316 - Monomelic Amyotrophy after neck injury: A case report

Case Diagnosis: Monomelic Amyotrophy

Case Description: A 40 year old male without significant past medical history presented with complaint of painless, chronic, nonprogressive weakness and atrophy of the bilateral intrinsic hand and right forearm muscles. Physical exam was consistent with the above. Additionally, isolated hypertrophy of the Right brachioradialis muscle was noted. No sensory deficits were appreciated.



The patient described a fall 20 years prior resulting in head and neck injury. Post injury, over a 2 year period, the findings noted above slowly progressed before plateauing. Cervical MRI demonstrated C6-T1 focal cord atrophy. Nerve conduction studies of the median and ulnar nerves exhibited reduced motor amplitudes. Sensory potentials were normal. A "reverse split hand syndrome" was present. Needle exam displayed spontaneous activity and giant motor unit potentials in C7-T1 muscle groups.



History, physical exam, imaging, and electrodiagnostic studies are consistent with monomelic amyotrophy.



Discussions: Monomelic amyotrophy (MMA), or Hirayama Disease, is an extremely rare lower motor neuron disease characterized by progressive degeneration and loss of motor neurons. There is insidious onset of asymmetric upper extremity distal weakness and muscle wasting. Symptoms progress slowly for one to two years before plateauing then remain stable for many years. Muscles with C8-T1 innervation are most affected, with some C7 involvement and C5-C6 sparing. There is no well understood etiology, though a handful of case reports link MMA to traumatic neck flexion injury. MRI may demonstrate spinal cord atrophy, most often at C8-T1 level. Electrodiagnostic studies demonstrate evidence of C8-T1 involvement and signs of acute, ongoing denervation.

Conclusions: This case illustrates the key clinical and diagnostic features for diagnosing monomelic amyotrophy. Despite this condition’s rarity, MMA profoundly affects the neuromuscular system and physiatrists must possess a thorough understanding for prompt recognition and management, as it must not be confused with other lower motor neuron syndromes with distinct treatments and prognoses.