Trigeminofacial reflex in lingual neuropathy

The blink reflex and its modulation - Part 1: Physiological mechanisms

The blink reflex (BR) is a protective eye-closure reflex mediated by brainstem circuits. The BR is usually evoked by electrical supraorbital nerve stimulation but can be elicited by a variety of sensory modalities. It has a long history in clinical neurophysiology practice. Less is known, however, about the many ways to modulate the BR. Various neurophysiological techniques can be applied to examine different aspects of afferent and efferent BR modulation. In this line, classical conditioning, prepulse and paired-pulse stimulation, and BR elicitation by self-stimulation may serve to investigate various aspects of brainstem connectivity. The BR may be used as a tool to quantify top-down modulation based on implicit assessment of the value of blinking in a given situation, e.g., depending on changes in stimulus location and probability of occurrence. Understanding the role of non-nociceptive and nociceptive fibers in eliciting a BR is important to get insight into the underlying neural circuitry. Finally, the use of BRs and other brainstem reflexes under general anesthesia may help to advance our knowledge of the brainstem in areas not amenable in awake intact humans. This review summarizes talks held by the Brainstem Special Interest Group of the International Federation of Clinical Neurophysiology at the International Congress of Clinical Neurophysiology 2022 in Geneva, Switzerland, and provides a state-of-the-art overview of the physiology of BR modulation. Understanding the principles of BR modulation is fundamental for a valid and thoughtful clinical application (reviewed in part 2) (Gunduz et al., submitted).

Mylohyoid late responses elicited with intraoral stimulation

Many disorders affect the trigeminal nerve, highlighting the need for electrodiagnostic methods to evaluate this cranial nerve. The purpose of this study was to develop and refine clinical nerve conduction techniques for examining mylohyoid late responses and to establish normative clinical data. Intraoral stimulation of the mylohyoid nerve was performed on normal volunteers using a pediatric stimulator taped to a tongue depressor. Mylohyoid direct and late responses were recorded with surface electrodes over the mylohyoid muscles. Mylohyoid late responses with supraorbital nerve stimulation and blink reflexes were also elicited. Fifty-four subjects, 65% females, with an average age of 39.2 (standard deviation, 13.4) yr, were studied. Direct mylohyoid responses were elicited in all subjects with a mean of 1.8 (standard deviation, 0.3) ms and amplitude of 4.1 (standard deviation, 1.4) mV. Late responses occurred in 94% ipsilaterally at an average latency of 32.4 (standard deviation, 6.5) ms and in 90% contralaterally at 32.4 (standard deviation, 5.6) ms. Two distinct late responses, the second occurring at an average of 72.3 (standard deviation, 12.4) ms with intraoral stimulation were noted in 25% of subjects. Only 11% of subjects had mylohyoid late responses with supraorbital stimulation. Blink (R1) reflexes occurred in 89% (10.9 ms; standard deviation, 0.7 ms), ipsilateral R2 responses in 88% (35.2 ms; standard deviation, 3.4 ms), and contralateral R2 responses in 69% (36.2 ms; standard deviation, 4.0 ms). Mylohyoid late responses are present in most normal subjects. The neuroanatomical pathways mediating these responses require delineation. These nerve conduction techniques may be useful in evaluating patients with suspected trigeminal nerve disorders.