Clinical characteristics of three subtypes of spinal muscular atrophy in children

The severity of spinal muscular atrophy (SMA) is highly variable and children with heterogeneous clinical features can be classified into three phenotypes (type I-III) on the basis of age of onset and maximum motor function achieved. The aim of this study was to compare the clinical characteristics of three phenotypes in children with SMA.

One hundred and thirty-two SMA patients were classified as type I, II or III according to the SMA classification criteria. The clinical features, deletion of survival motor neuron 1 (SMN1) gene and electrophysiology were analyzed and compared. The survival and functional status were obtained through telephone follow up.

In our study, 90.6% of the patients lacked both copies of SMNl. No difference in the deletion frequency among the 3 groups was observed. Although most of the neurophysiological parameters showed no differences among the groups, the amplitudes of compound muscle action potential (CMAP) was lower in type III SMA. Absent sensory nerve action potential (SNAP) amplitude of the sural nerve was observed in 26 (25.4%) of the patients. The survival pattern and functional status of 66 cases were obtained. Two type II SMA patients could walk unaided during follow-up. The functional ability of lower extremities improved in 4 patients with type III SMA.

In this study, we confirm that EMG examination and homozygous deletion of SMN1 do not correlate with the subtypes. Motor function of patients with SMA type II and III can improve. A period of follow-up is necessary before rendering accurate classification and prognosis.