Diagnosing Spinal Muscular Atrophy

Spinal muscular atrophy is a disease that is usually seen in babies and children. It makes their muscles weak and hard to move. It's a rare illness that is passed down in families.

A doctor might want to test your child for SMA if they show symptoms like:

• Weak or floppy arms or legs
• Delay in normal milestones like holding up their head, rolling over, sitting up, standing, or walking
• Trouble breathing
• Issues eating and swallowing
• Tongue twitching
• Curved spine (scoliosis)

SMA isn’t the only reason your child could have muscle weakness or any of these symptoms. So, to diagnose SMA, your doctor will likely do several things.

Exam and family history. The first step is a general physical exam. Your doctor will look for muscle weakness and any other signs of SMA. They'll also ask questions about all the symptoms your child has. You'll also be asked if anyone else in your family has problems with muscles and nerves, or if they have SMA.

Genetic testing. A simple blood test can check for SMA. Changes or defects in the SMN (survival motor neuron) gene are found in more than 95% of people who have this disorder. The other 5% have it because of a rare mutation. If that blood test shows one of those gene issues, your doctor will order more tests to confirm that SMA is there. If the results of the genetic testing aren’t clear, you'll have different tests to find out what's going on.

Electromyogram (EMG). This test measures how well muscles are working. Thin needles are inserted into a muscle or group of muscles. Then, the electrical activity of the muscles is measured. The test will also help your doctor see if nerves have been damaged.

Nerve conduction studies. You might get this along with an EMG. Nerve conduction studies measure how fast signals travel along nerves. Electrodes are placed on the skin over a nerve, and then a small electric shock is sent through the area. The mild shock stimulates motor and sensory nerves. How much of the electrical impulse reaches the nerve is recorded, as well as how fast the signals move.

Creatine kinase (CK) test. This blood test looks for an enzyme called creatine kinase. It's found when muscles are damaged. CK levels can be high from many things, but it's still a helpful test in diagnosing SMA. Levels are usually normal or just slightly higher in people with SMA. A CK test can help rule out similar diseases that involve muscles and nerves.

Muscle biopsy. In the past, this was the main way SMA was diagnosed. Now, it's used only in rare cases if other tests aren’t able to confirm the disease. A doctor will take a small sample of muscle tissue -- usually from the thigh -- and look at it under a microscope to search for signs of the disease.

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