Spinal muscular atrophy (SMA) refers to a group of inherited diseases that cause motor neurons to die.

They’re the nerve cells in the spinal cord and brain stem that control your muscle movements. SMA causes muscle weakness and wasting, affecting many bodily functions, such as swallowing and walking.

SMA is also quite rare, affecting only about 1 in 10,000 people, according to the Centers for Disease Control and Prevention (CDC). The condition is often diagnosed shortly after birth, but it can also onset in the teen years and later adulthood.

There are several different kinds of SMA resulting from a gene mutation. The most common form of the disease is SMA type 1, or Werdnig-Hoffmann disease, which accounts for 60 percent of all SMA diagnoses in infants.

Here, we’ll overview:

  • some of the early hallmark signs and symptoms of SMA
  • what causes it
  • ways the condition can be managed

Spinal muscular atrophy types are usually numbered 1 through 4. The lower the number, the earlier the onset of the disease and the more severe the symptoms. “Type 0” is sometimes used to refer to SMA so severe it is detectable in utero.

  • Type 1 (Werdnig-Hoffman disease) is usually noticeable in infants before they’re 6 months old. They almost immediately have trouble breathing, feeding, and moving. Untreated, children with type 1 SMA often die of respiratory failure before they reach 2 years old.
  • Type 2 (Dubowitz disease) usually presents in infants around 6 to 18 months old. They may be able to sit up, but they’re unable to stand or walk without assistance. They also have breathing difficulties, especially during sleep. Life expectancy is higher than type 1, and on average, babies with type 2 SMA can live to young adulthood.
  • Type 3 (Kugelberg-Welander disease) usually presents around or after 18 months old. It can also onset in the teenage years and is alternately called juvenile SMA. Children with type 3 SMA can stand and walk on their own, but they may experience challenges with more complex movements like climbing stairs. They’re more likely to develop respiratory infections, but most children with type 3 have a typical life expectancy.
  • Type 4 onsets in adulthood, in the 20s and 30s or later. This is the rarest type of SMA, and the mildest. People with type 4 can usually walk without challenges throughout their life and experience a typical life expectancy.

SMA symptoms can range from mild to severe. Certain symptoms are more common depending on the age at SMA onset.

When diagnosed in infancy, SMA symptoms tend to be much more serious than those in adult-onset SMA. They can even become life threatening.

Breathing difficulties

Breathing problems associated with SMA are more common in infants and can include:

  • weak or underdeveloped lungs
  • cough
  • shallow breathing during sleep
  • shortness of breath

Feeding difficulties

Weak swallowing muscles can affect a child’s ability to eat and drink safely. An infant or child with spinal muscular atrophy may be more likely to choke or have food and drink enter their lungs (aspiration). Malnutrition is another possible outcome of this symptom.

Swallowing specialists can sometimes help children practice eating and drinking safely. For older children, cutting up food into smaller pieces can help reduce the chance of choking. If physical therapy or adjusting food form does not work, a feeding tube may be necessary.

Muscle weakness

Muscles in the shoulders and legs are often among the first areas affected by SMA when it’s diagnosed in infancy. When SMA appears around age 1 year, muscles in the lower limbs tend to be affected first.

Spinal muscular atrophy can also often interfere with a baby’s ability to sit upright and walk, as well as other motor development milestones. Poor neck and head control has also been observed.

Muscle weakness usually worsens over time. Individuals who are able to walk during childhood may lose mobility as they get older.

Scoliosis

A common development in people who develop SMA in childhood is scoliosis, or irregular curvature of the spine.

Scoliosis occurs because the muscles that normally support your spine are too weak to hold it in place. Scoliosis can affect your posture and mobility, and may cause pain or numbness. In severe cases, spinal problems can make breathing more difficult.

Frequent respiratory infections

Breathing problems raise your risk of developing respiratory infections, including pneumonia. Respiratory difficulties and infections are often the reason why babies with early-onset SMA may only live a few months or years.

A 2017 study notes that SMA may be much more than a condition isolated to the motor neurons. Researchers suggest that SMA may involve multiple organs and the body’s immune system. This might help explain why individuals with spinal muscular atrophy face a heightened risk of serious infections.

A blood test given to infants can identify mutations of the SMN1 gene or determine if the gene is missing.

