A total of 270 subjects with a mean age of 8.70 ± 1.51 years (range: 6.15–14.05 years) had complete data for all measurements and were included in analysis. The subjects were 43.0% female. Boys and girls had a mean axial length of 23.46 ± 0.82 and 22.83 ± 0.80 mm, respectively. Additionally, boys and girls had a mean refractive error of 0.44 ± 1.28 diopters (D) and 0.40 ± 1.32 D, respectively. A total of 21 subjects had astigmatic refractive error greater than 1.00 D. Spectacle correction was worn by 51 subjects. No subjects wore contact lenses. The subjects were 78.0% non-Hispanic white, 12.9% African American, 2.7% Asian, 2.7% Hispanic, and 3.7% other. The vast majority of the subjects were healthy, with the following exceptions reported by parents: 3 cases of strabismus, 4 cases of amblyopia, 2 cases of diabetes, 58 cases of seasonal or food-related allergies, 18 cases of asthma, and 7 cases of attention-deficit hyperactivity disorder (ADHD). In one subject, the amblyopic/strabismic eye was the right eye, or the eye that was measured for this study. Because it is unknown how this factor could affect the relationship between refractive error and the ciliary muscle, this subject's data were excluded from all analyses.
The data for 269 of the 270 subjects were included in the following analyses, tables, and figures, thus providing useful models.
Table 1 summarizes ciliary muscle thickness, age, refractive error, and axial length distributions.
Tables 2 and
3 provide parameter estimates for the models of the relationship between each CMT measurement and spherical equivalent refractive error. Across all models, sex was not statistically significant; this indicated that a correction of the intercept term for sex was not necessary. Values of CMT did, however, increase with age at all locations (all
P ≤ 0.01). In the anterior region of the ciliary muscle (CMTMAX and CMT1) the relationship between CMT and spherical equivalent refractive error was quadratic (
Table 2: CMTMAX:
P = 0.005 and CMT1:
P = 0.0003). Thus, the ciliary muscle is thinner at CMTMAX and CMT1 with larger values of both hyperopia and myopia, which is depicted in
Figure 2, where the curve of the best-fit model shows the maximum ciliary muscle thickness occurring at low-to-moderate amounts of myopia.
Figure 2 was unadjusted for sex and age.
The relationship between spherical equivalent refractive error and ciliary muscle thickness in the posterior region of the muscle, however, was linear (
Table 3: CMT2:
P = 0.0008; CMT 3:
P = 0.007). For both CMT2 and CMT3, the slope was negative, which indicated that a thicker muscle was associated with more negative spherical equivalent refractive error. This relationship is depicted in
Figure 3, where the best-fit model shows the maximum ciliary muscle thickness occurring at the highest levels of myopia.
Figure 3 was also unadjusted for sex and age.
Table 4 provides parameter estimates for the fitted models of the thickness of apical fibers at CMT 1 (CMT1 − CMT2) and the apical fibers at CMTMAX (CMTMAX − CMT2), as a function of spherical equivalent refractive error. Age was statistically significant, with apical fiber thickness decreasing with age. For the apical fibers at CMTMAX, sex was statistically significant (
P = 0.003), with thickness greater in females. Apical fibers had a statistically significant linear relationship with spherical equivalent refractive error both at CMT1 (
P < 0.0001) and at CMTMAX (
P < 0.0001). Overall, these results indicated that thicker apical ciliary muscle fibers were associated with higher amounts of hyperopia.
Figure 4 provides the modeled projections of the apical fibers at CMTMAX and CMT1 as a function of spherical equivalent refractive error.
Figure 4 was also unadjusted for sex and age.