INTRODUCTION
Vertical misalignment of the eyes is considered a red flag in acute dizziness as it may indicate presence of a vestibular stroke. In that case, the vertical misalignement is termed skew deviation. Skew deviation can occur in patients with acute vestibular tone imbalance and is often part of the ocular tilt reaction (OTR). Its origin is derived from lesions of the graviceptive pathways leading to a classical trias of head tilt, skew deviation and ocular counter-roll4. OTR was first observed in animal models by Magendie-Hertwig in 1833 and 1855.5 The graviceptive pathways including otolithic and vertical semicircular canal pathways are responsible for postural stability but also for gaze stability during translational movements as well as head rotation and tilt in the roll plane. Peripheral lesions might also induce skew deviation if there is a complete vestibular loss,6 7,8 however, skew in vestibular strokes is believed to be larger and more sustained.9 The eye is lower on the ipsilateral side in peripheral lesions or pontomedullary lesions, however, central lesions at higher levels lead to a contraversive reaction since the graviceptive pathways are crossing to the contralateral side at the level of the pontomedullary junction. Vertical eye misalignement can also occur with many other lesions such as trochlear palsy.10
Skew deviations can be clinically assessed either qualitatively by the alternate cover test (ACT)1 or quantitatively by the additional application of prisms in combination with the ACT1. The alternate prism cover test (APCT), however, relies strongly on the experience of the examiner.2Currently there are no validated objective, quantitative tests of skew available in the emergency department (ED). Previously described semi-automated methods for skew measurements were suitable for neuro-ophtalmology laboratories and customized for the assessment of strabismus.3
We sought to test healthy subjects under standard conditions with quantitative recording of eye movements. The test of skew was assessed quantitatively by recording vertical eye positions with a portable Video - Oculography (VOG) device suitable for the assessment of vestibular reflexes in the ED. The diagnostic accuracy of such an eye tracking device measuring vertical misalignment of the eyes (skew deviation) is unknown.
In order to test the validity of VOG in detecting skew, we artificially induced skew deviation on healthy subjects using different self-adhesive Fresnel Prism (FP) foils on one eye while recording eye position with VOG. We then compared the system’s performance to that of a trained orthoptist using the conventional, semi-quantitative method of skew measurement (APCT) as a gold standard.