Feasibility and reliability of intraorally evoked “nociceptive-specific” blink reflexes

Cerebellar somatotopy of the trigemino-cervical complex during nociception

INTRODUCTION

The somatotopic organization of the human cerebellum processes somato-motoric input. Its role during pain perception for nociceptive input remains ambiguous. A standardized experimental trigeminal nociceptive input in functional imaging might clarify the role of the cerebellum in trigeminal nociception. Also of interest is the greater occipital nerve, which innervates the back of the head, and can influence the trigeminal perception due to functional coupling within the brainstem, forming the so-called trigemino-cervical complex.

METHODS

In our preregistered study (clinicaltrials.gov: NTC03999060), we stimulated the greater occipital as well as the three main branches of the trigeminal nerve during functional magnetic resonance imaging in two independent cohorts of young healthy volunteers without psychiatric, neurological or pain-related disorders to disentangle overlapping somatotopic cerebellar organization of the nerves innervating the human head.

RESULTS

We found a dominant effect of the first trigeminal branch in the cerebellum, underpinning its particular role for headache diseases, and somatotopic representations in bilateral cerebellar lobules I-IV, V, VIIb, VIIIa and Crus I as well as in the brainstem.

SIGNIFICANCE

The study expands the current knowledge on facial and head pain processing by the cerebellum and provides an initial somatotopic map of the trigemino-cervical complex in the human cerebellum with a predominant representation of the first trigeminal branch.

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).

Blink reflex monitoring in microvascular decompression for trigeminal neuralgia

Aim: To determine the value of the blink reflex in evaluating trigeminal sensory function during microvascular decompression for trigeminal neuralgia.Methods: The blink reflex (BR) in 103 patients with primary typical trigeminal neuralgia treated by microvascular decompression (MVD) was tested pre- and intraoperatively. The changes in BR were recorded. All patients underwent general anesthesia with intravenous propofol and fentanyl. Surgical efficacy and complications were evaluated after surgery. The relationship between intraoperative changes in the BR and postoperative trigeminal sensory function was analyzed.Results: The BR was elicited in all patients before surgery, and no significant difference was found between the affected side and the contralateral side. In 93 of the 103 cases, the BR was successfully elicited during MVD surgery. Therefore, the recordability of the BR was 90.29%. R1 latency on the affected side and the contralateral side were 11.62 ± 4.96 ms and 11.66 ± 4.37 ms, respectively. During MVD surgery, R1 of the BR disappeared on the affected side in 7 cases and remained in 86 cases. After the operation, 98 of the 103 patients had immediate and complete remission of trigeminal neuralgia symptoms, and 5 cases had partial remission. The 7 patients whose R1 disappeared during the surgery all experienced facial numbness postoperatively. Of the 86 patients whose R1 remained, only 2 patients had postoperative facial numbness. Of the 10 patients whose R1 was not recordable during the operation, one complained of postoperative facial numbness. No patients had complications such as facial paralysis, cerebrospinal fluid leakage, and death.Conclusions: Conclusion: The blink reflex may allow monitoring of trigeminal sensory function during microvascular decompression under general anesthesia.