdiagnosis.

Keywords: Biological assays; Monomelic amyotrophy; Pyramidal signs.

Fig. 1. Axial T2-weighted magnetic resonance imaging of the cervical spine at the C5-6 vertebral levels in neutral position. Spinal cord atrophy is apparent, especially on the left side (arrow).

Fig. 2. (A) Midline sagittal T2-weighted magnetic resonance imaging of the cervical spine in the neutral position reveals normal contour and intramedullary high signal intensity along the fourth to sixth cervical vertebral levels without cervical cord compression. (B) On sagittal T2-weighted images in the flexed neck position, there is anterior shifting of posterior dural sac below the third cervical vertebral level, compressing the spinal cord from third to sixth cervical vertebral levels (arrow).

Fig. 3. (A) On lateral view of cervical X-ray in the flexed neck position. Inter-segmental angles were measured by drawing lines through the center of each vertebra body perpendicular to the long axis of each one. (B) Overall range of cervical flexed motion was calculated as the angle between tangent lines of the C2 vertebral body and the C7 vertebral body.