Spinal muscular atrophy, or SMA, is an inherited condition that causes nerve cells in the lower part of the brain and spinal cord to break down and die. When this happens, the brain stops sending signals to the body's muscles.
Babies with SMA can have difficulty crawling, walking, or even breathing, and older kids can have trouble getting around and mastering simple tasks of everyday life, like combing their hair.
Much research has been done to identify the genes that cause SMA, but currently, there's still no cure for the disease. Despite the limitations SMA causes, many kids who have it learn to live as independently as possible with the help of physical and occupational therapy and by getting treatment for the disease's most troubling symptoms.
Normally, healthy nerve cells in the brain called motor neurons send messages to the body's muscles to tell them when and how to move. But SMA damages these brain cells and prevents those important messages from reaching the muscles.
When the muscles don't get direction on how to move, they become inactive, get smaller, and begin to waste away (a condition known as atrophy).
There are four types of SMA, categorized by the disease's severity and the age at which symptoms begin:
- Type I, sometimes called Werdnig-Hoffmann disease, begins to affect infants from birth up to 6 months of age, with most babies showing signs of the disease by 3 months. Some develop the disorder before birth. This is the most severe form of SMA.
- Type II, also called chronic infantile SMA, begins to affect children between 6 and 18 months old. This form can be moderate to more severe.
- Type III, also called Kugelberg-Welander disease or juvenile spinal muscular atrophy, begins to affect kids as early as 18 months of age or as late as adolescence. This is the mildest form of SMA in children.
- Type IV is the adult form of the disorder. Most people affected by this type start having symptoms after age 35, and these symptoms slowly get worse over time. Because it develops slowly, many people with type IV SMA don't know that they have it until years after symptoms begin.
A Genetic Cause
SMA is usually inherited. This means that both parents must have an altered (mutated) or missing copy of the gene involved in the disorder for a child to develop it.
Most cases of SMA are caused by a deficiency of a special protein called SMN ("survival of motor neurons"). Motor neurons need this protein to function. The gene that carries this protein is called SMN1. When each parent passes onto their child a chromosome with a mutated or missing SMN1 gene, the protein is not produced and motor neurons die, leading to SMA.
If a child receives the mutated SMN1 gene from only one parent, the child will be an SMA carrier. That means the child probably won't show any signs of having SMA, but later, could pass it on to his or her own child. If two SMA carriers have a child, there is a 1 in 4 chance that their child will have SMA. Being an SMA carrier is relatively common: About 1 in every 40 people carries the gene.
Signs & Symptoms
How SMA affects a child depends on when the disorder first causes symptoms. Typically, the later that symptoms first appear, the less severe a child's disabilities will be. Depending on the severity of the condition, long-term effects can be debilitating and even life threatening.
Many babies born with SMA have a tongue that quivers (called fasciculations). This symptom is characteristic of SMA and can lead to an early diagnosis. However, not all babies develop it, and the problems they do develop also can be caused by other diseases. This can make it difficult to determine whether SMA is responsible.
When SMA develops early, it can cause children to miss certain developmental milestones, such as rolling over, sitting up without support, and walking. One symptom of SMA that might not be as obvious is the muscle thinning that occurs because they aren't being used (this is the atrophy part of the disorder).
The problems associated with SMA continue to get worse as a child gets older. Progressive weakness can cause a child to begin to fall more often than other kids or to have trouble lifting or carrying items that he or she could previously carry with ease.
When the back muscles that normally support the spine become weak, children with SMA develop spinal deformities, such as scoliosis (sideways curvature of the spine) and kyphosis (front-to-back curvature of the spine, commonly known as a humpback). Over time, SMA continues to cause the body to waste away, leaving some children completely unable to walk, stand, and even move.
SMA sometimes affects the muscles used for breathing. This complication, along with a lack of mobility, puts children with SMA at risk for developing severe respiratory infections. These can be hard to treat and can occur over and over again.
Not all parts of the body are affected by SMA, however. Even kids with severe forms of the disease still have sensations in their face, arms, and legs. Intelligence is unaffected, and many children with SMA tend to be unusually alert, interactive, and gifted when it comes to making friends and socializing.
Doctors don't normally test babies for SMA. In fact, unless the parents know they are SMA carriers or a baby demonstrates significant muscle weakness or has difficulty breathing or feeding immediately after birth, SMA might not be detected right away.
