There is no single test that confirms a diagnosis of primary lateral sclerosis (PLS). In fact, because the disease can mimic signs and symptoms of other neurological diseases such as multiple sclerosis and ALS, your doctor may order several tests to rule out other diseases.

After taking a careful record of your medical history and family history and performing a neurological examination, your doctor may order the following tests:

  • Bloodwork. You'll have blood tests to check for infections or other possible causes of muscle weakness.
  • Magnetic resonance imaging (MRI). An MRI or other imaging tests of your brain or spine may reveal signs of nerve cell degeneration.

    Your doctor may also order an MRI to look for other causes of your symptoms, such as structural abnormalities, multiple sclerosis or spinal cord tumors.

  • Electromyogram (EMG). During an EMG, your doctor inserts a needle electrode through your skin into various muscles. The test evaluates the electrical activity of your muscles when they contract and when they're at rest.

    This test can measure the involvement of lower motor neurons, which can help to differentiate between PLS and ALS.

  • Nerve conduction studies. These tests use a low amount of electrical current to test and measure your nerves' ability to send impulses to muscles in different areas of your body. This test can determine if you have nerve damage.
  • Spinal tap (lumbar puncture). In this procedure, your doctor uses a thin, hollow needle to remove small samples of the cerebrospinal fluid — surrounding your brain and spinal cord — from within your spinal canal for laboratory analysis. A spinal tap can help rule out multiple sclerosis, infections and other conditions.

After other diseases are ruled out, your doctor may make a preliminary diagnosis of PLS.

Sometimes doctors wait three to four years before giving a diagnosis, because early amyotrophic lateral sclerosis (ALS) can look just like PLS until additional symptoms surface a few years later. You may be asked to return for repeat electromyography testing over three to four years before the PLS diagnosis is confirmed.