Relations Between Ocular Astigmatism and Corneal Astigmatism

Abstract

Purpose: : To investigate the relations between ocular (refractive) astigmatism and corneal (anterior and posterior) astigmatism in a normal population.

Methods: : We prospectively recorded ocular astigmatism (manifest refraction and autorefractometry data) and corneal astigmatism of 500 healthy eyes (including ametropia) of 250 patients by using Orbscan topography. Polar values (cylinder, axis) of regular astigmatism were transformed into rectangular coordinates (x,y) which respectively represent WTR/ATR component (0°/90° Jackson Crossed-Cylinder) and oblique component (45°/135°JCC). Total (anterior + posterior) corneal astigmatism and internal astigmatism were calculed by vectorial method.

Results: : Anterior corneal cylinder showed higher correlation with subjective cylinder (r: 0.84 to 0.89 p<0.001) than total cylinder (r=0.83 p<0.001) or posterior cylinder (r=0.64 p<0.001). Posterior corneal astigmatism data did not improve the prediction of subjective axis or cylinder. The relative proportion of both components of astigmatism (WTR/ATR and oblique) was not correlated with cylinder. Axis of subjective astigmatism was unaffected by corneal or ocular cylinder (p>0.05). The relative proportion of the anterior corneal astigmatism in the subjective astigmatism was correlated with the subjective cylinder (r=0.54 p<0.001; 83% when cylinder ≥3.00D; 52% when cylinder ≤0.50D). Autorefractometer was the best device to predict subjective cylinder in particularly when cylinder value is weak (r=0.90 p<0.001). The difference (in absolute value) between subjective and corneal axis decreased when the cylinder increased (r=-0.42 p<0.001).

Conclusions: : Anterior corneal astigmatism plays major role in ocular astigmatism especially when the magnitude of the last is important.

Keywords: cornea: clinical science • astigmatism • refraction