Increments, or increases in nerve responses, greater than 60% in electromyography (EMG) recordings upon short periods of exercise are better at diagnosing Lambert-Eaton myasthenic syndrome (LEMS) than the commonly used 100% increment, results from a recent study indicate.
The study, “Lowering the cut‐off value for increment increases the sensitivity for the diagnosis of Lambert‐Eaton myasthenic syndrome,” was published in the journal Muscle and Nerve.
LEMS is an autoimmune disease in which the immune system mistakenly attacks calcium channels in nerve cell endings — which are needed to control muscle movement — eventually leading to muscle weakness, mainly in the lower limbs.
EMG, a test used to diagnose the condition, records electrical signals coming from the muscle in response to a nerve signal. The initial response, called compound motor action potential (CMAP), is low, but increases when stimulated after exercise.
Usually, an increment in CMAP greater than 100% after short periods of exercise indicates LEMS, but one study showed that lowering that threshold to 60% could increase the rate of true positives (sensitivity), without increasing false negatives.
Despite that, only a few studies have incorporated the 60% threshold in their LEMS diagnosis.
To confirm whether the 60% threshold is indeed better than the traditional 100%, a team in the Netherlands compared both thresholds in a group of LEMS patients.
The group included 156 participants, 63 diagnosed with LEMS (median age 56) based on tests other than CMAP increments. Other patients had myasthenia gravis or other neuromuscular diseases and were used as the control group.
All participants received increment testing at the Leiden University Medical Center from 1999 to 2016. The tests were mainly conducted in muscles in the hands (97.5% of patients), but 11.7% also underwent testing in muscles of the nose — usually the patients with eye or facial weakness.
The team found that the 60% threshold was better at correctly identifying LEMS, with a true positive rate of 77.8% versus 58.7% for the 100% threshold. For both cut-off values, the proportion of true negatives, or of controls correctly identified as such (specificity), was very high — 98.9% with the proposed threshold and 100% with the standard value.
Notably, sensitivity using the lower cut-off was even higher among untreated LEMS patients and in those with LEMS but without associated lung cancer, the team found.
Overall, “we confirm that a 60% threshold for increment greatly increases sensitivity while maintaining a high specificity in a large group of LEMS patients and a different control group than previously studied,” the researchers wrote.
This suggests that lowering the threshold may help diagnose LEMS and prevent the need for additional, painful tests in some patients.
“We propose using a threshold for abnormal increment of 60%, as this should lead to improved diagnosis of patients with this rare neuromuscular disease,” the team said.
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