Spinal muscular atrophy (SMA) is a genetic disease characterized by the progressive loss of motor neurons, or nerve cells that control muscle movement. This loss causes muscle weakness and atrophy, among other symptoms.

There are several types of SMA, based on the affected gene, and the patient’s age at disease onset and severity of symptoms. Knowing the type of SMA can help to predict disease course of the disease and determine treatment options. However, each patient should be considered individually, as disease progression and symptoms can vary within a given type.

Primary types of SMA

The primary types of SMA are all caused by a mutation in the survival motor neuron 1 (SMN1) gene, which provides the instructions to make a protein involved in protecting motor neurons. As this gene is found on chromosome 5, it sometimes is referred to as chromosome 5-related SMA.

The primary types of SMA are numbered 0 to 4, based on the age of onset of symptoms.

SMA type 0

SMA type 0 can also be referred to as prenatal SMA, as it affects a baby while still in the womb. This is the most severe form of the disease and is characterized by poor fetal movement, joint abnormalities, difficulty swallowing, and respiratory failure. It can be fatal before birth, and is almost always fatal within a first year of life.

SMA type 1

SMA type 1 is also called infantile-onset SMA or Werdnig-Hoffmann disease. It is the most common type of SMA. Symptoms may be present at birth or appear within the first few months of life. Babies with SMA1 will have developmental delays, like failure to rollover or lift their heads, and difficulties breathing and swallowing from a young age.

SMA type 2

SMA type 2 can also be referred to as intermediate SMA, developing between 7 and 18 months of age. Children with this type of SMA may never be able to stand or walk unaided.

SMA type 3

SMA type 3 can also be referred to as juvenile SMA or Kugelberg-Welander syndrome. Symptoms are generally evident before age 3, but some children may not be diagnosed until their teenage years. Children with this type of SMA may be able to stand and walk, but often lose this ability as they age and require aids.

SMA type 4

SMA type 4, or adult-onset SMA, generally develops after age 30, and patients often experience only mild or moderate symptoms.

Other types of SMA

Other types of SMA tend to be rarer and involve mutations in genes other than SMN1.

Finkel type SMA appears in adulthood, generally after age 30. It is caused by a mutation in the VAPB gene and is inherited in an autosomal dominant manner. This means that if a parent has the condition, there is a 50 percent chance of child, regardless of gender, inheriting it.

SMA with respiratory distress syndrome (SMARD1) is generally present at birth, and causes severe breathing problems and weakness in the arms. It is caused by mutations in the IGHMBP2 gene.

X-linked infantile SMA (XL-SMA) tends to progress in a way similar to SMA type 0 or type 1, with symptoms starting before or shortly after birth. It is associated with mutations in UBA1, a gene located on the X chromosome.

Kennedy’s disease, or spinal and bulbar muscular atrophy, tends to only affect men as it is caused by mutations in the androgen receptor (AR) gene, involved in testosterone signaling and located on the X chromosome. Symptoms tend to begin later in adult life.

Spinal muscular atrophy lower extremity dominant (SMA-LED) can begin in early childhood, affecting the lower legs. It is caused by a mutation in either the DYNC1H1 or BICD2 gene. SMA-LED is one of more than 10 types of distal SMA (DSMA), which mainly affect the hands, feet, lower arms, and lower legs.

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