Distal Cervical Spondylotic Amyotrophy: Case Reports Demonstrating Clinical/Imaging Segmental Discrepancy

Monomelic pure motor amyotrophy may seem to be an ominous syndrome as it leads to consideration of motor neuron disease. We present a series of 3 very similar cases where unilateral pure distal lower motor neuron paresis and atrophy was limited to the C8-T1 myotomes, without long-tract signs. Electrodiagnostic studies were in keeping with a restricted anterior horn cell disorder. Neuroimaging showed very focal spinal cord compression at the C6-7 level. Two patients underwent surgical decompression. All 3 patients were improved or stable at follow-up. Distal spondylotic amyotrophy is characterized by equal involvement of thenar and hypothenar muscles, in contrast to amyotrophic lateral sclerosis or Hirayama disease. We discuss the striking 2-level discrepancy between imaging and clinical localization. Proposed explanations are arterial or venous compromise caudal to the site of compression. Anatomical variation such as a prefixed brachial plexus is unlikely. A similar imaging/clinical discrepancy has been documented in Hirayama disease and spondylotic myelopathy.