Influence of Head Position on Fundus-Based Ocular Torsion: Implications for Assessing Otolith-Ocular Function

Background: Ocular counter-roll (OCR) is an otolith-mediated vestibulo-ocular reflex that produces torsional rotation of the eyes in response to head tilt. Accordingly, measuring ocular torsion can provide an assessment of vestibular tone balance related to otolith function. This can be objectively quantified from fundus photographs using the disc-foveal angle (DFA). Because head tilt also physiologically alters ocular torsion through OCR, head position during fundus imaging may influence the measured DFA. Here, we examined this effect of head tilt and the resulting OCR on fundus-based measurements of ocular torsion. Methods: Ten healthy adults (age 28–45 y/o; 2 females) underwent bilateral fundus photography under three conditions: routine head placement, controlled upright (0°) positioning, and 10° lateral head tilt. Head orientation was continuously measured using a strap-mounted gyroscope. DFA was quantified from fundus images as the angle between the fovea–optic disc axis and horizontal meridian using ImageJ. Results: Mean DFA did not differ between standard and controlled head positioning (p = 0.23), although within-subject variability was greater with standard positioning (p = 0.020). In contrast, 10° head tilt significantly changed DFA (p < 0.01). Conclusions: Small, naturally occurring variations in head position do not significantly alter DFA but may increase measurement variability, whereas larger head tilts produce significant changes in DFA. Careful control of head position during fundus photography can improve the reliability of DFA measurements for assessment of otolith-ocular function.