How is amyotrophic lateral sclerosis (ALS) diagnosed?

Dr. Jeanne Morrison, PhD
Family Practitioner

Differential diagnosis of amyotrophic lateral sclerosis (ALS) sometimes requires a variety of tests to distinguish it from other diseases. Several tests using electrodes help doctors determine the level of electrical activity in the body's nerves and muscles. A muscle biopsy may be used to determine whether degeneration is being caused by ALS—a failure of motor neurons—or a separate muscle disease. An MRI is used for direct evaluation of the central nervous system. A spinal tap may be used to analyze spinal fluid directly. Blood and urine tests and a neurological exam are also used.

There are several kinds of tests that can be done to try to diagnose amyotrophic lateral sclerosis (ALS). These tests include:

  • Electromyogram (EMG) - This sees how well the muscles are working by inserting a thin electrode into the muscle which detects electrical activity.
  • Nerve conduction velocity (NCV) - This passes a small shock through a nerve, which measure the speed of the nerve signals.
  • Spinal tap - This inserts a needle into the spinal canal, removing cerebrospinal fluid for testing.
  • X-rays, CT scans and MRI - These create images of the spinal cord and the brain to look for any abnormalities.
  • Urine and blood tests - These are done to look for abnormal levels of certain substances in the urine and blood.
  • Muscle or nerve biopsy - This removes a small sample of tissue, which then is sent to a lab for testing and analysis.

There is no single test that exists today to specifically diagnose amyotrophic lateral sclerosis (ALS), or Lou Gehrig's disease. Doctors make the diagnosis when it is suggested by the history rendered by the person, his or her general and neurological examinations, electrodiagnostic studies (EMG) and exclusions provided by neuroimaging (MRI) and laboratory studies.

A common symptom is painless weakness in a limb. A physical exam can show any one of a number of findings including weakness, atrophy, brisk reflexes (when using the reflex hammer) and fasciculations (involuntary muscle twitching). EMG (electromyography) can be very helpful, but in and of itself, cannot confirm the diagnosis. Finally, laboratory studies can exclude conditions that mimic ALS.

It is only after a thorough evaluation as outlined above that a doctor can arrive at this diagnosis. It is important to note that a number of medical conditions can look very similar to ALS and must always be considered and excluded.

ALS is diagnosed by a neurologist and is based on several factors and diagnostic tests. One, the neurologist will take an indepth history and performed a physical examination looking for signs of difficulty breathing or swallowing, muscle wasting, weakness and hyperactive reflexes (reflexes are what the physician examines when tapping your knee or elbow with a small hammer). Then the neurologist will likely have you undergo an MRI of the brain and cervical spine, EMG/NCS (an electrical test which evalutes for nerve damage). and blood tests to exclude other disease which can mimic ALS.

If the the history, examination and EMG/NCS are consistent with ALS, and the MRI brain and cervical spine and blood work reveal no other causes for the patient's symptoms, a diagnosis of ALS is made. It should be noted that early on in the disease course, a definitive diagnosis cannot be made and so the patient is monitored for disease progression. The diagnosis may not be confirmed for many months.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.