Spinal Muscular Atrophy: Causes, Types, Symptoms, Diagnosis

Spinal muscular atrophy (SMA) is a genetic disorder characterized by weakness in the muscles used to control movement (atrophy is a medical term for wasting away). This muscle weakness can make it difficult or impossible to perform basic actions, such as walking, eating, or breathing. The disease affects motor neurons in the spinal cord—these are the nerve cells responsible for muscle movement and control.

young girl with spinal muscular atrophySpinal muscular atrophy can affect children causing muscles weakness making it difficult for some to walk. Photo Source: iStock.com.

SMA is the top genetic cause of infant death, and it affects all ethnic groups and genders. Although SMA is considered a rare disorder that affects 1 in 11,000 births, many more people are genetic carriers—about 1 in 50 Americans.1

The symptoms and severity of SMA run a wide spectrum. As such, the disorder has several “types”—ranging from 0 to 4—that are described below.

What Causes Spinal Muscular Atrophy? The Genetics in a Nutshell

SMA is a genetic disorder, so the cause of the disorder lies in the body’s genes. In the case of spinal muscular atrophy, a mutation in the survival motor neuron gene 1 (SMN1) causes SMA, with the survival motor neuron gene 2 (SMN2) playing a secondary role in symptom severity.

The SMN1 gene creates a protein that feeds the motor neurons—the nerve cells that communicate with the brain to stimulate muscle movement. Without that protein, the motor neurons will waste away and die. Eventually, muscles will also deteriorate, which can be fatal when those muscles support vital body systems (eg, respiratory muscles that control inhaling and exhaling).

Typically, people have two copies of the SMN1 gene and one to two copies of the SMN2 gene in each cell. In people with SMA, both copies of their SMN1 gene are either missing or mutated. This most often occurs from inheriting two problematic copies of the gene from each parent. In other words, each parent is a genetic carrier of SMA.

Spinal muscular atrophy carriers are people who have one healthy copy of the SMN1 gene and one missing/mutated copy of the SMN1 gene. Carriers do not have SMA, but they can pass the disorder on to their children.

Below is the child’s genetic picture for SMA if both parents are carriers:

  • 25% risk of having SMA
  • 25% chance of not having SMA
  • 50% risk of being a carrier

Below is the child’s genetic picture for SMA if one parent is a carrier:

  • 50% risk of being a carrier

Although most people have one or two copies of the SMN2 gene, some people have additional copies of the SMN2 gene. In people who have SMA, having more copies of the SMN2 gene is associated with reduced symptoms and disease severity. This is because the SMN2 also creates the protein necessary for motor nerve cell function, but it’s not as effective as the protein produced by the SMN1 gene.

People who have the most severe types of SMA typically have only one copy of the SMN2 gene, whereas those who have the least severe forms may have four or more copies of SMN2 gene in each cell. More copies of the SMN2 gene better compensates for the loss of the SMN1 gene’s protein-making power.

Spinal Muscular Atrophy Types and Symptoms

Spinal muscular atrophy can present at different ages and feature different symptoms and levels of severity. As such, doctors classify the disorder by type.

Of the five SMA types, type 1 is the most common—about 50% of people with SMA have it. Types 2 and 3 are the next most common. Type 0 is the second least common type of SMA. Type 4 is the least common, with 5% of cases.2

The Table below is an overview of the 5 types of SMA.3

table, 5 types of spinal muscular atrophyTable lays out 5 types of spinal muscular atrophy that can affect infants, children, adolescents and adults.(3) Source: SpineUniverse.

Although the different SMA types are relatively distinct, the general symptoms below may be experienced by anyone with the disorder, regardless of type:

  • Muscle weakness, and possibly paralysis (total loss of movement)
  • Poor muscle tone
  • Reduced reflexes in the tendons

Diagnosing Spinal Muscular Atrophy

A genetic disorder, spinal muscular atrophy is diagnosed using genetic testing. Below are three ways doctors usually diagnose SMA.

  1. Prenatal screening: Expecting parents have the option to pursue prenatal screening tests during pregnancy to learn whether their child has a genetic disorder. Two tests may be used to diagnose SMA prior to birth. The first and most common is amniocentesis, which captures the fluid surrounding the fetus using a tiny needle. This fluid contains the baby’s DNA, which can help diagnose SMA. The other test, chorionic villus sampling, extracts DNA from the placenta.
  1. Newborn screening: Shortly after a baby is born, doctors take a blood sample to test for several genetic conditions. If the screening indicates a likelihood for SMA, the doctor will conduct additional tests to confirm the diagnosis. The benefit of newborn blood screening is that it can reveal SMA before symptoms even appear, providing the child access to early treatment. Many states in the United States have made newborn screening mandatory.4
  1. Genetic testing: If a child isn’t meeting developmental milestones (eg, rolling over, holding head upright, sitting without support) or is visibly weak, your doctor may recommend genetic testing using a blood sample, which will confirm most cases of SMA.

What’s Next? Exploring Spinal Muscular Atrophy Treatment Options

If you or your child has received a spinal muscular atrophy (SMA) diagnosis, you will work with your doctor to understand next steps. Several treatments can help curb the progressive adverse effects of the disorder, and it’s especially important to start therapy as soon as possible to prevent motor nerve cell loss.

Updated on: 01/24/20
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