Short-Latency Covert Saccades - The Explanation for Good Dynamic Visual Performance After Unilateral Vestibular Loss?

[Research advances in the mechanism of vestibular compensation and treatment]

Unlike other sensory systems, since the vestibular system maintains the tension balance of the entire system in a"push-pull" mode, local dysfunction in the system will cause the balance of the entire system to collapse. Unilateral peripheral vestibular dysfunction will cause severe vestibular symptoms, but it can recover spontaneously within a few days to several weeks. This phenomenon is called "vestibular compensation"（VC）. Since the peripheral vestibular impact in most cases is irreversible, it is widely believed that the central mechanism plays a key role in the vestibular compensation process. Static symptom is fully compensated within a few weeks, which is in parallel with the restored balance in the resting discharge of the vestibular nucleus on both sides; the incomplete compensation of dynamic deficits takes longer and is achieved mainly through the mechanism of sensory substitution and behavioral substitution. Here we briefly reviewed the mechanism of vestibular compensation and treatment in order to provide an insight into further study and clinical treatment strategies.

Are Vestibuloocular Reflex Gain and Dynamic Visual Acuity Responsible of Oscillopsia After Complete Unilateral Vestibular Loss?

BACKGROUND

Acute and complete unilateral vestibular deafferentation induces a significant change in ipsilateral vestibuloocular reflex gain, making the patient unable to stabilize gaze during active or passive head movements. This inability creates the illusion that the visual environment is moving, resulting in persistent visual discomfort during rapid angular or linear acceleration of the head. This is known as oscillopsia. Our objective was to understand if the spontaneous sensation of oscillopsias after complete unilateral vestibular deafferentation by vestibular neurotomy at 5 days (D5) and at 3 months (M3) is correlated with the loss of vestibuloocular reflex gain and dynamic visual acuity.

METHODS

Retrospective cohort study was conducted in an otolaryngology tertiary care center (2019-2022) on patients with complete unilateral vestibular loss by vestibular neurotomy. They were divided into 2 groups according to the presence (group G1) or absence (group G2) of a spontaneous complaint of oscillopsia assessed at M3. Severity of oscillopsias evaluated by Oscillopsia Severity Questionnaire. Vestibuloocular reflex gain based on video head impulse test (vHIT) and the dynamic visual acuity were measured for each group at D5 and M3. Categorical variables were compared using χ2 test and quantitative variables using the nonparametric Wilcoxon-Mann-Whitney test.

RESULTS

All patients have a complete vestibular deafferentation at D5 and M3. At D5 (G1 = 8 patients, G2 = 5 patients), there is no significant difference for ipsilateral and contralateral vestibuloocular reflex gains and dynamic visual acuity losses. The Oscillopsia Severity Questionnaire was 2.68 ± 1.03 in G1 and 1.23 ± 1.03 in G2 (P < .05). At M3 (G1 = 9 patients, G2 = 6 patients), there is no significant difference between groups for epidemiologic and clinical data and for vestibuloocular reflex and dynamic visual acuity losses. The Oscillopsia Severity Questionnaire was 2.10 ± 0.63 in G1 and 1.24 ± 0.28 in G2 (P < .05).

CONCLUSIONS

The spontaneous disabling sensation of oscillopsia after complete unilateral vestibular loss is well assessed by the Oscillopsia Severity Questionnaire but cannot be explained by objective vestibular tests assessing vestibuloocular reflex gain (vHIT) or dynamic visual acuity loss at D5 or M3. Further studies are needed to measure the sensation of oscillopsia under real-life conditions and to identify the factors responsible for its persistence.

TRIAL REGISTRATION

Retrospectively registered.

Dynamic Visual Acuity Results in Otolith Electrical Stimulation in Bilateral Vestibular Dysfunction

(1) Background. Patients with bilateral vestibular disease (BVD) experience oscillopsia with a detriment to visual acuity (VA). This VA is driven mainly by the VOR that has two components: rotational and translational. VA can be tested by using dynamic visual acuity (DVA) on a treadmill because both systems are activated. The aim of this study is to compare VA before and after chronic electrical stimulation of the otolith organ. (2) Materials and Method. Five patients suffering from bilateral vestibular dysfunction (BVD), previously implanted with a new vestibular implant prototype, were included in this study with the aim to check VA with and without vestibular implant use (W and W/O) in static, 2 km/h and 4 km/h walking situations. DVAtreadmill was measured on a treadmill with a dynamic illegible E (DIE) test in static and dynamic conditions (while walking on the treadmill at 2 and 4 km/h). The DVA score was registered in a logarithm of the minimum angle of resolution (LogMAR) for each speed. In addition, every patient completed the oscillopsia severity questionnaire (OSQ) and video head impulse test (vHIT) before and after activation of the vestibular implant. (3) Results. The analysis shows a significant difference in OSQ scores and DVA with an improvement in dynamic conditions. Organized corrective saccades during the use of a vestibular implant with no changes in gain were also detected in the video head impulse tests (vHIT). (4) Conclusion. The vestibular implant with otolithic stimulation offers changes in the response of DVA, which makes this paper one of the first to address the possible restoration of it. It is not possible to rule out other contributing factors (presence of covert saccades, somatosensory system, …). More work seems necessary to understand the neurophysiological basis of these findings, but this implant is added as a therapeutic alternative for the improvement of oscillopsia.

Enhanced Eye Velocity With Backup Saccades in vHIT Tests of a Menière Disease Patient: A Case Report

Reduced eye velocity and overt or covert compensatory saccades during horizontal head impulse testing are the signs of reduced vestibular function. However, here we report the unusual case of a patient who had enhanced eye velocity during horizontal head impulses followed by a corrective saccade. We term this saccade a "backup saccade" because it acts to compensate for the gaze position error caused by the enhanced velocity (and enhanced VOR gain) and acts to return gaze directly to the fixation target as shown by eye position records. We distinguish backup saccades from overt or covert compensatory saccades or the anticompensatory quick eye movement (ACQEM) of Heuberger et al. (1) ACQEMs are anticompensatory in that they are in the same direction as head velocity and so, act to take gaze off the target and thus require later compensatory (overt) saccades to return gaze to the target. Neither of these responses were found in this patient. The patient here was diagnosed with unilateral definite Meniere's disease (MD) on the right and had enhanced VOR (gain of 1.17) for rightward head impulses followed by backup saccades. For leftwards head impulses eye velocity and VOR gain were in the normal range (VOR gain of 0.89). As further confirmation, testing with 1.84 Hz horizontal sinusoidal head movements in the visual-vestibular (VVOR) paradigm also showed these backup saccades for rightwards head turns but normal slow phase eye velocity responses without backup saccades for leftwards had turns. This evidence shows that backup saccades can be observed in some MD patients who show enhanced eye velocity responses during vHIT and that these backup saccades act to correct for gaze position error caused by the enhanced eye velocity during the head impulse and so have a compensatory effect on gaze stabilization.