Testing Muscle Electrical Activity After Exercise Helps Diagnose LEMS

Report shows importance of post-exercise facilitation test in diagnosis

Marisa Wexler, MS avatar

by Marisa Wexler, MS |

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Repeated testing of electrical signaling in muscles before and after exercise is a key test for diagnosing Lambert-Eaton myasthenic syndrome (LEMS), a new case report highlights.

The case was discussed at the 32nd International Congress of Clinical Neurophysiology (ICCN) of the IFCN, Sept. 4–8 in Geneva, Switzerland, in a poster titled, “Post-Exercise Facilitation Test as key in early diagnosis of Lambert Eaton Myasthenic Syndrome case treated with amifampridine.”

Scientists from Hospital General Universitario Gregorio Marañón, in Spain, described the case of a 75-year-old woman who sought medical attention due to weakness in her limbs and neck, difficulty swallowing, and weight loss.

She had a prior history of autoimmune diseases, namely limited scleroderma and Sjögren’s syndrome.

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Electromyography, which can be used to assess a muscle’s electrical activity in response to nerve stimulation, showed a “diffuse proximal myopathic pattern,” or evidence of reduced electrical activity in muscles throughout her body, especially closer to the trunk.

Clinicians stimulated the right ulnar nerve, which runs down the arm and controls the movement of muscles in the hands, and then measured the compound motor action potential (CMAP) in the abductor digiti minimi — a muscle on the outside of the palm under the pinkie finger. CMAP is a measure that averages the electrical signals of many muscle fibers in a particular piece of muscle tissue.

After taking initial measurements, doctors asked the patient to flex her hand for 10 seconds, activating the abductor digiti minimi muscle. CMAP was again measured. This experimental setup is known as a post-exercise facilitation test, or PEFT. An increase in CMAP after PEFT is a telltale sign of LEMS.

In initial measurements, CMAP had an amplitude of 0.39 millivolts (mV). After PEFT, it increased to 7.94 mV.

“The first right ulnar nerve PEFT showed a +2035% CMAP increase, which, together with the patient’s clinical findings, proves the presence of LEMS,” the researchers wrote.

The investigation was repeated after the patient took a dose of 10 mg of Firdapse (amifampridine), an approved therapy for LEMS.

This time, CMAP at rest was 0.57 mV and after PEFT it was 8.9 mV. After a recovery period and Firdapse treatment, CMAP was at 1.2 mV at rest and increased to 9.1 mV after PEFT.

“Baseline CMAP after amifampridine administration showed a +217% increase, compared with the pre-treatment study. Then, with PEFT a +739% CMAP increase was obtained,” the researchers wrote. “These neurophysiological findings show a severe presynaptic neuromuscular transmission disorder, which improves after the administration of 10 mg of amifampridine.”

This report highlights the importance of PEFT in diagnosing LEMS, the researchers noted.

“Although LEMS remains a rare disease, it should be suspected in any patient with proximal muscle weakness,” they wrote. “The detection of diffusely reduced CMAPs in patients with clinical suspicion should raise the alarm.”