Axial length, corneal shape and optical aberrations in myopic versus hyperopic eyes

Purpose: To compare the optical features of the components of myopic and hyperopic eyes. Methods: Axial length, corneal topographic data (radius and asphericity) and corneal and total aberrations were obtained for 10 myopic (age 28.9±4.1 years) and 11 hyperopic eyes (age 29.9±4.8 years). The spherical equivalent ranged from -1.2 to -7.6 D (-3.8±2.4 D) for the myopic, and from 0.6 to 7.4 D (3.4±2.3 D) for the hyperopic group. Cylinder was <2.50 D. Axial length was measured with optical biometry. Corneal topographic data were obtained from a videokeratographic system and fit to a biconic function to obtain corneal radius and asphericity. Corneal aberrations were estimated from virtual ray tracing on corneal elevation maps, using custom algorithms. Total aberrations were measured with Laser Ray Tracing (sequentially scanning the pupil with a 786 nm diode laser and recording the corresponding retinal image on CCD). Pupil size was 6.5 mm. Results: 1) Axial length was significantly longer (p<0.0001) in the myopic group (25.30±1.32 mm vs 22.62±0.53), and statistically significantly correlated with spherical equivalent (p=0.007), 2) Corneal radius was not significantly different between both groups. Asphericity was significantly (p=0.004) more negative in the myopic group (-0.19±0.10 vs 0.050.09 in hyperopes) Corneal spherical aberration was significantly lower (p<0.001) in the myopic (0.21±0.16micron) than in the hyperopic group (0.44±0.08 micron) for the group), 3) Total spherical aberration was significantly (p=0.02) lower for the myopic (0.09±0.16micron) than for the hyperopic group (0.28±0.20micron). 4) Internal aberrations were not significantly different between both groups (-0.14±0.17 and 0.12±0.10micron respectively). However, we observed a marked age-related change in the hyperopic eyes (with nearly null internal spherical aberration in eyes >30 years). Conclusions: Hyperopic eyes are shorter than myopic eyes and show higher spherical aberration, mainly due to the cornea. Hyperopic eyes may show earlier loss of corneal/internal spherical aberration balance.Peer Reviewe