Nerve excitability testing: Technical pitfalls and threshold norms using absolute values

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Abstract
Percutaneous stimulation of the facial nerve is used widely in tests to judge the severity and prognosis of facial paralysis. Several test paradigms are used including nerve excitability threshold (NET), the maximum stimulation test (MST), and electroneuronography (EnoG). Consistent technique and careful control of variables are essential to achieve accurate test results. The sources of variability examined in this study were age, gender, body weight, and the use of electrode paste; the NET was used as the test method. The facial NET in 120 adults without a history of facial paralysis increased linearly with age (P = .0004) and with body weight (P < .0001) and was higher in men than in women adjusted for age and weight (P = .0001). The mean NET ± SD was 0.7 ± 0.27 mA in the upper division using the eyelid twitch as an end point, and 1.2 ± 0.40 mA in the lower division. There was no statistically significant difference in the results between sides. The NET was falsely elevated by the use of electrode paste, presumably due to current shunting away from the nerve.Based on the technique described herein, an absolute NET of ≥ 1.25 mA in the upper division or an absolute NET ≥ 2.0 mA in the lower division of the human facial nerve is statistically abnormal. These norms are not applicable to grossly obese patients or patients with facial edema or inflammation. Statistical norms allow the NET results to be reported on a continuous scale rather than the dichotomous scale used in the past. The predictive power of the NET will be greatly enhanced by basing test interpretation on both statistical and clinical significance.