Lens power loss in early adulthood

Since the time of Sorsby, the lens has been shown to lose power while the eye grows during school years. This loss of power compensates for axial elongation in eyes that remain nearly emmetropic during infancy and adolescence. But during school myopia development, the lens has been shown to alter its rate of lens power loss (Iribarren et al. 2012; Mutti et al. 2012). Three studies in selected groups developing myopia during early adulthood have reported prospectively on myopic shifts and biometry. These studies have been performed in microscopists and medical or engineering students, who have been shown to develop adult onset myopia (Lin et al. 1996; McBrien & Adams 1997; Kinge et al. 1999), but authors did not report on lens power change with age, although those studies showed that the myopic shifts found were smaller than expected for the increase in axial length, thus pointing to possible loss of lens power during those adult years. Full prospective data of refractive change and biometry, including anterior chamber depth and lens thickness, were available in one of these studies (Kinge et al. 1999), so the lens power could be calculated with Bennett's equation (Bennett 1988). As can be seen in the Table 1, the 3-year growth of approximately 0.4 mm in axial length that would represent a myopic shift in spherical equivalent of about −1.2 dioptres did not match the actual myopic shift found of −0.50 dioptres because the lens lost about 0.40–0.80 dioptres of power. It can then be seen that the lens continues to compensate for axial elongation in early adult years. Interestingly, the compensation is less evident in progressing myopes, as emmetropes or hyperopes show greater loss of lens power than progressing myopes in the same period. The lens thins and flattens curvatures in children up to age 10, and from then on, the lens increases in axial thickness, steepens front and back curvatures and loses internal effective refractive index (Brown's lens paradox). The increase in axial thickness can be seen in the follow-up of the current study, and interestingly, although curvatures may be steepening slightly with age in these engineering students, the lens is losing power probably because of changes in its internal structure, as has been discussed before (Iribarren et al. 2012). The fact that the lens seems to be losing more power in baseline emmetropes or hyperopes than in myopic subjects is possibly related to the changes seen in lens power loss during myopia development in previous studies (Iribarren et al. 2012; Mutti et al. 2012). A −0.77 dioptres change in 3 years represents a rate of lens power loss of −0.26 dioptres per year, which compares well with the rate of lens power loss in SCORM emmetropic schoolchildren of −0.29 dioptres per year (Iribarren et al. 2012). It could then be that the rate of lens power loss does not slow up much after age 10 and is still present during early adult life in subjects prone to develop myopic shifts, like these engineering students in Norway. It would be interesting to analyse lens power loss prospectively in population-based studies in younger and older adults according to refractive groups as has been done in CLEERE and SCORM studies in schoolchildren.