The Effect of Vestibulo-Ocular Reflex Deficits and Covert Saccades on Dynamic Vision in Opioid-Induced Vestibular Dysfunction

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.

Head impulse compensatory saccades: Visual dependence is most evident in bilateral vestibular loss

“…After VOR loss short latency compensatory saccades can improve visual acuity [79][80][81][82], so the VOR and saccades provide complimentary and differential roles in gaze by generating compensatory velocity and position changes respectively [7,53]. The neuro-physiology of saccade modification after loss of the VOR could be explained by saccade motor learning paradigms, in man [83,84] and monkey [85,86], by which saccade characteristics are modulated over time by visual error feedback loops through the super colliculus and cerebellum [87][88][89][90][91][92].…”

Video Head Impulse Testing (vHIT) and the Assessment of Horizontal Semicircular Canal Function

“…Patients with a relatively wide corrective saccade latency distribution had significantly higher scores on the Dizziness Handicap Inventory than patients with a relatively narrow latency distribution and suggested an association between the latency characteristics of corrective saccades and self-perceived handicap. Recently, Ramaioli et al (2014) examined reading ability during head impulses in subjects with pharmacologically reduced VOR gain. One individual was able to generate consistent covert saccades and had minimal degradation in reading ability during head rotation.…”

Moderne Schwindeldiagnostik

“…Aktuelle Studien haben gezeigt, dass z. B. auch Alkohol und Medikamente das Ergebnis beeinflussen können [178][179][180]. Darüber hinaus zeigt sich, dass der hochfrequente hVOR durch Gentamicin frühzeitig beeinträchtigt wird.…”

“…Bei der Bewertung der Befunde muss auch im Rahmen der HNO-Begutachtung berücksichtigt werden, dass z. B. Alkohol und Medikamente das Ergebnis moderner Diagnostik beeinflussen können [178][179][180].…”