Patient Guide to Spinal Muscular Atrophy

Adult Spinal Muscular Atrophy: Symptoms

Fatigue, muscle weakness and trouble breathing could be signs of spinal muscular atrophy type 4. Learn more about what causes this condition and its symptoms.

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Have you been easily fatigued for most of your life? Do you wonder how people around you keep their balance so effortless when you have to work so hard sometimes to keep from falling? Are you more of a sports fan than a participant because you get so winded so quickly?

Woman experiencing adult spinal muscular atrophy Fatigue, trouble breathing, and leg weakness are symptoms of adult-onset SMA.

For many people, these symptoms are nothing more than the genetic luck of the draw. But for others, they could be signs of a lifelong congenital condition. They could be signs of spinal muscular atrophy type 4.

What Is Spinal Muscular Atrophy Type 4?

Spinal muscular atrophy (SMA) is a genetic degenerative neuromuscular condition. There are multiple forms of the disease, which are classified by the age of onset and the severity of symptoms. The earlier you experience symptoms, the more severe they will be.

Unlike other forms of the disease, which show up in utero, during infancy, or childhood, SMA 4 is diagnosed in adulthood and is therefore the mildest of all types. The hallmarks of SMA 4 include weakness that generally begins in the legs and may spread to the arms, overall fatigue, and tremors in the fingers.

Spinal Muscular Atrophy Type 4 Causes

SMA 4 is a genetic condition; in other words, it has nothing to do with lifestyle or environmental factors. Due to a genetic anomaly, people with SMA (including those with SMA 4) are born with a faulty gene sequence that results in poor muscle function and eventually leads to weakness, respiratory problems, and, in some cases, death.

The longer story is a bit more complicated. The SMN1 gene is responsible for producing survival motor neuron (SMN) protein, which plays a vital role in creating motor neurons, cells essential to your ability to move.

Motor neurons enable your brain to signal your muscles so quickly and effectively that, most of the time, we don’t even acknowledge it. Whenever you scratch an itch, brush your teeth, or climb the stairs without even thinking about it, you have your motor neurons to thank.

People with SMA have a mutation (change) in their SMN1 gene, which prevents them from producing adequate levels of SMN protein. With SMA, your SMN1 gene is either deleted or defective (though in most cases it’s the former). Without a functional SMN1 gene, your body relies on another gene, SMN2, for “backup” SMN protein production. The problem is, SMN2 can’t produce enough of it.

This is why people with SMA experience weakness and limited mobility. With inadequate motor neurons, the nerve impulse that normally starts in your brain and travels to your muscles in a split second gets “stuck.” This deficiency makes it impossible to use your muscles at their normal capacity, and, as a result, they atrophy (break down) and grow increasingly weaker.

Onset of Spinal Muscular Atrophy Type 4 Symptoms

Your genetics are responsible for the onset and degree of symptoms.  You’re born with a certain number of copies of the SMN2 gene; the more copies you have, the greater your capacity to produce SMN protein, and the less severe your condition will be. Because SMA4 is the least severe form of SMA, people with SMA4 generally have the most copies of SMN2.

“If you had two copies [of SMN2], then you're likely to be a baby with early onset and fatal progression relatively quickly,” explains Thomas Crawford, MD, co-director of the Muscular Dystrophy Association clinic and Professor of Neurology at Johns Hopkins University. Meanwhile, “Three and then four and even five copies, in some cases have progressively milder manifestation.”

SMA4 is diagnosed in adulthood, typically in your 20’s or 30’s.

Common Spinal Muscular Atrophy Type 4 Symptoms

The most common initial symptoms of SMA 4 include leg weakness and a tremor in the fingers. If left untreated, SMA 4 may lead to weakness throughout the body. While most people maintain the ability to walk, some may require the use of a wheelchair and/or assistance with activities of daily living.

According to Wildon Farwell, MD, MPH, Global Medical Head of Neuromuscular Diseases at Biogen, the onset of symptoms in adults with SMA 4 can be subtle. He often sees patients who report “slowing down. They’re not able to do as much. They are more fatigued, they're not able to walk as far, they're not able to walk as fast, they may start tripping,” he says. Overall, they “may not be able to keep up in ways they were previously able to.”

The subtle nature of symptoms in adults can make SMA 4 relatively difficult to diagnose, whereas the symptoms of infant and childhood onset paint a clearer picture of the disease.

According to Dr. Crawford, one of the barriers to early diagnosis of SMA 4 in adults is that patients have likely always been somewhat weaker than their peers, and the gradual nature of the disease’s progression makes it hard to pinpoint a specific issue.

He says adults with SMA 4 have probably always experienced some level of weakness. Adults with SMA 4 “are the ones [who say], ‘I always avoided the stairs,’ or ‘I was never really that athletic but I didn’t think too much of it,'” says Dr. Crawford. “I don’t know of any 50-year-old people who were completely normal and athletic and doing vigorous athletic pursuits, who then weaken at that point.”

Why are some cases milder than others?

Again, the severity of SMA, including SMA 4, depends entirely on your genes. According to Dr. Crawford, it’s simply a matter of “just how many motor neurons'' you have—and that’s a function of how much SMN protein your SMN1 and SMN2 genes can make. Lifestyle choices such as diet and exercise have no effect on the severity of your SMA. “I will say very, very vigorously, no, your lifestyle had no influence on it,” says Dr. Crawford. “I don't want somebody to feel like it's your fault, in any way.”

Spinal Muscular Atrophy Type 4 Complications

According to Dr. Crawford, the “single most important” complication people with SMA 4 experience is difficulty breathing due to the physical weakness associated with SMA 4.

While adults with SMA 4 have less complex medical issues than babies and children with more severe forms, many adults cope with life-long respiratory issues.

Though SMA 4 patients’ lungs are more robust than younger patients with more serious forms of SMA, Dr. Crawford says many of his adult patients require bi-pap support at night and sometimes during the day. Unfortunately, he explains, even the new medications cannot reverse the damage that’s already been done.

Nutrition is also an issue with SMA 4, according to Dr. Crawford. The low muscle mass that comes with SMA causes some patients to be “way underweight.” Other patients, however, experience significant weight gain. Higher weight is associated with diabetes, he explains, which brings its own complications, including neuropathy, cardiovascular disease, skin problems, and visual impairment.

Limited mobility is also a risk factor for joint contractures. Prolonged periods of immobility often leave people with limited range of motion in their joints. Decreased range of motion makes it harder to be active, leading to worsening contractures, creating a downward spiral. As Dr. Crawford explains, joint contractures contribute to stiffness in your limbs, which then predisposes you to fractures.

Constipation can also be an issue, says Dr. Crawford. Proximal muscle weakness, he explains, can interfere with a patient’s ability to perform the Valsalva maneuver (e.g. “pushing” or “bearing down”).

What we really want you to take away from this is: Your SMA is not your fault. You couldn’t have prevented it. You didn’t do anything wrong, but there’s plenty you can do right now to slow your SMA down and help you keep living the life you want.

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

Hearing you have a genetic disease like spinal muscular atrophy type 4 can be jarring, but new treatments and some lifestyle modifications can help.
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