Spinal Muscular Atrophy (SMA) refers to a group of diseases which affect the motor neurons of the spinal cord and brain stem. These critically important cells are responsible for supplying electrical and chemical messages to muscle cells. Without the proper input from the motor neurons, muscle cells can not function properly. The muscle cells will, therefore, become much smaller (atrophy) and will produce symptoms of muscle weakness. There are dozens of diseases which affect the motor neuron.
*Spinal Muscular Atrophy kills more babies than any other genetic disease.
Degeneration and death of the motor neurons (also called Anterior Horn Cells) in the brain stem and spinal cord produces weakness in the muscles of swallowing, breathing, and limbs. This disease afflicts infants, children, and adults worldwide. It is estimated that spinal muscular atrophy occurs in between one-in-6,000 and one-in-20,000 births. Advances in our understanding of the genetics of this disorder confirm that the majority of children and adults afflicted with SMA, have inherited this disorder by receiving one gene from both their mother and their father. This is termed “autosomal recessive genetic transmission.”
Between one-in-40 and one-in-80 “normal” men and women carry the gene for spinal muscular atrophy. If both a man and woman carry the gene, the chances are 25% that any of their children will manifest SMA.
There are many types of SMA, and some of them are fatal. Life expectancy depends on the type you have and how it affects your breathing. There is no cure. Medicines and physical therapy help treat symptoms
Symptoms of Spinal Muscular Atrophy
Infants with SMA type I are born with very little muscle tone, weak muscles, and feeding and breathing problems. With SMA type III, symptoms may not appear until the second year of life.
Often, weakness is first noted in the shoulder muscles and proximal leg muscles. Weakness gets worse over time and will eventually become severe.
Symptoms in an infant:
- Breathing difficulty
- Feeding difficulty
- Floppy infant (poor muscle tone)
- Lack of head control
- Little spontaneous movement
- Progressive weakness (older infant to toddler)
- Very weak infant
Symptoms in a child:
- Frequent, increasingly severe respiratory infections
- Nasal speech
- Worsening posture
What Cause Spinal Muscular Atrophy
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.
More: Spinal Muscular Atrophy can mimic child abuse
More: Spinal Muscular Atrophy can mimic child abuse
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.
Treatment for Spinal Muscular Atrophy
There is no treatment for the progressive weakness caused by the disease. Supportive care is important. Attention must be paid to the respiratory system because affected people have difficulty protecting themselves from choking. Breathing complications are common.
Physical therapy is important to prevent contractions of muscles and tendons and abnormal curvature of the spine (scoliosis). Bracing may be necessary.
Contractions of muscles and tendons
Respiratory infections
Scoliosis
Physical therapy is important to prevent contractions of muscles and tendons and abnormal curvature of the spine (scoliosis). Bracing may be necessary.
Prognosis and Life Expectancy for Spinal Muscular Atrophy
The lifespan in SMA type I is seldom longer than 2 - 3 years. Survival time with type II is longer, but the disease kills most of those affected while they are still children. Children with type III disease may survive into early adulthood. However, people with all forms of the disease have worsening weakness and debility.Complications of SMA
AspirationContractions of muscles and tendons
Respiratory infections
Scoliosis
Sources:
http://www.fightsma.org/sma-guidebook/what-is-sma/
http://www.fightsma.org/sma-guidebook/what-is-sma/
http://www.fsma.org
http://health.nytimes.com/health/guides/disease/spinal-muscular-trophy/overview.html
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