Spinal muscular atrophy (SMA) is an inherited disease that affects nerves and muscles, causing muscles to become increasingly weak. It mostly affects infants and children but can also develop in adults. Symptoms and prognosis vary depending on SMA type. Gene replacement and disease-modifying therapies offer hope.
Spinal muscular atrophy (SMA) is a genetic (inherited) neuromuscular disease that causes muscles to become weak and waste away. People with SMA lose a specific type of nerve cell in the spinal cord (called motor neurons) that control muscle movement. Without these motor neurons, muscles don’t receive nerve signals that make muscles move. The word atrophy is a medical term that means smaller. With SMA, certain muscles become smaller and weaker due to lack of use.
Approximately 10,000 to 25,000 children and adults are living with SMA in the United States. It’s a rare disease that affects one out of 6,000 to 10,000 children.
A person with SMA inherits two copies of a missing or faulty (mutated) survival motor neuron 1 (SMN1) gene. One faulty gene comes from the mother and the other comes from the father. An adult can have a single copy of the defective gene that causes SMA and not know it.
About six million Americans (1 in 50) carry the mutated SMN1 gene. These carriers have one healthy SMN1 gene and one missing or defective SMN1 gene. Carriers don’t develop SMA. There's a 1 in 4 chance that two carriers will have a child with SMA.
There are four primary types of SMA:
People with SMA are either missing part of the SMN1 gene or have a changed (mutated) gene. A healthy SMN1 gene produces SMN protein. Motor neurons need this protein to survive and function properly.
People with SMA don’t make enough SMN protein, and so the motor neurons shrink and die. As a result, the brain can’t control voluntary movements, especially motion in the head, neck, arms and legs.
People also have SMN2 genes that produce a small amount of SMN protein. A person may have up to eight copies of an SMN2 gene. Having multiple copies of the SMN2 gene typically leads to less severe SMA symptoms because the extra genes make up for the missing SMN1 protein. Rarely, non-SMN gene mutations (non-chromosome 5) cause SMA.
SMA symptoms vary depending on the type. In general, people with SMA experience a progressive loss of muscle control, movement and strength. Muscle loss gets worse with age. The disease tends to severely affect the muscles closest to the torso and neck. Some people with SMA never walk, sit or stand. Others gradually lose their ability to do these actions.
Some SMA symptoms resemble those resulting from neuromuscular disorders like muscular dystrophy. To find the cause of symptoms, your healthcare provider will perform a physical exam and get a medical history. Your physician may also order one or more of these tests to diagnose SMA:
If you’re pregnant and have a family history of SMA, prenatal tests can determine if the developing fetus has the disease. These tests slightly increase the risk of miscarriage or pregnancy loss. Prenatal tests for SMA include:
There isn’t a cure for SMA. Treatments depend upon the type of SMA and symptoms. Many people with SMA benefit from physical and occupational therapy and assistive devices, such as orthopaedic braces, crutches, walkers and wheelchairs.
These treatments may also help:
Over time, people with SMA experience progressive muscle weakness and loss of muscle control. Potential complications include:
SMA is an inherited disease. If you or your partner carries the mutated gene that causes SMA, a genetic counselor can explain the chances of your child having SMA or being a carrier.
You may be able to take steps before pregnancy to lower the risk of passing on SMA. A process called preimplantation genetic diagnosis (PGD) identifies embryos that don’t have the mutated gene. Your doctor implants healthy embryos during in vitro fertilization (IVF). PGD ensures your child will have two healthy SMN1 genes and not get SMA.
The quality of life and life expectancy for people with SMA varies depending on the type. Infants with type 1 SMA usually die before their second birthday. Children with type 2 or type 3 SMA may live full lives depending on the severity of symptoms. People who develop SMA during adulthood (type 4) often remain active and enjoy a normal life expectancy.
You should call your doctor if someone with SMA experiences:
You may want to ask:
A note from Cleveland Clinic
SMA is a genetic neuromuscular disease that can significantly affect quality of life and life expectancy. It’s a progressive disease that gets worse over time. Symptoms may be present at birth (type 1), or develop during childhood (type 2 or 3) or in adulthood (type 4). Newer disease-modifying and gene replacement therapies offer promise. It’s possible to carry the gene that causes SMA and not know it. If SMA runs in your family, talk to your doctor about ways to lower your future baby’s chance of getting SMA.
Last reviewed by a Cleveland Clinic medical professional on 04/26/2021.
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