Doctors diagnose SMA by taking a blood sample and performing other tests. These include a genetic test and a muscle biopsy.
In SMA, a mutation in the SMN1 gene prevents motor neurons from sending signals to muscles. This causes weakness that gets worse over time. Some forms of SMA are more severe than others.
Symptoms
SMA is a genetic disease that causes the nerve cells (called motor neurons) in the spinal cord and lower part of the brain to break down. Without the motor neurons, muscles can’t get stronger or move. They also become smaller, which is called atrophy. Most of the types of SMA are caused by problems with a gene called SMN1. Mutations (changes) in this gene prevent the body from making enough SMN protein to keep motor neurons working properly.
The symptoms of SMA vary, depending on which type of SMA a person has. All types of SMA cause weakness and difficulty moving, but how severe the symptoms are varies from person to person. The different types of SMA usually affect the muscles closest to the center of the body, such as those in the shoulders, hips, and thighs. The arms and legs are usually affected later. Children with SMA type 1 show the most serious symptoms. They cannot sit up, crawl or walk and have trouble breathing, eating, and swallowing. They have a very short life expectancy unless they are given treatment early.
Some people with SMA have a form that’s not linked to chromosome 5. They show less severe symptoms. People with this type of SMA may still need a wheelchair as they grow older, but they can usually stand and walk.
Other forms of SMA that are not related to a deficiency in the SMN protein occur when variants (changes) in other genes on different chromosomes cause the condition. These forms of SMA, including Kennedy’s disease, vary in their severity and the muscles affected.
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To diagnose SMA, doctors use a blood test to look for abnormal levels of a protein that’s made by the motor neurons. They also do a test to measure the strength of muscle contractions, known as electromyography. A muscle biopsy — taking a small piece of muscle for testing — is usually done only when other tests can’t provide a diagnosis. This test enables doctors to see the damage to muscle cells. Doctors also use X-rays, ultrasound, and CT scans to check for other health problems.
Diagnosis
Spinal muscular atrophy (SMA) is a group of genetic disorders that weakens muscles used for movement. The disorder affects the motor neurons in the spinal cord, which send messages from the brain to muscles. The symptoms vary depending on the type of SMA. Some forms of the disease are more severe than others. The most common type is called infantile-onset SMA or Werdnig-Hoffmann disease, and it strikes babies within their first six months of life. It’s the most serious form of SMA, and it may cause death in early infancy. But aggressive treatment can prolong life.
Other types of SMA may appear after the first six months of life, but they’re less severe and have a better prognosis. The condition progressively worsens over time, and it eventually makes it difficult or impossible to walk and eat. Children with SMA can also develop a curve in their spine (scoliosis).
Most forms of SMA are caused by a change (mutation) in the gene SMN1. Mutations lead to insufficient levels of the protein needed for normal muscle function. The gene is passed down from parents to their children, and if both copies of the gene have mutations, the child will have SMA.
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A doctor can diagnose SMA by taking a blood sample and checking for high levels of an enzyme that breaks down muscle tissue, creatine kinase. Another test looks for abnormal electrical activity in the muscles, which can show problems with nerve and muscle function. An electromyogram, or EMG, can check for problems with the muscles’ ability to respond to signals from the brain.
The best way to detect SMA is to have a genetic test before a baby is born. An obstetrician can use a needle to draw out a small amount of fluid from the amniotic sac, and a lab specialist can check the sample for the faulty SMN1 gene. This procedure, called amniocentesis, is typically done in the 14th week of pregnancy.
Other blood tests can help identify SMA, as well. An MRI or CT scan of the spinal cord can show signs of damage, including atrophy in the area where the motor neurons are located.
Treatment
SMA is an inherited neuromuscular disorder. It affects the lower motor neurons (also called anterior horn cells) in the spinal cord. These neurons are responsible for relaying nerve impulses from upper motor neurons in the brain to the muscles they control. In SMA, the loss of lower motor neurons causes weakness and muscle wasting that worsens over time. It weakens the muscles closest to the center of the body, including the shoulders, hips, thighs and upper back. It also weakens the muscles that control breathing, swallowing and moving the eyes. The muscles that are farther from the center of the body, such as the arms, legs and feet, tend to weaken more slowly or not at all. The heart, blood vessels and the muscles that move the digestive tract are not affected.
SMA can be diagnosed in infants and children by a thorough physical examination and tests of muscle strength, function and movement. A doctor with special training in the diagnosis and treatment of SMA may perform these tests. These tests include a blood test to look for high levels of an enzyme called creatine kinase, which indicates damage to muscles. A neurologist with expertise in SMA may also perform nerve conduction velocity studies to measure the speed at which electrical signals travel through your child’s nerves.
Genetic testing can show if your child has a deletion or mutation in the SMN1 gene. It can also tell whether your child is a carrier of the gene variant that causes SMA and how likely it is that your child will pass the faulty gene to future children.
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Several treatments are being studied to help people with SMA. Some work to enhance the amount of surviving SMN protein, while others replace or repair the faulty gene. Some of these therapies are in clinical trials.
A gene replacement therapy, onasemnogene abeparvovac-xioi (Zolgensma) by AveXis, has been approved for use in babies younger than 2 years with SMA type 1 and type 2. This treatment gives your child’s body a functioning copy of the SMN1 gene. It is delivered as a one-time intravenous infusion.
Preventing complications
Spinal Muscular Atrophy is a genetic (passed down from parents) disease that affects the motor neurons-nerve cells in the spinal cord that control muscle movement. When these cells die, it prevents muscles from receiving the signals they need to grow stronger and move. As the muscles weaken, they become smaller and waste away (atrophy). The types of SMA vary in severity and how early symptoms appear. But the underlying weakness that causes them is always present.
People with SMA inherit two copies of the faulty SMN1 gene, which provides instructions for making the protein called survival motor neuron 1 (SMN1). If not enough SMN1 protein is produced, motor neurons die and cannot carry out their function, and the condition becomes severe. The weakened muscles also can’t receive the signals that cause them to grow stronger and move, so the muscle fibers slowly waste away.
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The symptoms of SMA can resemble other health conditions, so it’s important to see a doctor if you or your child have them. Newborns are screened for SMA in most states, and this test can help detect the condition before symptoms appear. Other tests can include a blood test that shows how much a protein called creatine kinase is in the body. This test may show less than normal amounts of the protein in people with SMA type 1 or type 3. A genetic test can identify at least 95 percent of all cases of SMA. Other tests that may be done include an electromyogram (EMG), which records the electrical activity of muscles during contraction and at rest, and nerve conduction velocity studies, which measure how quickly a nerve sends an electrical signal to the muscle.
The good news is that treatments are available to treat Spinal Muscular Atrophy. One is a gene therapy that replaces the faulty SMN1 gene. Another is a drug called nusinersen (Spinraza), which helps the body make more of the SMN1 protein. Both of these drugs were developed at Boston Children’s and tested in clinical trials in patients with SMA before being approved by the FDA.
