444 - Monomelic Amyotrophy

A 19-year-old male with a history of possible right ulnar neuropathy at the elbow on prior EMG presented with progressive right-hand weakness and atrophy over 2 years. He had no pain or sensory changes throughout his course but noted increased weakness when exposed to cold. A prior ulnar nerve transposition failed to improve his symptoms. Exam revealed forearm atrophy as well as hand atrophy and weakness. A repeat NCS/EMG was performed. The right ulnar nerve motor study showed prolonged distal onset latency and markedly reduced amplitude with normal conduction velocities.  The right ulnar sensory study was normal. EMG showed chronic denervation in the right EIP, APB, FDI, ADM, and FCU muscles without any evidence of acute denervation.

Discussions:

While ulnar neuropathy is a common cause of hand weakness, it is atypical for axonal ulnar neuropathy at the elbow to present with markedly abnormal motor study but normal sensory findings. A chronic right C8-T1 motor radiculopathy was considered but deemed unlikely given a normal MRI. Given these findings and his symptoms, the possibility of an alternative diagnosis was explored. Monomelic amyotrophy, or Hirayama Disease, is a rare motor neuron disease that presents in young males with hand weakness without sensory changes. The disease is thought to be caused by a tight dural sac in the cervical canal leading to chronic anterior horn cell ischemia. Given the suspicion, an MRI of the C-spine with flexion was obtained, revealing increased prominence of the posterior epidural space suggestive of engorged epidural vessels contributing to the narrowing of the thecal sac, strongly supporting the diagnosis of Hirayama Disease.

Conclusions:

In young males presenting with hand weakness and no sensory abnormalities, monomelic amyotrophy should be considered in the differential diagnosis during NCS/EMG studies. Early recognition may prevent unnecessary procedures and ensure appropriate treatment.