Split phenomena in amyotrophic lateral sclerosis: Current evidences, pathogenetic hypotheses and diagnostic implications

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron disease and has emerged among the disorders with the largest increasing incidence in Western countries. Although the diagnosis is based on clinical grounds, electromyography (EMG), and nerve conduction studies (NCS) play a crucial role to exclude other potential etiologies of lower motor neuron (LMN) dysfunction. Based on clinical grounds, a peculiar pattern of dissociated atrophy of the intrinsic hand and foot muscles, termed the "split-hand" (SH) and "split-leg" (SL) signs, has been described in a significant proportion of subjects with ALS, even at the early stages of the disease, when symptoms are focal. These signs are rare in neurological and non-neurological diseases other than ALS. In this review, we discussed current evidences concerning SH and SL signs, their pathogenetic hypotheses and neurophysiological findings. We also analyze whether SH and SL signs can be reliable markers in the differential diagnosis and in the prognosis of ALS.

The senile hand: age effects on intrinsic hand muscle CMAP amplitudes influence split-hand index calculations

INTRODUCTION/AIMS

Age can affect hand muscles non-uniformly. We investigated the influence of age on the compound muscle action potential (CMAP) amplitude of the hand muscles and the derived split-hand index (SHI).

METHODS

We studied 244 subjects investigated for myasthenia gravis but without neuromuscular disorders. Abductor pollicis brevis (APB), first dorsal interosseous (FDI), and abductor digiti minimi (ADM) CMAPs were obtained by supramaximal stimulation at the wrist, recording with surface electrodes while checking the best recording site. We applied Tukey's HSD and Kruskal-Wallis one-way analysis of variance for comparing age groups defined by median and inter-quantile ranges (IQR). Spearman's rank correlation coefficient and linear regression were used for testing age-dependence of measurements.

RESULTS

Median age was 61.5 years (1st IQR 44.5, 3rd IQR 72.0, range 18-89). Age and neurophysiological measurements were similar between genders. APB_CMAP , FDI_CMAP , ADM_CMAP and SHI were correlated with age (p < 0.001). Median and cut-off values were significantly different between age groups. APB_CMAP , FDI_CMAP and ADM_CMAP decreased by 0.8/0.7/0.3 mV/year, respectively, and SHI decreased 0.15/year.

DISCUSSION

The CMAP amplitudes of hand muscles and derived SHI were strongly age-dependent, although this effect was less in ADM. This represents a physiological phenomenon. Future studies using the SHI should consider age effects. This article is protected by copyright. All rights reserved.

A prospective study on split-hand index as a biomarker for the diagnosis of amyotrophic lateral sclerosis

OBJECTIVE

To determine the practical diagnostic utility of split-hand index (SI) values calculated from F-wave persistence (SI_FP) and the F/M amplitude ratio (SI_F/M) for differentiating patients with amyotrophic lateral sclerosis (ALS) from other conditions. Methods: This prospective study recruited consecutive patients from Peking Union Medical College Hospital, China, between June 2019 and December 2019. Patients 18-80 years old who had clinical neuromuscular symptoms affecting the upper limbs and required electrophysiological examinations to aid diagnosis were eligible. Compound muscle action potentials (CMAPs) and F-waves recorded from the abductor pollicis brevis (APB), first dorsal interosseous muscle (FDI), and abductor digiti minimi (ADM) were examined. SI_FP and SI_F/M were calculated as: SI = (APB × FDI)/ADM. The sensitivity and specificity of SI_FP and SI_F/M in differentiating ALS from non-ALS conditions were derived using receiver operating characteristic (ROC) curves. Results: A total of 309 participants, comprising 91 (29.4%) with ALS and 218 (70.6%) with other neuromuscular disorders, were enrolled after 54 were excluded. SI_FP was significantly reduced and SI_F/M increased in the ALS group compared with the non-ALS group (p < 0.001). By ROC curve analysis, an SI_FP cutoff of 73.3 showed 85.7% sensitivity and 80.7% specificity for differentiating ALS from non-ALS. SI_F/M and SI_CMAP showed lower sensitivity (67% and 75.8%, respectively, p < 0.001) than SI_FP for ALS diagnosis. SI_FP and SI_F/M combined did not outperform SI_FP alone. Conclusion: SI_FP could be a sensitive, noninvasive neurophysiological diagnostic marker for ALS patients with affected upper limbs. In particular, an SI_FP value of 73.3 might be the optimal cutoff for diagnosing ALS.

Utility of dissociated intrinsic hand muscle atrophy in the diagnosis of amyotrophic lateral sclerosis

The split hand phenomenon refers to predominant wasting of thenar muscles and is an early and specific feature of amyotrophic lateral sclerosis (ALS). A novel split hand index (SI) was developed to quantify the split hand phenomenon, and its diagnostic utility was assessed in ALS patients. The split hand index was derived by dividing the product of the compound muscle action potential (CMAP) amplitude recorded over the abductor pollicis brevis and first dorsal interosseous muscles by the CMAP amplitude recorded over the abductor digiti minimi muscle. In order to assess the diagnostic utility of the split hand index, ALS patients were prospectively assessed and their results were compared to neuromuscular disorder patients. The split hand index was significantly reduced in ALS when compared to neuromuscular disorder patients (P<0.0001). Limb-onset ALS patients exhibited the greatest reduction in the split hand index, and a value of 5.2 or less reliably differentiated ALS from other neuromuscular disorders. Consequently, the split hand index appears to be a novel diagnostic biomarker for ALS, perhaps facilitating an earlier diagnosis.