Effects of physical exercise on phoria: a pilot study.

Purpose: Diplopia or heterotropia may occur during physical exercise. However, the influence of exercise on phoria is still unknown. The purpose of this study is to evaluate the effects of a physical activity involving both aerobic and anaerobic exercise on horizontal phoria. Methods: Twenty young adult competitive soccer players (19 males, 1 female) were included and divided equally in two groups by manifest refraction: ametropic (n = 10) and emmetropic (n = 10) subjects. Horizontal phoria was evaluated by a Risley-Maddox rotating prism after a complete dissociation with a 5 Δ prism placed base-up on the non-dominant eye during five different times before (at rest) and after an aerobic-anaerobic workout (immediately, 5 min, 10 min, 25 min). Results: Ametropic subjects had myopia in either eye (median spherical equivalent of -0.63 D [interquartile range from -4.45 to -0.47 D]). Emmetropic subjects were at baseline significantly more esophoric than ametropic subjects (+0.90 ± 1.37 vs. -2.20 ± 2.57 Δ, p = .003). On average emmetropic subjects showed a quadratic course of increasing esophoria, peaking at 10 min (+1.65 ± 2.03 Δ) and then decreasing (+1.25 ± 1.78 Δ), returning to baseline in 50% of cases; while ametropic subjects had a quadratic course of increasing exophoria, peaking at 5 min (-4.00 ± 2.71 Δ) and then decreasing (-2.40 ± 2.80 Δ), returning to baseline in 80% of cases. The variations of phoria induced by physical exercise were significant throughout the time (repeated measures ANOVA p < .001, η_p²  = 0.42). The interaction of time with refractive state on phoria was significant for quadratic term (p = .001, η_p²  = 0.48). Ametropic subjects showed on average higher angles (p = .049, η_p²  = 0.20) and higher variations from baseline phoria (quadratic interaction p = .015, η_p²  = 0.29). Conclusions: Phoria is significantly affected by physical exercise, following a quadratic course of increasing and then decreasing angle. Myopic subjects were on average exophoric, displayed higher angles and variations from baseline, peaked before but recovered at last assessment more than emmetropic subjects. Refractive state may have a major role on predicting the progression; however, whether it is due to manifest refraction itself, accommodation or baseline phoria must be ascertained. Any reliable conclusion cannot be drawn due to small sample size and missing physiological measurements. Fluctuating phoria may provide a basis for decreased stereopsis and altered motion perception, thus affecting sports performance.