Hirayama disease: an uncommon cause of motor neuron disease.

We herein report the case of a 23-year-old man with a 3-year history of progressive right-hand weakness, leading to functional impairment. A neurological examination revealed signs of lower cervical motor neuron involvement, including the reverse split hand sign and the Wartenberg's sign. Electrophysiological studies showed reduced compound muscle action potentials (CMAPs) in the right ulnar and median nerves, with preserved sensory conduction and neurogenic changes in the myotomes from C7 to T1. A conventional cervical spine magnetic resonance imaging (MRI) scan was unremarkable, but a flexion MRI scan revealed anterior displacement of the posterior dura and spinal cord compression, confirming the diagnosis of Hirayama disease (HD), which is a rare, self-limiting cervical myelopathy in young male patients caused by dynamic compression during neck flexion. The diagnosis requires a high index of suspicion and flexion MRI scans. While cervical stabilization remains controversial, it may help prevent progression in selected cases. The current report highlights the clinical and radiological features of HD, discusses differential diagnoses, and underscores the importance of dynamic imaging in young patients with asymmetric upper-limb weakness.