When an adult visits their doctor complaining of muscle weakness that is gradually getting worse, and may be accompanied by fatigue and muscle twitching, their doctor may run a test for adult-onset spinal muscular atrophy (SMA).
But what is SMA, really? How does it impact adults versus children? And what can be done?
“Spinal muscular atrophy is a rare disorder causing severe weakness in the body, often in the swallowing and breathing muscles,” explained Lawrence Zeidman, MD, Director of the Neuromuscular and Autonomic Section of the Department of Neurology for Endeavor Health.
While it’s relatively rare — affecting approximately 1 in 10,000 live births — SMA is the second most common severe hereditary disease for babies and children after cystic fibrosis. Adults tend to have milder cases than children.
“It is a genetic or hereditary disorder that can range in severity. The infantile forms of SMA are fatal without supportive care, whereas the adult form is more mild, but still disabling,” explained Dr. Zeidman.
What are the types?
Spinal muscular atrophy is not one disease, but rather a group of several types of inherited neuromuscular disorders that cause muscle weakness and atrophy.
There are four different types of SMA. Type 4 is the mildest form and does not typically appear until after age 21. Because the symptoms — primarily muscle weakness — progress slowly, most people with type 4 can move on their own and it typically does not impact life expectancy.
Types 1-3 occur in young children and infants:
- SMA type 1. This is a severe form of SMA, also called Werdnig-Hoffman disease. The symptoms, which appear during the first 6 months of life, include difficulty controlling head movement, weak muscle tone, and difficulty swallowing and breathing. Type 1 represents about 60% of SMA cases.
- SMA type 2. This is an intermediate form of SMA, also called Dubowitz disease. The symptoms appear between 6-18 months of life and include decreasing muscle tone and weakness. Around 70% of people with type 2 survive until approximately age 25, with respiratory issues usually being the cause of death.
- SMA type 3. This is a mild form of SMA, also called Kugelbert-Welander disease. Symptoms, which appear after the first 18 months of life, include leg weakness. This type does not tend to affect the respiratory system and does not usually impact life expectancy.
How is it diagnosed?
“SMA is a genetic disorder that requires two copies of an abnormal gene from both parents,” said Dr. Zeidman. “This causes an abnormal function of a protein called the survival motor neuron (SMN) protein, which leads to impaired function in the motor neurons of the spinal cord and brainstem.”
Some individuals with the genetic predisposition for SMA don't develop symptoms until adulthood. A provider might suspect that a patient has SMA if they’re showing signs of severe weakness, trouble breathing or difficulty swallowing.
In cases where a provider suspects SMA, genetic blood testing is used to determine if there are issues with the SMN1 gene, which is responsible for making the SMN protein. Genetic testing can accurately diagnose SMA in 95% of cases.
“At Endeavor Health, we have medical genetic counselors and work closely with them to confirm the diagnosis of SMA,” said Dr. Zeidman. Medical genetic counselors are healthcare professionals with advanced genetics training who can help support and guide patients through the diagnosis of an inherited disease.
What are the treatments?
When it comes to managing SMA, there are several supportive care options like physical or speech therapy, feeding tubes or ventilation for cases where SMA affects the patient’s breathing and swallowing.
There are also exciting new developments for treating the disorder. In recent years, several new medications have shown promise in being able to treat the underlying genetic issue.
“There are new medications that improve the function of the SMN gene, including nusinersen, risdiplam, and onasemnogene,” said Dr. Zeidman. “Risdiplam and nusinersen are approved for children over 2 and adults. For infants who are not already ventilator-dependent, onasemnogene is an option.”
Dr. Zeidman noted that these medications can be expensive, and nusinersen requires injection directly into the spinal space.
Because SMA varies so much in severity, it’s critical to get a diagnosis and start working with an expert medical team.
While it does depend on the type and severity of the disease, through supportive therapies and cutting-edge medications, people with SMA can experience improved survival odds and a better quality of life.
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