Abstract
Patients with bilateral vestibular dysfunction cannot fully compensate passive head rotations with eye movements, and experience disturbing oscillopsia. To compensate for the deficient vestibulo-ocular reflex (VOR), they have to rely on re-fixation saccades. Some can trigger “covert” saccades while the head still moves; others only initiate saccades afterwards. Due to their shorter latency, it has been hypothesized that covert saccades are particularly beneficial to improve dynamic visual acuity, reducing oscillopsia. Here, we investigate the combined effect of covert saccades and the VOR on clear vision, using the Head Impulse Testing Device – Functional Test (HITD-FT), which quantifies reading ability during passive high-acceleration head movements. To reversibly decrease VOR function, fourteen healthy men (median age 26 years, range 21–31) were continuously administrated the opioid remifentanil intravenously (0.15 µg/kg/min). VOR gain was assessed with the video head-impulse test, functional performance (i.e. reading) with the HITD-FT. Before opioid application, VOR and dynamic reading were intact (head-impulse gain: 0.87±0.08, mean±SD; HITD-FT rate of correct answers: 90±9%). Remifentanil induced impairment in dynamic reading (HITD-FT 26±15%) in 12/14 subjects, with transient bilateral vestibular dysfunction (head-impulse gain 0.63±0.19). HITD-FT score correlated with head-impulse gain (R = 0.63, p = 0.03) and with gain difference (before/with remifentanil, R = −0.64, p = 0.02). One subject had a non-pathological head-impulse gain (0.82±0.03) and a high HITD-FT score (92%). One subject triggered covert saccades in 60% of the head movements and could read during passive head movements (HITD-FT 93%) despite a pathological head-impulse gain (0.59±0.03) whereas none of the 12 subjects without covert saccades reached such high performance. In summary, early catch-up saccades may improve dynamic visual function. HITD-FT is an appropriate method to assess the combined gaze stabilization effect of both VOR and covert saccades (overall dynamic vision), e.g., to document performance and progress during vestibular rehabilitation.
Highlights
Patients with bilateral vestibular dysfunction cannot produce eye movements that are fully compensatory of passive head rotations, for example during walking [1] or driving [1,2]
The vestibulo-ocular reflex (VOR) was intact with the eyes compensating for the head movement
Using the fact that remifentanil acutely and reversibly decreases VOR-function, we show that the Head Impulse Testing Device – Functional Test (HITD-FT) reflects the combined effects of VOR and catch-up saccades on dynamic reading ability, i.e., it is a measure of overall dynamic visual function
Summary
Patients with bilateral vestibular dysfunction cannot produce eye movements that are fully compensatory of passive head rotations, for example during walking [1] or driving [1,2]. Some can trigger shorter-latency re-fixation saccades occurring while the head is still moving (‘‘covert’’ saccades, [6]) whereas others only initiate ‘‘overt’’ saccades after head movement end Due to their shorter latency, it has been hypothesized that covert saccades are advantageous to stabilize gaze, reduce oscillopsia [6,7] and improve visual performances, e.g., reading ability during head movements. Vestibular dysfunction and the presence of covert and overt saccades can be quantified with the search-coil [8,9] and the video [10,11] head-impulse test [12] Vestibular dysfunction was transiently introduced pharmacologically in healthy subjects by the short-acting opioid remifentanil, to investigate whether the HITD-FT was useful to assess dynamic vision, i.e., the effect of VOR and early catch-up saccades on visual perception during physiologically relevant head movements.
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