Spinal Muscular Atrophy (SMA) is an autosomal recessive neuromuscular disease affecting the part of the nervous system that controls voluntary muscle movement. SMA is characterized by a loss of specialized nerve cells – motor neurons, in the spinal cord and the part of the brain that is connected to the spinal cord (the brainstem). The loss of motor neurons leads to progressive muscle weakness and muscle wasting (atrophy) in muscles used for activities such as crawling, walking, sitting up, and controlling head movement. In severe cases of spinal muscular atrophy, the muscles used for breathing and swallowing are affected.
Chromosome 5 SMA is the most common form of SMA caused by an abnormal or missing gene known as the survival motor neuron gene 1 (SMN1) on chromosome 5, which is responsible for the production of a protein essential to motor neurons. Without this protein, lower motor neurons in the spinal cord degenerate and die. Neighboring SMN2 genes can in part compensate for nonfunctional SMN1 genes. Other rare forms of SMA (non-chromosome 5) are caused by mutations in genes besides SMN.
SMA can affect any race or gender. SMA is the second most common fatal autosomal recessive disorder after cystic fibrosis, with an estimated incidence of 1 in 6,000 to 1 in 10,000 live births, with a carrier frequency of 1/40-1/60.
SMA is clinical classified into four phenotypes on the basis of age of onset and highest physical milestone achieved.
- Type I (Werdnig-Hoffmann disease) – the most severe type. Symptoms appear in babies less than six months old, and they never develop the ability to sit unsupported. This type accounts for about 50-60% of patients diagnosed with SMA.
- Type II (intermediate) – less severe than type I. Symptoms usually appear in babies aged 7 to 18 months. Children with Type II can sit unsupported and some are able to stand, but they can’t walk independently.
- Type III (mild, Kugelberg-Welander disease) in adulthood – the mildest type affecting children. Symptoms usually appear after 18 months of age, and children are usually able to reach all the major motor milestones, including independent walking.
- Type IV – patients with adult onset (> 18 years) and mild course. This group includes patients who are able to walk in adulthood and without respiratory and nutritional problems
A blood test is available that can indicate whether there are deletions or mutations of the SMN1 gene. This test identifies at least 95 percent of SMA Types I, II, and III. Other diagnostic tests may include electromyography (which records the electrical activity from the brain and/or spinal cord to a peripheral nerve root found in the arms and legs that controls muscles during contraction and at rest), nerve conduction velocity studies (which measure electrical energy by assessing the nerve’s ability to send a signal), muscle biopsy (used to diagnose neuromuscular disorders and may also reveal if a person is a carrier of a defective gene that could be passed on to children), and laboratory tests of blood, urine, and other substances.
There is no cure for SMA. Treatment consists of managing the symptoms and preventing complications. Physical therapy, occupational therapy, and rehabilitation may help to improve posture, prevent joint immobility, and slow muscle weakness and atrophy. When nutrition is a concern in SMA, placement of a gastrostomy tube is appropriate. As respiratory function deteriorates, tracheotomy or noninvasive respiratory support is offered. Muscle relaxants such as baclofen, tizanidine, and the benzodiazepines may reduce spasticity.
Research is currently being carried out into possible treatments for spinal muscular atrophy (SMA).
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