Background: Compensatory ocular counter-torsion (COCT) is supposed to maintain the eyes aligned with the visual environment following head tilt. Because of some recent controversies the functional capacity of this phenomenon was defined according to the extent of induced astigmatic axis error following head tilt.
Materials and methods: Objective autorefractometry was performed on 70 eyes with a regular astigmatism of ≥2D at vertical, right head tilt and left head tilt positions of 5°, 10°, 15°, 20° and 25°. Astigmatic axis error was calculated according to the difference between the defined axis at each tilted head position and the defined axis at the vertical head position. A tiltometer was used for this purpose to show the angle of head tilt without disturbing the process of refractometry.
Results: The mean astigmatic axis error was 3.2° ± 1.5° and 18.4° ± 4.2° at the head tilt angles of 5° and 25° respectively. The mean percentage of tilt angle compensation by COCT was 36% and 26% at the head tilt angles of 5° and 25° respectively. There was a direct relation between the head tilt angle and the induced astigmatic axis error (ANOVA, P < 0.001, 95% of confidence interval [CI]). Astigmatic axis error values at right head tilt were significantly lower than their corresponding values at left head tilt (ANOVA, P = 0.04 95% CI).
Conclusion: Any minimal angle of head tilt may cause erroneous measurement of astigmatic axis and should be avoided during refraction. One cannot rely on the compensatory function of ocular counter-torsion during the refraction.
Keywords: Astigmatism; axis; compensatory ocular counter-torsion; head tilt; ocular torsion.