Abstract
Purpose: :
Corneal topography maps often appear to be shifted nasally from the pupil center in emmetropic eyes. This displacement, videokeratoscope shift, is consistent with angle alpha, the deviation between the eye’s optical and visual axes. We have shown that videokeratoscope shift decreases with increasing myopia (Bansal et al., ARVO, 2004) and axial length (Coletta et al., OSA Vision Meeting, 2005). These findings imply that emmetropic eyes have decentered optical systems relative to the visual axis, which should induce lateral coma. However lateral coma in the eye may be less than coma induced by the cornea if the corneal coma is compensated by the internal optics (Kelly et al., JOV, 2004). We examined the roles of pupil displacement and refractive error in the amount of corneal and ocular lateral coma aberration.
Methods: :
Measurements were obtained from one eye of 37 adults whose spherical equivalent refractions ranged from +1.25 D to –9.00 D. Ocular aberrations were obtained with a Wavefront Sciences COAS aberrometer and corneal aberrations were calculated from Humphrey Atlas corneal topography maps, using VolCT software (Sarver Associates). The pupil center was located in the raw topographer images and pupil displacement was determined from the scaling coordinates of the topography map. Both ocular and corneal aberrations were calculated for a 6 mm pupil and referenced to the pupil center. Lateral coma coefficients (C31) were reversed in sign for left eyes. Internal coma was obtained by subtracting the corneal from the ocular coma aberration.
Results: :
Corneal coma was significantly correlated with pupil displacement (r=0.46; p=0.004) and trended toward zero with increasing myopia (r=0.29; p=0.08). Ocular lateral coma values were smaller than the corneal values and did not vary significantly with either pupil displacement or refractive error. The absolute value of ocular lateral coma was 0.073±0.01 micron (mean±se), which is 10 times greater than the measurement accuracy of the COAS for this aberration (Cheng et al., OVS, 2003). Internal lateral coma was significantly correlated with pupil displacement (r=0.497; p=0.002) and decreased significantly with increasing myopia (r=0.394; p=0.016). The mean internal lateral coma coefficient was 0.198±0.045 micron in emmetropic eyes and 0.101±0.019 micron in eyes with more than 1D of myopia (p=0.03; t–test).
Conclusions: :
Ocular lateral coma aberration is small, even though emmetropic eyes have decentered optics. The compensation of corneal coma by internal coma of opposite sign is a feature of emmetropic eyes, and both corneal and internal coma aberrations decrease with increasing myopia.
Keywords: myopia • refraction • cornea: clinical science