Axial length changes in healthy adult eyes

Abstract

Purpose : It is generally assumed that, barring pathological myopia, axial length in healthy adults remains constant. Several authors, however, reported an axial length decrease with age. Since a stable outcome for laser surgery and intraocular lens calculations require that a constant axial length, this work analyses the factors that influence axial length measurements over the age of 20.

Methods :
Based on a literature review several influences were identified: cohort effects (myopization of young people), measurement bias (changes in the lens gradient index and thickness), and physiological processes. The importance of each effect on axial length measurement was calculated using the Atchison aging eye model.

Results : In the literature we identified 15 cross-sectional studies of axial length changes with age, of which 10 showed a significant decrease, 4 showed no change and 1 showed a significant increase. Combined, these studies suggest an age-related change in axial length by –6.4 µm/year. This cannot be due to measurement bias, since, assuming a fixed axial length, calculations with the Atchison eye model suggest that between 20–80 years of age, axial length measurements will appear to increase by +1.07 µm/year due to lens index and thickness changes. In the presence of cataract this bias would increase further by +1 µm per increment of 0.01 in refractive index. At the same time the choroidal thickness shrinks by 2.98 µm/year (Wakatsuki et al. PLoS One, 2015;10:e0144156), increasing the axial length by the same amount. Considering the axial length change as a function of year of birth, however, the axial growth curves of Western countries (which had a similar economic development during the 20^th Century) lined up to one another. This fit improved considerably when including the mean education of the countries of the studies (based on UN data) and the curves of all data sets, both Western and non-Western, lined up even further (r² = 0.681; p < 0.001).

Conclusions : Given the cross-sectional nature of all studies considered, the strong correlation between an increase in education level and an increase in axial length (as well as myopia), the reported axial length decreases with age are almost certainly the result of a cohort effect.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.

 

Axial length changes plotted as (a) a function of year of birth and (b) of the mean education of 25-34-year olds hin the countries of the studies. Black lines: Western countries; blue: non-Western countries.