The clinical significance of fundus magnification produced during direct ophthalmoscopy of the corrected eye has not been fully established. Based on paraxial ray tracing, fundus magnification (M) can be defined by a simple equation, M = (K'/4) x (Fs/K), where K' is the dioptric axial power of the eye, Fs is the correcting thin lens power and K is the ocular ametropia. Refractive myopes produce greater fundus magnification than axial myopes, whereas refractive hyperopes produce lower fundus magnification than axial hyperopes. If we assume 15 x fundus magnification as our standard magnification for an emmetropic reduced eye, then wearing glasses or putting the focusing lens at or close to the anterior focus of the eye is able to achieve the standard magnification for axial myope and axial hyperope, whereas wearing contact lenses is able to achieve the standard magnification for refractive myope and refractive hyperope. Vertex distance has greater influence on fundus magnification produced during direct ophthalmoscopy than other funduscopic techniques. In conclusion, the newly defined formula has clinical applications during direct ophthalmoscopy.
Studies of optical defocus on refractive development and ocular growth in animals are presented and discussed in relation to the accommodation hypothesis. None of these studies fully support the accommodation hypothesis. The problems encountered in these studies are also discussed.
The effect of myopic defocus on myopia progression was assessed in a two-year prospective study on 94 myopes aged 9-14 years, randomly allocated to an undercorrected group or a fully corrected control group. The 47 experimental subjects were blurred by approximately +0.75 D (blurring VA to 6/12), while the controls were fully corrected. Undercorrection produced more rapid myopia progression and axial elongation (ANOVA, F(1,374)=14.32, p<0.01). Contrary to animal studies, myopic defocus speeds up myopia development in already myopic humans. Myopia could be caused by a failure to detect the direction of defocus rather than by a mechanism exhibiting a zero-point error.
We performed a vision screening of 1883 Chinese schoolchildren from 4 schools around Kuala Lumpur in June 1990. The group contained 1083-males and 800 females. Visual acuity, refractive error, oculomotor balance, and axial length were measured. The prevalence of myopia in Chinese schoolchildren was found to be 37% in the 6- to 12-year age group and 50% in the 13- to 18-year age group. Approximately 63% of the sample had unaided visual acuity of 6/6 or better and 24% had unaided acuity of 6/12 or worse. Six hundred twenty-five students (33%) failed the vision screening test and were referred for further examinations. The group which failed the vision screening test and had the highest rate of referral (46%) was the 11- to 12-year-old age group. The most common visual disorder was uncorrected myopia, accounting for 38% of the referrals (235 students). Only 26% of the sample were wearing a spectacle correction.
A cross-sectional study was undertaken to determine and compare the refractive status of premature children without retinopathy of prematurity (ROP) and full term children below the age of three years.