Hirayama disease: a rare cause of cervical myelopathy.

Hirayama disease (HD) is a rare cervical myelopathy involving lower cervical myotomes, causing asymmetric or unilateral distal upper extremity muscle atrophy. Diagnosis relies on clinical findings, nerve conduction studies, and cervical spine MRI in neutral and flexion positions. We present a 17-year-old male with painless, progressive weakness and atrophy of the left forearm and hand muscles, along with hyperhidrosis. Initial cervical spine MRI revealed loss of cervical lordosis, lower cervical hemicord atrophy, and increased T2 signal intensity in the anterior horn cells. Flexion MRI demonstrated anterior shifting of the posterior dura, cervical cord compression, venous plexus congestion, and posterior epidural space widening with enhancement, confirming Hirayama disease. In adolescent males with cervical cord atrophy and asymmetric flattening on routine MRI, Hirayama disease should be suspected. Flexion MRI is crucial for identifying anterior dural shifting and related changes, ensuring accurate diagnosis.