Spinal Muscular Atrophy (SMA) is an inherited disease characterized by muscle atrophy and loss of motor function, caused by the absence of or defect in the Survival Motor Neuron 1 (SMN1) gene. The SMN1 gene encodes for survival of motor neuron (SMN) protein. This protein is critical to the survival and health of α-motor neurons, which are nerve cells in the spinal cord responsible for muscle contraction. As the motor neurons become unhealthy due to the reduced SMN levels, muscles weaken and become atrophic.
What are the different forms of SMA?
Type I, Acute Form of SMA (Werdnig-Hoffmann Disease): Patients typically exhibit limited movement and have difficulty holding their head straight, feeding, and swallowing. Reduced strength in the chest muscles often results in labored breathing with the chest appearing sunken. The progressive weakening of the muscles leads to respiratory infections, lung collapse and eventual death, usually by the age of two years. About 60% of patients with SMA are born with this form of the disease.
Type II, Intermediate Form of SMA: Symptoms usually emerge in patients between six and eighteen months, and the progression of symptoms varies greatly. Infants and children with this form of the disease are at one time able to sit unassisted, but do not walk independently. Due to the varied progression of symptoms, life expectancy ranges from early childhood to adulthood. The majority of Type II patients live into adulthood.
Type III, Mild Form of SMA (Kugelberg-Welander or Juvenile Spinal Muscular Atrophy): Symptoms typically appear between eighteen months and early adulthood. People with SMA Type III often exhibit difficulty walking, have mild muscle weakness and are at risk for respiratory infections. These patients have a normal life expectancy.
Type IV, Adult Form of SMA: A less common form of SMA that afflicts adults and is characterized by a slower progression of symptoms that particularly affect walking. Symptoms typically emerge after age 35.
What is the life expectancy for people who have SMA?
SMA is a genetic disease characterized by progressive muscle weakening and loss. Because the muscles controlling breathing are affected by the disease, SMA can cause premature death. Life expectancy of SMA tends to vary by SMA type, which is generally associated with age of onset of symptoms. Children diagnosed with SMA Type I may survive for up to two years or longer, depending on their individual strength. Children with moderate to mild forms of SMA (SMA Types II and III) generally live into adulthood and could have normal life expectancy. Good multidisciplinary care, including physical therapy, occupational therapy, respiratory therapy, and nutritional support, can improve quality and length of life for people with SMA and is recommended. Planning for medical emergencies is also strongly suggested.
How is SMAdiagnosed?
A simple blood test designed to identify deletions and mutations in the Survival Motor Neuron 1 (SMN1) gene is typically used to confirm a diagnosis of SMA. For a listing of clinics and laboratories offering the genetic test for SMA, please visit the NCBI website. If symptoms are present and there is no indication of a typical SMA gene mutation, additional genetic studies, a muscle biopsy, and/or electromyography (EMG) may be necessary to confirm the diagnosis.
Where can I find information on carrier screening for SMA?
A carrier of SMA is someone who has one defective copy of the SMN1 gene and one normal SMN1 copy. Carriers do not experience the symptoms of the disease because their single, normal SMN1 copy is enough to make all SMN protein the body needs. However, if two carriers of SMA have a child together, there is a 1 in 4 chance that the child will inherit a defective copy of SMN1 from both parents and will have SMA. Carrier screening is a type of genetic testing that couples may choose to take if they want to find out whether they have certain deletions or mutations that cause some autosomal recessive diseases, such as SMA. In about 97% of cases, carrier screening can determine if someone is a carrier of SMA. Individuals and couples considering carrier screening for SMA, may find it helpful to talk with an obstetrician/gynecologist or a genetic counselor to gather more information.
What are the genetics of SMA?
SMA is an autosomal recessive disease, caused by a deletion or mutation in the Survival Motor Neuron 1 (SMN1) gene. Most people have two copies of the SMN1 gene, one inherited from their mother and the other from their father. However, carriers of SMA have one normal copy of SMN1 and one mutated or defective copy. Having at least one normal SMN1 copy will allow a person to produce more than enough SMN protein to prevent any symptoms of SMA, so carriers do not show any symptoms of the disease.
Though all people with SMA have two mutated copies of SMN1, not all patients are affected at the same severity. Other genes, such as SMN2, are modifiers of the disease and can make the symptoms of SMA less severe.
– 25% chance of inheriting two normal copies of SMN1. These children will not have SMA and will not be carriers of SMA.
– 50% chance of inheriting one normal and one defective copy of SMN1. These children will not have SMA but will be carriers of SMA.
– 25% chance of inheriting 2 defective copies of SMN1. These children will have SMA.