Introduction: In upper motor neuron (UMN) syndromes, F-waves are of higher amplitude, increased duration, and increased latency. Increased F-wave frequency in UMN syndromes is not well documented. We have studied the effect of corticospinal lesion on F-wave frequency, M-wave amplitude, distal motor latency and on the Neurophysiological Index (CMAP amplitude/DML x % F-wave frequency) which we have described previously in amyotrophic lateral sclerosis (ALS).
Method: We studied 145 abductor digiti minimi (ADM) muscles in 75 patients with ALS. Six groups of hands were defined, based on strength of the ADM, as assessed clinically by the MRC scale, and the presence or absence of UMN signs in the same upper limb.
Results: CMAP amplitude and area were decreased, and DML increased, in weaker muscles (MRC < 3) causing a change in the ALS Neurophysiological Index. In general, UMN signs were not correlated with increased F-wave frequency. In five weak hands F-responses were studied in individual motor units. A larger number of responding motor units in these weak limbs with UMN signs was observed.
Conclusion: F-wave excitability is largely dependent on the size of the lower motor neuron pool.