Spinal Muscular Atrophy
The first steps in diagnosis of a neuromuscular disease are usually an in-office physical examination and family history, with some simple tests to distinguish spinal muscular atrophy (SMA) from similar conditions (such as muscular dystrophy). Muscle weakness and hypotonia should be the first signs that raise suspicion for SMA in babies. Other signs can help to confirm the diagnosis, such as a history of motor difficulties, loss of motor skills, proximal muscle weakness, hyporeflexia (absence of reflexes), tongue fasciculations (involuntary twitches), and signs for low motor neuron disease.
A doctor may order a blood test for an enzyme called creatine kinase (CK), which leaks out of muscles that are deteriorating. This is a nonspecific test because CK levels are elevated in many neuromuscular diseases, but it’s often a useful test. High blood CK levels aren’t harmful in and of themselves, but they do indicate that muscle damage has occurred. Usually, CK levels are normal in patients diagnosed with SMA type 1 but can be a slightly elevated in patients diagnosed with other types of SMA, such as types 2 and 3. As a result, SMA types 2 and 3 might be confused with other neuromuscular disease, such as Duchenne muscular dystrophy.1,2,3
A doctor probably will recommend genetic testing if SMA is suspected because this is the least invasive and most accurate way to diagnose chromosome 5-related SMA (types 1-4). Genetic testing requires only a blood sample. However, it has implications for the whole family that must be considered (see Causes/Inheritance). Reliability and specificity of genetic tests are improving, and the number of tests available is expanding rapidly as knowledge and technology improve. For more on getting a definitive genetic diagnosis, see The Genie's Out of the Bottle: Genetic testing in the 21st century. Your MDA Care Center team can guide you toward the right type of genetic testing for your situation.
In rare cases, doctors may order a muscle biopsy, which involves taking a small sample of muscle tissue, usually from the thigh, and looking at it under a microscope.
Other tests sometimes used to diagnose SMA include one that measures nerve conduction velocity — the speed with which signals travel along nerves — and one that measures the electrical activity in muscle, called an electromyogram, or EMG. Nerve conduction velocity tests involve sensations that feel like mild electric shocks, and EMGs require that short needles be inserted in the muscles.
- Muqit, M. M. K., Moss, J., Sewry, C. & Lane, R. J. M. Phenotypic variability in siblings with type III spinal muscular atrophy. J. Neurol. Neurosurg. Psychiatry (2004). doi:10.1136/jnnp.2003.018614
- Zhang, Y., Huang, J. J., Wang, Z. Q., Wang, N. & Wu, Z. Y. Value of muscle enzyme measurement in evaluating different neuromuscular diseases. Clin. Chim. Acta (2012). doi:10.1016/j.cca.2011.11.016
- Fraidakis, M. J. et al. Genotype-phenotype relationship in 2 SMA III patients with novel mutations in the Tudor domain. Neurology (2012). doi:10.1212/WNL.0b013e318247ca69