Spinal muscular atrophy (SMA) refers to a group of hereditary diseases that progressively destroy motor neurons. Motor neurons are a type of nerve cell found in the brainstem and spinal cord.
Motor neurons are responsible for skeletal muscle activity that can include breathing, speaking, walking, and swallowing. The destruction of the nerve cells leads to muscle weakness and atrophy.
The most common form occurs due to changes to the survival motor neuron 1 gene (SMN1) on chromosome 5q. The gene is responsible for making survival motor neuron (SMN) proteins. Without these proteins, muscle neurons disappear, which leads to muscle weakness and wasting away of the muscles.
This article reviews the different types, symptoms, complications, and treatments for SMA.
There are four common types and one rare one of SMA due to a mutation in the SMN1 gene. The types are based on the age of onset of the symptoms and highest milestone achieved.
Even within each type, your or a loved one’s abilities can vary greatly. There may also be additional loss of function over time.
This is the rarest form and very severe. While in the womb, the fetus will show reduced movement. After birth, infants typically require feeding and breathing care due to weakness. They may also have joint issues and heart defects.
They typically live for only a few months without treatment.
Doctors can often identify type 1 SMA within 6 months of birth. Infants develop:
- low muscle tone
- reduced limb movements
- fasciculations (muscle twitching)
- feeding difficulties
- impaired breathing
- lack of tendon reflexes
Without treatment, infants with type 1 won’t reach milestones like sitting or standing and may die within 2 years due to breathing complications.
An intermediate form, type 2 often develops between 6 and 18 months of age. It typically causes a reduced life expectancy, with mortality between adolescence and young adulthood.
Disease progression can vary greatly, but treatments can help improve outcomes.
Also called Kugelberg-Welander disease, symptoms of type 3 SMA develop after 18 months of age. Signs typically include trouble with:
- standing from a chair
- climbing steps
It can often cause increased risk of respiratory infections, but with treatment, those affected have a normal lifespan.
This is a milder form of SMA that typically develops after the age of 21 and is associated with mild to moderate proximal muscle weakness.
Other, less common forms of SMA exist, but they occur due to mutations in other genes.
Symptoms range from severe to mild and are generally based on the amount of functional SMN protein there is in the motor neurons, a type of nerve cell. Symptoms for each type include:
- reduced fetal movement
- severe weakness
- decreased muscle tone
- inability to reach milestones
- heart defects
- profound weakness
- never reaching milestone of sitting on their own
- trouble sucking, swallowing, or breathing
- muscle weakness that may prevent standing or walking but generally doesn’t affect the face
- ability to stand and walk, but muscle weakness slowly decreases this ability later in life
- all milestones reached, and symptoms of weakness in proximal muscles in adulthood
In severe types of SMA, a child will not live long past birth. A baby born with type 0 has a very limited life expectancy between a few weeks and 6 months. They typically die from respiratory failure.
Children born with type 1 may also die from respiratory failure by the age of 2, but modern treatments can help increase their lifespan. They may also reach some milestones, such as walking or sitting.
Children with type 2 often live into their teenage years or young adulthood. They may have respiratory issues and may not hit all milestones related to movement.
With treatment, children with type 3 often live normal lifespans. However, they may develop conditions such as scoliosis, joint contractures, or respiratory conditions.
Type 4 is the mildest form of SMA and is associated with adult onset and weakened central muscles. A person will generally not have any major complications and may live a full life.
Treatment does not cure SMA. But it can address symptoms and help with complications.
The first medication the Food and Drug Administration (FDA) approved in 2016 is nusinersen (Spinraza). The injection is safe for both children and adults. It helps to increase the production of SMN proteins.
In 2020, a similar oral medication, ri0.25sdiplam (Evrysdi), became available for children over the age of 2.
Other treatments and therapies may include:
- physical therapy
- assistive devices, such as braces or wheelchairs
- stretching exercises
- providing adequate nutrition
SMA can cause severe, life threatening complications. Children born with the most severe types of SMA typically have a shortened life span and may die from respiratory failure.
Children and adults with milder forms may need assistive devices to aid movement and may develop scoliosis, more frequent respiratory infections, and joint conjectures.
Treatments can help reduce the chances of developing complications and improve overall outcomes.