Spinal muscular atrophy

Introduction
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By Barry S Russman MD and Erika Hedderick MD

Spinal muscular atrophy is also known as or subsumes Kugelberg-Welander disease and Werdnig-Hoffman disease. -ed

Spinal muscular atrophy presents with proximal muscle weakness of the upper and lower extremities, the latter being weaker than the former, at least initially. The condition is caused by a deletion of exon 7 on chromosome 5. Other SMAs exist that are not linked to 5Q. Current classification is based on clinical criteria, including age of onset and maximum function attained. The diagnosis is established by a DNA test for the SMN gene. Further testing, including EMG and muscle biopsy are unnecessary. To date, therapeutic trials have failed to reveal a specific treatment for this condition.

Key Points

  • Slowly progressive proximal muscle weakness starting as early as birth and as late as the teenage years, which is associated with a peculiar tremor of the fingers and is especially noted in patients older than the age of 1 year, is almost pathognomonic of spinal muscular atrophy 5q.
  • It is an autosomal recessive condition with a carrier rate of approximately 1 in 50 to 1 in 100.
  • A diagnosis can be made by genetic testing for the 5q deletion or sequence alteration.
  • There is no disease-directed therapy, but prognosis is improved by management of associated complications (ie, pulmonary failure, scoliosis).