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.