SMA Fast Facts (video)

An overview of SMA and current research, including video interviews with researchers

by Margaret Wahl on April 1, 2009 - 10:42am

QUEST Vol. 16, No. 2

SMA is caused by a deficiency of a motor neuron protein called SMN (survival of motor neurons). This deficiency results when there is a faulty gene on chromosome 5. The disease causes a loss of nerve cells (motor neurons) in the spinal cord.

One in every 40 people carries the gene flaw that causes SMA. Both parents must carry the gene flaw for SMA to be passed on; each of their children has a 25 percent change of developing SMA.

SMA is most commonly divided into types 1 through 4. (Sometimes further subdivisions, such as type 2a, 2b, etc., are used to indicate gradations of severity.) These types vary in age of onset and intensity of symptoms, but all are due to mutations in the SMN gene on chromosome 5. SMA types are defined, in part, by a person’s age at symptom onset and the highest motor milestone achieved (i.e., sitting, standing, walking).

Types of SMA

SMA type 1
(infantile-onset, Werdnig-Hoffmann disease)

The most severe form of SMA, type 1 is evident in infants between birth and 6 months old. These children never achieve independent sitting. At present, 50 percent of babies with type 1 SMA die before their second birthday.

Some physicians refer to the most severe cases of infantile-onset SMA as type 0. they reserve this designation for babies who are born with severe weakness and die during the first month of life.

SMA Type 2
(intermediate SMA)

Type 2 typically affects babies before 18 months of age. Children may be able to sit unaided or stand with support. They are at increased risk for complications from respiratory infections, and life span is often shortened.

SMA Type 3
(juvenile SMA, Kugelberg-Welander disease)

Type 3 is the mildest form of childhood-onset SMA. It strikes as early as 18 months and as late as adolescence. Children with type 3 have variable degrees of weakness but are able to stand and walk independently for some period of time.

SMA Type 4
(adult SMA)

Type 4 is the adult form of the disease. Symptoms generally begin after age 35.

SMA type 1 and SMA type 2 are most prevalent.

One in every 6,000 babies is born with SMA, making it one of the most prevalent genetic disorders. It is the most common genetic cause of death in infancy.

Although SMA has not been shown to affect a person’s intellect, it is often observed that many people with SMA are highly intelligent.

More About SMA

Video Watch videos of SMA experts who spoke at an MDA conference in January: Susan Iannaccone, John Kissel and Tom Crawford.

Watch a video of SMA expert Susan Iannaccone

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Watch a video of SMA expert John Kissel

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Watch a video of SMA expert Tom Crawford

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