F waves in amyotrophic lateral sclerosis: A systematic review and meta-analysis

This systematic review and meta-analysis aimed to determine the pattern of F-wave abnormalities and their potential utility in the early diagnosis of amyotrophic lateral sclerosis (ALS).

Medline and Embase were thoroughly searched. We primarily emphasized F-wave recordings from the abductor digiti minimi, following stimulation of the ulnar nerve at the wrist. Data from case-control studies involving individuals with ALS versus healthy controls (HC) or other well-defined patient groups were reviewed and -if appropriate- quantitatively synthesized.

Twenty-nine studies were included in this systematic review and 17 of them in the analytic part. The pattern of F-abnormalities in ALS compared to HC was as follows: decreased persistence (MD=20.25 %,15.67-24.84 %), mildly prolonged minimum latency (MD=1.59msec,1.11-2.06msec), increased maximum amplitude (MD=196μV,106-287μV) and elevated Index total Freps (MD=33.9 %,26.0-41.8 %). Affected limbs (with substantial weakness in clinical examination and/or muscle wasting and/or abnormal nerve conduction studies) exhibited more marked abnormalities in persistence, minimum latency, and Index total Freps, whereas abnormalities in these parameters were very mild in clinically unaffected limbs. More prominent increases in maximum amplitude accompanied pyramidal dysfunction. Of note, isolated upper motor neuron (UMN) disorders exhibited a comparable increase in Index total Freps without a decrease in persistence.

The pattern of F wave abnormalities may raise suspicion of involvement of the under-study lower motor neuron (LMN) pool in ALS. These findings may identify LMN dysfunction even at a preclinical stage and prompt extensive electromyographic investigations. UMN involvement may to some extent differentiate the profile of F wave abnormalities in ALS.