In March 2005, the U.S. Preventive Services Task Force (USPSTF) found insufficient evidence to recommend for or against screening adults for glaucoma.1The recommendation and associated evidence review2 updated the 1996 recommendation on this topic, for which the USPSTF also concluded that there was insufficient evidence for a general recommendation.
Here is what the evidence tells us about glaucoma, screening for this condition, and treatment: (1) glaucoma is an important cause of visual impairment in older patients; (2) screening with formal visual field testing and direct ophthalmoscopy accurately identifies persons with primary open-angle glaucoma, whereas measurement of intraocular pressure is not a good screening tool for glaucoma because 25 to 50 percent of patients with glaucoma will have normal intraocular pressure, and many patients with increased intraocular pressure will not develop glaucoma; (3) treating persons with primary open-angle glaucoma results in fewer patients with small visual defects as measured by specialized visual field testing; and (4) treatments such as laser therapy may lead to potential harms, including cataract formation, whereas medical treatments may lead to ocular dryness, tearing, itching, and, rarely, psychiatric harm such as depression.2
In the face of all we know about glaucoma, what led the USPSTF to conclude there is insufficient evidence to recommend for or against screening? There are no studies that make it possible to extrapolate the relevance of the measured benefit of treatment. The level I recommendation was based on the lack of evidence that early detection through screening and early treatment leads to meaningful health improvements for patients, such as improved quality of life or better function related to vision.
In 1996, the USPSTF concluded that there was insufficient evidence to recommend for or against routine screening for glaucoma based on a lack of evidence that early treatment is effective in improving vision-related outcomes. At that time, the USPSTF stated that a controlled trial was needed to compare vision-related outcomes in treated and untreated groups. Since 1996, three studies have provided evidence about the effect of early treatment on intermediate outcomes in persons with increased intraocular pressure or early primary open-angle glaucoma: the Ocular Hypertension Treatment Study,3 Collaborative Normal-Tension Glaucoma Study,4 and Early Manifest Glaucoma Trial.5 Two of the three trials3,5 showed that early treatment results in reduced progression of visual field defects; one trial4 showed no difference. Although the evidence shows that these primary treatments for increased intraocular pressure or early glaucoma reduce development and progression of small visual field cuts (i.e., intermediate outcomes), the different methods used to define visual field progression in the three trials lacked a consistent reference standard. The practical implications of these studies may be modest, because the tests used to identify visual field cuts are sensitive to small changes in visual field defects. More importantly, no studies have linked prevention of these small visual field cuts to clinically meaningful outcomes in patients (e.g., improvement in vision-related function). It is uncertain whether small reductions in visual field cut progression would, in the long term, lead to important reductions in vision-related function. For these reasons, the USPSTF could not determine the magnitude of benefits of screening adults for glaucoma.
Glaucoma screening has become standard practice in older adults. So what does the USPSTF recommendation mean for family physicians? It does not mean that screening and earlier treatment for glaucoma produce no benefits. Patients most likely to benefit are those at greatest risk for the disease (e.g., older adults, blacks, and patients with a family history of glaucoma). In the absence of clear evidence showing an important benefit of screening, the decision about whether to screen for glaucoma is left to the individual physician and patient . A definitive answer will require future research standardizing measurement of visual field defects and correlating them to visual impairment. Until research is completed, family physicians must make decisions based on imperfect science. With limited time and resources, it is critical for family physicians to first offer and provide preventive services for which there is evidence of benefit; these services correspond to grade A and B recommendations from the USPSTF.