Spinal muscular atrophy (SMA) is one of several hereditary diseases that progressively destroy lower motor neurons—nerve cells in the brain stem and spinal cord that control essential voluntary muscle activity such as speaking, walking, breathing, and swallowing. Lower motor neurons control movement in the arms, legs, chest, face, throat, and tongue.
When there are disruptions in the signals between lower motor neurons and muscles, the muscles gradually weaken and may begin wasting away and develop uncontrollable twitching (called fasciculations). When there are disruptions in the signals between the upper motor neurons (located in the brain) and the lower motor neurons, the limb muscles develop stiffness (called spasticity), movements become slow and effortful, and tendon reflexes such as knee and ankle jerks become overactive. Over time, the ability to control voluntary movement can be lost.
SMA is caused by defects in the gene SMN1, which makes a protein that is important for the survival of motor neurons (SMN protein). In SMA, insufficient levels of the SMN protein lead to degeneration of the lower motor neurons, producing weakness and wasting of the skeletal muscles. This weakness is often more severe in the trunk and upper leg and arm muscles than in muscles of the hands and feet.
SMA disorders in children are inherited in an autosomal recessive manner. Autosomal recessive means the child must inherit a copy of the defective gene from both parents. These parents are likely to be asymptomatic (without symptoms of the disease). Autosomal recessive diseases often affect more than one person in the same generation (siblings or cousins).
Kennedy’s disease, an adult form of SMA is X-linked inherited, which means the mother carries the defective gene on one of her X chromosomes and passes the disorder along to her sons. Males inherit an X chromosome from their mother and a Y chromosome from their father, while females inherit an X chromosome from each parent. Daughters have a 50 percent chance of inheriting their mother's faulty X chromosome and a safe X chromosome from their father, which would make them asymptomatic carriers of the mutation.
SMA in children is classified into three types, based on ages of onset, severity, and progression of symptoms. All three types are caused by defects in the SMN1 gene.
Other forms of SMA include:
A blood test is available that can indicate whether there are deletions or mutations of the SMN1 gene. This test identifies at least 95 percent of SMA Types I, II, and III. Other diagnostic tests may include electromyography (which records the electrical activity from the brain and/or spinal cord to a peripheral nerve root found in the arms and legs that controls muscles during contraction and at rest), nerve conduction velocity studies (which measure electrical energy by assessing the nerve’s ability to send a signal), muscle biopsy (used to diagnose neuromuscular disorders and may also reveal if a person is a carrier of a defective gene that could be passed on to children), and laboratory tests of blood, urine, and other substances.
There is no cure for SMA. Treatment consists of managing the symptoms and preventing complications.
Muscle relaxants such as baclofen, tizanidine, and the benzodiazepines may reduce spasticity. Botulinum toxin may be used to treat jaw spasms or drooling. Excessive saliva can be treated with amitriptyline, glycopyolate, and atropine or by botulinum injections into the salivary glands. Antidepressants may be helpful in treating depression.
Physical therapy, occupational therapy, and rehabilitation may help to improve posture, prevent joint immobility, and slow muscle weakness and atrophy. Stretching and strengthening exercises may help reduce spasticity, increase range of motion, and keeps circulation flowing. Some individuals require additional therapy for speech, chewing, and swallowing difficulties. Applying heat may relieve muscle pain. Assistive devices such as supports or braces, orthotics, speech synthesizers, and wheelchairs may help some people retain independence.
Proper nutrition and a balanced diet are essential to maintaining weight and strength. People who cannot chew or swallow may require insertion of a feeding tube. Non-invasive ventilation at night can prevent apnea in sleep, and some individuals may also require assisted ventilation due to muscle weakness in the neck, throat, and chest during daytime.
Prognosis varies depending on the type of SMA. Some forms of SMA are fatal.
The course of Kennedy’s disease varies but is generally slowly progressive. Individuals tend to remain ambulatory until late in the disease. The life expectancy for individuals with Kennedy disease is usually normal.
The National Institute of Neurological Disorders and Stroke (NINDS), a component of the National Institutes of Health (NIH), conducts basic, translational, and clinical research on SMA in laboratories at the NIH and also supports research through grants to major medical institutions across the country.
Cellular and molecular studies seek to understand the mechanisms that trigger motor neurons to degenerate.
Scientists are developing a broad range of model systems in animals and cells to investigate disease processes and expedite the testing of potential therapies. Among these efforts:
NINDS has established the NeuroNext clinical trials network to promote the rapid development and implementation of trials for neurological disorders that affect adults and/or children. Among its goals, the network is designed to develop early-phase trials aimed at identifying biomarkers—usually a physical trait or substance in the blood or other bodily fluids that can be measured to determine the presence and severity of a disease—and testing promising, emerging therapies. One of the first projects in this new network will hope to identify biomarkers for SMA, which could speed the development of effective treatments for the disease.
For more information on neurological disorders or research programs funded by the National Institute of Neurological Disorders and Stroke, contact the Institute's Brain Resources and Information Network (BRAIN) at:
P.O. Box 5801
Bethesda, MD 20824
Information also is available from the following organizations:
1321 Duke Street
Alexandria, VA 22134
925 Busse Road
Elk Grove Village, IL 60007
Spinal Muscular Atrophy Foundation
888 Seventh Avenue
New York, NY 10019
Tel: 877-FUND-SMA (877-386-3762) 646-253-7100
Muscular Dystrophy Association
National Office - 222 S. Riverside Plaza
Chicago, IL 60606
"Spinal Muscular AtrophyFact Sheet", NINDS, Publication date May 15, 2015.
NIH Publication No. 12-5597
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National Institute of Neurological Disorders and Stroke
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Last Modified May 15, 2015