Abstract
Purpose: :
Wearing progressive addition lenses has been recently found to slow down myopia progression, especially for children with near esophoria and large accommodative lag. The purpose of this study was to measure accommodative lag and near heterophoria (phoria) for Chinese myopic children in order to obtain information for effectively slowing down myopia progression.
Methods: :
250 children (aged from 5 to 13 yrs) with myopia ranged from –0.37 to –5.25D spherical equivalent with a mean value of –2.01D (±0.95) participated in this study. Refractive errors for myopic children were corrected to reach binocular distance vision at least 20/25. Accommodative lag was measured for the right eye using the Monocular Estimate Method (MEM) dynamic retinoscopy with a visual target at 33 cm from the eye, and near phoria was measured in random sequence using three standardized clinical procedures: (1) Cover test, (2) Howell test, (3) Modified Thorington test. Each measurement was repeated three times, and the mean was used for analysis.
Results: :
Mean lag of myopic children was 0.97D (± 0.43), and 23.2% of myopic children with two myopic parents had significantly larger accommodative lags than the others (F=3.53, p<0.05). For this group of children, the accommodative lag increased with refractive error (p<0.01). The mean values of near phoria for myopic children were –0.94Δ (± 4.55) with the Howell test and –1.75Δ (± 4.95) with the Modified Thorington test. For the Cover test, percentages of near exophoria, near orthophoria and near esophoria for myopic children were 31.2%, 68.0% and 0.8% respectively. While 41.6% exophoria, 30.4% orthophoria and 28.0% esophoria were observed for the Howell test, a similar distribution (49.2% exophoria, 28.0% orthophoria and 22.8% esophoria) was found for the Modified Thorington test. Myopic children with near orthophoria or esophoria showed larger accommodative lags than those with near exophoria (F= 7.97, p<0.025 for the Howell test; F= 3.53, p<0.05 for the Modified Thorington test).
Conclusions: :
Chinese myopic children tend to have a larger mean lag of accommodation than Caucasian children (median value is 0.43D in COMET), especially for children with two myopic parents. The high proportion of children with large accommodative lag and near esophoria may be a factor contributing to the high prevalence of myopia in Chinese school children. It is a significant guideline to ask if the two parents are myopes (inducing higher lag in their children) and to measure near phoria for myopic children in clinic.
Keywords: myopia • refraction • refractive error development