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

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

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.

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.

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.

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