Commentary on: Evaluation of viewing distance on vision tasks using virtual reality technology for children with intermittent exotropia

Intermittent exotropia (IXT) is the most common type of exotropia. Binocular vision is present in intermittent cases, but if untreated, the condition usually increases in its frequency of manifestation to become a constant divergent squint. This happens as the binocular fusional capacity decreases. However, not all cases of IXT progress, and some may remain stable. Abnormal stereopsis, especially for distance, has been noted to be an early indicator of intermittent exotropia becoming manifest; moreover, studies show improvement of the stereopsis after surgery.[1] It is therefore useful to understand the intricate mechanisms of normal three-dimensional perception.

For a better understanding of spatial abilities at different distances, one should familiarize themselves with the natural history of intermittent exodeviations. The progression of exodeviations goes through the following stages:

1. Exotropia at distance, orthophoria at near, asymptomatic

2. Intermittent exotropia for distance, orthophoria/exophoria at near, symptomatic for distance

3. Exotropia at distance, exotropia or intermittent exotropia at near, binocular vision for near only

4. Exotropia at distance as well as near, lack of binocularity.

As also mentioned earlier, stereoacuity in patients with IXT was found by the authors to be worse for distance than near. Similarly, spherical equivalent (SE) decreases when patients with IXT change from distant to near viewing. The novelty of this study lies in that newer parameters like dynamic visual acuity (DVA), perceptual eye position (PEP), and contour integration were also evaluated simultaneously – this is a modernized state-of-the-art approach to evaluation of exodeviations.

Conventional methods of strabismus evaluation are mostly based on static visual acuity (SVA) testing in addition to assessment of stereopsis and fusional vergences. However, our daily lives require observation of moving objects as well as viewing at different distances. DVA is defined as the ability to identify the details of visual targets when there are relative movements between the subjects and objects. Dynamic and static visual stimuli have different neural conduction pathways, so evaluation of DVA is an important visual parameter. It should preferably be incorporated into routine practice for a more comprehensive evaluation. DVA testing has already found applications in studies evaluating the progression of optic neuritis and glaucoma. Commonly used DVA tests include static optotypes with movements of the subject’s head, moving optotypes, and motion perception behavior tests. In the present study, the authors have used the moving optotypes.

The concept of PEP has already been explained by the authors. Different from traditional methods of evaluating the horizontal and vertical deviations with prisms, PEP is measured under dichoptic conditions (like Hess charting, wherein different stimuli are presented to each of the eyes), thereby evaluating fusional capacity under dichoptic conditions. Unlike the Hess charting, the recordings are done by a computer, which enables precise quantification. The minimum unit of binocular misalignment measured by a computerized system is 1 pixel, compared to 1 prism measured by the Synoptophore, which equals 25 pixels.[2]

Visual acuity can be influenced by the presence of nearby targets. Contour interaction refers to the impairment of visual resolution produced by different types of flanking stimuli.[3] Similar to crowding (seen in amblyopia), it is a method of assessment of spatial ability, albeit more specific to the part of the cortex that disrupts ocular stability in IXT patients.

In the present study, virtual reality (VR)-based testing has shown an efficient method for detecting visuomotor deficits in children with IXT. Previously, the angle of ocular deviation has been measured accurately by a VR-based system and shown to have excellent correlation with the alternate prism cover test.[4] These technologies would enable the digitization of strabismus examination and seem promising in the new era of artificial intelligence, AI, we are currently witnessing.