Spherical Aberration, Accommodation and Myopia

Myopia should not be considered a benign condition because of the potential risks of vision loss associated not only with high myopia but also with the procedures used to correct myopia. Results from animal studies demonstrating that emmetropization is an active rather than a passive process and the induction of myopic refractive errors with imposed hyperopic defocus underlie the hypothesis that hyperopic defocus resulting from under-accommodation during near work is the driving factor in the development and progression of myopia in susceptible individuals. Based on this premise both bifocal spectacles and progressive addition lenses have been investigated, with limited success, as potential treatments to slow myopia progression. However, alternative treatment modalities such as bifocal and multifocal contact lenses and orthokeratology have proved more effective in slowing myopic eye growth. An understanding of the aberrations induced by the latter treatments and their effects on retinal image quality and accommodative responses can significantly increase our understanding of the optical factors driving myopia progression and guide the design parameters of future optical treatments. 1.1 Etiology of Myopia Myopia is often considered to be a benign condition that can be corrected as needed with spectacles or contact lenses, or permanently, with refractive surgery. However, it is more appropriately viewed as a failure in both structure and function, a product of the eye growing too long to match its optical focal length. The myopic eye has an increased risk of developing ocular pathology that may lead to vision loss, including retinal detachment, glaucoma, cataract and myopic degeneration with the potential sequela of choroidal neovascularization (Curtin, 1985). The odds ratio for rhegmatogenous retinal detachment with myopia of 1.33/Diopter reported in a recent study (Chou et al., 2007) implies that even moderate levels of myopia carry a significantly increased risk of detachment, i.e. by 4 times for a -3.00 D myope compared with someone with no refractive error. Myopes may also lose vision as a consequence of complications of refractive surgery and more rarely, contact lens use. Epidemiological studies provide strong evidence that the prevalence of myopia is increasing worldwide, with the most rapid increase being in East Asian countries. Morgan and Rose (2005) have written a comprehensive review of this subject, providing the following data for 12 to 13 year old children: Japan 39% in 1984 and 59% in 1996 (Matsumura and Hirai, 1999) Taiwan 36.7% in 1983 and 60.7% in 2000 (Lin et al., 2004, Lin et al., 2001) Hong Kong 53% in 1991 and 83% in 2001 (Lam and Goh, 1991, Lam et al., 2004) United States 12.3% in 1953-54 and 28% in 1993 (Blum et al., 1959, Zadnik, 1997) Sweden 49.7% in 2000 (Villarreal et al., 2000) Not only is the prevalence of myopia increasing in teenagers, it is also increasing in younger age groups, implying that the age of onset of myopia is also decreasing. Earlier onset is also associated with progression to higher levels, with increased prevalence of high myopia (> -6.00 D) being the salient feature. Animal studies have demonstrated that emmetropization is an active rather than a passive process (Wallman and Winawer, 2004). External optical influences can affect eye growth; for example, when a minus lens is placed in front of the eye it imposes hyperopic defocus, causing the eye to grow longer to compensate for the decrease in overall optical power. When the minus lens is removed, the focal plane of the eye is now in front of the retina, i.e. the eye is now myopic. This lens paradigm involving imposed hyperopic defocus to induce myopic refractive errors is widely used in animal myopia research. One theory proposed to explain the development of myopia in humans is based on the above observation from animal experiments. Specifically, that hyperopic defocus resulting from reduced accommodation during near tasks is the driving factor underlying the development and progression of myopia in susceptible individuals. Accommodation serves to bring into focus objects at close distances. However, if the eyes under-accommodate, the focal plane conjugate to the near object of regard will lie behind the retina. Because of accommodative lag, extended periods of near work will be associated with extended exposure to hyperopic defocus, a myopiainducing stimulus. While subjects report seeing clearly and thus the lag of accommodation is assumed to match the ocular depth of focus, it is possible that this defocus error is detectible by the retina, the presumed source of ocular regulating eye growth (Wallman and Winawer, 2004). The etiology of myopia has been studied extensively, with the relative importance of heredity versus environmental influences being the subject of much debate. However, the preponderance of evidence suggests that environmental influences are more important,