Patient Guide to Spinal Muscular Atrophy

Adult Spinal Muscular Atrophy: What You Need to Know

Spinal muscular atrophy usually affects babies and small children, but SMA type 4 strikes in adulthood. Learn more about causes, symptoms, diagnosis and treatment of SMA type 4.

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As a genetic disease, spinal muscular atrophy (SMA) is something you’re born with. The good news is, it’s now easier to live with SMA than ever before, as recent scientific developments have brought about three new treatments since 2016. If left untreated, SMA causes weakness, respiratory problems, immobility and, in some cases, death. The new medications, however, can stop the disease in its tracks. Though they can’t reverse damage that’s already been done, they can keep it from progressing.

Woman studying spinal muscular atrophy type 4Great strides have been made in treating all forms of spinal muscular atrophy, including adult onset (type 4).

In 2018, the federal government put SMA on the national recommended newborn screening panel. As newborn testing grows more widespread, we should expect to see a significant drop in the number of people affected by the disease. According to Thomas Crawford, MD, co-director of the Muscular Dystrophy Association clinic and Professor of Neurology at Johns Hopkins University, “We can now identify the genetic defect at birth before the kids show any weakness and start [treatment], and effectively prevent this disease from occurring.”

Spinal Muscular Atrophy Types

There are five types of SMA:

  • SMA Type 0 presents in utero and is characterized by low-quality fetal movement. At birth, a newborn with SMA type 0 typically demonstrates abnormal breathing, issues with swallowing, respiratory failure, significant weakness, and abnormal joints.
  • SMA Type 1 presents in infancy, usually before the six-month mark. Symptoms include limited muscle tone and movement, swallowing and feeding issues, and difficulty breathing. Without treatment, most babies with Type 1 die of respiratory failure by age two.
  • SMA Type 2 generally shows up in babies between six and 18 months old. They have similar issues to those seen in children with Type 1 (limited mobility and breathing difficulties); however, their cases are less severe. If left untreated, the course of the disease varies, but most children with Type 2 reach their teens or 20’s.
  • SMA Type 3 first presents as leg muscle weakness after the 18-month mark. Initial symptoms may include problems with running, climbing stairs, or standing up from a seated position. Most people with Type 3 have a normal lifespan with treatment.
  • SMA Type 4 is characterized by the onset of symptoms after 21 years of age. While the severity varies, most people with SMA 4 maintain the ability to walk. Although treatment cannot reverse muscle atrophy that has already occurred, it can keep it from worsening.  

Causes of Spinal Muscular Atrophy Type 4

Affecting about one to two of every 100,000 people, SMA 4 presents in adults and is the rarest and least severe of all the forms.

According to Dr. Crawford, SMA is “a two-gene disorder.” He explains, “Everyone with SMA has a ‘broken’ SMN1 gene.” SMN1 is responsible for manufacturing survival motor neuron (SMN) protein, without which we're unable to move our muscles.

Meanwhile, says Dr. Crawford, “there’s a second copy of the SMN gene that's nicely labeled SMN2,” which is capable of making some SMN protein “but not very much—not nearly enough to compensate for the non-functional SMN1 gene.”

The severity of the disease depends on how many copies of SMN2 you have; more copies are typically associated with milder forms of the disease, says Dr. Crawford. According to a 2016 study, the early, severe onset of SMA types 0 and 1 are likely to have one or two copy of the SMN2 gene, respectively, while SMA 4, with its later onset and milder symptoms, is associated with more than four copies of SMN2.

Spinal Muscular Atrophy Type 4 Symptoms

The initial symptoms of SMA 4 typically include hip and leg weakness. To compensate for that lower body weakness, people with SMA 4 may adopt a gait pattern or style of walking that resembles a waddle, as a wider base of support makes it easier to maintain balance. Lower body weakness may be followed by shoulder and arm weakness. Other symptoms include finger tremors and muscle fasciculation (twitching).

These symptoms are generally accompanied by a general decrease in endurance and energy. This might look like difficulty climbing stairs, needing to take a break and sit down while shopping, or just generally feeling more tired.

Unfortunately, the non-specific, relatively mild symptoms often preclude them from being linked to a specific diagnosis right away.  Unlike the forms of SMA that show up in childhood with a distinct set of hallmark symptoms—including serious problems with breathing, strength, and swallowing, along with the developmental delays—adult onset SMA is far more subtle.

In addition to the gradual nature of the symptoms with SMA 4, some of them, particularly muscle weakness, may have already been present to some degree, which can make a diagnosis difficult to pinpoint. Dr. Crawford explains that many people who receive an SMA diagnosis later in life have never been athletic or particularly interested in vigorous physical activities. What they’d always chalked up to lack of interest or aptitude was probably a function of their biology.

He describes a patient in her 20’s who reported being athletic as a 12-year-old. “But she realized that she wasn’t keeping up with the trajectory that she thought she was going to have when she was 14 and 15. She didn't really make too much of it, since it sort of changed her interests from athletics to other things.” By the time she was 26, however, as her function declined more significantly, she knew something was wrong, sought a diagnosis, and learned she had SMA 4.

Spinal Muscular Atrophy Type 4 Diagnosis

Unfortunately, it can take years to get an accurate SMA diagnosis, says Dr. Crawford, due to its relatively subtle constellation of symptoms. SMA 4 might be mistaken for other “look-alike” genetic diseases that also cause muscle atrophy, such as limb-girdle muscular dystrophy and Duchenne muscular dystrophy.

If your doctors have ruled out other diseases and/or your symptoms lead them to suspect SMA, they’ll generally do genetic testing via a blood draw. According to a recent Neuromuscular Disorders study, there is typically no need for further testing, such as muscle biopsy or EMG.

Spinal Musculat Atrophy Type 4 Treatment

Two drug therapies are available for SMA 4 treatment: risidiplam and nusinersen. Both work by increasing the SMN2 gene’s capacity for SMN protein production.

  • Risidiplam (Everysdi) is taken orally on a daily basis.
  • Nusinersen (Spinraza) is administered via a lumbar injection (spinal tap) every four months.

Gene therapy is also available; however, it’s for babies only and not indicated for individuals over 23 months old.

While medications can’t restore muscle function you’ve already lost, they are very effective in ensuring your condition won’t deteriorate further. Therefore, the earlier you get a diagnosis and begin treatment, the better your prognosis.

In addition to medication, outpatient rehabilitation (physical and/or occupational therapy) can help you maximize your function. Your therapists can prescribe exercises to help you maintain strength, flexibility, mobility, and overall function, as well as collaborate with you on energy conservation strategies and suggest adaptive equipment and home modifications to ensure you can keep doing the activities that are most important to you.

Meanwhile, it’s important that you stay as active as possible to help prevent contractures. Contractures happen when, due to chronic immobility, your muscles shorten and cause stiffening at your joints. Limited range of motion at your joints can not only limit your functional abilities, but can also be dangerous; decreased flexibility in your hips, ankles, or knees can impair your balance and put you at serious risk for falls.

Now that you know about SMA 4, read on to learn more about causes, symptoms, and how to treat SMA 4. Then, hear from someone who lives with the condition. 

Updated on: 02/26/21
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Adult Spinal Muscular Atrophy: Symptoms
Khoi D. Than, MD
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Adult Spinal Muscular Atrophy: Symptoms

Adult-onset spinal muscular atrophy (SMA 4) can cause fatigue, muscle weakness, trouble breathing and more. Learn about causes and symptoms.
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