title: Compensatory saccade differences between outward versus inward head impulses in chronic unilateral vestibular hypofunction.
abstract: The horizontal head impulse test (HIT) is a valuable clinical tool that can help identify peripheral vestibular hypofunction by the refixation (compensatory) saccade that returns the eyes to the target of interest after the head has stopped. We asked if there were differences in the compensatory saccade responses during the HIT when the head was rotated away or toward straight ahead (outward versus inward). We also investigated the influence of a fixation target. Using scleral search-coils, we tested five patients with chronic unilateral vestibular hypofunction (UVH) and three healthy control subjects. In UVH patients, the latencies of both overt and covert saccades were longer when the head was rotated inward from an initially eccentric position, regardless of a visual target. The proportion of HIT with covert saccades was independent of a visual target. In control subjects no compensatory saccades were observed and there were no differences in either angular vestibulo-ocular reflex gain or latency between inward and outward HIT. Our data suggest that inward applied HIT in chronic UVH is more likely to include an overt compensatory saccade based on its lengthened latency. Neither latency nor the occurrence of covert compensatory saccades during HIT depended on a visual target, suggesting they have become a learned behavior in response to chronic UVH.
authors.affiliation: Department of Neurology, Johns Hopkins School of Medicine, Baltimore, MD, USA; Department of Neurology, Chonnam National University Medical School, Gwangju, South Korea; Research Institute of Clinical Medicine of Chonnam National University Hospital, Gwangju, South Korea.
authors.forename: Seung-Han
authors.initials: SH
authors.lastname: Lee
authors.affiliation: Department of Neurology, Johns Hopkins School of Medicine, Baltimore, MD, USA; Department of Otolaryngology Head and Neck Surgery, Johns Hopkins School of Medicine, 601 North Caroline Street, Room 6245, Baltimore, MD 21287, USA.
authors.forename: David E
authors.initials: DE
authors.lastname: Newman-Toker
authors.affiliation: Department of Neurology, Johns Hopkins School of Medicine, Baltimore, MD, USA; Department of Otolaryngology Head and Neck Surgery, Johns Hopkins School of Medicine, 601 North Caroline Street, Room 6245, Baltimore, MD 21287, USA.
authors.forename: David S
authors.initials: DS
authors.lastname: Zee
authors.affiliation: Department of Otolaryngology Head and Neck Surgery, Johns Hopkins School of Medicine, 601 North Caroline Street, Room 6245, Baltimore, MD 21287, USA; Department of Physical Medicine and Rehabilitation, Johns Hopkins School of Medicine, Baltimore, MD, USA. Electronic address: mschube1@jhmi.edu.
authors.forename: Michael C
authors.initials: MC
authors.lastname: Schubert
mesh_terms: D000368:Aged; D002908:Chronic Disease; D053483:Eye Movement Measurements; D005260:Female; D007839:Functional Laterality; D064087:Head Impulse Test; D006801:Humans; D008297:Male; D008875:Middle Aged; D010775:Photic Stimulation; D011930:Reaction Time; D012027:Reflex, Vestibulo-Ocular; D012438:Saccades; D015837:Vestibular Diseases / Q000503:physiopathology*
keywords: Compensatory saccade; Covert saccade; Head impulse test; Overt saccade; Vestibulo-ocular reflex