Focal nerve enlargement is not the cause for increased distal motor latency in ALS: Sonographic evaluation.

Objective: To elucidate the mechanism of focal conduction slowing in the median nerve in ALS.

Methods: The patients with ALS and CTS and normal control subjects were tested with sonography of the median and ulnar nerves. The cross-sectional areas (CSAs) and the wrist-forearm CSA ratios were compared with the parameters of nerve conduction study.

Results: The median motor distal latency was frequently prolonged in ALS and CTS. CSA and the wrist-forearm ratio of the median nerve were smaller in ALS than in CTS. The ulnar nerve sonography was similar in all the groups.

Conclusions: Selective conduction slowing of the median nerve at the wrist in ALS is unlikely due to secondary compressive neuropathy, as seen in carpal tunnel syndrome.

Significance: Unique vulnerability of the median nerve in ALS may explain the selective conduction slowing.