Abstract
This study aimed to determine differences in spinal motoneuron dysfunction between the abductor pollicis brevis (APB) and the abductor digiti minimi (ADM) in amyotrophic lateral sclerosis (ALS) patients based on studying F-waves. Forty ALS patients and 20 normal controls (NCs) underwent motor nerve conduction studies on both median and ulnar nerves, including F-waves elicited by 100 electrical stimuli. The F-wave persistence (P < 0.05), index repeating neuron (RN; P < 0.001), and index repeater F-waves (Freps; P < 0.001) significantly differed between the APB and the ADM in the NC participants. For the hands of the ALS patients that lacked detectable wasting or weakness and exhibited either no or mild impairment of discrete finger movements, significantly reduced F-wave persistence (P < 0.001), increased index RN (P < 0.001), and increased index Freps (P < 0.001) were observed in APB in comparison with the normal participants, with relatively normal ADM F-wave parameters. For the hands of ALS patients that exhibited wasting and weakness, the mean F-wave amplitude (P < 0.05), the F/M amplitude ratio (P < 0.05), F-wave persistence (P < 0.001), index RN (P < 0.05), and index Freps (P < 0.05) significantly differed between APB and ADM. The differences in the dysfunction of motoneurons innervating APB and ADM are unique manifestations in ALS patients. The F-wave persistence (P = 0.002), index RN (P < 0.001), and index Freps (P < 0.001) in the APB seemed to differentiate ALS from the NCs more robustly than the ADM/APB Compound muscle action potential (CMAP) amplitude ratio. Thus, F-waves may reveal subclinical alterations in anterior horn cells, and may potentially help to distinguish ALS from mimic disorders.
Highlights
Amyotrophic lateral sclerosis (ALS) is a progressive disorder characterized by the involvement of both upper and lower motor neurons (UMNs and LMNs)
Corticomotoneuronal projections to the thenar complex are preferentially affected in ALS, which suggests that corticomotoneuronal dysfunction contributes to the split-hand sign in ALS (Weber et al, 2000)
The index repeating neuron (RN) and index Freps of the abductor pollicis brevis (APB) appeared to be reliable variables for differentiating ALS patients from normal controls (NCs), as the area under the curve (AUC) for the index RN (0.998, 95% confidence interval (CI) 0.937–1.000) and index Freps (1.000, 95% CI 0.940–1.000) showed ‘‘very good’’ diagnostic utility
Summary
Amyotrophic lateral sclerosis (ALS) is a progressive disorder characterized by the involvement of both upper and lower motor neurons (UMNs and LMNs). The extent of motor unit loss is significantly greater in the APB than in the ADM in patients with ALS (Kuwabara et al, 1999) Both cortical and peripheral mechanisms have been proposed to underlie the different levels of atrophy among the small hand muscles in ALS (Weber et al, 2000; Shibuya et al, 2013). The findings of transcranial magnetic stimulation studies have shown that corticomotoneuronal input to the spinal motoneurons innervating the thenar complex is more extensive in normal participants (Macdonell et al, 1999; Menon et al, 2014) Such a difference in inputs might cause these spinal motoneurons to preferentially degenerate in ALS through a transsynaptic anterograde excitotoxic mechanism. The primary purpose of this study was to examine the differences in dysfunction between spinal motoneurons innervating the APB and ADM in ALS using F-waves
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