Instead, a doctor might realize that a child has SMA only after the child misses certain developmental milestones, such as being able to support his or her head or stand without help. In many cases, progressive muscle weakness and atrophy, parental concerns, and repeated doctor visits because of respiratory problems prompt further investigation that reveals SMA as the cause of a child's difficulties.
A diagnosis usually comes only after the child undergoes several tests that rule out other diseases that cause similar symptoms. These tests usually include:
- nerve conduction tests, such as an electromyogram (EMG)
- computed tomography (CT) scan
- magnetic resonance imaging (MRI)
- muscle tissue biopsy
To confirm an SMA diagnosis, the doctor will usually recommend a blood test to look at the child's genes. If the SMN1 gene is missing or imperfect, it will confirm the diagnosis of SMA. The doctor might also recommend that parents and other members be screened for the disorder, even if they've never had any symptoms.
Unfortunately, there is no cure for SMA. The treatment that children receive for SMA varies, depending on their age and the severity of symptoms. The goal of treatment is to relieve specific symptoms, maintain function and enhance a child's mobility for as long as possible, and maximize the child's independence and quality of life.
Children with SMA usually require ongoing care throughout their lives from many different types of doctors, including pulmonologists, neurologists, orthopedists and orthopedic surgeons, gastroenterologists, nutritionists, and physical and occupational therapists.
Counseling and support groups can help children, parents, and family members cope with the devastating aspects of SMA and the ongoing medical care required. In addition, parents may want to consider genetic counseling, especially if they plan to have more children.
Children with SMA commonly need help breathing when muscle weakness begins to affect the respiratory muscles. Different therapies can help a child breathe. Children who are less severely affected may just need to breathe through a mask or mouthpiece. Others may be connected to a breathing machine (called a negative-pressure ventilation system), which helps the lungs expand and contract. Air gets into the lungs through a tube placed into the windpipe (tracheostomy).
A critical factor in respiratory care is preventing infection, especially pneumonia. Pneumonia is common in children with SMA and is life threatening.
SMA can affect the muscles used for chewing, sucking, and swallowing. This can cause a child to become malnourished or develop pneumonia if the child inhales food or liquids while eating. Some children do better by eating small, frequent meals throughout the day instead of having three large meals.
Some children with SMA are at risk for obesity if they consume too many calories for their activity levels and they can't exercise to effectively burn the calories. Ongoing consultation with a nutritionist is necessary to ensure adequate nutrition that doesn't overload a child with unnecessary calories.
Children who can't swallow or suck must be fed in another way to ensure that they're receiving enough nutrition. Sometimes, they have a tube inserted into the stomach to help them eat. Through this tube, called a gastrostomy tube, they can receive a nutritionally sound liquid diet.
Function and Mobility
Many children with SMA benefit greatly from physical and occupational therapies, which help to maintain function and mobility and enhance quality of life for as long as possible.
During physical therapy, kids engage in exercises that help to protect the joints from stiffness and injury. Physical therapy also helps to keep the muscles strong, maintain flexibility, enhance the circulation of blood in the limbs, and relieve pain that's associated with muscle weakness and immobility. Exercises are also performed to strengthen the respiratory muscles that assist in breathing.
In occupational therapy, children do exercises that can help them perform typical daily activities, such as walking, dressing themselves, combing their hair, and brushing their teeth as independently as possible. In some cases, tools can help make these tasks easier, such as:
- leg braces, standing frames, canes, and walkers, which provide stability and improve mobility
- electric wheelchairs with customized controls
- specialized seats to use in school
- tools for using computers and phones and controlling other home electronics, such as the TVs and lights
- tools to aid educational activities, such as writing and drawing
The goal of physical and occupational therapy is to help kids with SMA — and their parents — find ways to live as normally as possible despite the limitations caused by the disease.
Children who develop spinal deformities need to wear a splint, brace, or corset to straighten their backs as toddlers or adolescents. Sometimes, surgery (called a spinal fusion) is done to correct the spinal deformity in children who are done growing and whose respiratory systems can tolerate sedation with anesthesia.
Scientists who are researching SMA have made dramatic breakthroughs in the past few years, including identifying the genes and the protein associated with the disorder.
Research is now focused on finding drugs and other therapies that can help to keep motor neurons functioning as long as possible, enhance muscle tone and function, and even modify gene function. Many scientists are optimistic that ongoing research will lead to better treatment of SMA.
Reviewed by: Elana Pearl Ben-Joseph, MD
Date reviewed: July 2012