Purpose: To investigate the refractive and biometric changes associated with adult-onset and adult-progression of myopia in an occupational group.
Methods: The sample population consisted of 251 clinical microscopists aged 21 to 63 years. Subjects had their refraction and ocular dimensions measured on four occasions during a 2-year period, and a total of 166 subjects (332 eyes) completed the longitudinal aspect of the study. Refraction was measured objectively with a Canon R-1 autorefractor and subjectively by an optometrist using standard procedures. Corneal curvature and axial ocular dimensions were measured with a keratometer and A-scan ultrasonography, respectively.
Results: Of eyes emmetropic at the start of the study, a total of 39% underwent a myopic change in refraction greater than 0.37 diopter (D), with a mean change of -0.58 +/- 0.04 D (mean +/- standard error of the mean; n = 37). This was associated with an elongation of the vitreous chamber of 0.26 +/- 0.05 mm (P < 0.01). Eyes emmetropic at the start of the study that did not undergo a refractive change > 0.37 D (n = 58) during the 2-year study period had a mean change in refraction of 0.02 +/- 0.03 D (P = 0.69) associated with a change in vitreous chamber depth of 0.05 +/- 0.02 mm. Changes in corneal curvature, anterior chamber depth, or lens thickness between the initially emmetropic groups were not significant. The median age of onset of myopia in initially emmetropic eyes was 26.3 years. Of eyes that were myopic at the start of the study, 48% progressed further into myopia by 0.37 D or more during the 2-year period. The mean increase in myopia for the "myopia progressor" group was 0.77 +/- 0.03 D (n = 108 eyes) compared to -0.01 +/- 0.02 D (n = 115 eyes; P = 0.49) for myopes who did not undergo a refractive change > 0.37 D during the study period. The only significant difference in ocular component dimension changes during the study period for these two initially myopic groups was elongation of the vitreous chamber depth (0.24 +/- 0.04 mm versus 0.03 +/- 0.03 mm, P < 0.01). The average age of the myopes who progressed further into myopia during the study was 29.3 years. Axial length-corneal radius ratio at the start of the study was not significantly different between initially emmetropic eyes in which adult onset myopia developed or emmetropic eyes that remained refractively stable. The incidence of adult myopia development during a 2-year period in this occupational group was 45%.
Conclusions: The structural cause of adult-onset and adult-progression of myopia is vitreous chamber elongation.