Discrimination of spinal and bulbar muscular atrophy from amyotrophic lateral sclerosis using sensory nerve action potentials

Abstract

Spinal and bulbar muscular atrophy (SBMA) and amyotrophic lateral sclerosis (ALS) are motor neuron diseases. Sensory impairment is sometimes observed, and electrophysiological involvement has been described in the sensory nerves of SBMA patients. We hypothesized that a sensory nerve conduction study (NCS) could be used to discriminate SBMA from ALS. We compared the results from NCSs in a total of 120 SBMA cases confirmed by genetic analysis, 188 ALS cases, and 50 normal subjects. Sensory nerve action potential (SNAP) amplitudes of the SBMA group were significantly lower than in the ALS and control groups. In addition, receiver-operating characteristic curve analysis for SNAP amplitude showed that a cut-off value of 13.8 μV for median, 10.7 μV for ulnar, and 9.9 μV for sural nerve best discriminated SBMA from ALS. The specific decrease of SNAP amplitude in SBMA provides another useful tool for the differential diagnosis of motor neuron diseases.