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B.A. Francis, M.–Y. Lai, S.P. Azen, V. Chopra, R. Varma; Relationship of the Prevalence of Open–Angle Glaucoma and Intraocular Pressure and the Effects of Adjusting for Central Corneal Thickness: The Los Angeles Latino Eye Study (LALES) . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2345.
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To examine the relationship between prevalence of open angle glaucoma (OAG) and intraocular pressure (IOP) before and after adjusting for and stratifying by central corneal thickness (CCT).
LALES is a population–based study of eye disease in adult Latinos. Participants completed an eye exam with grading of visual fields (Humphrey SITA Standard) and optic nerves (stereoscopic disc photos) to determine OAG prevalence. Those with current IOP lowering therapy or previous glaucoma surgery were excluded. The prevalence of OAG was calculated for each Goldmann IOP value, and IOP adjusted for CCT effect using three correction algorithms (± 1, 2.5 or 3.5 mmHg to Goldmann IOP per 50µ CCT below or above 545µ). This is done linearly by the formula: Corrected IOP = Measured IOP – (CCT – 545)/50 x [X] mmHg. The data then underwent natural log transformation and linear regression analysis was performed. The final comparison of the correlation coefficients (R2) after transformation indicates the degree to which each plot fits an exponential curve. Glaucoma prevalence was also plotted against unadjusted IOP while stratifying by CCT (<510, 511–579, >580).
The overall prevalence of undiagnosed and untreated glaucoma was 3.88% (232/5972). The frequency of glaucoma for different IOP strata were: <15mmHg=2.2%, 15–21=5.1%, 22–29=18.2%, and ≥30=69.2%. The mean IOP in persons with and without OAG was 17.2 mmHg (95% CI 16.5, 17.9) and 14.3 (14.2, 14.4), respectively. The R2 for the relationship between IOPs and OAG prevalence were 0.89, 0.89, 0.93 and 0.94 in order of magnitude of correction (none, 1, 2.5, 3.5). All four plots exhibited an exponential increase in OAG prevalence beginning at IOP=22 mmHg. At this same point, there is a divergence between the adjusted and non–adjusted curves as those with a greater magnitude of correction rise more steeply as expected for an exponential increase. When stratified by CCT, there was a stepwise increase in the slopes of glaucoma prevalence as a function of IOP as CCT went from thick to normal to thin.
OAG prevalence in LALES shows an exponential relationship to IOP, beginning to rise at IOP ≥22 mmHg. Adjusting IOP for CCT using three algorithms more closely approximated an ideal exponential curve, indicating that CCT adjusted IOP may provide a better assessment of the relationship between applanation IOP and OAG prevalence. Furthermore, CCT showed a clear effect on the prevalence of OAG as a function of unadjusted IOP.
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