S102. Repetitive nerve stimulation: ROC curve analysis

Introduction Repetitive nerve stimulation (RNS) is the standard procedure to evaluate neuromuscular transmission disorders. The cut-off level is often arbitrary and is usually the same for every test, despite the peculiarities of each nerve-muscle setting. Our objective was to perform a ROC curve analysis of four different RNS techniques and define the best cut-off level for them. Methods We did a retrospective study of RNS with 20 patients with positive serologic confirmation of Myasthenia Gravis and 20 patients with negative acetylcholine receptor binding antibodies and normal jitter studies of both orbicularis oculi and of another symptomatic muscle. All patients were submitted to RNS with six stimuli at 3 or 2 Hz of the following nerves (muscles): ulnar (abductor digiti minimi), accessory (trapezius), facial (nasal), and axillary (deltoid). The decrement was calculated using the negative amplitude of the fourth potential in relation to the first. Recordings after maximal contraction were not considered for analysis. We calculated sensitivity and specificity at four different cut-off levels for each technique: 10%, 7.5%, 5%, and >0%. We constructed the ROC curve, compared the techniques, and suggested the best cut-off point for each test. Results The specificity was 100% for all tests using the classical decrement cut-off of 10%. Using this, sensitivity was: 5% (ulnar), 30% (accessory), 65% (facial), and 60% (axillary). The area bellow curve was 0.52 (ulnar), 0.58 (accessory), 0.83 (facial), and 0.86 (axillary). The best cut-off was a decrement above 7.5% for all tests, keeping specificity at 100% for all except accessory, which was 95%. Using this, sensitivity was: 20% (ulnar), 40% (accessory), 65% (facial), and 70% (axillary). Conclusion RNS of facial and axillary nerves had a better performance than ulnar and accessory nerves. The cut-off level suggested by our study is a decrement above 7.5% for all tests. Larger studies can propose different cut-off levels for each technique.