Astigmatism and Amblyopia among Native American Children (AANAC): design and methods.

The overall goal of the AANAC study is to improve detection of astigmatism and prevention of amblyopia in populations with a high prevalence of astigmatism. To meet this goal, the study will evaluate four methods of screening for astigmatism in preschool children and will assess both the short-term and long-term benefits of early correction of astigmatism in improving acuity and preventing amblyopia. This paper presents an overview of the design and methodology of the AANAC study. Subjects are members of the Tohono O'Odham Nation, a Native American tribe with a high prevalence of astigmatism. Preschool-age children who attend Head Start are screened with four tools: the Marco Nidek KM-500 autokeratometer, the MTI photoscreener, the Nikon Retinomax K-Plus autorefractor, and the Lea Symbols acuity chart. Sensitivity and specificity for detection of significant astigmatism, as measured by a technique that uses both cycloplegic retinoscopy and cycloplegic autorefraction, is determined for each of the four screening tools. Presence of amblyopia is evaluated by measurement of best-corrected recognition acuity and acuity for orthogonal gratings. Spectacles are provided to all 3-year-old children with > or =2.00 diopters (D) of astigmatism and all 4- and 5-year-old children with > or =1.50 D of astigmatism. Persistence of amblyopia after glasses wearing is evaluated by follow-up measurement of best-corrected recognition acuity and acuity for orthogonal gratings, conducted 2-5 months after glasses are prescribed. Long-term effectiveness of early screening and glasses prescription is evaluated through measurement of recognition acuity in two groups of first-grade children: one group who participated in the Head Start program before the intensive vision screening program was initiated, and a second group who participated in the study's Head Start vision screening program.