Diagnosing Spinal Muscular Atrophy

SMA is diagnosed primarily through a blood test, which looks for the presence or absence of the SMN1 gene, in conjunction with a suggestive history and physical examination.
Normally, individuals have two genes called Survival Motor Neuron 1 and 2. In approximately 95% of patients with SMA there is an absence of the SMN gene sequence, which is present in normal individuals. Sometimes the SMN1 gene is not missing, but mutated. The numbers of copies of SMN2, a near identical backup copy of the SMN1 gene, is related to the severity of the disease, but does not reliably predict a specific SMA type in a given individual. SMA type is generally determined from the clinical examination evaluating the child’s degree of weakness and ability to achieve major motor milestones such as sitting independently or walking.

Occasionally, doctors may request muscle biopsy or EMG (electromyography) testing. Since the genetic blood test became available, a muscle biopsy is almost never indicated and is valuable mainly in cases where the blood DNA test is negative.

EMG measures the electrical activity of muscle. Sometimes this test is performed to help distinguish other disorders of nerve or muscle, which can mimic SMA. Small recording electrodes (needles) are inserted into the patient's muscles, usually the arms and thighs, while an electrical pattern is observed and recorded. In addition, a nerve conduction velocity test (NCV) is performed to help assess how well the nerves are functioning in response to an electrical stimulus. Small shocks are repeatedly administered to help assess nerve integrity and function. When performing this test on a child, if at all possible, it should be performed by a doctor experienced in caring for children.