A LONGITUDINAL INVESTIGATION OF ADULT-ONSET AND ADULT-PROGRESSION OF MYOPIA IN AN OCCUPATIONAL GROUP - REFRACTIVE AND BIOMETRIC FINDINGS

Purpose. To investigate the refractive and biometric changes associate d with adult-onset and adult-progression of myopia in an occupational group. Methods. The sample population consisted of 251 clinical micros copists aged 21 to 63 years. Subjects had their refraction and ocular dimensions measured on four occasions during a 2-year period, and a to tal of 166 subjects (332 eyes) completed the longitudinal aspect of th e study. Refraction was measured objectively with a Canon R-l autorefr actor and subjectively by an optometrist using standard procedures. Co rneal curvature and axial ocular dimensions were measured with a kerat ometer and A-scan ultrasonography, respectively. Results. Of eyes emme tropic at the start of the study, a total of 39% underwent a myopic ch ange in refraction greater than 0.37 diopter (D), with a mean change o f -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 di d not undergo a refractive change > 0.37D (n = 58) during the 2-year s tudy 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 +/- 0.0 2 mm. Changes in corneal curvature, anterior chamber depth, or lens th ickness 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% progr essed further into myopia by 0.37 D or more during the 2-year period. The mean increase in myopia for the ''myopic progressor'' group was 0. 77 +/- 0.03 D (n = 108 eyes) compared to -0.01 +/- 0.02 D (n = 115 eye s; 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 init ially 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 th e 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 whic h adult-onset myopia developed or emmetropic eyes that remained refrac tively stable. The incidence of adult myopia development during a 2-ye ar period in this occupational group was 45%. Conclusions. The structu ral cause of adult-onset and adult-progression of myopia is vitreous c hamber elongation.