Spinal muscular atrophy (SMA) is a genetic disease characterized by the loss of motor neurons, or nerve cells that control the movement of voluntary muscles.
There are several types of SMA based on the gene affected, age of onset, and severity of symptoms. SMA types 0 to 4 are all caused by mutations in the SMN1 gene, which provides instructions for the making of a protein essential for the survival of motor neurons, called SMN. The disease develops only if both copies of the SMN1 gene, one inherited from the mother and one from the father, are mutated. It is thought that the severity of the disease depends on the number of copies of a second gene, SMN2, from which some SMN protein can be produced.
What is SMA type 0?
SMA type 0, also called prenatal onset SMA, is the most severe form of SMA. It affects a baby that is still in the womb. It can be fatal before birth and is almost always fatal within the first year of life.
In contrast to SMA type 1 in which symptoms appear at birth or within the first few months of life, symptoms of SMA type 0 are usually present before birth in the later stages of pregnancy. The fetus is less active inside the womb than expected. After birth, babies’ ability to move, breathe, and swallow is significantly lower than that of healthy newborns. The affected babies do not develop any of the motor skills typically seen in infants their age.
The standard tool for the diagnosis of SMA is genetic testing to look for mutations in the SMN1 gene. Other diagnostic methods such as muscle biopsy and magnetic resonance imaging (MRI) are usually unnecessary.
There is currently no cure for any type of SMA, but the symptoms of SMA type 0 can be managed with breathing aids or feeding tubes. Patients and their families or caregiver should be offered genetic counseling to help them understand the genetics of the disease and help with future family planning.
Infants with SMA type 0 often succumb to the disease before 6 months of age (usually, between 2 and 6 months old).
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