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Spinal muscular atrophy

Definition

Spinal muscular atrophy is a group of inherited diseases that cause progressive muscle degeneration and weakness, eventually leading to death.

Alternative Names

Werdnig-Hoffmann disease

Causes, incidence, and risk factors

Spinal muscular atrophy (SMA) is a collection of different muscle diseases. Grouped together, it is the second leading cause of neuromuscular disease. Most of the time, a person must get the defective gene from both parents to be affected. Approximately 4 out of every 100,000 people have the condition.

The most severe form is SMA type I, also called Werdnig-Hoffman disease. Infants with SMA type II have less severe symptoms during early infancy, but they become progressively weaker with time. SMA type III is the least severe form of the disease.

Rarely, SMA may begin in adulthood. This is usually a milder form of the disease.

A family history of spinal muscular atrophy is a risk factor for all types of the disorder.

Symptoms

Infants with SMA type I are born with very little muscle tone, weak muscles, and feeding and breathing problems. With SMA type III, symptoms may not appear until the second year of life.

Often, weakness is first noted in the shoulder muscles and proximal leg muscles. Weakness gets worse over time and will eventually become severe.

Symptoms in an infant:

• Breathing difficulty
• Feeding difficulty
• Floppy infant (poor muscle tone)
• Lack of head control
• Little spontaneous movement
• Progressive weakness (older infant to toddler)
• Very weak infant

Symptoms in a child:

• Frequent, increasingly severe respiratory infections
• Nasal speech
• Worsening posture

Signs and tests

The health care provider will take a careful history and perform a brain/nervous system (neurologic) examination to find out if there is:

• A family history of neuromuscular disease
• Floppy (flaccid) muscles
• No deep tendon reflexes
• Twitches (muscle fasciculation) of tongue muscle

Tests:

• CPK levels
• DNA testing to confirm diagnosis
• Electromyography
• MRI of the spine
• Muscle biopsy

Treatment

There is no treatment for the progressive weakness caused by the disease. Supportive care is important. Attention must be paid to the respiratory system because affected people have difficulty protecting themselves from choking. Breathing complications are common.

Physical therapy is important to prevent contractions of muscles and tendons and abnormal curvature of the spine (scoliosis). Bracing may be necessary.

Support Groups

Expectations (prognosis)

The lifespan in SMA type I is seldom longer than 2 - 3 years. Survival time with type II is longer, but the disease kills most of those affected while they are still children. Children with type III disease may survive into early adulthood. However, people with all forms of the disease have worsening weakness and debility.

Complications

• Aspiration
• Contractions of muscles and tendons
• Respiratory infections
• Scoliosis

Calling your health care provider

Call your health care provider if your child:

• Appears weak
• Develops any other symptoms of spinal muscular atrophy
• Has difficulty feeding

Breathing difficulty can rapidly become an emergency condition.

Prevention

Genetic counseling is recommended for prospective parents with a family history of spinal muscular atrophy.

References

Kliegman RM, Behrman RE, Jenson HB, Stanton BF. The hip. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF. Nelson Textbook of Pediatrics. 18th ed. Philadelphia, Pa: Elsevier; 2007:chap 606.