Distal motor axonal dysfunction in amyotrophic lateral sclerosis

Area of Science:

• Neurology
• Neurophysiology

Background:

• Nerve conduction slowing in amyotrophic lateral sclerosis (ALS) is typically attributed to the loss of fast motor axons.
• Understanding the specific patterns of nerve conduction abnormalities is crucial for diagnosing and managing ALS.

Purpose of the Study:

• To investigate the frequency, extent, and distribution of prolonged distal motor latencies in patients with ALS.
• To differentiate nerve conduction abnormalities in ALS from other neurological disorders.

Main Methods:

• Retrospective review of median, ulnar, and tibial nerve conduction studies in 91 ALS patients, 24 with lower motor neuron disorders, and 36 with axonal neuropathy.
• Exclusion of patients with coincidental carpal tunnel syndrome.
• Analysis of distal latencies, with marked prolongation defined as >125% of the upper limit of normal.

Main Results:

• Markedly prolonged distal latencies were exclusively observed in the median nerve of ALS patients (9%), not in control groups.
• No cases of marked distal latency prolongation were found in disease control groups.
• Excitability studies indicated potential membrane depolarization in some ALS patients.

Conclusions:

• About 10% of ALS patients present with significantly prolonged median distal motor latency, unexplained by axonal loss or carpal tunnel syndrome.
• This distal nerve conduction slowing may be partly due to membrane depolarization linked to motor neuronal degeneration in ALS.
• Identifying this specific pattern of median nerve dysfunction is clinically significant for understanding ALS pathophysiology.