Introduction
Vertigo is a common complain of patients that visiting
otolaryngologist’s office. With a lifetime prevalence of 7.8%, patients
experienced vertiginous symptoms, such as dizziness, imbalance, sense of
spinning, and loss of coordination, which have large impact on their
daily life (1, 2). Not only to individuals, these imbalance problems
also bring considerable cost to healthcare system and society. As an
apparatus of inner ear, the vestibular system is composed by two otolith
organs and three semicircular canals (3). Semicircular canals are
perpendicular oriented to each other to sense angular acceleration in
three dimensions (Figure 1). In the clinics, since the beginning of the
20th century, efforts were made to evaluate the
function of semicircular canals. Caloric test (4, 5),
electronystagmography (ENG) (6), and head-impulse test (HIT) (7) are
introduced during the years that still widely used in today’s practices
to locate the defected vestibular organ.
Among the different tests, recently, the vHIT has been considered as the
standard even the initial test since it is applicable repetitively in a
short interval to all six semicircular canals (1). A passive,
unpredictable, fast angular head movement (>150°/s) is
applied to activate a specific pair of semicircular canals (8) (Figure
1). Coordinately, eyes are able to move against head through the
vestibular-ocular reflex (VOR), to stabilize the image on the retina.
During the tests, the relative of eye movements to head movements is
recorded as the gain of VOR to assess the vestibular system. Damaged
semicircular canal will not be able to move eyes against to head during
the test, then the patient has to make a corrective saccade at the end
of head impulse. During the practices, the results of VOR gains for
vertical canals usually arise controversy due to the technical
complications. First of all, the recording is binarization of a 3-D
movement that need carefully align the testing parameters for vertical
and torsional components to avoid noises (Figure 1A and C) (9). The
pupil was driven toward the eyelids while performing the vertical
impulses, in which case, eyelid obscures part of the pupil that
diminished the vertical gain recorded. Efforts were made to find an
optimal staringposition to minimize the adverse effects. Previous paper
found that with the increased of gaze angle on the horizontal plane away
from right-ahead, the measured vertical VOR gain decreased (10).
However, this is not we found in daily practices that with one eye open
and head tilted 45 degrees to ensure impulses at vertical canal planes,
patients are hard to stare at the targetduring head torsions, which
attributes to mixed outcomes that obscure the real defects (Figure 1D).
In addition to the gaze angle, another concern about the frequently-used
vHIT systems was raised. The goggle that was used in some vHIT systems
could only record VOR from the right eye. The inconvenience raised from
some patients hard to see the straight targets when head tilted
especially to the right. Besides, it is even harder to record when any
patient suffered a right eye damaged or defected. In this case, a goggle
that can record VOR gain from either eye is needed. The new system,
VertiGoggles ZT-VNG-I (VG) provides this kind of google to eliminate
the inconveniences. To improve the protocol of vHIT, we first tried to
find a more suitable gaze angle on horizontal plane to obtain a better
VOR gain to perform the vertical impulses. Furthermore, to make sure the
new goggle and vHIT system has as good effectiveness as the existed
popular systems under various conditions, we started a two-center
project to compare between VG system to the long-standing vHIT systems.
Our results demonstrated that staring at 25 degree has a higher VOR gain
in vertical canal vHIT. Besides, the new kind of goggle offered
comparable or even better vHIT results, especially when head tilted to
right.