Introduction: Split-hand sign in amyotrophic lateral sclerosis (ALS) is characterized by preferential wasting of the lateral, with relative preservation of the medial hand. Split-hand indices (SI) can be based on compound muscle action potentials (SICMAP), F-wave persistences (SIFP), neurophysiological indices (SINI) and hand-held dynamometry (SIHHD). Purpose: To evaluate the diagnostic accuracy of the SI, the divide by 0 problem and correlation among the SI and between the SI and the ALS Functional Rating Scale-Revised (ALSFRS-R) upper-limb score. To determine the proportion of patients with split-hand syndrome, the prognostic usefulness of the SICMAP and correlation between the SI and prognostic biomarkers. Methods: We included 42 patients and 43 healthy participants. Bilateral motor nerve conduction study for ulnar and median nerve and HHD were carried out to calculate SICMAP, SIFP, SINI and SIHHD. We used receiver operating characteristic curves to determine the diagnostic accuracy, Spearman's and Pearson's correlation coefficients to examine relationship between the variables, Kaplan-Meier analysis and log-rank test for survival analysis and Cox' proportional hazards model to identify the independent predictors of survival. Results: SICMAP had an excellent (AUC = 0,91), SINI (AUC = 0,87) and SIHHD (AUC = 0,80) good and SIFP moderate (AUC = 0,71) diagnostic accuracy. AUC and sensitivity were higher when acknowledging the SI value of 0. ALS patients had moderate to very strong correlation among the SI (rs = 0,58–0,95; p < 0,0001) and weak to moderate correlation between the SI and ALSFRS-R upper limb score (rs = 0,34–0,57; p < 0,05). Split-hand sign was present in 79,5–95,2 % of patients. Median survival time was 43 months with three predictors of survival being age at disease onset (HR = 1,12; p = 0,0237), regression coefficient slope (HR = 0,15; p = 0,0023) and split-hand prognostic index (SHpi) (HR = 2,89; p = 0,0211). NIADM had the highest correlation with the SI (rs = 0,76–0,81; p < 0,0001). Discussion and conclusion: SICMAP had the highest diagnostic accuracy. Acknowledging the SI value of 0 improved diagnostic accuracy and sensitivity. Age at disease onset, regression coefficient slope and SHpi were identified predictors of survival.
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