A post-exercise facilitation test, which measures muscle activity following voluntary muscle contraction or electrical stimulation, can help identify the effects of carpal tunnel syndrome (CTS) on nerves and muscles in people with Lambert-Eaton myasthenic syndrome (LEMS), a case report shows.
The study, “Use of Post-Exercise Facilitation to Assess Carpal Tunnel Syndrome with Concurrent Lambert-Eaton Myasthenic Syndrome,” was published in the American Journal of Physical Medicine & Rehabilitation.
LEMS is a rare autoimmune disease that affects the neuromuscular junction — the place where nerve cells and muscle fibers communicate — leading to muscle weakness, reduced reflexes, and extreme fatigue.
CTS is a condition caused by compression of the median nerve — the nerve that travels from the arm to the hand — that can cause numbness, tingling, and muscle weakness in the hands and fingers.
The similarity of symptoms may make it difficult to distinguish the two disorders.
Investigators at the University of Michigan documented the case of a patient for whom a post-exercise facilitation test enabled the distinction between CTS and LEMS based on the effects on the muscles and nerves of the hands.
The 64-year-old man had a history of LEMS, and chronic neuropathy (nerve pain) at the elbow. He presented with acute pain and swelling of both hands and feet.
He also had been diagnosed with remitting seronegative symmetrical synovitis with pitting edema (RS3PE), a rare syndrome associated with arthritis in multiple joints, fluid accumulation at the joints, and swelling of hands and feet.
After two months of treatment with oral prednisone, his pain and swelling eased, but numbness and tingling in his hands remained.
At this point, physicians started suspecting the patient might have CTS, which was backed up by a physical examination showing he had mild muscle weakness on the muscle controlling the movement of his thumbs.
A post-exercise facilitation test performed after the patient was asked to contract his arm muscles for 20 seconds showed that muscle activity on his left arm increased by more than 150%. However, no changes in muscle activity were observed on his right arm.
According to his care team, the findings suggest that CTS accounts for muscle activity reduction on the right arm due to lack of nerve signal propagation caused by the compression of the median nerve at the wrist, or by the loss of nerve endings (axons).
“In contrast, on the left, repair of amplitude to within normal range with exercise-induced facilitation indicates neuromuscular junction dysfunction, rather than CTS, as the primary cause,” the scientists wrote.
This hypothesis was supported by additional tests that showed greater compression and a longer time to respond to electrical stimulation in the median nerve on the right arm compared to the left arm.
“This case illustrates the use of post-exercise facilitation test to distinguish the reversible neuromuscular junction transmission defect from the fixed low amplitudes seen with a focal nerve lesion,” the team concluded.
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