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.