Spinal Muscular Atrophy (SMA)
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[edit] Name of Condition
Spinal Muscular Atrophy (SMA), also known as SMA1, Infantile Spinal Muscular Atrophy, Werdnig-Hoffman Disease
[edit] Definition
SMA is an inherited genetic condition that affects the nervous system, particularly the parts that control voluntary muscle movements, causing the muscles to weaken and shrink. There are four types of SMA which differ in severity, ranging from SMA 1 to SMA 4. SMA 1 is the most extreme, while 2 and 3 are less severe. SMA 4 is a late-onset version of the disease which only affects adults.
[edit] Description, Signs and Symptoms
Spinal Muscular Atrophy varies in severity. There are four types of SMA, 1-4.
SMA 1 is early onset and severe. Type I is also known as Wednig-Hoffmann Disease. Children with this form of the condition are usually diagnosed before 3 months of age when they show a lack of expected infant behavior, such as having poor head control, less vigorous leg kicking, and the inability to sit up. The chest muscles of these children are also underdeveloped, and the chest may appear sunken in due to the child breathing mostly with the diaphragm rather than the lungs.
SMA 2 is usually diagnosed before age 2, often by 15 months. Children with Type 2 have limited muscle control, and may be able to sit up unsupported. They can sometimes stand with the help of a person or something to hold on to. Some may have tongue fasciculations or experience slight tremors in their hands. Chest muscles are weak, causing difficulty with coughing and deep breathing during sleep. Type 2 children almost always develop scoliosis, resulting in the need for spinal surgery or bracing, and decreased bone density can cause the bones to be fragile and more susceptible to breakage.
SMA 3 is also known as Kugelberg-Welander or Juvenile Spinal Muscular Atrophy. This form of the condition may present itself as early as one year of age to as late as the teenage years. However, typical diagnosis is around age 3 or earlier. Type 3 children can usually stand alone and walk, but may develop difficulty walking and not be able to run. Over time, the disease progresses, and they may lose the ability to walk as they begin to fall frequently and have trouble standing or getting up from a sitting position.
SMA 4 is the adult onset form of this condition. Symptoms are commonly seen after age 35, and rarely between 18 and 30. SMA 4 is the least common form of SMA, and progresses very slowly. It almost strictly affects the muscles used in the arms, legs, and abdomen, which control active movement, and rarely affects the bulbar muscles which are those controlling swallowing and breathing.
Individuals with SMA typically lose function over time. Loss of function can occur rapidly in the context of a growth spurt or illness, or much more gradually. Research has not been able to find an explanation for this loss. It has been observed that patients with SMA may often be very stable in terms of their functional abilities for prolonged periods of time, even years, although the almost universal tendency is for continued loss of function as the patients age.
[edit] Inheritance
SMA is an autosomal recessive inherited disease. Both parents must be carriers of the mutation and pass it on to their child. The mutation is in a gene called SMN1 (survival motor neuron 1) that, when missing or mutated, fails to produce a working protein called Survival Motor Neuron Protein, which affects the strength of motor neurons. Without the protein, nerve cells may atrophy, shrink, and eventually die, resulting in muscle weakness.
An associated gene called SMN2 produces another type of protein that affects the course of the disease. If the child has only one or two copies of the SMN2 gene, the disease will be severe, but if the child has three or more copies, it will run a more mild course.
[edit] Demographics
[edit] Diagnosis
When physical symptoms like muscle weakness begin to manifest themselves, a doctor may order a creatine kinase test (ck test) which would test for levels of creatine kinase in the blood. This is an enzyme that leaks from the muscles when they deteriorate, which would be indicative of muscle atrophy. A genetic test would confirm that it was SMA and not a different form of a muscle disease, although genetic testing for this condition carries implications for the rest of the family, such as indicating that the parents are carriers and could potentially have another child with the same condition, that would have to be considered before going through with testing.
[edit] Treatment and Management
There are a number of physical therapy and occupational therapy options available for children with SMA. In the case of difficulty with feeding and swallowing, children should be monitored while they eat because they can easily choke on their food. In some cases, assistive feeding may be necessary with a feeding tube. Due to weak chest muscles that prevent children from taking deep breaths, babies with type 1 SMA may need assistance breathing while they sleep. Artificial respiration machines are available that can assist a child with breathing openly, especially if the child becomes sick with a cold. With Type 2, scoliosis is a common problem which may need to be remedied with surgery or a brace. Type 3 children may need a walker or other supportive device to walk with, and the diet should be carefully monitored, such as with the help of a dietician.
[edit] Prognosis
The prognosis for this condition is variable due to the spectrum of symptoms that may be experienced by any individual. Type 1 is usually fatal around the age of 2, due to complications from the weakness of the chest muscles and the lungs. Types 2 and 3 depend on the age of onset and the severity of the symptoms. Adult-onset SMA individuals may live complete lives depending on the severity of their condition and the treatment they receive.
[edit] Resources
AccessDNA.com - SMA[1]
http://www.mda.org/publications/fa-sma-qa.html
