Reassessing the Diagnostic Utility of the Split Hand Index in Amyotrophic Lateral Sclerosis Patients—The Divide by Zero Problem

Abstract

We set out to assess the diagnostic utility of the split hand index (SHI) for amyotrophic lateral sclerosis (ALS) and also to see if and how it can be applied to severely atrophied muscles, a frequent finding in this setting. We enrolled 38 patients from our clinic, 19 diagnosed with ALS and 19 controls, matched for age and sex. The SHI was calculated, on both sides, for all the patients. We calculated a SHI of 0 when the abductor pollicis brevis muscle (ABD) or first dorsal interosseous muscle (FDI) compound muscle action potentials (CMAPs) were unobtainable, and we allotted a value of 0.1 mV to abductor digiti minimi muscle (ADM) CMAP, for mathematical purposes, when the value would have been 0. The means differences were large between groups, with a significant variance heterogeneity. We performed a ROC analysis and obtained an accuracy of 0.83 for a SHI of 7.2, p-value < 0.0001. In conclusion, we reaffirm the utility of the SHI in the diagnosis of ALS, especially in limb onset cases, and we think that it can be safely extended to severely atrophied muscles with absent or very low CMAP values, without endangering the sensitivity or specificity.