The purpose of our study was to measure the objective angle of ocular deviation using VOG with alternate cover in subjects with exotropia. The principle of the VOG device used in our study was that light was transmitted through the tilted semi-transparent glass and the subject could look at the target. As the light was being reflected, the two cameras could record the movement of the eyes without blocking the visual axes. The pupil was detected in real time, and the deviation of the eyeball was assessed by measuring the change in the reference point of the center of the pupil.
Although APCT may represent a typical test for measuring angles of deviation, the results can differ because of differences in measurements using prisms made by individual examiners. Therefore, error involved in the measurements are dependent on the skill of the examiner and the cooperation of the subject [11,12,13]. In particular, the Pediatric Eye Disease Investigator group  suggested that two skilled observers may have an error ≥ 12 PD in the measurement of esotropia exceeding 20 PD, and an error ≥ 6 PD in the measurement of 10 to 20 PD esotropia. The angle of ocular deviation measured by one observer demonstrated that the 95% limit of agreement on a measurement was ±7.3 PD for esotropia exceeding 20 PD, and ± 4.1 PD for 10–20 PD esotropia at distance. In addition, APCT could not record eye movement itself; therefore, it depends on the record of the examiner. For this reason, methods using photography were used for measurements that are more objective. Among them, Yang et al.  took pictures at a distance of 1 m, and the corneal light reflex points and limbus locations were extracted from two-dimensional photographs and analyzed using a three-dimensional strabismus photo analyzer (R & DB Foundation, Seoul, Republic of Korea). The results demonstrated high correlation with the Krimsky test. This is useful to examine cases of manifest strabismus. However, because this test did not dissociate the two eyes, the angle of ocular deviation would be variable according fusion of both eyes in intermittent exotropia, or changeable depending on dominant eye in incomitant strabismus. Therefore, there are restrictions to its use in intermittent exotropia and incomitant strabismus. Additionally, there are limitations in measuring the angle of ocular deviation for retinopathy of prematurity with macular dragging because the angle kappa cannot be considered. In another improvement study, the use of an infrared ray filter and an infrared camera with this method was proposed to observe the deviation angle in patients with latent strabismus .
An alternative for objectively measuring eye movements is to use a VOG device equipped with a camera at a sample rate of 200 to 250 Hz. In previous studies, it was reported to have high correlation with the scleral search coil in the fixation position [6, 7]. The VOG device used in our study had a frequency of 150 Hz; hence, it was less accurate than the scleral search coil used to assess rapid eye movements. However, because our study was intended to measure the distance moved during an eyeball deviation, the results were not greatly influenced by camera frequency. In our study, we needed a device verification step to accurately measure the amount of eyeball rotation. The results obtained using VOG and the amount of eyeball rotation showed statistically high correlation (linear correlation coefficient = 1.000; p < 0.001), and a linear regression equation of the eyeball rotation angle with the angle obtained using VOG was derived. Using this equation, the degree of ocular change in the subject group measured using VOG was converted to actual eyeball rotation angle values. These converted values were compared with the angles of ocular deviation obtained using APCT. The Bland-Altman plot between the APCT values and the calculated VOG values showed consistent variability, except for two subjects. As for the outliers, one subject exhibited ocular deviation values of 20, 20, and 25 PD from the three APCTs, and 19.92 PD from VOG testing. In our study setting, ocular deviation was analyzed based on the last visit, and was judged to be located outside the 95% limits of agreement. The other subject had exotropia with inferior oblique overaction in both eyes. It was V-pattern exotropia with 55 PD in the upward gaze, 40 PD in the primary position, and 35 PD in the downward gaze. We presume that the upward gaze may have been the cause of the positive difference despite maintenance of head position.
The advantage of using VOG is that all eye movements can be recorded as video recordings, and the eye tracking system can record both eye movements and the angle of ocular deviation. Another advantage is the dissociation of the two eyes using alternate covering, and measurement of the distance moved by the eye by recording a video. Hence, VOG is not influenced by the angle kappa. Several studies reporting the objective measurement of strabismus were influenced by the angle kappa because their assessments were based on corneal light reflex points. In addition, the measurement of the manifest strabismus alone was performed without dissociating the two eyes [2, 5, 14,15,16]. In contrast, in a study measuring the angle of ocular deviation by dissociating the two eyes, Yang et al.  used an infrared transmission filter for dissociation and measured the ocular deviation using photographs. However, because photographs―unlike video recordings―do not reflect the continuity of time, it is different from the method used in our study in that it does not record eye movement in real time. Therefore, if we analyze the angle of ocular deviation by the method used in our study with the aid of VOG, we can analyze deviation patterns in dissociated strabismus cases and use it for screening in intermittent exotropia with good convergence. Second, it is also useful for measuring the maximum angle of ocular deviation, which is significant in intermittent exotropia. Third, it can measure the ocular deviation in a short time. It took approximately 2 min during the VOG test (1 min for wearing goggles, 1 min for performing VOG with alternate cover). Finally, because VOG can quantitatively record and compare eye movements in both eyes separately, it is possible to accurately analyze the difference between secondary and primary deviations in incomitant strabismus. This may be useful for follow-up observations. The purpose of our study was to evaluate the accuracy of VOG and to include the inter-visit variability of a single examiner in the process of comparing and analyzing the degree of variability of APCT measurements. However, paralysis, a type of incomitant strabismus, was excluded from our study because there can be changes in ocular deviation during recovery. In the future, we will perform a comparative analysis of both eyes in cases of incomitant strabismus such as paralysis.
The first limitation of our study was that children < 4 years of age were excluded from the evaluation of the accuracy of VOG. We suspect that younger children have a lower attention span and ability to fixate well. Moreover, subjects with claustrophobia could not be assessed because the video goggles had to be worn at the time of the test. Second, subjects did not wear glasses, even if there was a refractive error, although the author found that the VOG goggle supported the use of glasses and video camera could detect the center of the pupil beyond the glasses, the results were affected by the prism effect of glasses. The aim of our study was to evaluate a method using VOG with alternate cover to measure ocular deviation in lieu of standard tests. Therefore, it was important to demonstrate the accuracy of VOG. Subjects did not wear glasses to eliminate variables caused by the prism effect. Third, the target was a red light (50 MOA) for the VOG test and a black-on-white optotype (50 MOA) for the APCT. For measuring the angle of ocular deviation, we did not use a light source as the target; instead, the visual acuity chart was used because the accommodation levels are different. If a light is used as the target, the accommodation is less than that with the acuity chart; hence, the deviation will be lesser for esotropia and vice versa for exotropia . The VOG goggle is constructed from semi-transparent glass, which can degrade contrast of letter targets. The same a black-on-white optotype in VOG and APCT may be different. Surmising that red light would better attract subject attention, it was used instead of a black-on-white optotype for evaluating VOG. However, no difference between red light and a black-on-white optotype was observed. A possible reason is that subjects may perceive the red light as a red dot with higher resolution through the semi-transparent glass. In addition, a recent study using a prism cover test for far distance in intermittent exotropia reported no significant variability between light and a black-on-white optotype target , supporting our trial to use red light for the VOG test. Finally, the distance of the fixation target in VOG was different from that in APCT for measuring ocular deviation because the distance was set up by a meter in the software of VOG test. For reducing the variability caused the distance, we limited our study to a group of subjects with < 3 PD difference between near and far distance ocular deviation.