A clinician may suspect a child has SMA (spinal muscular atrophy) 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.
A doctor may want to test for creatine kinase (CK), an enzyme that leaks out of muscles that are deteriorating, according to the MDA. This test is not SMA-specific because CK levels can be elevated in several neuromuscular diseases, including muscular dystrophy, but it is a starting point. High CK levels indicate that muscles has been damaged.
Genetic testing through a blood sample is the least invasive and most accurate way to diagnose SMA because the specific mutation that causes the disease has been identifed. According to Cure SMA, a blood test can identify about 95 percent of all SMA cases.
Gene mutations in the SMN1 (survival motor neuron) genes cause SMA. Most people have two copies of this gene; when they have one faulty copy, they are called carriers. When two carriers pass on the mutation in the SMN1 gene, there is a 25 percent chance their child will have SMA.
Another gene also plays a role in SMA: the SMN2 gene. According to Cure SMA, “Most of the SMN protein produced by SMN2 lacks a key building block that is normally produced by SMN1. This means that SMN2 cannot fully make up for the mutated SMN1 gene.”
Some people with SMA have three or more copies of the SMN2 gene. These extra copies can help modify the course of the disease, and typically people with more copies of SMN2 will have less severe symptoms.
Genetic testing for mutations in the VAPB gene is done to diagnose Finkel type SMA.
Sometimes, other tests are also needed, such as an electromyography (EMG, to assess the health of muscles and the nerve cells that control them — motor neurons), which tests the speed that signals travel along nerves. Nerve conduction velocity tests involve sensations that feel like mild electric shocks.
Sometimes a muscle biopsy is needed (which involves taking a small sample of muscle tissue, usually from the thigh) in cases where it is not possible to perform a genetic test or no abnormality is identified.
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