Exploring the split hand phenomenon with the neurophysiological index.

In 164 subjects of different age groups, we studied the neurophysiological index (NI) ([CMAP amplitude/Distal motor latency] *[F-wave frequency]; CMAP=compound muscle action potential) for three hand muscles (APB= abductor pollicis brevis; FDI= first dorsal interosseous; ADM= abductor digiti minimi). A split hand index based on CMAP amplitude (SHI_CMAP) and NI (SHI_NI) were calculated ([APB CMAP amplitude or NI * FDI CMAP amplitude or NI]/[ADM CMAP amplitude or NI]). All these neurophysiological measurements differed between age groups (p<0.001). Hand muscle NIs, as well as SHI_NI and SHI_CMAP were age dependent. This may be relevant for diagnostic purposes in motor neuron diseases.