Spinal Muscular Atrophies

Spinal muscular atrophies are hereditary disorders in which nerve cells in the spinal cord and brain stem degenerate, causing progressive muscle weakness and wasting.

The disorders are usually inherited as a recessive autosomal (not sex-linked) trait: two genes are required, one from each parent (see Section 1, Chapter 2). There are three main types of spinal muscular atrophy.

Symptoms

Symptoms first appear during infancy and childhood. In acute (type I) spinal muscular atrophy (Werdnig-Hoffmann disease), muscle weakness is apparent at or within a few days of birth. Death occurs in 95% of children by age 1½ years and in all by age 4 years, usually due to respiratory failure.

In children with intermediate (type II) spinal muscular atrophy, weakness develops by age 6 months. Most children are confined to a wheelchair by age 2 to 3 years. The disorder is often fatal in early life, usually because of respiratory problems, but some children survive with permanent weakness that does not continue to worsen.

Chronic (type III) spinal muscular atrophy (Wohlfart-Kugelberg-Welander disease) begins in children between the ages of 5 and 15 years and worsens slowly. Consequently, people with this disorder usually live longer than those with type I or II spinal muscular atrophy. Weakness and wasting of muscles begin in the legs and later spread to the arms.

Diagnosis and Treatment

Doctors test for these rare disorders when unexplained weakness and muscle wasting occur in young children. Because these disorders are inherited, a family history may help doctors make the diagnosis. Electromyography (see Section 6, Chapter 78) also helps. The specific defective gene has been identified for some of the types and can be detected by blood tests. If there is a family history of one of the disorders, amniocentesis can be performed to help determine whether an unborn child has the defective gene.

No specific treatments are available. Physical therapy and wearing braces can sometimes help.