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Spinal muscular atrophy (SMA) is a neuromuscular disorder defined by muscle weakness that progresses over time. Depending on type, symptoms of SMA can include breathing problems, lack of motor function, inability to stand or walk, and difficulty chewing and swallowing.
Scientific advances into the genetic causes of spinal muscular atrophy have made it easier to diagnose. The process can be as straightforward as a physical exam and blood test. However, because SMA is a rare disease generally, with some extremely rare subtypes, the path to diagnosis can sometimes be long.
The path to diagnosis of spinal muscular atrophy will vary depending on the type of SMA a person has and age of onset. For babies and children with more severe forms of SMA, the path to diagnosis often begins when a parent or health care professional notices unusual muscle weakness (hypotonia). Individuals with adult-onset SMA types like type 4 or Kennedy's disease (X-linked spinal and bulbar muscular atrophy) may start the journey to diagnosis after noticing mild symptoms like hand tremors.
Recent changes to the United States Department of Health and Human Services (HHS) Recommended Uniform Screening Panel (RUSP) for newborns may change the process of SMA diagnosis. HHS added spinal muscular atrophy to the RUSP in July 2018. Newborn screening is a standard part of care at every American hospital and generally occurs in the first 24 to 48 hours of life. Newborn screening includes drawing a blood sample (often via a heel stick) that will be evaluated for the conditions on each state’s screening panel. As of January 2020, 17 states routinely screen infants for the genetic mutation that causes SMA types 0 through 4. Other states are expected to add SMA genetic screening in the future. The adoption of newborn screening has the potential to greatly impact early interventions for infant and adult-onset SMA.
In cases where a newborn is not screened for SMA at birth, the first step to diagnosing spinal muscular atrophy is a physical exam to determine if symptoms like muscle weakness or lack of reflexes are present. This may be done by a primary care physician or a neurologist. In many cases, a primary care doctor will refer someone with suspected symptoms of SMA to a specialist.
As part of your or your child’s physical examination, the physician will take a thorough family history to determine if there are instances of neuromuscular disease in the family. If SMA is suspected based on the physical exam and family history, the next step will likely be genetic testing.
SMA is diagnosed through molecular genetic testing, which is done via a blood sample. In molecular genetic testing, a single gene is evaluated for mutations associated with a genetic disease. After providing the blood sample, you may have to wait a month or longer for results to be available.
You may meet with a genetic counselor as part of the genetic testing process. A genetic counselor is a health care professional trained to help people at increased risk for or diagnosed with inherited conditions. If you work with a genetic counselor, they can help you understand the testing process and results and help you determine next steps.
When diagnosing SMA, molecular genetic testing focuses on the survival motor neuron 1 (SMN1) gene on chromosome 5. Mutations in the SMN1 gene cause SMA types 0 through 4, which account for nearly all cases of SMA.
The SMN1 gene is responsible for creating survival motor neuron (SMN) proteins. These proteins are important for motor neuron function. Motor neurons are nerve cells that transmit signals from the brain and spinal cord to the muscles. Without these proteins, the brain and the muscles can’t communicate and the muscles weaken.
Other types of SMA are caused by mutations on different genes. Specialized genetic testing for other SMA types, such as SMA with respiratory distress (SMARD), are also available.
Before genetic testing was available for diagnosing SMA, doctors performed a muscle biopsy, removing a tiny piece of muscle tissue for evaluation in a lab. Electromyography was also conducted to evaluate the electrical activity of muscles. Together, these tests were previously used to diagnose SMA.
Today, a genetic test is sufficient to diagnose the vast majority of SMA cases. However, if genetic testing was inconclusive, your or your child’s doctor may order an electromyogram (also called EMG) or muscle biopsy to confirm an SMA diagnosis.
Genetic screening for SMA is routinely available to women planning to become pregnant or who are already pregnant. Screening is particularly recommended for men and women with a known family history of SMA.
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