Time constant of nystagmus slow-phase velocity to yaw-axis rotation as a function of the severity of unilateral caloric paresis.
OBJECTIVE
Complete unilateral loss of vestibular function results in a phase advance (reduced time constant) of the horizontal slow-phase nystagmus response to yaw-axis rotation. The objective of this study was to determine whether partial losses of lateral semicircular canal function would result in proportional reductions in the time constant. SETTING AND STUDY DESIGN: This was a retrospective study of consecutive patients' records at two tertiary referral centers for vestibular disorders.


METHODS
Four hundred fifty-four patients who presented for evaluation of vertigo or imbalance or both and who were found to have partial or complete unilateral canal paresis on caloric testing.


METHODS
In 372 patients, the gain and time constant of the horizontal nystagmus response was measured using a 5-second velocity ramp of constant yaw-axis acceleration. Caloric responses to standard bithermal irrigations at 30 degrees and 44 degrees were obtained using an open-loop irrigation system. In a second group of 82 patients, the gain and time constant of the horizontal vestibulo-ocular reflex were measured using a sum-of-sines (pseudorandom) yaw-axis acceleration. The caloric response was measured using a closed-loop system.


RESULTS
In both groups, the peak gain of the nystagmus response was independent of the level of the canal paresis. However, the time constant of the response both toward and away from the lesioned side decreased proportionally with increasing canal paresis.


CONCLUSIONS
This result supports the hypothesis that bilateral symmetrical peripheral vestibular input is a necessary condition for the mechanisms or processes underlying normal horizontal slow-phase velocity storage.
Wade
S W
SW
Department of Neuro-otology, Royal Prince Alfred Hospital, Sydney, Australia.
Halmagyi
G M
GM
Black
F O
FO
McGarvie
L A
LA
D002150:Caloric Tests / Q000379:methods*; D004585:Electrooculography / Q000379:methods; D006801:Humans; D008954:Models, Biological; D009759:Nystagmus, Pathologic / Q000175:diagnosis*; D010291:Paresis / Q000175:diagnosis / Q000503:physiopathology*; D012027:Reflex, Vestibulo-Ocular / Q000502:physiology; D012189:Retrospective Studies; D012399:Rotation; D012720:Severity of Illness Index; D013997:Time Factors; D014717:Vertigo / Q000175:diagnosis*; D015837:Vestibular Diseases / Q000175:diagnosis / Q000503:physiopathology*