Study participants were examined according to a prescheduled date established at the time of enumeration. Identity of the subjects was verified using the subjects' official photo identity cards. A standard questionnaire was administered by a trained interviewer to collect details of ophthalmic history, general medical history, income, and education. Distance visual acuity (VA) was measured employing an Early Treatment Diabetic Retinopathy Study logMAR E-chart (Precision Vision, Villa Park, IL) with a standard illumination box. VA measurement began at a distance of 4 m with the top line (20/200). If the orientation of at least four of the five optotypes was correctly identified, the subject was then tested by dropping down to line 4 (20/100) line, to line 7 (20/50), to line 10 (20/25), and finally to line 11 (20/20). If the individual failed to identify the top line at 4 m, the subject was advanced to 2 m and then to 1 m, progressing down the chart as described earlier. The lowest line read successfully was assigned as the VA for the eye. Testing for counting fingers, hand movement, light perception, or no light perception was checked on those who unable to identify any optotypes on the chart. The VA was measured in each eye initially without refractive correction or with distance glasses if worn. All the participants with a presenting VA (PVA) ≤20/25 in either eye received subjective refractive measurements and the best-corrected VA (BCVA) was noted. Subjective refraction was performed by a trained optometrist for those subjects, and autorefractor (RM-8000; Topcon, Tokyo, Japan) readings were used as the starting point for subjective refraction carried out without cycloplegia. For those in whom subjective refraction was not performed, particularly the elderly examined in their homes, BCVA was assumed to be the same as pinhole vision. Intraocular pressure (IOP) was measured by a study ophthalmolgist using a handheld tonometer (Tono-Pen AVIA; Reichert Inc., Depew, NY) device after instilling topical anesthesia (0.4% Benoxil [oxybuprocaine]; Santen Pharmaceuticals, Osaka, Japan). Goldmann applanation tonometry was performed on an optional basis for all glaucoma suspects. The detailed examination of the eyelid, globe, pupillary reflex, lens, and fundi was carried out by an experienced ophthalmologist using a slitlamp (model SL-1E; Topcon), a +90-diopter (D) lens at ×16 magnification and direct ophthalmoscopy. The participants were examined at local community facilities; those who failed to come to the examination site were examined in their home using portable equipment.
Patients with BCVA ≤20/40 had their pupils dilated for examination. Similarly, if participants' lens and fundus status could not be examined satisfactorily, their pupils were also dilated. Individuals with shallow anterior chambers did not have their pupils dilated. Eyes presenting with PVA ≤20/40 were assigned a principal cause of visual impairment by the same experienced oculist. Refractive error was considered the cause of visual impairment for those with a VA ≤20/40 that could be subsequently improved to >20/40 after correction. Cataract was regarded as the main cause of visual impairment if there was no evidence of retinal abnormality in an eye with significant cataract that obscured the vision. Myopic maculopathy was considered only in those with a refractive error exceeding −6.0 D in either eye, in conjunction with one or more of the following ophthalmologic findings: tessellated fundus with yellowish white diffuse or grayish white patchy chorioretinal atrophy, macular hemorrhage, or posterior staphyloma.
12 Age-related macular degeneration (AMD) as a reason for visual impairment was characterized by soft drusen of the retinal pigment epithelium, subfoveal hemorrhage, subretinal and intraretinal edema without any retinal reason detected for it, or a subfoveal disciform scar. Glaucoma was defined according to the International Society for Geographical and Epidemiological Ophthalmology Classification.
13 Diabetic retinopathy was present if the macula showed cystoid macular edema, hard exudates, intraretinal hemorrhages, and microaneurysms. A self-reported diagnosis of diabetes mellitus was a relatively important but not necessary part of the diagnosis of diabetic retinopathy. Other causes for visual impairment were determined according to clinical routine diagnosis.