All eligible individuals were invited and scheduled for a detailed eye examination and questionnaire interview, which were performed in a standardized manner at the Handan Eye Study (HES) centralized clinic (located in the Yongnian County Hospital). The examination included visual acuity, autorefraction, and subjective refraction; questionnaires, anterior segment examination with a slit lamp; intraocular pressure (IOP), gonioscopy, dilated examination, and photography of the fundus including sequential stereoscopic optic nerve photographs, and Heidelberg retinal tomography. Other tests carried out in the clinic have been described in detail previously.
11 Subjects who did not present for a complete examination at the central clinic had all the tests performed at a clinic that was set-up in the local community for this purpose.
IOP was measured using Perkins handheld applanation tonometry (HA-2; Kowa, Tokyo, Japan) after topical anesthesia. IOP was measured twice, and a third measurement was performed if the difference between two measurements was > 2 mm Hg with the mean of the two closest results used as the IOP measurement. The tonometer was calibrated daily before examination.
Central corneal thickness (CCT) was obtained using ultrasound pachymetry (UP1000; Nidek, Inc., Tokyo, Japan), after instillation of 2 drops of 0.5% proparacaine for topical anesthesia (Alcon Laboratories, Inc., Fort Worth, TX). The pachymeter probe was placed on the center of the cornea, and the average of five readings was used. Axial length, anterior chamber depth, and lens thickness were measured using a 10 MHz A/B-mode ultrasound device (Cine Scan, Quantel Medical, Cedex, France), using a hard-tipped, corneal contact probe mounted on a slit-lamp. The optic disc was imaged with the a retinal tomography instrument (Heidelberg Retina Tomograph; HRT II; Heidelberg Engineering, Heidelberg, Germany) after cylinder lens correction was determined without pupil dilation. When the image quality was poor (SD > 30 μm), a repeat image was obtained after pupil dilation. All disc contours were drawn by a certified ophthalmologist (DZ).