Diagnosis of Amyotrophic Lateral Sclerosis

ALS is difficult to diagnose because the symptoms are similar to those of other neuromuscular disorders, many of which are treatable. The diagnosis is usually based on a complete neurological examination and clinical tests.

A neurological exam usually shows evidence of muscle weakness (localized or widespread, depending on the extent of the disease). The exam also reveals muscle atrophy. The muscles may be so stiff that when the neurologist moves them, they continue to move abnormally afterward. When the neurologist tests the "knee jerk" reaction, the movement is abnormally quick.

Because ALS affects the skeletal, voluntary muscles, the neurological exam usually does not reveal abnormalities in the sensory reflexes (i.e., vision, hearing, taste, smell, touch, or bowel and bladder control).

Diagnostic Tests for ALS

Nerve conduction velocity (NCV) and electromyography (EMG) help diagnose nerve and muscle disorders. These tests, which can be used in combination, are often referred to as EMG/NCV studies. NCV is administered before EMG and measures the speed at which nerves transmit electrical signals.

During NCV, electrodes are placed on the skin over a nerve that supplies a specific muscle or muscle group. A mild, brief electrical stimulus is delivered through the electrode and the response of the muscle is detected, amplified, and displayed. The strength of the signal is also measured.

Neurological conditions can cause the NCV to slow down or to be slower on one side of the body. The strength of the response also provides clues to help with diagnosis and to determine the extent of the disease. In ALS, the speed of the nerves is usually slowed, but compound muscle action potentials (CMAPs) may be decreased.

EMG measures nerve impulses within the muscles. Tiny electrodes are placed in the muscles in the arms and legs and the electronic responses are observed using an instrument that displays movement of an electric current (oscilloscope). As muscles contract, they emit a weak electrical signal that can be detected, amplified, and tracked, providing information about how well the muscles are working. These responses are abnormal in cases of ALS.

Tests may be performed to rule out other neurological disorders. Magnetic resonance imaging (MRI scan) may be used to rule out spinal cord or brainstem disease. Blood tests may be done to detect the presence of heavy metals such as lead in the blood. Laboratory tests may detect abnormal proteins or hormone levels associated with other neurological diseases. A lumbar puncture or spinal tap may be performed to analyze the cerebrospinal fluid for genetic abnormalities (e.g., viral, autoimmune, neurotoxic).

Publication Review By: Gordon R. Kelley, M.D., Stanley J. Swierzewski, III, M.D.

Published: 02 Jan 2000

Last Modified: 02 Oct 2014