Clinical study of proximal spinal muscular atrophy. Report on 89 cases

Clinical characteristics of three subtypes of spinal muscular atrophy in children

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Deletions in the SMN gene in infantile and adult spinal muscular atrophy patients from the same family

Recently, a gene determining spinal muscular atrophy (SMA), termed survival motor neuron (SMN) gene, has been isolated from the 5q13 region. This gene has been found to be deleted in most patients with childhood-onset SMA. We have studied the SMN gene in a clinically heterogeneous family, including one patient affected by infantile chronic SMA and three subjects with mild adult-onset muscle weakness. Deletions in the SMN gene were detected in all of these patients, indicating that the childhood and adult SMAs are genetically homogeneous in this family. Genotyping of the family members established that the three mildly affected individuals were homozygous for the same haplotype from the SMA region, whereas the more severely affected patient was heterozygous with one different haplotype.

Autosomal recessive proximal spinal muscular atrophy in 101 sibs out of 48 families: clinical picture, influence of gender, and genetic implications

We analysed the clinical picture of 101 sibs (43 sib pairs, 5 triplets) with autosomal recessive proximal spinal muscular atrophy (SMA). Linkage data of 20 sibships, which were available for analysis, were in agreement with chromosome 5q linkage. The patients were classified according to the motor development into SMA I (never sat), SMA II (sitting without support), and SMA III (walking without aids). Three sibs with adult onset (> 30 years = SMA IV) were discussed as a separate entity. Age-of-onset of the 101 patients showed a wide spectrum (prenatal to 47 years). Among sib pairs with SMA I and SMA II the ages-of-onset appeared to be very similar except of one atypically discordant sib pair. With regard to SMA III, 3 out of 13 sibships (23%) showed a marked variation in age-of-onset ranging from 5-15 years within a family. Concerning acquired motor development (ability to sit and walk), 7 sibships (15%) belonged to different SMA types. Ages of death in 29 sib pairs in whom at least one sib had died before the age of 20 years were strikingly discordant. Neither the degree of disability nor the respiratory deficits are reliable predictors of life expectancy. Although a predominance of males can be observed, no significant effect of gender has been established in familial cases. The existence of multiple allelism seems to be the most suitable explanation for the high interfamilial variability considering the clinical concordance in most affected sib pairs.