Eyeseecam

vHIT was performed with a video-oculography device (Interacoustics, EyeSeeCam, Denmark). The subject was instructed to maintain the fixation on an earth-fixed target, which is usually straight ahead. Experienced otologists An experienced laboratory technician delivered at least 20 brief, abrupt, and unpredictable head impulses per side (10–20° angle, duration 150–200 ms, peak velocity of >150°/s). The VOR gain was defined as the ratio of the eye velocity (°/s) over the head velocity (°/s). Individual VOR gains were automatically calculated using the device software. vHIT testing was considered to be abnormal for horizontal canal if VOR gain at 60 ms was <0.8 (18 (link)).

HSC function was assessed using caloric reflex testing and vHIT. For bilateral caloric reflex testing, the patient was placed supine with their heads lifted 30° to place the HSC in the vertical position. Each external auditory canal was alternately irrigated with a constant flow of cold (30°C) and warm (44°C) water for ≈40 seconds to induce caloric nystagmus. Nystagmus was recorded using infrared videonystagmography (Interacoustics, VNG VO425). The sum of the maximum slow phase velocity (SPV_max, in °/s) at 30°C and 44°C after irrigation of the healthy ear was used as the outcome measure. For vHIT, the patient’s head was rapidly (velocity between 150 and 250 °/s) rotated (10° to 20°) in the horizontal plane. Compensating eye movements were measured using videography (Interacoustics EyeSeeCam). The VOR gain measured at 60 ms (Gain_{60 ms}) when rotating the head toward the healthy side was used as the outcome measure. Because vHIT was not implemented until 2016 (2 years after caloric reflex testing was implemented) and because patients with neck problems did not undergo vHIT, not all patients included in this study underwent assessment by both caloric reflex testing and vHIT.

Vestibular function testing included video head impulse tests (VHITs) and cVEMPs. The VHIT (EyeSeeCam, Interacoustics, Middelfart, Denmark or ICS impulse, Otometrics, Taastrup, Denmark) is performed by applying rapid horizontal head thrusts randomly to each side, with gaze fixation (5 (link)). The VHIT outcome for the lateral semicircular canals was determined by the vestibulo-ocular reflex (VOR) median gain value (LVOR gain) and compensatory saccades (covert or overt). VOR vestibulopathy (uni- or bilateral) was defined as an ipsilateral gain value below 0.70 or presence of saccades (5 (link)). Complete loss of VOR function was defined as a gain value <0.25. Gain values below zero were defined as 0. CVEMPs evaluate the vestibulo-cervical reflex (and thus the function of the saccule in the vestibule of the inner ear) and were performed by applying air-conducted click sounds of 100 dB nHL to the external ear canal and recording myogenic potentials evoked in the ipsilateral sternocleidomastoid muscle (Eclipse, Interacoustics, Middelfart, Denmark) (6 (link)). The outcome was evaluated binarily: presence or absence of a potential. VEMP vestibulopathy (uni- or bilateral) was defined as absence of an ipsilateral potential.

vHIT was performed using EyeSeeCam (Interacoustics, Middelfart, Denmark) to assess vestibulo-ocular reflex (VOR). vHIT evaluates VOR by testing all three pairs of SCCs, and calculates the average gain value and the asymmetry ratio of the two semicircular canals in the conjugate plane for each semicircular canal. Reduced SCC function is defined as a horizontal SCC gain less than 0.8 and a vertical SCC gain less than 0.7.

The horizontal vHIT and the vHIT in the Right-Anterior-Left-Posterior (RALP) and Left-Anterior-Right-Posterior (LARP) canal planes were performed using the Video-Head Impulse Test device from Otometrics at center 1 and 2 (Otometrics, Taastrup, Denmark). At center 3, horizontal vHIT was performed using the Eye-SeeCam (Interacoustics, Munich, Germany). The testing method was described previously (45 (link), 46 (link)). In brief, the technician stood behind the subject (who was sitting on a static chair) and held their head firmly without touching the goggles. The subject was instructed to maintain visual fixation on an earth-fixed target at a distance of 2 m at centers 1 and 2 and 1.8 m at center 3. Head impulses comprised fast unpredictable, low-amplitude (±20°) head movements in the horizontal plane (all three centers, peak head velocity > 150°/s) and in the RALP and LARP planes (center 1 and 2, peak head velocity > 100°/s). The Otometrics system defines the VOR gain as the ratio of the area under the eye velocity curve to the area under the head velocity curve from the impulse onset until the head velocity drops to zero again (47 (link)). The inter-acoustics system divides the eye and head velocity at a certain point in time (around 60 ms after impulse onset) (46 (link)).