Background: Amyotrophic lateral sclerosis (ALS) is characterized by gradual disturbance of both upper and lower
motor neurons (LMN). In ALS, muscle wasting favors the abductor pollicis brevis (APB) and first dorsal interosseous
(FDI), with relative preservation of abductor digiti minimi (ADM).
Objectives: To interpret F wave changes in the context of upper and LMN dysfunction and the differences in
dysfunction between spinal motoneurons innervating the APB and ADM.
Patients and methods: Forty-four subjects were studied (22 patients with ALS and 22 controls). F wave was
elicited by 50 electrical stimuli from the median and ulnar nerves, and the split hand index (SHI) was measured.
Results: F latency mean, median, and maximum and F amplitude mean, median, and maximum F/M amplitude
ratio were increased in patients with versus those without pyramidal signs. Limb-onset ALS patients showed the
biggest reduction in SHI. The APB muscle of patients with no detectable wasting and upper MN (UMN) signs
showed reduced F wave persistence, mean F wave latency and amplitudes, increased index repeater neuron and
index F repeater, and mean F/M amplitude ratio.
Conclusion: There is enhanced segmental motoneuronal excitability following UMN dysfunctions. SHI appears to
be a diagnostic biomarker for ALS. Abnormal F parameters recorded from APB muscle can distinct patients with
ALS from the normal controls to a greater extent than do the APB/ADM and FDI/ADM compound muscle action
potential amplitude ratios.
Keywords: ALS, F wave, Split hand index
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17th Jun. 2020
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