The Role of Neck Input in Producing Corrective Saccades in the Head Impulse Test

Shinichi Iwasaki; Teru Kamogashira; Chisato Fujimoto; Kayoko Kabaya; Makoto Kinoshita; Tatsuya Yamasoba

doi:10.3389/fneur.2022.881411

The Role of Neck Input in Producing Corrective Saccades in the Head Impulse Test

Front Neurol. 2022 May 17:13:881411. doi: 10.3389/fneur.2022.881411. eCollection 2022.

Authors

Shinichi Iwasaki¹, Teru Kamogashira², Chisato Fujimoto², Kayoko Kabaya¹, Makoto Kinoshita², Tatsuya Yamasoba²

Affiliations

¹ Department of Otolaryngology & Head and Neck Surgery, Nagoya City University Graduate School of Medicine, Nagoya, Japan.
² Department of Otolaryngology and Head and Neck Surgery, Faculty of Medicine, The University of Tokyo, Tokyo, Japan.

Abstract

Background: The head impulse test is a valuable clinical test that can help identify peripheral vestibular dysfunction by observing corrective saccades that return the eyes to the target of interest. Corrective saccades have been classified as covert if the onset occurs before the end of the head impulse and as overt if they occur afterwards. However, the mechanism that trigger these saccades remain unclear.

Objective: The objective of this study was to examine the role of neck input in generating overt as well as covert saccades.

Methods: Sixteen patients (9 males and 7 females: age 35-80 years, average 62.7 years old) who showed corrective saccades during the head impulse test were included. Twelve patients had unilateral vestibular dysfunction, and 4 patients had bilateral vestibular dysfunction. Patients underwent both the head impulse test (HIT) and the body impulse test (BIT) in a randomized order. While the head is rotated horizontally in HIT, the body is rotated horizontally in BIT. During BIT, the neck is fixed by a cervical collar (neck lock extrication collar) to reduce somatosensory input from the neck. The head movements and eye movements were recorded and analyzed by the video HIT recording system.

Results: In all 16 patients, corrective saccades were observed in HIT as well as in BIT. While there were no significant differences in peak head velocities between HIT and BIT (p = 0.33, paired t-test), the VOR gain in BIT was significantly smaller than that in HIT (p = 0.011, paired t-test). The number of overt saccades per trial in BIT was significantly decreased compared to that in HIT (p < 0.001, paired t-test) whereas there were no significant differences in the number of covert saccades between the two tests. The proportion of overt saccades among all corrective saccades in BIT was significantly lower than the proportion in HIT (p < 0.001, paired t-test).

Conclusions: Somatosensory input from the neck contributes to the generation of overt saccades and reinforces the vestibulo-ocular reflex complementing the retinal slip during high frequency head movements.

Keywords: covert saccades; head impulse test; neck input; overt saccades; vestibular.