A 2021 study suggests that newborn screening for SMA has been successful in leading to treatment before the loss of an excessive number of motor neurons. This allows for improved motor development and generally better outcomes.

Spinal muscular atrophy was added to the United States’ list of recommended screening tests for newborns in 2018. The Recommended Uniform Screening Panel (RUSP) identifies serious health conditions to help ensure babies can receive prompt treatment and have the best possible health outcomes.

Other tests may be performed to detect SMA to make a concrete diagnosis.

However, with genetic testing, these methods are usually no longer needed:

Spinal muscular atrophy is a group of hereditary disorders, meaning they are determined by your genes.

SMA is caused by a specific gene mutation in one of your chromosomes. This gene, SMN1, produces an important protein called the “survival motor neuron” that’s essential for healthy nerve function and control of various muscle groups.

Without enough SMN protein, the motor neurons around the spinal cord die, causing muscle weakness and loss of mass (wasting).

SMA typically occurs in individuals who are missing both copies of the SMN1 gene or who inherit an abnormal SMN1 gene. Having a parent with SMA significantly increases the likelihood that you may be an SMA carrier.

What is a “carrier”?

A carrier is someone who has the gene for a particular disease or disorder but usually does not have symptoms of it.

Carriers can pass the gene to their children. For example, if both parents are carriers of the SMN1 gene mutation, their child will likely have symptomatic SMA. If only one parent has the SMN1 gene mutation, the child may also be a carrier, but likely have no symptoms.

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There is currently no cure for SMA, but there are various treatments available to lessen the severity of and manage symptoms.

Medications

FDA-approved drugs used to treat SMA include:

MedicationHow it worksAdministrationPossible side effects
Nusinersen (Spinraza)Boosts SMN protein production. In 2016, this was the first drug to be FDA approved to treat SMA.injected into the lumbar spinal canal (inthrathetcal administration)upper and lower respiratory infections, constipation, kidney toxicity
Risdiplam (Evrysdi)Boosts SMN protein production. This is the first oral medication to be FDA approved to treat SMA.taken orallyfever, diarrhea, rash, mouth ulcers, joint pain, urinary tract infections
Onsemnogene abeparvovec-xioli (Zolgensma)Replaces the mutated SMN1 gene with a functional gene. It is FDA approved for children under age 2 with SMA.taken intravenously (injected into a vein)vomiting, elevated liver enzymes, risk of liver injury

Other therapies

Surgery, other medical procedures, physical therapy, and caregiving can all play important roles in treating people with spinal muscular atrophy.

Scoliosis may be treated successfully with surgery to straighten the spine and fuse two or more bones in the spine to give the backbone a longer, more supportive structure. Some people also wear special braces.

Physical and occupational therapies may be used to strengthen muscles and improve coordination. This includes practicing swallowing techniques.

A feeding tube, where liquid nutrition is delivered directly to the stomach, may be necessary for those who can no longer swallow.

Mobility aids such as walkers or wheelchairs can help adults and older children with SMA move around and feel more independent.

Some people with SMA may require the support of a ventilator. This happens when SMA affects your lungs’ ability to function, causing you to need help breathing. Noninvasive ventilation is when air is delivered through a mask or mouthpiece. It can be used as needed and taken off to eat and talk.

If this isn’t enough, more invasive types of ventilation may be necessary. Doctors may have to surgically insert a tube into your windpipe to deliver air, which is a process called a tracheostomy.

Those with more severe types of spinal muscular atrophy may require part or full-time caregivers to help with daily functions, such as getting dressed and eating.

SMA is a genetic condition affecting the nerves that control certain muscle groups throughout the body. In severe cases, SMA can limit a child’s motor development and limit their life expectancy.

This disease is categorized by type, ranging from 0 to 4, with higher numbers indicating a later onset and milder disease progression. People with types 3 and 4 are often able to walk without assistance and have a typical life expectancy.

Getting a timely diagnosis and starting treatment can help with symptom management. Adults with SMA may need caregivers. Pain medications, physical therapy, and mobility aids can help people with SMA make adaptations and have a better quality of life.

Currently, there’s no cure for SMA, but research is ongoing. Experts have already made progress on promising new medications and forms of gene therapy.