Spinal Muscular Atrophy

Spinal Muscular Atrophy

Description, Causes and Risk Factors:

Spinal Muscular Atrophy (SMA) is a genetic neuromuscular disease characterized by muscle atrophy and weakness. The disease generally manifests early in life and is the leading genetic cause of death in infants and toddlers. SMA is caused by defects in the Survival Motor Neuron 1 (SMN1) gene that encodes the SMN protein. The SMN protein is critical to the health and survival of the nerve cells in the spinal cord responsible for muscle contraction (motor neurons).

SMA has generally been believed to affect as many as 10,000 to 25,000 children and adults in the United States, and therefore it is one of the most common rare diseases. One in 6,000 to one in 10,000 children are born with the disease. One in 50 people (approximately 6 million Americans) are carriers of the SMA gene.


    Type I SMA: Patients typically exhibit limited movement and have difficulty holding their head straight, feeding, and swallowing. Reduced strength in the chest muscles often results in labored breathing with the chest appearing sunken. The progressive weakening of the muscles leads to respiratory infections, lung collapse and eventual death, usually by the age of two years. About 60% of patients with SMA are born with this form of the disease.

  • Type II SMA: Symptoms usually emerge in patients between six and eighteen months, and the progression of symptoms varies greatly. Infants and children with this form of the disease are at one time able to sit unassisted, but do not walk independently. Due to the varied progression of symptoms, life expectancy ranges from early childhood to adulthood. The majority of Type II patients live into adulthood.

  • Type III SMA: Symptoms typically appear between eighteen months and early adulthood. People with SMA Type III often exhibit difficulty walking, have mild muscle weakness and are at risk for respiratory infections. These patients have a normal life expectancy.

  • Type IV SMA: A less common form of SMA that afflicts adults and is characterized by a slower progression of symptoms that particularly affect walking. Symptoms typically emerge after age 35.

Spinal muscular atrophy is an autosomal recessive disease. This means that both males and females are equally affected, and that two copies of the gene, one inherited from each parent, are necessary to have the condition.

A gene called survival motor neuron (or SMN) is found to have an abnormal area (either a deletion or extra copy of the gene) in over 95 percent of cases of SMA. Symptomatic individuals of all ages can be tested through DNA studies typically done from a blood sample.

When both parents are carriers, there is a one in four, or 25 percent, chance, with each pregnancy, to have a child with SMA. Carrier testing for the general population is technically difficult and usually available only if a previously affected child in the family has been studied.


SMA is characterized by a progressive loss of muscle control and movement and increasing weakness due to the loss of motor neurons in the spinal cord. Proximal muscles (muscles closest to the center of the body, such as those in the trunk and neck) are more severely affected than distal muscles (muscles furthest from the center of the body, such as those in the hands and feet). Normal growth and development can place additional demands on already weakened muscles. There is a wide range of severity of SMA; however, even in its moderate form, SMA can limit function and mobility. People with SMA either never acquire, or progressively lose, the ability to walk, stand, sit and eventually move. Although the disease varies in terms of age of onset and severity, most patients start to show symptoms during infancy or as toddlers. People with SMA often develop bone and/or spinal deformities which may require surgical treatment. Respiratory illnesses may be more common for people living with SMA and can be severe.


The diagnosis of spinal muscular atrophy is made after the sudden or gradual onset of specific symptoms and after diagnostic testing. During the physical examination, your child's physician will obtain a complete medical history of your child, and he/she may also ask if there is a family history of any medical problems.

Diagnostic tests that may be performed to confirm the diagnosis of spinal muscular atrophy include the following:

    Blood tests.

  • Muscle biopsy - a small sample of the muscle is removed and examined to determine and confirm a diagnosis or condition.

  • Genetic tests - diagnostic tests that evaluate for conditions that have a tendency to run in families.

  • Electromyogram (EMG) - a test that measures the electrical activity of a muscle or a group of muscles. An EMG can detect abnormal electrical muscle activity due to diseases and neuromuscular conditions.


There is no known cure for spinal muscular atrophy. The key to medically managing spinal muscular atrophy is through early detection. The goal of treatment is to prevent respiratory problems and provide adequate nutritional care to the child since the swallowing and breathing muscles can be affected by this condition. Good multidisciplinary care, including physical therapy, occupational therapy, respiratory therapy, and nutritional support, can improve quality and length of life for people with SMA and is recommended. It is important to note that children with SMA undergo normal intellectual and emotional development and, with help and support, can participate in many childhood activities.

Specific treatment for spinal muscular atrophy will be determined by your physician based on:

    Age, overall health, and medical history.

  • The extent of the condition.

  • The type of spinal muscular atrophy.

  • Tolerance for specific medications, procedures, or therapies.

  • Expectations for the course of the condition.

  • Your opinion or preference.

The extent of the problem is dependent on the severity of the condition and the presence of other problems that could affect the child. In severe cases, a breathing machine may be required to help the child breathe easier.

The healthcare team educates the family after hospitalization on how to best care for their child at home and outlines specific clinical problems that require immediate medical attention by his/her physician. A child with spinal muscular atrophy requires frequent medical evaluations throughout his/her lifespan.

NOTE: The above information is for processing purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.


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