Single intraocular pressure measurement for glaucoma detection: Beijing Eye Study

Editor, The three pillars of glaucoma diagnosis have been glaucomatous abnormalities of the optic nerve head, elevated intraocular pressure (IOP) and glaucomatous visual field defects. However, as perimetric defects have been reported to occur after the glaucomatous loss of optic nerve fibres has begun, it was the purpose of the present study to evaluate in a population-based investigation how many patients with glaucomatous optic nerve damage as defined by the appearance of the optic nerve head have a single normal IOP measurement. The results may provide information about the predictive value of IOP measurements in screening examinations for glaucoma, and may hint atthe proportions of subjects with normal-pressure glaucoma and high-pressure glaucoma within the whole glaucoma group. The Beijing Eye Study, a population-based study of subjects aged ≥ 40 years in Greater Beijing, included 4439 of 5324 subjects invited to participate (response rate 83.4%). The present investigation consisted of 8637 eyes (99.3%) in 4318 (97.3%) subjects for whom readable optic disc photographs and a single IOP measurement obtained by pneumotonometry were available. Intraocular pressure was measured using a non-contact pneumotonometer (CT-60 computerized tonometer; Topcon Ltd, Tokyo, Japan) by an experienced technician. Three measurements were taken and the mean of these was used for further statistical analysis. If the measurements were > 25 mmHg, tonometry was repeated. Glaucoma was defined by the appearance of the optic nerve head (optic disc glaucoma) or by additional visual field defects (perimetric glaucoma) as has recently been described in detail (Xu et al. 2007). Absolute criteria for a glaucomatous optic disc appearance, each of which were sufficient for the diagnosis of glaucoma, were a neuroretinal rim notch (in eyes with optic cups sufficiently large for rim assessment); a localized retinal nerve fibre layer defect which could not be explained by any cause other than glaucoma; and an abnormally large cup in relation to optic disc size (Jonas et al. 1999; Xu et al. 2007). Relative criteria included a neuroretinal rim that was markedly thinner in the inferior disc region compared with the superior disc region, even if the smallest part of the neuroretinal rim was located in the temporal horizontal disc region; a diffuse decrease in the visibility of the retinal nerve fibre layer (particularly in eyes with small optic discs), if the background pigmentation of the eye allowed an assessment of the retinal nerve fibre layer and if there were no causes other than glaucoma for retinal nerve fibre layer loss; a marked diffuse thinning and/or focal thinning of the retinal arteries if there were no causes other than glaucoma for retinal vessel thinning, and an optic disc haemorrhage, if there were no other reasons for disc bleeding, such as retinal vessel occlusions. If none of the absolute glaucoma criteria were observed, at least two relative criteria had to be present, including a suspicious neuroretinal rim shape in eyes with an optic cup large enough to assess rim shape, or an optic cup in eyes with a small optic disc that would not usually show cupping. In the total study population, 212 (2.5%) eyes showed a glaucomatous appearance of the optic nerve head, of which 148 (69.8%) had IOP ≤ 21 mmHg. A total of 130 (6.5%) eyes showed a glaucomatous appearance of the optic nerve head and exhibited at least one defect in frequency doubling perimetry. Of these 130 eyes, 89 (68.5%) had an IOP measurement of ≤ 21 mmHg. In partial agreement with previous population-based studies (Mitchell et al. 1996; Dandona et al. 2000; Varma et al. 2004), the results suggest that a single IOP measurement taken in a screening examination for glaucoma may overlook about 70% of all glaucoma patients if glaucoma is defined by the appearance of the optic nerve head and if no additional parameters are taken into account. The study was supported by the National Key Laboratory Fund, Beijing, China.