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D.O. Mutti, G.L. Mitchell, J.R. Hayes, L.A. Jones, M.L. Moeschberger, S.A. Cotter, R.N. Kleinstein, R.E. Manny, J.D. Twelker, K. Zadnik; The Response AC/A Ratio Before and After the Onset of Myopia . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5455.
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© ARVO (1962-2015); The Authors (2016-present)
We previously found that increased accommodative lag appears in the year of myopia onset or one year after onset rather than prior to myopia onset (Mutti et al., ARVO 2004). The current analysis investigates the hypothesis that alterations in accommodative convergence in children who will develop myopia are detectable before changes in the accommodative response. This question is addressed by assessing the response AC/A ratio before, during, and after the onset of myopia.
The age of onset of myopia (defined as –0.75 D or more myopic in each principal meridian) was identified in 434 children between the ages of 6 and 14 years participating in the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error between 1996 and 2003. The AC/A ratio (using a 4.00 D stimulus) measured annually before and after the onset of myopia was compared to age–matched predicted values of the AC/A ratio from a model derived from 467 children who were emmetropic at all visits (between –0.25 D and +1.00 D in each meridian). Measurements were excluded from the analysis if the accommodative response was less than 1 D (7.9% of data excluded), if the AC/A ratio was negative (2.1% of data excluded), or if it was more than 20 Δ/D (0.4% of data excluded).
The response AC/A ratio from emmetropic model values fell within a narrow range from 2.8 Δ/D to 3.2 Δ/D across visits. There was no difference between the response AC/A ratio from emmetropic model values and pre–myopic AC/A ratios at five years before the onset of myopia (P = 0.52). Children who went on to become myopic had higher response AC/A ratios compared to emmetropic model values from four years before onset through five years after onset (elevated by 1.4 Δ/D 4 years before onset, by 3.7 Δ/D at onset, and by 4.0 Δ/D 5 years after onset; all P<0.001).
The response AC/A ratio was elevated in children who went on to become myopic at least four years before increases in accommodative lag appeared using the CLEERE protocol. The response AC/A ratio therefore seems to be a more sensitive indicator of altered accommodative function in future myopes than accommodative lag. The open loop nature of the AC/A ratio may remove a ceiling effect that constrains closed loop accommodative responses. The response AC/A ratio may also have value as a predictor of the onset of myopia.
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