The Response AC/A Ratio Before and After the Onset of Myopia

Abstract

Purpose: To investigate the ratio of accommodative convergence per diopter of accommodative response (AC/A ratio) before, during, and after myopia onset.

Methods: Subjects were 698 children aged 6 to 14 years who became myopic and 430 emmetropic children participating in the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error. Refractive error was measured using cycloplegic autorefraction, near work by parent survey, and the AC/A ratio by simultaneously monitoring convergence and accommodative response. The response AC/A ratios of children who became myopic were compared with age-, sex-, and ethnicity-matched model estimates for emmetropic children from 5 years before through 5 years after the onset of myopia.

Results: The response AC/A ratio was not significantly different between the two groups 5 years before onset, then increased monotonically in children who became myopic until reaching a plateau at myopia onset of about 7 Δ/D compared to about 4 Δ/D for children who remained emmetropic (differences between groups significant at P < 0.01 from 4 years before onset through 5 years after onset). A higher AC/A ratio was associated with greater accommodative lag but not with the rate of myopia progression regardless of the level of near work.

Conclusions: An increasing AC/A ratio is an early sign of becoming myopic, is related to greater accommodative lag, but does not affect the rate of myopia progression. The association with accommodative lag suggests that the AC/A ratio increase is from greater neural effort needed per diopter of accommodation rather than change in the accommodative convergence crosslink gain relationship.

Figure 1

The response AC/A ratio as a function of annual visit relative to the onset of myopia (−5 years before to +5 years after onset, which is designated as visit 0). Data are from children who became myopic (▪) and from emmetrope model values (○). All error bars are ±SEM. Error bars for the emmetrope model values are smaller than the symbols. *Significant differences between groups (P < 0.01).

Figure 2

(A) The difference in response AC/A ratio between children who became myopic and age-, sex-, and ethnicity-matched emmetropic model values by visit relative to the onset of myopia. For example, Hispanic children at visit +3 years after myopia onset have a response ACA ratio that is approximately 3.2 Δ/D higher than their emmetropic model. Symbols for ethnic group are the same in the two panels. Instead of differences, panel (B) shows the average response AC/A ratio in children who became myopic (dark symbols and solid lines) and the corresponding age-, sex-, and ethnicity-matched emmetropic model values (gray symbols and dashed lines). Data are plotted where there were at least 20 children per visit for each ethnic group. For clarity, SE bars are shown for Hispanic children but were similar across ethnic groups.

Figure 3

The proportion of children who became myopic with near esophoria ≥2 Δ/D by cover test as a function of visit relative to the onset of myopia. The proportion of emmetropic children with near esophoria ≥2 Δ/D was approximately 4% across the range of ages from 6 to 14 years.

Figure 4

The difference in response AC/A ratio between children who became myopic and age-, sex-, and ethnicity-matched emmetropic values as a function of visit relative to the onset of myopia by refractive error correction status. Due to sample size considerations, only visits from −2 to +2 are plotted. All error bars are ±SEM.