Diagnosis of Spinal Muscular Atrophy
Overview
Spinal muscular atrophy (SMA) is a genetic neurodegenerative disease characterized by progressive muscle weakness caused by the loss of specialized nerve cells called motor neurons in the spinal cord and the part of the brain connected to the spinal cord.
A clinician may suspect a child has SMA when there are early symptoms, such as noticeable weakness or a delay in meeting certain developmental milestones. A diagnosis starts with a physical examination and family history. Other tests may include electromyography, muscle biopsy, six-minute walk test, and genetic testing, the latter of which is the most accurate way to confirm a diagnosis.
Electromyography
An electromyography (EMG) is a test used to assess the health of muscles and the nerve cells that control them – the motor neurons. An EMG is mainly performed to help diagnose diseases that cause muscle weakness, as is the case in SMA. EMG results can help determine whether symptoms are due to a muscle disease or a neurological disorder, and when combined with clinical findings, may be able to confirm a diagnosis.
Muscle Biopsy
Along with electromyography and nerve conduction velocities test, a muscle or nerve biopsy can be used to diagnose SMA if genetic testing does not identify mutations. The biopsy is used to check if the muscles are degenerating or have characteristics unique to SMA, such as group atrophy of type 1 and type 2 muscle fibers instead of the normal checkerboard pattern seen in healthy muscles.
Six-Minute Walk Test
The six-minute walk test (6MWT) is an easily administered, reliable, and standard test that can be used to assess the status of motor function in patients with neuromuscular disorders including SMA. The 6MWT can be performed in a small area such as a hallway. It does not require any advanced equipment or training for technicians. The walking course is generally 30 meters in length and marked by cones so that the test taker can easily identify the course.
Genetic Testing
Genetic testing is one of the most accurate ways of diagnosing SMA. It involves taking a blood sample from the patient and checking for known mutations associated with this disorder. The test is generally non-invasive, as it only involves taking a small sample of blood from the patient to be sent to a laboratory for testing. It can detect 95% of all SMA cases, and results are usually available in about a month.
