title: Head impulse test in unilateral vestibular loss: vestibulo-ocular reflex and catch-up saccades.
abstract: BACKGROUND
  Quantitative head impulse test (HIT) measures the gain of the angular vestibulo-ocular reflex (VOR) during head rotation as the ratio of eye to head acceleration. Bedside HIT identifies subsequent catch-up saccades after the head rotation as indirect signs of VOR deficit.
  
  
  OBJECTIVE
  To determine the VOR deficit and catch-up saccade characteristics in unilateral vestibular disease in response to HIT of varying accelerations.
  
  
  METHODS
  Eye and head rotations were measured with search coils during manually applied horizontal HITs of varying accelerations in patients after vestibular neuritis (VN, n = 13) and unilateral vestibular deafferentation (UVD, n = 15) compared to normal subjects (n = 12).
  
  
  RESULTS
  Normal VOR gain was close to unity and symmetric over the entire head-acceleration range. Patients with VN and UVD showed VOR gain asymmetry, with larger ipsilesional than contralesional deficits. As accelerations increased from 750 to 6,000 degrees /sec(2), ipsilesional gains decreased from 0.59 to 0.29 in VN and from 0.47 to 0.13 in UVD producing increasing asymmetry. Initial catch-up saccades can occur during or after head rotation. Covert saccades during head rotation are most likely imperceptible, while overt saccades after head rotation are detectable by clinicians. With increasing acceleration, the amplitude of overt saccades in patients became larger; however, initial covert saccades also became increasingly common, occurring in up to about 70% of trials.
  
  
  CONCLUSIONS
  Head impulse test (HIT) with high acceleration reveals vestibulo-ocular reflex deficits better and elicits larger overt catch-up saccades in unilateral vestibular patients. Covert saccades during head rotation, however, occur more frequently with higher acceleration and may be missed by clinicians. To avoid false-negative results, bedside HIT should be repeated to improve chances of detection.
authors.affiliation: Neurology Department, Royal Prince Alfred Hospital, Camperdown NSW 2050, Sydney, Australia.
authors.forename: K P
authors.initials: KP
authors.lastname: Weber
authors.forename: S T
authors.initials: ST
authors.lastname: Aw
authors.forename: M J
authors.initials: MJ
authors.lastname: Todd
authors.forename: L A
authors.initials: LA
authors.lastname: McGarvie
authors.forename: I S
authors.initials: IS
authors.lastname: Curthoys
authors.forename: G M
authors.initials: GM
authors.lastname: Halmagyi
mesh_terms: D000054:Acceleration / Q000009:adverse effects; D000328:Adult; D000368:Aged; D005188:False Negative Reactions; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D008875:Middle Aged; D015835:Ocular Motility Disorders / Q000175:diagnosis* / Q000209:etiology / Q000503:physiopathology; D011237:Predictive Value of Tests; D012027:Reflex, Vestibulo-Ocular / Q000502:physiology; D012399:Rotation / Q000009:adverse effects; D012438:Saccades / Q000502:physiology; D012680:Sensitivity and Specificity; D015837:Vestibular Diseases / Q000175:diagnosis* / Q000209:etiology / Q000503:physiopathology; D014724:Vestibular Function Tests / Q000379:methods* / Q000592:standards; D014722:Vestibule, Labyrinth / Q000294:innervation / Q000503:physiopathology