Glaucoma is a chronic progressive disease that, left untreated, can lead to blindness. Glaucoma represents a range of conditions in which the flow of aqueous humour in the eye is blocked, causing an increase in intraocular pressure (IOP). An impaired blood flow to the optic disc may also cause glaucoma, even without an abnormally high IOP. Even with careful monitoring and lowering of IOP, approximately 25% of patients lose their sight.1 Consequently, glaucoma is the second most common cause of blindness worldwide.
The global prevalence of glaucoma was estimated to be 67 million in 2001.2 Approximately 13% of these were estimated to be in Europe, of which half were undiagnosed and untreated. The most prevalent type of glaucoma, primary open-angle glaucoma (POAG), has subtle symptoms early in its course and, consequently, often remains undetected. Late diagnosis of glaucoma significantly increases the cost of treatment and raises the risk of visual impairment.3 Historically, an increased level of IOP was presumed to be the most accurate means of diagnosis; however, it is now recognised that high IOP is a risk factor for glaucoma, but is not an accurate diagnostic. Therefore, there is a need for an accurate diagnosis system.
Glaucoma Diagnosis – The ‘Gold Standard’ Tests
In glaucoma, structural changes usually occur first and are followed by functional deficits. There are three well-established ‘gold standard’ tests used by ophthalmologists to diagnose glaucoma: IOP measurements, visual field tests and (stereoscopic) assessments of the optic nerve. Together, these methods provide information on both structural and functional defects.
A gradual increase in IOP has long been recognised as the major risk factor for glaucoma, and the lowering of IOP serves to impede progression of optic nerve damage. Therefore, IOP measurements are usually documented over time.4 The Goldmann applanation tonometer is the standard device used by physicians to detect alterations in IOP. The device can accurately measure IOP in the eye with a small deviation of 0.5mmHg.5
Although the device is fairly accurate, the measurement of IOP is not always a precise indication of glaucoma. Individually, IOP is highly variable, and even low IOP cannot be ruled out for risk of glaucoma. Additionally, IOP does not indicate the extent of damage or, indeed, that damage has actually been done to the optic nerve. Thus, in the diagnosis of glaucoma this variable can be used only alongside other evidence for a positive outcome.
Peripheral vision is usually the first to deteriorate in glaucoma; hence, tests of the visual field have been used to diagnose the disease. Perimetry is a systematic measurement of light sensitivity in the visual field by the detection of targets presented on a defined background. The standard diagnostic tool for visual field examination is the computerised field analyser. Responses are statistically analysed and compared with a database of normal responses. Even with this comparison it is challenging to state definitively whether a patient has glaucoma. Previously, for detecting any progression the test results were judged by comparing print-outs from visual field tests, which was time-consuming and often inaccurate. Today, new software provides ophthalmologists with automated visual-field-progression analysis; however, it may take three examinations before an accurate baseline is obtained. Also, long-term fluctuations in the field tests can often occur, thus the accuracy of this method of diagnosis is still in question.