The purpose of this study was to investigate accommodation
, accommodative convergence, and AC/A ratios before and at the onset of myopia
Methods. Refractive error
, and phorias were measured annually over a period of 3 years in 80 6- to 18-year-old children
(mean age at first visit = 11.1 years), including 26 who acquired myopia
of at least -0.50 D and 54 who remained emmetropic (-0.25 to + 0.75 D). Refraction was measured by noncycloplegic distance retinoscopy. Concomitant measures of accommodation
and phorias were taken for letter targets at 4.0 m and 0.33 m using the Canon R-1 open field-of-view autorefractor with an attached motorized Risley prism and Maddox rod. The accommodation
and phoria measurements were used to calculate response AC/A ratios.
Compared with children
who remained emmetropic, those who became myopic had elevated response AC/A ratios at 1 and 2 years before the onset of myopia
, in addition to at onset and 1 year later (t’s = -2.97 to -4.04, p < 0.01 at all times). The significantly higher AC/A ratios in the children
who became myopic are a result of significantly reduced accommodation
. Accommodative convergence was significantly greater in myopes only at onset.
These findings suggest that the abnormal oculomotor factors found before the onset of myopia
may contribute to myopigenesis by producing hyperopic retinal defocus when a child is engaged in near-viewing tasks.