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

Causes

Spinal muscular atrophy (SMA) is the second leading cause of neuromuscular disease. It is inherited as an autosomal recessive trait (a person must get the defective gene from both parents to be affected) and has an incidence of approximately 4 per 100,000 people.

In its most severe form (SMA type I, also called Werdnig-Hoffman disease), infants are born floppy with weak, thin muscles and feeding and breathing problems. Their lifespan seldom exceeds 2 to 3 years. Infants with SMA type II have less severe symptoms during early infancy, but they become progressively weaker with time. Survival time with type II is longer, but the disease kills most of those affected while they are still children.

SMA type III is the least severe form of the disease, and symptoms may not appear until the second year of life. Often, weakness is first noted in the shoulder muscles and proximal leg muscles. Weakness is progressive and will eventually become profound, but children with type III disease may survive into early adulthood.

Rarely, SMA may begin in adulthood. This is usually a milder form of the disease. This form may be inherited in an autosomal dominant (only one copy of the gene is needed for the disease to occur) or autosomal recessive manner.

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

Symptoms

In an infant:

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

In a child:

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

Exams and Tests

• A family history of neuromuscular disease
• Absent deep tendon reflexes
• Flaccid muscles
• Muscle fasciculation
• Fasciculation of tongue muscle
• Elevated CPK levels

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

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 clearing secretions. Respiratory complications are common.

Physiotherapy is important to prevent contractures and scoliosis. Bracing may be necessary.

Outlook (Prognosis)

The most severe form of spinal muscular atrophy is fatal early on. The less severe form is compatible with a longer lifespan. However, progressive weakness and debility are present in all forms.

Possible Complications

• Contractures
• Scoliosis
• Respiratory infections
• Aspiration

When to Contact a Medical Professional

Call your health care provider if your child appears weak, has difficulty feeding, or develops any other symptoms of spinal muscular atrophy. Breathing difficulty can rapidly become an emergency condition.

Prevention

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

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