Head-gaze related influences on heterophorias revealed by an HMD-based cover test

Strabismus is a disorder in which the visual axes of the eyes are misaligned and affects 3-5 percent of children; it can be latent or visible and, if visible, it may be persistent or intermittent. Angle of deviation may change at different gaze positions for incomitant (non-comitant) strabismus or may not change for comitant (concomitant) strabismus, which accounts for most forms of childhood strabismus. Common clinical practice prioritizes the primary position of gaze and a fixed distance, since testing other gaze directions and variable distances would require a longer and more difficult procedure. Employing Head-Mounted Displays (HMD) with embedded eye-trackers in clinical practice would allow to generalize standard procedures, like the cover test, to eventually quantify strabismus angle in natural conditions (free head movement). To this end an HTC Vive Pro Eye was used to administer an alternating cover test: subjects begin by fixating at a 1.6° visual angle grey cross with a rotating grey ellipse at its center, placed 40 cm within the virtual HMD environment. The initial binocular fixation established a 2s baseline, followed by monocular covering, virtually occluding each eye view for 2.25s, alternating for 10 trials each lasting 8s. Specifically, in this work we investigate the extent of heterophorias under different head rotations (45° of yaw and 45° of pitch). The ocular deviation is quantified in ten subjects who have never been diagnosed with strabismus. We conducted an analysis to explore potential trends associated with head rotations. Results revealed patterns of deviation across different head rotations, independently of the specific direction of rotation. Individual differences suggest complex relationships between head movements and ocular alignment. Preliminary results show heterogeneous behaviors for different eye-head relative positions, which would pave the way to a systematic assessment of multiple and varying parameters captured under complex, ecological conditions.