Strengths and limitations
This is the first study reporting the diagnostic accuracy of automated
skew deviation measurements by a portable VOG system designed for the
assessment of dizzy patients in the ED. We did not test, however, the
specificity of the VOG device in measuring misalignments in subjects
without inducing any skew. Unlike the APCT, it allows a precise
quantification of eye misalignment with an objective measure of eye
excursion.
Our investigative setup is not generalizable to all other systems on the
market. We used VOG goggles, designed and approved for measuring static
and dynamic vestibular reflexes such as the VOR. Any test using
certified eye tracking systems for that purpose can be adapted using an
alternating manual or automated cover to obtain skew deviation
measurements. In order to assure meaningful saccade detection we
recommend a frame rate of 250 Hz.
It has been shown in previous studies that skew deviation varies with
upright and supine body position in patients with a vestibular
imbalance.21 In our study, we tested participants
solely in an upright sitting position.
We studied only healthy subjects and did not test patients with
spontaneous nystagmus. Such vestibular nystagmus is predominately
horizontal, however, a mixed torsional and upbeat component in
incomplete, superior vestibular neuritis, might bias the clinical
assessment. Small vertical eye movements from the underlying nystagmus
might be falsely interpreted as vertical skew during the clinical APCT.
Physiological skew deviation (<= 0.30
degrees4) in healthy participants, however, is usually
not discernable.
We were not able to induce small skews in all healthy participants due
to central adaptation (see condition 1- 4 PD in Figure 5). We therefore
asked participants to keep their eyes closed before the measurements
began in order to prevent as much as possible prior adaptation and to
maximize the success of obtaining a valid skew measurement. Those
participants with small induced skews (< 2 PD (1.14°)) were
still detectable by the eye tracker, provided that the recorded eye
traces were clean and free from artifacts (Figure 2).
Finally, during the measurements of large skews (10PD (5.7°)), there was
selection bias due to a limited ordinal scale from 1-10PD (0.57-5.7°)
compared to continuous results supplied by the VOG. Our current study
investigated artificially induced skew on healthy participants. Thus, it
has not yet been tested on patients with pathological skews. Future
studies must be validated with patients suffering from conditions such
as heterophoria, acute vestibular imbalance or other
neuro-ophthalmological ailments affecting the alignment of the eyes.
Additionally, it would be interesting to use this quantitative
measurement of skew alongside with other vestibular exams such as
oVEMPs, for example, to confirm the involvement of the utricular pathway
in transitory skews observed in patients with an acute vestibular
syndrome.