Recording the trigemino-facial inhibitory reflex: technique and normal findings

The aim of this study was to propose a standardized technique for recording the trigemino-facial inhibitory reflex (TFIR) and to provide reference data of its components. The TFIR was recorded with concentric needle electrodes from the voluntarily activated depressor anguli oris muscle bilaterally in 26 healthy subjects (23-83 years). A vertical line was drawn from the corner of the mouth to the mandible margin, from which the needle insertion point was localized moving 1 to 2 cm upward and 1 to 2 cm laterally. The mentalis nerve was stimulated by surface electrodes at an intensity of 10x the perceptive threshold. The recovery cycle was studied at interstimulus intervals of 200, 350, and 500 milliseconds. The TFIR was constantly elicited in all participants. Upper normal limits (mean +/- 3 SD) of latency of the TFIR both ipsilateral and contralateral to stimulation were 65 milliseconds. Recovery of the reflex was of 71% at 200 milliseconds, 84% at 350 milliseconds, and 98% at 500 milliseconds interstimulus intervals. The recovery at the 200 milliseconds interstimulus intervals increased significantly with participant's age. The TFIR can be consistently and reliably recorded from the depressor anguli oris muscle using a standard technique. The reflex is robust showing complete recovery at 500 milliseconds interstimulus intervals.