Spinal muscular atrophy

Causes

Spinal muscular atrophy (SMA) is a collection of different muscle diseases. Grouped together, it is the second leading cause of neuromuscular disease, after Duchenne muscular dystrophy.

Most of the time, a person must get the defective gene from both parents to be affected. About 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 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 II, symptoms may not appear until age 6 months to 2 years.
• Type III SMA is a milder disease that starts in childhood or adolescence and slowly gets worse.
• Type IV is even milder, with weakness starting in adulthood.

Often, weakness is first felt in the shoulder and leg muscles. Weakness gets worse over time and eventually becomes severe.

Symptoms in an infant:

• Breathing difficulty, leading to a lack of oxygen
• Feeding difficulty (food may go into the windpipe instead of the stomach)
• Floppy infant (poor muscle tone)
• Lack of head control
• Little movement
• Weakness that gets worse

Symptoms in a child:

• Frequent, increasingly severe respiratory infections
• Nasal speech
• Posture that gets worse

Exams 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 the tongue muscle

Tests:

• Aldolase
• Erythrocyte sedimentation rate (ESR)
• CPK levels
• DNA testing to confirm diagnosis
• Electromyography (EMG)
• Lactate/pyruvate
• MRI of the spine
• Muscle biopsy
• Nerve conduction
• Serum amino acids
• Thyroid-stimulating hormone (TSH)

Treatment

There is no treatment for the weakness caused by the disease. Supportive care is important. Attention must be paid to the respiratory system because affected people have trouble 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 needed.

Outlook (Prognosis)

People with SMA type I rarely live longer than 2 - 3 years because of respiratory problems and infections. Survival time with type II is longer, but the disease kills most of those who are 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 weakness and debility that gets worse over time.

Possible Complications

• Aspiration (food and fluids get into the lungs, causing pneumonia)
• Contractions of muscles and tendons
• Heart failure
• Scoliosis

When to Contact a Medical Professional

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 people with a family history of spinal muscular atrophy who want to have children.

Alternative Names

Werdnig-Hoffmann disease

References

Sarnat HB. Spinal muscular atrophies. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF. Nelson Textbook of Pediatrics. 19th ed. Philadelphia, Pa: Elsevier; 2011:chap 604.2.

Update Date: 2/1/2012

Updated by: Neil K. Kaneshiro, MD, MHA, Clinical Assistant Professor of Pediatrics, University of Washington School of Medicine; Luc Jasmin, MD, PhD, Department of Neurosurgery at Cedars-Sinai Medical Center, Los Angeles, and Department of Anatomy at UCSF, San Francisco, CA. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix Inc.