7 - Spinal muscular atrophies (SMAs)

Abstract

This chapter discusses the clinical features and genetics of spinal muscular atrophies (SMAs). These disorders clinically resemble the muscular dystrophies and the chronic forms have often been so misdiagnosed in the past. Although there are distinguishing features, namely the presence of fasiculations, tremor, and the early loss of tendon reflexes, if these are absent the distinction between a muscular dystrophy and a neurogenic atrophy is difficult and must be assisted by electromyography and muscle biopsy. Even then there may be difficulty as some patients display a mixture of neurogenic and myopathic features. The creatine kinase level in SMA is often mildly increased, however not to the gross elevations seen in Duchenne and Becker muscular dystrophy. Tendon reflexes are nearly always reduced or absent in SMA; in 5 to 7% of patients they are normal; and in about 2% some tendon reflexes are hyperactive, but in such patients other reflexes are diminished or absent. Therefore, the patients with neurogenic muscular atrophy, who also have widespread brisk reflexes and extensor plantar responses, are classified as having the progressive muscular atrophy variety of motor neuron disease.