Spinal Muscular Atrophy (SMA type 1, SMA type 2, SMA type 3, SMA type 4)
If you have just found out through carrier testing that you and/or your partner are carriers of spinal muscular atrophy (SMA) or through prenatal testing that your baby has SMA or may have SMA and are looking for more information, the Genetic Support Foundation is a good starting point.
There are other important resources out there, including your doctor and genetic counselor. We have also compiled a list of resources that can provide you with additional information and support.
After reviewing the information below, if you have a question that you are having difficulty finding an answer for, please feel free to contact us.
Spinal Muscular Atrophy (SMA) is an autosomal recessive genetic condition that affects an individual’s motor neurons and ultimately, their muscles. The motor neurons affect the voluntary muscles that are used for activities such as crawling, walking, head and neck control, and swallowing. SMA does not cause problems with cognitive ability or intellect. There are 4 types of SMA that are based on maximum functional ability (i.e. if the individual can learn to sit or walk). SMA type 1 is the most severe and SMA type IV the most mild. There is no cure for SMA but symptoms are treated as needed. SMA occurs in approximately 1 in 6000 babies born; approximately 1 in 40 individuals are carriers of SMA. According to the American College of Genetics & Genomics, carrier screening for SMA should be offered to all women who are pregnant or considering pregnancy; other professional organizations suggest offering carrier screening ONLY if there is a suggestive family history or upon patient request.
SMA is what we call an autosomal recessive condition. Autosomal means that the gene or instruction involved in SMA is found on one of the first 22 chromosomes (called the autosomes), not the X or Y (sex chromosomes). We know that the gene involved in the vast majority of SMA is found on chromosome 5 and is called Spinal Motor Neuron 1 (SMN1) (there is a rare form of SMA that is caused by a mutation in a gene found on the X chromosome, which does not follow the same autosomal recessive pattern of inheritance discussed here).
Autosomal chromosomes come in pairs; we inherit one from our mom and one from our dad. Along the chromosomes are the individual instructions or genes. So, we get one chromosome 5 from our mom and one chromosome 5 from our dad, therefore, we get a copy of the genes involved in SMA from each parent, as well (since it is found on chromosome 5).
Recessive means that in order to have SMA, both of the SMA genes must be not working as usual (mutated), the one that we inherit from our mom and the one that we inherit from our dad. If only one of our SMA genes is not working as usual, we are called “carriers” of SMA but we do not typically have symptoms of the disease.
There is nothing mom and dad can do to cause or prevent SMA. If both mom and dad are carriers of SMA, there is a 25% chance for each of their children to inherit the condition (therefore, a 75% chance they will not get the condition). To read more about autosomal recessive inheritance, click here.
Of note: There are rare cases of SMA in which only one parent is a carrier and the child ends up having SMA due to a new (de novo) mutation or change in the gene involved in SMA that occurs in the egg or the sperm (usually the sperm) prior to conception.
- SMA type 1– Individuals with SMA type 1 (also known as Werdnig-Hoffman disease, or infantile-onset SMA) have symptoms at birth or within the first few months, including floppy limbs and trunk, swallowing and feeding difficulties, and breathing difficulties.
- SMA type 2– Individuals with SMA Type 2 usually start showing symptoms at 6-18 months of age. Children with this type have delayed motor milestones and there is a range of physical abilities. Individuals with SMA type 2 can sit unsupported and with assistance and bracing these children may be able to stand but are unable to walk and require a wheelchair to get around.
- SMA type 3– Individuals with SMA type 3 (also called Kugelberg-Welander disease) usually show symptoms between 2 and 17 years of age. Individuals with type 3 can sit and walk, although some individuals may require wheelchair assistance at different times in their life.
- SMA type 4– SMA type 4 is the mildest form of SMA and people do not usually have symptoms until adulthood. Symptoms of SMA type IV include mild motor impairment such as muscle weakness, tremor, and twitching; individuals may or may not have respiratory problems. Life expectancy is normal.
|SMA type||Age of Onset/When symptoms start||Maximum (at best) Mobility/Functional ability
|Type 1 (Werdnig-Hoffmann disease)||0-6 months||Will not be able to sit or stand|
|Type 2||7-18 months||Will be able to sit but will not be able to walk|
|Type 3 (Kugelberg-Welander disease)||After 18 months||Most able to stand and walk without help|
|Type 4 (adult-onset)||Over 18 years of age||Able to walk without help, normal life expectancy|
The long-term outlook or prognosis for individuals with SMA really depends on what type of SMA they have. Historically, individuals with SMA type 1 passed away within the first 2 years of life. With advances in treatment of symptoms, people with SMA can live longer. For long-term outlook information for types 1, 2, & 3 it is important to talk with your doctor to get information based on your unique circumstance. Individuals with type 4 SMA typically have a normal life expectancy.
- Muscular Dystrophy Association (MDA) patient information on SMA
- CURE SMA- Patient informational booklets about SMA
- National Institutes of Health/National Institute of Neurological Disorders and Stroke information on spinal muscular atrophy
- American College of Genetics and Genomics SMA carrier screening guideline
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