Glossary of SMA Terms

Reviewed by: HU Medical Review Board | Last reviewed: August 2021



Progressive weakness and wasting away of muscles. In SMA, it happens because of a genetic mutation leading to a loss of nerves that communicate with muscles.1

Autosomal recessive

Inheritance pattern where the gene is located on a non-sex chromosome (autosome) and causes disease when both copies of the gene are mutated (recessive). SMA is autosomal recessive because the SMN1 gene is located on chromosome 5, and both copies must be altered.1



Someone who inherits only 1 copy of an altered gene for a recessive disease. Carriers do not show symptoms but can pass on the mutated gene to their children. Carriers of SMA inherit only 1 copy of the mutated SMN1 gene.2


Evrysdi™ (risdiplam)

Drug approved in August 2020 to treat adults and children over 2 months old with SMA. It is taken by mouth. It works by increasing the amount of SMN protein made by the SMN2 gene.3,4



Involuntary muscle twitches. Tongue fasciculations are a common symptom of SMA.2



The number of new cases of a disease within a certain time period. The incidence of SMA is about 1 out of 6,000 to 10,000 newborns worldwide.5


Joint contracture

Permanent shortening and stiffening of a joint. This causes a loss of movement around the joint. Joint contractures are common complications of SMA. Physical therapy tries to prevent joint contractures in people with SMA.6


Motor neuron

Nerve cell in the brain or spinal cord that controls muscle movements. Without motor neurons, muscles that depend on them become inactive and weak. In SMA, the motor neurons in the spinal cord are affected.7



Doctor who diagnoses and treats neurological disorders, such as SMA. Pediatric neurologists often manage the healthcare team for children with SMA.6


Involving the communication between the nervous system and the muscular system. In SMA, a loss of motor neurons causes muscles to weaken and waste away.2

Non-invasive ventilation (NIV)

Technique to support breathing, where oxygen is given through a face mask. NIV should be used in all infants with symptoms of SMA and all infants with type 1 SMA.8



The number of people currently living with the disease. The prevalence of SMA is about 1 in 100,000 people worldwide.5


Scapuloperoneal spinal muscular atrophy (SPSMA)

Rare form of SMA caused by mutations in the TRPV4 gene. Symptoms start in childhood or adulthood and slowly get worse over time.9,10


Abnormal curvature of the spine that reduces mobility. It is a common complication of SMA. Braces and other postural supports are commonly used to prevent scoliosis.6,11

Spinal and bulbar muscular atrophy (SBMA)

Rare form of X-linked SMA also known as Kennedy’s disease. Symptoms appear between 30 and 50 years old. It is caused by mutations in the AR gene.10,12

Spinal muscular atrophy with respiratory distress 1 (SMARD1)

Rare form of SMA caused by mutations in the IGHMBP2 gene. Respiratory distress and muscle weakness begin before 6 months old and worsen until age 2.9,10,13

Spinraza® (nusinersen)

First drug approved by the U.S. Food and Drug Administration (FDA) to treat children and adults with SMA. It is given by injection into the fluid around the spine. It works by improving the ability of SMN2 to make SMN protein.3,4,14,15

Survival of motor neuron 1 gene (SMN1 gene)

Gene altered in about 95 percent of SMA cases. The gene provides instructions for cells to make the SMN protein.1

Survival of motor neuron 2 gene (SMN2 gene)

Back-up gene that also provides instructions for cells to make the SMN protein. Most of the variation in SMA types is due to the SMN2 gene. It does not work as well as SMN1, but people with SMA rely on SMN2 to make up for the lack of functional SMN1. More copies of the SMN2 gene means the disease will be milder.1

Survival of motor neuron protein (SMN protein)

Protein important for motor neurons. It is made with instructions from the SMN1 and SMN2 genes. In SMA, mutations in the SMN1 gene reduce the production of the SMN protein.1,16


Type 0 SMA

Most severe and rarest SMA form caused by altered SMN1 genes. Symptoms appear before birth, and infants have severe weakness and muscle wasting. Life expectancy is less than a month.10,15

Type 1 SMA (Werdnig-Hoffmann disease)

Most common type of SMA. Symptoms appear within 6 months of life, and infants can never sit independently. Life expectancy is 2 years.10,15

Type 2 SMA (Dubowitz disease)

Intermediate form of SMA. Symptoms appear between 6 and 18 months old. Children can sit independently but usually cannot walk. Life expectancy is 20 to 40 years.10,15

Type 3 SMA (Kugelberg-Welander disease)

Mild form of SMA. Symptoms appear between 18 months old and adulthood. People may lose the ability to walk over time. Life expectancy is normal.10,15

Type 4 SMA

Mildest form of SMA. Symptoms appear after 30 years old. People maintain the ability to walk throughout life. Life expectancy is normal.10,15


X-linked SMA

Forms of SMA caused by mutations in genes on the X chromosome. X-linked SMA forms are much more common in men than women. One example is X-linked infantile spinal muscular atrophy, which is caused by alterations in the UBA1 gene.10


Zolgensma (onasemnogene abeparovec-xioi)

Gene therapy approved by the U.S. Food and Drug Administration (FDA) in 2019 to treat children under 2 years old with SMA. It is given by intravenous (into a vein) injection. It delivers a functional SMN1 gene to motor neurons.3,4,17,18

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