The study design was a cross-sectional study. The sampling and recruitment methods for this study have been described in details in our previous papers [6
]. Sample size for the study was calculated with 99% confidence, within an error bound of 5%. The sample size requirement was 661, as calculated by:
where Z = 2.57 for 99% confidence interval and d = 0.05 for 5% error bound. After allowing for an arbitrary 50% increase in sample size to accommodate possible inefficiencies associated with the sample design, the sample size requirement increased to 991 subjects [6
]. We decided to define an older adult as person aged ≥35 years because in our previous reports conducted on young males in the military population, we considered young adults as persons aged 18–34 years [29
]. We used simple systematic sampling to select our study population. In total 14,110 outpatients were examined in the Department of Ophthalmology and Visual Rehabilitation of the Medical University of Lodz in year 2012 and we included into the study every tenth subject aged 35 years and older [28
]. Based on age, the study subjects were divided into two groups; group I, aged 35–59 years, and group II, aged 60 years and older. All participants were interviewed and information regarding brief details of the eye conditions, age, sex and socioeconomic status was collected. Comprehensive ophthalmic examination included: distance visual acuity (VA) testing, a cover test, binocular and color vision assessments, intraocular pressure (IOP) measurements as well as slit lamp and indirect ophthalmoscopic evaluation of the anterior and posterior segments and other examinations where needed. Distance visual acuity (VA) was tested monocularly, using a retroilluminated Snellen chart placed at 4 m. Because the present study was a continuation of our previous reports as mentioned earlier, we used the methodology of refraction measurements and definitions of refractive errors from the Polish Army national regulations for ophthalmic examination [29
]. Autorefraction data were obtained in all study subjects using a Topcon KR 8900 autorefractometer (supplied by Topcon Corporation, Tokyo, Japan). Cycloplegic refraction data were obtained only in eyes presenting with distance visual acuity <20/40 Snellen (0.3 logMAR). Based on this, subjective refraction tests using only spherical or cylindrical glasses were performed to achieve best corrected visual acuity (BCVA). Spherical equivalent (SER) refractive error, defined as sphere plus half cylinder, was applied for myopia and hyperopia calculations. According to the Polish Army regulations myopia was defined as spherical equivalent (SER) refraction ≤−0.5 dioptres (D), hyperopia was defined as SER ≥+0.5 dioptres (D) and emmetropia as SER between −0.5 and +0.5 diopters (D). Astigmatism was considered if the cylinder was ≥0.5 dioptres [29
]. Anisometropia was defined as difference of SER greater than 1.0 dioptres (D) between the right and the left eyes. The distribution of refractive errors was presented binocularly. If the study subject had one myopic and a fellow hyperopic eye, the refractive error of the eye with larger spherical equivalent was taken into account. Eyes with previous history of cataract surgery, which underwent corneal transplantation and with ocular conditions which precluded autorefraction measurements were excluded from statistical analysis.
For this report the presence of cataract, aphakia or pseudophakia was determined on the slit lamp examination. Glaucoma was diagnosed when characteristic morphological changes of the optic nerve head and retinal nerve fiber layer (RNFL) not related to other ocular disease or congenital anomalies were present, associated with typical glaucomatous visual filed loss. The ocular hypertension (OHT) was diagnosed if the intraocular pressure was elevated with all other ocular findings within normal limits [28
]. In a few subjects with large media opacities, when results of optic nerve head examinations and visual field were unavailable, glaucoma was diagnosed basing on previous evidence of glaucoma treatment.
Because of the nature of the survey, verbal informed consent was obtained from all study participants. The institutional review board waived the need for written informed consent from the participants, but otherwise the work was conducted in accordance with the provisions of the Declaration of Helsinki for research involving human subjects and was approved by the ethic committee of the Medical University of Lodz (Ethical Approval Code RNN/848/12/KB).