In our study, 44.3% of intermittent exotropia patients had transient eye closure with bright light. Even the TEC under low-intensity light had Phi correlation coefficient of 0.67 with self-reporting photosensitivity, while some of the patients had different responses to TEC test. The amount of exodevaition, stereopsis, the presence of diplopia, and pupil dynamic had no significance with TEC’s presence except normal sensory status at a distance in the patients with IXT.
Lew et al. suggested that photosensitivity and eye closure were more likely to occur in patients with a distance angle of strabismus > 25 prism diopters. Moreover, the study revealed that these phenomena were found to disappear after strabismus surgery, even when the surgery was deemed under-corrected [4]. However, in the study by Oh et al., the preoperative angle of deviation was not significantly different between those with a presence of squinting and those without squinting [6]. In the previous study, the term ‘squinting’ has been used as the same meaning as ‘photophobia.’ Our present study found that, by individual analysis, the smaller angle of deviation at near was associated with the presence of TEC and the distance angle was not significantly associated with the presence of TEC, even though the motor status was not significant in multivariate analysis. A previous study recruited patients who underwent surgical correction while our study involved all patients with IXT who came to our clinic without any treatment [4]. These discrepancies may be related to selection bias, as patients who required surgical treatment might have had more severe symptoms.
A study analyzing 162 patients who underwent surgical correction also showed that squinting and photosensitivity were more likely to occur in patients with stereopsis worse than 60 s [4]. However, Oh et al. revealed that stereopsis was not significantly associated with photosensitivity [6]. Our study also demonstrated that the degree of stereopsis was not related to the presence of TEC evoked by bright light.
The iris is the primary organs controlling retinal luminance, and abnormal pupil dynamics may cause photosensitivity. Dulop reported that there was abnormal pupil dynamic in patients with IXT; about one-third of patients with IXT have paradoxical pupil changes, and in these patients, pupil dilation occurs when the eyes are aligned immediately prior to exotropia [14]. However, our study revealed that pupil dynamics, pupil size, and response to light stimulation were within normal ranges. In addition, there was no significant association between pupil dynamics and the presence of transient eye closure evoked by bright light.
The presence of TEC evoked by bright light intensity showed a strong correlation with photosensitivity. Taken together, this study suggests that TEC may be a part of the light-modulating mechanism and an avoidance mechanism of photosensitivity. Therefore, we propose that TEC under bright light is a form of photic blink reflex. A photosensitivity grading study using a synoptophore, which measured subjective discomfort on a numeric scale, reported that the binocular photophobia threshold was significantly lower in participants reporting eye closure compared with those who did not [7]. Campos et al. reported that a deterioration of fusional amplitude and a weakening of binocular sensory status were shown in patients with IXT during light exposure [15]. They suggested that since bright light lowered the threshold of binocular photosensitivity, this impairment, although not conscious, may determine a subjective disturbance. To avoid this inconvenience, the patient may close one or both eyes, thereby eliminating any binocular demand [15]. However, the study was performed by comparing the IXT group to a control group, which involved people with orthophoria, esophoria, and exophoria. Some IXT patients may have more deteriorated sensory status than normal people, and therefore, the IXT group investigations may show us more significant factors related to TEC. Our study demonstrated that normal sensory status was the only significant factor in the presence of TEC in patients with IXT. These findings suggest that the TEC is independent of motor alignment, stereopsis, and pupil reflex pathway in patients with IXT. We suppose that the different pathway, such as neural circuits in the brainstem may play a role in TEC. In addition, our study demonstrated that test also provides data regarding TEC under different light intensity conditions; therefore, it may help grade the TEC in IXT patients and be an analytic tool when objective comparison of TEC is required.
From a practical clinical standpoint, our new test method helps identify the presence of TEC in patients with IXT. Near-infrared camera vision monitor system is noninvasive test method that use a Ganzfeld (Entire field) environment with a stimulator. A near-infrared camera vision monitor system can monitor pupil changes under different light intensities and observe eyelid movement simultaneously with pupil changes. This test method is easy to perform, even in children, without patient discomfort. The presence of TEC had a strong agreement with self-reporting photosensitivity in IXT patients. Although we could not investigate the physiologic mechanism between transient eye closure and photosensitivity, the evaluation of TEC using a near-infrared camera vision monitor system can be an objective and repeatable test method to assess photosensitivity in patients with IXT.
Our method has some limitations. First, it is a retrospective analysis and there could be a selection bias. However, we believe the chance of selection bias is small since all the patients visited clinics were included as source population. In second, we made up the questionnaire for the evaluation of subjective photosensitivity, and it was not validated by test-retest. However, the questionnaire included major symptoms of photosensitivity, and we believe that it is relevant for the study. Test-retest should be done in the future study. Thirdly, we did not complete the method with a normal control group to find an association of TEC and IXT, and further study is necessary. Fourth, we could not calibrate the light source’s intensity, although there is a possibility that each patient has a different TEC threshold. Further research should be conducted to find the customized luminance intensity that accounts for more precise results. Fifth, subgroup analysis was not performed. The TEC-positive group’s subgroup analysis without photosensitivity and the TEC-negative group with photosensitivity would provide us further information about this rather contradictive phenomenon. Sixth, possible correlation of TEC measured with or without glasses were not considered. There might be a glaring effect of glasses which might increase TEC. Further study should be done on the effect of glasses. Seventh, our TEC test was performed in the Ganzfeld environment, which might have caused the difference in the results compared to the real world. Finally, although the Ganzfield based infrared camera may not be easy to equip in each clinic, we expect to get help from the recent technologic development of virtual reality and head-mounted display.
In conclusion, this new test method using a near-infrared vision monitoring system with different light intensity helped evaluate the presence of TEC. The presence of TEC had an statistically significant agreement with photosensitivity in patients with IXT. In addition, our study suggested that the TEC in patients with IXT is related to photic blink reflex, which modulated independently with the angle of deviation, subnormal sensory status, and pupil dynamics.
