Glaucoma screening

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Rohan Bir Singh, M.B.B.S.[2]

Overview

The results of prior studies need to be read cautiously, however, in light of the current definition of primary open-angle glaucoma (POAG), first promulgated by the American Academy of Ophthalmology (AAO) in 1996: [15] “a multifactorial optic neuropathy” with “a characteristic acquired loss of optic nerve fibers”; the definitive characteristics of glaucoma are based on visual field loss or the appearance of the disc or retinal NFL. Early or mild glaucoma is characterized by optic nerve abnormalities with normal visual fields. [1] Thus, visual field defects are no longer part of the case definition of glaucoma. Moderate glaucoma is defined as visual field abnormalities in one hemifield, not within 5° of fixation, whereas severe glaucoma involves visual field abnormalities in both hemifields or loss within 5° of fixation. [1] By using this definition, up to 15 million persons in the United States could potentially have glaucoma presently, well above the current estimate by Prevent Blindness America (PBA) of 2.2 million. [16] PURPOSE OF THE TEST The purpose of a glaucoma-screening test is to detect and then treat the disease before it significantly reduces function. The current definition raises four important questions: (1) What are the likelihood and rate of progressive loss in persons with early glaucoma (i.e., those with only optic nerve or retinal NFL loss)? (2) Do currently available treatments slow, stop, or reverse NFL loss and the consequent loss of visual functioning? (3) At what point does NFL loss cause functional loss of significance to patients, and what degree of visual field loss (or any other physiologic or psychometric measure), if any, is required before patients notice a decrease in visual functioning or quality of life (QOL)? (4) Is treatment success compromised if the therapy is initiated later in the disease course? These essential issues address key concepts that were presupposed in prior screening efforts – namely, that even early loss adversely affects patients (or that later loss is harder to control), that treatment effectively reduces the rate of anatomic and functional loss, and that a sufficiently high number of patients progress without treatment to make screening for early stages worthwhile. Estimates of the likelihood of progression from early optic nerve loss to subsequent additional loss depend on the stage of disease. [17] [18] [19] Among untreated persons with elevated IOP in the Ocular Hypertension Treatment Study (OHTS), 9% had progression of optic nerve changes or visual field loss over 6 years. [19] Risk factors for incremental progression included an increased cup-to-disc ratio, indicating the possibility of subtle prior glaucomatous damage (early glaucoma, by definition). For those with manifest glaucoma (including early visual field loss), the Early Manifest Glaucoma Trial showed that 62% of untreated patients had worsening of their visual field over 5 years. [20] [21] On the basis of a meta-analysis of these and other data, Maier and colleagues concluded that reducing IOP “in patients with ocular hypertension or manifest glaucoma is beneficial in reducing the risk of visual field loss in the long term.” [22] The second issue has been effectively answered by OHTS, EMGT, and other studies. The EMGT reported that patients with POAG who achieved IOP reduction generally had subsequent decreased progression of visual field loss. [20] [21] Although these studies are ongoing, the evidence to date indicates that treatment can, indeed, effectively retard the rate of vision loss due to glaucoma. Our understanding of the effect of suboptimal vision on patients’ function has increased considerably in recent years. For patients with “trouble seeing,” the QOL impact is commensurate with that of several major systemic illnesses. [23] [24] [25] Yet glaucoma patients traditionally have been thought not to have noticeable vision problems until relatively late in the disease course. A prospective case-control study reported that patients with glaucoma had significantly lower general functional status than those without glaucoma, [26] although results of other studies contradict this finding. [27] [28] Notably, significant visual field loss is associated with reduced activities of daily vision [29] and elevated rates of automobile accidents. [30] Studies have shown that visual function and field loss are associated with vision-related QOL. [31] [32] [33] Hyman et al. assessed the impact of different severities of glaucoma on quality of life by comparing the mean deviation (MD) on visual field testing with mean Visual Functioning Questionnaire (VFQ) scores. [32] These investigators found that under -4.15 MD, there was no significant affect of glaucoma on quality of life. Patients with MD scores of -4.16 to -19.08 started developing VFQ deficits. These findings are important because these justify screening efforts to detect moderate glaucoma (with field loss > 4.15MD). However, further studies are necessary to justify allocating resources to screen patients with earlier glaucoma (< 4.15 MD). [32] The final question is whether early treatment is related to a better outcome over the course of the disease. A study from Olmstead County, MN, looked at the rates of glaucoma progression to blindness in patients undergoing filtration surgery. [34] One of the key findings from this study was that patients who had more severe visual field loss at presentation had an increased risk of blindness following surgery. This suggests that intervening earlier in the course of the disease, prior to the development of significant field loss, is beneficial. Considering that most patients will demonstrate disease worsening during their lifetimes despite therapy, if patients are identified earlier through screening, they may not progress to blindness or significant visual impairment. Thus, the importance, value, and timing of screening for glaucoma has become more positive, but it remains an open question, pending additional information on the functional impact of early visual loss and o