Blink reflex

17.1

Introduction

The closing of eyelids in response to various stimuli is a blink reflex (BR). In humans and primates this closing of eyelids is bilateral while in other animals, especially those with lateralized position of the eyes, it is frequently unilateral. Clinically, the BR is elicited after a gentle brush to the cornea or the eyelashes or after a glabelar percusion .

I n 1952, Kugelberg was the first to report the presence of two components of the electrically elicited BR: R1 and R2 . The R1 response corresponds to the oligosynaptic reflex arc, which includes trigeminal afferents, brainstem connections between the sensory part of the trigeminal nucleus, the motor nucleus of the facial nerve, the facial nerve proper, and the orbicularis oculi muscle. The R2 component is more complex in its central, polysynaptic connections within the brainstem, but it has the same afferent and efferent parts of the reflex arc ( Fig. 17.1 ).

This reflex has become an important tool in neurophysiology . It has also found a place as an additional diagnostic tool in investigation of pathology involving the reflex arc. The BR has been studied extensively in adults, infants, and patients with different diseases of the peripheral and central nervous system . Some extensive BR reviews have been published .

Attempts were made to evoke this reflex in anesthetized patients but with limited results .

Mourisse et al. demonstrated the effect of hypnotic drugs on elicitability of the BR and concluded that it is a good marker of depth of anesthesia due to its sensitivity especially to sevoflurane but also to propofol .

Previous studies have reported the inability to elicit the BR in unconscious patients who had received doses of anesthetics compatible with those used for surgery . The exception was in patients with hemifacial spasm in whom the R1 component could be elicited . Montero et al. presented evidence that previous report of BRs elicited during general anesthesia in patients with hemifacial spasm, reported by Moller and Jannetta was in fact a phenomenon of “lateral spreading,” and was initiated by antidromic activation of the facial nerve instead of trigeminal afferents.

In this chapter, we describe the method for eliciting the R1 component of the BR in patients under general anesthesia using a short train of electrical stimuli applied over the supraorbital nerve .

The discovery that a short train of stimuli can elicit an R1 component of the BR in most patients under general anesthesia, when a single stimulus cannot, is very similar to the history of motor evoked potentials (MEPs). The methodology for eliciting MEPs in awake patients or healthy subjects by applying a single stimulus transcranially was not successful in the patient under general anesthesia. Once a short train of stimuli was used, it became the standard for eliciting MEPs under general anesthesia . Furthermore, the ability of a short train of stimuli to overcome the inhibitory effects of anesthetics has been shown in other intraoperative neurophysiological methods such as bulbocavernosus reflex monitoring or for mapping Broca’s area in patients under general anesthesia .

The efficacy of a short train of stimuli to overcome the inhibitory action of anesthetics is very likely due to temporal summation and building-up of excitatory postsynaptic potentials (EPSPs) in the interneuronal chain involved in the BR in the brainstem.

The possibility of recording BR intraoperatively opens a new door at the intraoperative neuromonitoring field, since it eventually allows assessing the functional integrity of all anatomical structures involved in the reflex arc.

17.2

Methodology