Spinal muscular atrophy (SMA) Types I, II, and III belong to a group of hereditary diseases that cause weakness and wasting of the voluntary muscles in the arms and legs of infants and children.
The disorders are caused by an abnormal or missing gene known as the survival motor neuron gene (SMN1), which is responsible for the production of a protein essential to motor neurons. Without this protein, lower motor neurons in the spinal cord degenerate and die.
The type of SMA (I, II, or III) is determined by the age of onset and the severity of symptoms. Type I has the earliest onset, usually at birth, and the most severe symptoms. Type II usually happens in early childhood and is less severe but still disabling. Type III can happen as late as adolescence and may be only moderately disabling.
There are other types of SMA disorders with similar symptoms, but different causes. Infantile SMA disorders, such as X-linked infantile SMA, SMA with cerebellar hypoplasia, diaphragmatic SMA, and SMA with congenital bone fractures are linked to genes other than SMN1. Kennedy syndrome (X-linked spinal and bulbar muscular atrophy, SMAX1), a disease of adult males, has symptoms similar to the childhood SMAs, but is caused by a different gene and genetic mutation.
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 electrodiagnosis with nerve conduction velocities (EMG), and muscle biopsy.
There is no cure for SMA. Treatment consists of managing the symptoms and preventing complications. Treatment for specific symptoms and common complications are described below.
Breathing issues: Babies with SMA (especially those with Type I) may need help breathing, especially at night, using non-invasive methods that include negative pressure ventilators and bi-level positive airway pressure support, which direct air through the nostrils via a small, gently-fitted mask. Children who survive their first two years are at risk for complications involving the lungs, which may not be fully developed. A regular program of respiratory therapy and breathing exercises is helpful. Parents should be instructed in chest physiotherapy (CPT), a series of physical maneuvers that clear the lungs and airway.
Failure to thrive: Infants with SMA I may have difficulties getting adequate nutrition because they have a weak sucking reflex and tendency to tire easily. Their unprotected air passage makes it difficult for older babies to chew and swallow; they may inhale and choke on their food. Some babies may require feeding with naso-gastric or gastric tubes.
Weak arms and legs: Children with SMA Types I and II are not likely to stand or walk on their own. They can be taught to operate a power wheelchair at two to three years of age. Less handicapped children may benefit from a standing frame, vertical stander, or standing wheelchair. Physical therapy and exercise may also help improve mobility and joint movement, brighten mood, and improve sleep patterns. Stretching exercises can preserve and increase flexibility.
Orthopedic complications. Scoliosis (curvature of the spine) occurs at some point in the majority of children with SMA Types I and II, and some with Type III. Custom seating systems, seating aids, and a body jacket can be used to prevent severe scoliosis. Spinal fusion surgery may be necessary for some children.
The prognosis is poor for babies with SMA Type I. Most die within the first two years. For children with SMA Type II, the prognosis for life expectancy or for independent standing or walking roughly correlates with how old they are when they first begin to experience symptoms - older children tend to have less severe symptoms. Some children may have a normal life expectancy and learn to walk with the aid of a brace. Others may die from respiratory infections, such as pneumonia. Children with onset after 18 months are often able to walk and are fully functional for years before they need assistance. They may have a normal life expectancy.
The National Institute for Neurological Disorders and Stroke (NINDS) conducts research on SMA in laboratories at the NIH and also supports research through grants to major medical institutions across the country.
Between 2003 and 2012, the NINDS piloted the Spinal Muscular Atrophy Project to expedite therapeutics development for this hereditary neurodegenerative disease. The Project was designed to accelerate the research process by identifying drugs already in use that increase the level of SMN protein in cultured cells, so that they could be used as potential leads for further drug discovery and clinical testing. Read more about the history of this pioneering effort and how it led to collaboration with several pharmaceutical and biotechnology companies. (LINK TO SUMMARY)
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
|Families of Spinal Muscular Atrophy
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
NIH Publication No. 12-5597
Office of Communications and Public Liaison
National Institute of Neurological Disorders and Stroke
National Institutes of Health
Bethesda, MD 20892
NINDS health-related material is provided for information purposes only and does not necessarily represent endorsement by or an official position of the National Institute of Neurological Disorders and Stroke or any other Federal agency. Advice on the treatment or care of an individual patient should be obtained through consultation with a physician who has examined that patient or is familiar with that patient's medical history.
All NINDS-prepared information is in the public domain and may be freely copied. Credit to the NINDS or the NIH is appreciated.
Last updated May 2, 2014