Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a group of genetic diseases that affects the part of the nervous system that controls voluntary muscle movements, such as crawling, walking, head and neck control, and swallowing. The disease causes weakness and wasting of the voluntary muscles.

SMA affects infants, children and adults. The condition occurs at an estimated rate of 1 in every 6,000 births. Childhood SMA is an autosomal recessive disease, meaning it runs in families. Children inherit a gene from both their mother and father, although the parents may have no symptoms. An estimated 1 in 40 "normal" people are carriers of the SMA gene. If both a man and woman carry the gene, their child has a 25 percent chance of developing the condition.

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Most nerve cells that control muscles, called motor neurons, are located in the spinal cord. Motor neurons send electrical and chemical messages to the muscles. In SMA, the motor neurons don't send enough signals and the muscles don't function properly and deteriorate.

The Neuromuscular Clinic at UCSF Benioff Children's Hospital is sponsored by the Muscular Dystrophy Association. It specializes in the diagnosis, treatment and research of SMA and other neuromuscular disorders that affect muscles and nerves. Our pediatric neurologists and orthopedic surgeons are among the nation’s leading SMA experts.

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Each child with spinal muscular atrophy (SMA) may experience symptoms differently. There are three main types of SMA, which are defined by their symptoms and the time symptoms first develop.

Acute Infantile SMA (Type I)

Also known as Werdnig-Hoffman disease, this form of SMA is the most severe. Some children develop the disease before birth. Mothers may notice that during the last three months of pregnancy, fetal movements are very weak. The majority of children with this form of the disease will experience symptoms before 8 months of age.

The condition primarily affects the muscles that control chewing and swallowing, chest wall muscles, and arm and leg muscles. Symptoms are typically severe and may include hypotonia or diminished muscle tone, muscle weakness, respiratory problems, pneumonia, and swallowing and feeding difficulties.

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Your doctor will conduct a thorough physical exam of your child, as well as record a family medical history. These are very important steps in making a diagnosis of spinal muscular atrophy (SMA) and in understanding your child's pattern of symptoms.

To make a definite diagnosis and rule out other disorders, your doctor will recommend the following tests:

Genetic Testing

Two genes that contribute to SMA are the "survival motor neuron gene (SMN)" and the "neuronal apoptosis inhibitory protein gene (NAIP)." In over 95 percent of patients with SMA, genetic defects or changes in the SMN gene are detected.

Although we don't fully understand how the gene abnormality causes the disease, the discovery of the SMN gene has allowed doctors to make a definite diagnosis of SMA and provide specific genetic counseling to those affected as well as to SMA carriers. It has led to a number of research projects, including developing and testing new treatments in patients.

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Children with spinal muscular atrophy (SMA) are treated at our Pediatric Muscular Dystrophy Center. They require ongoing, specialized care from experts, including neurologists, orthopedists, pulmonologists and surgeons. Although there is no cure for the disease, its symptoms and complications can be successfully managed.

Several medications are being studied for this disease. Treatments may include:


Riluzole blocks certain proteins in the central nervous system that may cause damage to nerve cells. This drug is prescribed for adults with amyotrophic lateral sclerosis (ALS), also called Lou Gerhig's disease, which is similar to SMA. Preliminary studies have suggested a benefit for some children with SMA, but doctors believe that more studies must be completed before the benefits for children are confirmed.

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UCSF Research & Clinical Trials

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Reviewed by health care specialists at UCSF Benioff Children's Hospital.

Related Information

UCSF Clinics & Centers

Pediatric Brain Center

Neuromuscular Clinic
1825 Fourth St., Sixth Floor
San Francisco, CA 94158
Phone: (415) 353-7596
Fax: (415) 353-2400
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