Subjects for the BIBS were recruited from several sources, including advertisements in diaper service newsletters, word-of-mouth, and letters sent to parents of newborns identified from birth records in Contra Costa County, California. Parents provided written informed consent according to the tenets of the Declaration of Helsinki after all procedures were explained. The BIBS protocol was reviewed and approved by the Institutional Review Boards for The Ohio State University, the University of California, Berkeley, and the State of California. Inclusion criteria were both genders, all refractive errors (including emmetropia), birthweight over 2500 g, and confirmation that the infant was under the general care of a pediatrician. Existing strabismus was allowed, although no infant entered the study with strabismus. Infants were excluded if they had a history of difficulty with pupil dilation, a history of cardiac, liver, asthma, or other respiratory disease, or a history of ocular disease or active ocular inflammation. This report analyzes 222 (53% female) of 302 subjects with examination data for both the 3-month (±1 month) and the 9-month (±1 month) visit. Subjects were excluded from analysis because of age outside the visit window (
n = 60), losses to follow-up (
n = 15), inability to test (
n = 3), low birthweight (
n = 1), and incorrect cycloplegia (tropicamide instead of cyclopentolate (
n = 1). The 15 subjects lost to follow-up were not significantly different from the sample as a whole in initial refractive error, axial length, corneal or lenticular power, gender, family income, or any ocular component measure. Ethnicity was unknown in 13 of the 15. Subject participation by parent-reported ethnicity and family income is given in
Table 1 .
Refractive error was measured by two observers performing retinoscopy 25 minutes after instillation of 1 drop of proparacaine 0.5%, followed by 2 drops of cyclopentolate 1.0% in each eye with 5 minutes between cyclopentolate drops. The second examiner’s measurement was made without knowledge of the first examiner’s findings. Refractive error is reported as the average of the two examiners’ retinoscopy results for the cycloplegic spherical equivalent refractive error of the right eye. The 95% limits of agreement between the two examiners have been reported as −0.71 to +0.98 D for spherical equivalent.
9 Changes in refractive error and ocular components were defined as the values at 9 months of age minus the values at 3 months.
All reported biometric measurements were performed on the right eye only. Keratometry and phakometry were performed with a custom, hand-held, video-based phakometer described in detail elsewhere.
7 An equivalent refractive index and radii of curvature for the crystalline lens were determined by using an iterative procedure that produced agreement between the measured refractive error and that calculated from ocular component values.
10 This hand-held method produces measurements comparable to the conventional slit lamp mounted phakometer used in a large-scale longitudinal study of school-aged children.
10 The average difference between the two techniques was 0.03 ± 0.22 D in a validation study of 35 6-year-old children. The lens and corneal dimensions analyzed were the averages of the two meridians. Ocular axial dimensions were measured with an A-scan ultrasound (model 820; Carl Zeiss Meditec, Dublin, CA). Measurements were taken through the closed eyelid in semiautomatic mode, with the “dense cataract” setting at 100% gain. This method has been shown to produce results comparable to the standard corneal contact technique.
11 12 One comparison found a 0.05-mm difference in axial length,
12 whereas another found that the through-the-lid technique resulted in thicker lenses by 0.12 mm and longer vitreous chambers by 0.18 mm.
11 Any small bias present in the technique would be expected to cancel out as differences between examinations were taken to calculate longitudinal change. The repeatability (95% limits of agreement) of the through-the-lid measurement technique on adults was ±0.32 mm,
12 similar to that with the corneal contact technique.
13 The repeatability between two examinations of infants 3 to 7 months in age was ±6.22 D for lenticular power and ±0.84 mm for vitreous chamber depth,
14 roughly twice that in children.
15 Given these estimates for repeatability, a sample size of 210 was calculated to provide power of at least 0.90 to find differences of 0.13 mm in axial growth and 1 D in change in lenticular power between hyperopes above compared with below the upper tertile for refractive error at 3 months (+2.50 D).
Data were transmitted to the Optometry Coordinating Center at The Ohio State University for dual data entry. The Optometry Coordinating Center verified that all forms were accounted for. A computer running commercial software (SAS ver. 8.0; SAS Institute, Cary, NC) was used for verification of ranges and missing information, as well as for data analysis. Regression analyses were used to assess the relationship between ocular components (SAS JMP, ver. 3.1.5; SAS Institute). Paired Student’s
t-tests were used to compare mean refractive errors between examinations. The correlated variances for refractive error were compared using the method described by Cox and Hinkley.
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