With advances in genetics, invasive muscle biopsies are becoming less common in diagnosing spinal muscular atrophy (SMA). Due to an increase in newborn screenings, more people are diagnosed with SMA at birth rather than later on as symptoms appear. However, if your child was not screened at birth, their pediatrician or a specialist may diagnose SMA using a physical exam, family history review, genetic testing or electrodiagnostic tests, and possibly a muscle biopsy.
Here’s why muscle biopsies used to be the gold standard and what has changed about the path to a diagnosis of SMA.
The two types of muscle biopsies are needle biopsies and open biopsies. In a needle biopsy, a needle is inserted into the muscle to remove a small tissue sample that can then be examined under a microscope. For an open biopsy, a cut is made in the skin and then a tiny piece of muscle is removed. Usually, local anesthesia (freezing injected into a specific part of the skin) is all that’s needed to numb the area, so you can expect to be awake during the procedure.
Once the sample is taken, it’s sent off to the lab for testing. The biopsied area may be a little sore after the procedure, but otherwise, there’s a very low risk of any complications.
A physician may order a muscle biopsy to look for a variety of conditions:
Muscle biopsies can show the difference between muscle tissue that’s paralyzed, dying, shrinking, inflamed, injured, or experiencing nerve problems.
When doctors use muscle biopsies to diagnose SMA, they’re looking directly at the muscle for signs of atrophy. Muscle atrophy describes when muscle tissues start to thin or waste away. There are three general types of muscle atrophy:
People with SMA experience neurogenic atrophy that may cause rapid muscle weakness and significantly affect muscle movement. Other potential causes of neurogenic atrophy include:
To differentiate between SMA and other neuromuscular disorders, a specialist may consider the results of a muscle biopsy along with other factors, like the person’s age, current symptoms, and family history.
Today, doctors can order better tests to confirm an SMA diagnosis. Genetic testing (performed through a blood test) is typically the least invasive and most accurate way to identify underlying markers of SMA. Additionally, if needed, electromyography can replace biopsies by measuring muscle function. Rather than removing muscle tissue for analysis in the lab, electromyography tests muscle tissue while it’s still in the body.
Different types of SMA are associated with unique genetic mutations. However, up to 95 percent of cases are caused by autosomal recessive proximal SMA or 5q-SMA. Genetic testing identifies this type of SMA by detecting abnormalities in the survival motor neuron 1 (SMN1) gene. One in every 11,000 births possesses a mutation or deletion in this gene and it’s the leading cause of infant death related to genetics.
About 1 in every 50 Americans is a carrier of SMN1 gene mutations or deletions. Carriers don’t have symptoms, so most people are unaware. When two carriers have a child, there’s a 25 percent chance of both parents passing on the defective gene and producing a child with SMA. Additionally, the SMN2 gene is part of the SMN complex that can contribute to the development of SMA. SMN2 doesn’t cause SMA, but it can help predict the severity of the disease.
Because genetic testing can have implications for the whole family, it’s a good idea for those considering genetic testing to undergo genetic counseling and determine which relatives may also benefit from hearing the results.
Electromyography (EMG) measures electrical signals received by muscles from the nerves during exercise and at rest. An EMG shows when muscles react abnormally to nerve signals. Unlike genetic testing and muscle biopsies, which need to be analyzed in the lab, an EMG offers immediate results, helping identify the type and severity of SMA.
During an EMG, you’ll be instructed to sit or lie down. After cleaning the area, your provider will insert a needle electrode into the muscle and record electrical activity on a video screen and sometimes an audio speaker. Then, to show muscle activity during exercise, you’ll need to contract and relax the muscle. The electrode may be moved to various muscles for testing during your appointment.
Although genetic testing is an accurate way to diagnose most forms of SMA, there’s a possibility that someone may have a rare SMA type that wasn’t identified in their DNA sample. If symptoms persist and there’s still a suspicion of SMA, muscle and nerve biopsies can still be a viable diagnostic tool.
As with most rare diseases, the journey to a diagnosis of SMA isn’t always straightforward. Experienced health professionals may need to use various diagnostic tools and a process of elimination to settle on the most accurate assessment of your (or your child’s) condition. Although muscle biopsies are unlikely to be the first step in diagnosing SMA, they may be included along the way depending on the results of other tests. Talk to your health care provider about their rationale for suggesting a muscle biopsy, especially if less invasive options have yet to be explored.
On mySMAteam, the social network for people with SMA, members come together to ask questions, give advice, and share their stories with others who understand life with spinal muscular atrophy.
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