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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. That is, two genes are required, one from each parent (see Genetics: Sex Chromosomes). These disorders may also affect the central nervous system. There are four main types of spinal muscular atrophy.
Symptoms
Symptoms of four main types first appear during infancy and childhood.
In acute (type I) spinal muscular atrophy (Werdnig-Hoffmann disease), muscle weakness is usually apparent at or within a few days of birth. It is virtually always apparent by age 6 months. Infants lack muscle tone and reflexes and have difficulty sucking, swallowing, and eventually breathing. Death occurs in 95% of children within the first year and in all by age 4 years, usually due to respiratory failure.
In children with intermediate (type II) spinal muscular atrophy, weakness typically develops between age 6 and 15 months. Fewer than one fourth of them learn to sit. None can crawl or walk. Muscles are weak, and swallowing may be difficult. 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. These children often have severe curvature of the spine (scoliosis).
Chronic (type III) spinal muscular atrophy (Wohlfart-Kugelberg-Welander disease) begins in children between age 15 months and 19 years and worsens slowly. Consequently, people with this disorder usually live longer than those with type I or II spinal muscular atrophy. Some of them have a normal life span. Weakness and wasting of muscles begin in the hips and thighs and later spread to the arms, feet, and hands.
Type IV spinal muscular atrophy first appears during adulthood, usually between age 30 and 60 years. Muscles, mainly in the hips, thighs, and shoulders, become weak and waste away.
Diagnosis and
Treatment
Doctors usually 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 and nerve conduction studies (see Diagnosis of Brain, Spinal Cord, and Nerve Disorders: Electromyography and Nerve Conduction Studies) help confirm the diagnosis. The specific defective gene has been identified for some of the types and can be detected by blood tests. Occasionally, biopsy of a muscle is done. If there is a family history of one of the disorders, amniocentesis can be done to help determine whether an unborn child has the defective gene.
No specific treatments are available. Physical therapy and wearing braces can sometimes help. Physical and occupational therapists can provide adaptive devices to enable children to feed themselves, write, or use a computer.
Last full review/revision February 2008 by Michael Rubin, MD
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