Spinal muscle atrophy (SMA) is a serious heritable condition characterized by the loss of motor neurons, or nerve cells responsible for carrying instructions from the brain to the muscles to control their movement. Without these nerve cells, muscles weaken and atrophy. Patients with SMA may have difficulty standing, walking, and dressing. Swallowing, speaking, and breathing may also be affected.
SMA is divided into types based on the age of onset of the symptoms, type of mutation, and disease progression.
What is SMA type 4?
Most types of SMA, including SMA type 4, are caused by mutations in the SMN1 (survival motor neuron 1) gene. The protein for which this gene provides instructions is involved in protecting motor neurons. Mutations cause no SMN protein to be made from this gene. Patients are, therefore, reliant on a related gene, SMN2, to protect nerve function. However, this gene is only able to produce some functional SMN protein. The severity of the disease usually correlates with how many copies of the SMN2 gene patients have.
SMA type 4, also called adult-onset SMA, is usually diagnosed in the second or third decade of life, usually after the age of 35, although some people may have symptoms as early as their late teens. Patients with SMA type 4 generally have four or six copies of the SMA2 gene, meaning they are less affected by mutations in the SMN1 gene.
Symptoms of SMA type 4 include mild muscle weakness in the legs and hips that may progress to the arms and shoulders. Despite a set pattern of weakness, patients differ in the extent to which they are affected; most patients are usually able to walk, and only a small number of them will require wheelchair assistance.
Muscle tremors and twitches are common. Cognition is not affected.
Diagnosing SMA type 4 can be difficult as the initial symptoms are fairly mild. Moreover, the disease is rare, affecting about 1 in 300,000 people. This means many physicians may not be familiar with the disease.
Following a preliminary diagnosis by a physician, a genetic test can confirm a final diagnosis.
Physiotherapy and occupational therapy can be useful to determine appropriate aids and adaptive equipment as well as home, school, or work modifications that can help patients maintain their mobility and independence.
There are also many experimental treatments under development for SMA.
Many patients with type 4 SMA will likely no longer be able to walk unassisted by age 60, and most will probably need a wheelchair or motorized chair to get around. However, life expectancy is normal, and swallowing and breathing functions are rarely affected.
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