About SMA

Spinal Muscular Atrophy (SMA) is a motor neuron disease. The motor neurons affect the voluntary muscles that are used for activities such as crawling, walking, head and neck control, and swallowing. It is a relatively common "rare disorder"; approximately 1 in 6000 babies born are affected, and about 1 in 40 people are genetic carriers.

SMA affects muscles throughout the body, although the proximal muscles (those closest to the trunk of one’s body - i.e. shoulders, hips, and back) are often most severely affected. Weakness in the legs is generally greater than in the arms. Sometimes feeding and swallowing can be affected. Involvement of respiratory muscles (muscles involved in breathing and coughing) can lead to an increased tendency for pneumonia and other lung problems. Sensation and the ability to feel are not affected. Intellectual activity is normal and it is often observed that patients with SMA are unusually bright and sociable. Patients are generally grouped into one of four categories, based on certain key motor function milestones.

What Causes Spinal Muscular Atrophy?

SMA is an autosomal recessive genetic disease. In order for a child to be affected by SMA, both parents must be carriers of the abnormal gene and both must pass this gene on to their child. Although both parents are carriers the likelihood of a child inheriting the disorder is 25%, or 1 in 4.

An individual with SMA has a missing or mutated gene (SMN1, or survival motor neuron 1) that produces a protein in the body called Survival Motor Neuron (SMN) protein. This protein deficiency has its most severe affect on motor neurons. Motor neurons are nerve cells in the spinal cord which send out nerve fibers to muscles throughout the body. Since SMN protein is critical to the survival and health of motor neurons, without this protein nerve cells may atrophy, shrink and eventually die, resulting in muscle weakness.

As a child with SMA grows their bodies are doubly stressed, first by the decrease in motor neurons and then by the increased demands on the nerve and muscle cells as their bodies grow larger. The resulting muscle atrophy can cause weakness and bone and spinal deformities that may lead to further loss of function, as well as additional compromise of the respiratory (breathing) system.

There are four types of SMA, SMA Type I, II, III, IV. The determination of the type of SMA is based upon the physical milestones achieved. It is important to note that the course of the disease may be different for each child.

Diagnosing Spinal Muscular Atrophy

SMA is diagnosed primarily through a blood test, which looks for the presence or absence of the SMN1 gene, in conjunction with a suggestive history and physical examination.

Normally, individuals have two genes called Survival Motor Neuron 1 and 2. In approximately 95% of patients with SMA there is an absence of the SMN gene sequence, which is present in normal individuals. Sometimes the SMN1 gene is not missing, but mutated. The numbers of copies of SMN2, a near identical backup copy of the SMN1 gene, is related to the severity of the disease, but does not reliably predict a specific SMA type in a given individual. SMA type is generally determined from the clinical examination evaluating the child’s degree of weakness and ability to achieve major motor milestones such as sitting independently or walking.

Occasionally, doctors may request muscle biopsy or EMG (electromyography) testing. Since the genetic blood test became available, a muscle biopsy is almost never indicated and is valuable mainly in cases where the blood DNA test is negative.

EMG measures the electrical activity of muscle. Sometimes this test is performed to help distinguish other disorders of nerve or muscle, which can mimic SMA. Small recording electrodes (needles) are inserted into the patient's muscles, usually the arms and thighs, while an electrical pattern is observed and recorded. In addition, a nerve conduction velocity test (NCV) is performed to help assess how well the nerves are functioning in response to an electrical stimulus. Small shocks are repeatedly administered to help assess nerve integrity and function. When performing this test on a child, if at all possible, it should be performed by a doctor experienced in caring for children.

For more information visit our national website at www.curesma.org.