Decremental Responses to Repetitive Nerve Stimulation in X-Linked Bulbospinal Muscular Atrophy

BACKGROUND AND PURPOSE: X-linked bulbospinal muscular atrophy (X-BSMA) is characterized by bulbar and spinal muscular weakness and fasciculations. Although X-BSMA is a motor neuronopathy, there are several reports of myasthenic symptoms or decremental responses to repetitive nerve stimulation (RNS). We report the results of applying the RNS test to 15 patients among 41 with genetically confirmed X-BSMA; these 15 patients complained of fatigue, ease of becoming tired, or early muscular exhaustion. METHODS: The 3-Hz RNS test was performed on the trapezius, nasalis, orbicularis oculi, flexor carpi ulnaris, and abductor digiti quinti muscles. A decrement greater than 10% was considered abnormal. Additionally, a pharmacologic response to neostigmine was identified in three patients. RESULTS: A significant decrement was observed in 67% of patients, and was most common in the trapezius muscle (nine cases). The decrement of the trapezius muscle response ranged from 15.9% to 36.9%. The decrement was inversely correlated with the amplitude of compound muscle action potentials at rest. Neostigmine injection markedly improved the decrement in three patients, who showed noticeable decremental responses to 3-Hz RNS. CONCLUSIONS: This study shows that myasthenic symptoms and abnormal decremental responses to low-rate RNS are common in X-BSMA.