Myopia is a condition in which the eye grows too long to match its optical focal length and represents a failure in both structure and function. Because of the potential risks of vision loss associated with high myopia, and also with corrective treatments such as refractive surgery and occasionally also with contact lenses, myopia should not be considered a benign condition. The hypothesis that hyperopic defocus from under-accommodation during near work is the driving stimulus in the progression of myopia, motivated studies of bifocal spectacles and progressive addition lenses, as optical treatments for the control of myopia progression, with limited success. In contrast, multifocal (MF) soft contact lenses (SCL) and orthokeratology (ortho-k) have proven to be more beneficial although the mechanisms underlying their anti-myopia effects are not understood. This dissertation, which describes 4 main studies, represents efforts to understand how spherical aberration influences the accommodative response and examines as an explanation for the myopia control effects of MF SCLs and ortho-k the possibility that induced changes in ocular spherical aberration decreases the lag of accommodation.
First the effects of bifocal (BF) SCLs on the accommodative responses of young adult emmetropes and myopes were measured using a refractometer. Interpretation of these results proved to be problematic because direct measurements could not be made through the BF SCLs necessitating an assumption to be made about the effective add provided by the lenses.
To address the above issues, in a follow-up study MF SCLs were used in conjunction with a wavefront sensor, allowing direct measurements of accommodative responses through the lenses. To analyze the collected data, the problem of determining a suitable method for calculating accommodative responses from wavefront aberrations had to be solved. Thus a second complementary study evaluated some of the methods used to calculate objective refractions from wavefront aberrations. The best results were obtained with a through-focus procedure, which used an optical quality metric to determine the best image plane and then calculated the accommodative error relative to this plane. The latter findings enabled a comprehensive analysis of the accommodative response data obtained in the MF SCL study, which demonstrated that spherical aberration and pupil diameter independently influence the accommodative response. Both center-distance and center-near MF lenses produced myopic shifts in the best image plane, the former by adding positive spherical aberration and the latter with the added power of the near addition. For pupils larger than approximately 5 mm, both MF lenses resulted in increased accommodative responses determined by a neural sharpness metric compared with those for a single vision distance lens.
A fourth study measured the change in ocular aberrations induced by ortho-k and assessed the long term effect of ortho-k on the accommodative response of young adult myopes. This study found that ortho-k had similar effects to the center-distance MF SCL on aberrations and accommodative responses. An intriguing long-term outcome of this treatment was a dramatic increase in pupil size for all tested vergences. Explanations in terms of changes in both the pupillary light and near reflexes were considered.
In summary, the studies reported in this dissertation point to complex interactions between spherical aberration, pupil size and the accommodative response, which may be deliberately manipulated in designing novel optical treatments for the control of myopia progression.