Peripheral nerve abnormalities in pediatric patients with spinal muscular atrophy

We examined the specific nerve conduction deficits distinguishing spinal muscular atrophy (SMA) subtypes I and II. Five SMA I patients (age, 0.2–1.1 years) and 10 SMA II patients (age, 1.0–2.8 years) were examined. Patients were compared to age-matched controls for motor and sensory conduction velocity (MCV and SCV) changes, compound muscle and sensory nerve action potential amplitudes (CMAP and SNAP), and F-wave occurrence (FO). Slower MCVs were found in three of five SMA I patients; all five exhibited markedly decreased CMAP amplitudes. Tibial nerve CMAP amplitudes significantly reduced in SMA II patients (p < 0.01). Slower SCVs and decreased SNAP amplitudes were observed in three of five SMA I patients but not in SMA II patients. Although FOs were reduced in both extremities of SMA I patients, the reduction was prominent in the tibial nerve of SMA II patients (p = 0.031). Loss of motor units may be widespread in the early stage of SMA I, while specific to the legs in young SMA II patients. SMA I showed sensory nerve degeneration, especially of large myelinated fibers. SMA II showed no sensory nerve abnormalities.