Importance of sample size for the estimation of repeater F waves in amyotrophic lateral sclerosis

Abstract

Background: In amyotrophic lateral sclerosis (ALS), repeater F waves are increased. Accurate assessment of repeater F waves requires an adequate sample size.

Methods: We studied the F waves of left ulnar nerves in ALS patients. Based on the presence or absence of pyramidal signs in the left upper limb, the ALS patients were divided into two groups: One group with pyramidal signs designated as P group and the other without pyramidal signs designated as NP group. The Index repeating neurons (RN) and Index repeater F waves (Freps) were compared among the P, NP and control groups following 20 and 100 stimuli respectively. For each group, the Index RN and Index Freps obtained from 20 and 100 stimuli were compared.

Results: In the P group, the Index RN (P = 0.004) and Index Freps (P = 0.001) obtained from 100 stimuli were significantly higher than from 20 stimuli. For F waves obtained from 20 stimuli, no significant differences were identified between the P and NP groups for Index RN (P = 0.052) and Index Freps (P = 0.079); The Index RN (P < 0.001) and Index Freps (P < 0.001) of the P group were significantly higher than the control group; The Index RN (P = 0.002) of the NP group was significantly higher than the control group. For F waves obtained from 100 stimuli, the Index RN (P < 0.001) and Index Freps (P < 0.001) of the P group were significantly higher than the NP group; The Index RN (P < 0.001) and Index Freps (P < 0.001) of the P and NP groups were significantly higher than the control group.

Conclusions: Increased repeater F waves reflect increased excitability of motor neuron pool and indicate upper motor neuron dysfunction in ALS. For an accurate evaluation of repeater F waves in ALS patients especially those with moderate to severe muscle atrophy, 100 stimuli would be required.

Figure 1

The comparison of repeater F waves among the P, NP and control groups and the comparison of repeater F waves obtained from 20 with those from 100 stimuli for each group. The edges indicate the 25^th and 75^th percentile. The open circle (o) designates values below or above 1.5 box-lengths from the box-edges, and the asterisk (*) designate values below or above 3.0 box-lengths from the box-edges. (a) Index RN; (b) Index Freps (*P < 0.05, **P < 0.01). P: Limbs with pyramidal signs; NP: Limbs without pyramidal signs; C: Control group; RN: Repeating neuron.

Figure 2

Twenty consecutive traces exhibiting absence of F wave during a train of 20 supramaxiamal stimuli to the left ulnar nerve at the wrist in a 50-year-old male patient of the P group. A number of repeater F waves were obtained during the following 80 supramaximal stimuli. Letters to the right of record identify waves on the basis of waveform and latency. (a) F waves recorded from 20 stimuli; (b) F waves recorded from 100 stimuli. Calibration: M response: 5 mV/division, 5 ms/division; F waves: 500 μV/division, 5 ms/division.

Figure 3

Twenty consecutive traces exhibiting absence of F wave during a train of 20 supramaximal stimuli to the ulnar nerve at the wrist in a 61-year-old male patient of the NP group. Only one F wave was recorded during the following 80 supramaximal stimuli. (a) F waves recorded from 20 stimuli; (b) F waves recorded from 100 stimuli. Calibration: M response: 5 mV/division, 5 ms/division; F waves: 500 μV/division, 5 ms/division.