Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disease, and the most common cause of mortality in infants linked to a genetic mutation. It affects 1 in between 8,000 and 10,000 people.
The disease is characterized by progressive muscle weakness caused by the loss of specialized nerve cells called motor neurons in the spinal cord and the part of the brain connected to the spinal cord.
Motor neurons control voluntary muscle movements, including those of the arms, legs, chest, face, throat, and tongue. The loss of motor neurons leads to weakness and atrophy in these muscles. Fundamental activities using muscles, such as walking, sitting up, controlling head movement, breathing, and swallowing can be affected by the disease. Movement progressively becomes slower and the ability to control voluntary movement may be lost completely in the later stages of the disease.
The disease is divided into subtypes based on the age of onset and maximum function achieved. SMA types 0, 1, 2, 3, and 4 are associated with mutations in the SMN1 gene, which is located on chromosome 5. This gene provides the instructions for the making of the SMN protein, which is important for the survival of motor neurons.
Some people have extra copies of a second gene called the SMN2 gene, and the number of copies of this second gene can modify the course of SMA. The more copies there are, the less severe the symptoms tend to be.
Other types of SMA not linked to mutations in the SMN1 gene include Finkel type SMA, SMA with respiratory distress syndrome (SMARD1), SMA lower extremity dominant (SMA-LED), X-linked infantile SMA, and Kennedy’s disease.
Some of the common symptoms of SMA include:
- poor muscle tone that leads to lack of motor development.
- breathing difficulties that may cause respiratory complications, including underdeveloped lungs and weak cough.
- problems with feeding and swallowing.
- scoliosis, or an abnormal spinal curvature, due to the weakness of the muscles that normally support the spinal column.
SMA diagnosis may start with a physical examination by a physician who will record the patient’s family history. He or she may also want to check the level of creatine kinase (CK) in the blood. CK is an enzyme that leaks out of muscles as they deteriorate. Although this is not an SMA-specific test, high CK levels indicate that muscles have been damaged.
When symptoms are present, diagnosis can be made through genetic testing since different forms of SMA are linked to mutations in specific genes (e.g., SMA types 0 to 4 are linked to mutations in the SMN1 gene, and Finkel type SMA to mutations in the VAPB gene). Extra copies of the SMN2 gene can also be identified in a genetic test.
Electromyography assesses the health of the muscles and motor neurons that control them. This test determines whether muscles are responding to electrical stimulations.
In a muscle biopsy, a small section of muscle tissue is removed, usually from the upper thigh, and examined to see whether it is degenerating or has characteristics unique to SMA.
SMA has no cure yet, and existing treatments consist of managing the symptoms and preventing complications. These include:
- palliative care, which is aimed at improving the quality of life of both the patient and their family, focusing on relief from the symptoms and stress caused by the disease.
- physical therapy, which may help improve posture, prevent joint immobility, and slow muscle weakness and atrophy in some forms of SMA.
- occupational therapy, which focuses on adjustments that can help patients manage their everyday lives with the aim of improving quality of life.
The only treatment addressing the underlying genetic cause of SMA is Spinraza (nusinersen), which was approved by the U.S. Food and Drug Administration in December 2016. The therapy is intended to increase the production of SMN protein from the SMN2 gene.
There are also promising advances in gene therapy where a functional copy of the SMN1 gene is delivered to the motor neurons using a modified virus. Zolgensma is one such therapy currently under review by the FDA.
Additionally, there are a number of experimental therapies being tested to treat SMA.
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