GUIDELINES FOR GLAUCOMA DIAGNOSIS

Despite the availability of treatments which can delay or prevent the progression of glaucomatous optic neuropathy, patients still go blind from the disease because of inaccurate diagnoses and incorrect treatment, said George Spaeth MD, at the 10th European Glaucoma Society Congress. “An accurate diagnosis is important, but it is not achieved by extensive testing; it is achieved from accurate testing with minimal bias,†said Dr Spaeth, Wills Eye Institute, Philadelphia, Pennsylvania, US.

He noted that of the patients referred to him, almost half of them have been overtreated, about a quarter of them have been under-treated, others are inappropriately treated and only a very small percentage appear to be getting correct treatment. There are a range of factors that can lead to misdiagnoses and incorrect treatment, he said. Such factors include drawing conclusions from indirect evidence, failure to perform appropriate examinations and unconscious bias. Tonometry is an example of an indirect measurement that can lead to false-positive and false-negative diagnoses of glaucoma, he noted. Some ophthalmologists continue to regard IOPs in excess of 20.0 mmHg and 24.0 mmHg as diagnostic of the disease. Conversely, some will fail to make the diagnosis of glaucoma in patients with loss of vision because their IOP is in the lower ranges.

Failure to look at the optic disc or evaluate it properly also gives rise to a high proportion of misdiagnoses, Dr Spaeth said. He cited a study conducted in the US that showed that there was mention of the optic disc in only half of the glaucoma patients’ charts surveyed. As an example of a false negative diagnosis based on IOP, Dr Spaeth described the case of a man referred to him by his ophthalmologist because he complained of failing vision. The patient was in general good health and his pressure was low, at 13.0 mmHg. However, one look at the patient’s optic disc was enough for an unequivocal diagnosis of glaucoma, Dr Spaeth said. “Had his ophthalmologist looked at the disc without paying attention to pressure, he would have seen this much earlier. As to where he is on the glaucomatous neuropathy scale, he is way down in the disabled area already. He is in serious trouble,†he added.

Cup/disc ratios can be misleading

Another common mistake is to use large cup/disc ratios as being diagnostic of glaucoma at initial examination. However, it is only when cup/disc ratios are about 0.8 or 0.9 at initial presentation that they have any diagnostic value at all, Dr Spaeth said. Otherwise, it is only when the ratio increases over time that it becomes indicative of glaucomatous optic neuropathy, he added. Similarly, a large optic disc size may mislead the physician to believe that glaucoma is present, Dr Spaeth said. He described the case of a 63-year-old engineer who was in good health, had no symptoms and was in doubt over his ophthalmologist’s recommendation for surgery. When Dr Spaeth looked at his optic discs he saw that while they were very large, they were symmetrical and lacking in pathological features. “He has no visual field loss, he does not have glaucoma, he just has huge disks. Regarding his level of disability, he is way up in the normal patients range,†Dr Spaeth said.

Another case he cited was that of a 64-year-old beautician who was very unhappy because the drops she was receiving for glaucoma were making her eyes uncomfortable. She came to Dr Spaeth for a second opinion. Her IOP was 26.0 mmHg in one eye and 22.0 mmHg in the other eye, he noted. However, although her optic discs had some asymmetry of size and cupping, the cupping was greatest in the eye with the lowest pressure. “Not only does she not need surgery but she probably doesn't need any treatment at all,†he added.

Unconscious bias

Another factor that can lead to the inaccurate diagnosis of glaucoma is a failure to eliminate unconscious bias. Research has shown that trained observers are three times more likely to say a disc photograph was abnormal if they were previously shown an abnormal field testing results and told they belonged to the same patient than if they were shown a normal field. Similarly, research has also shown that trained observers are three times more likely to say a disc has changed for the worse if they believe that the second photograph was taken after the first. “That biases you, because you know that glaucoma tends to get worse. So if you want to compare photographs without bias, don’t tell the person making the judgment which one is first which is second. You have to look at them masked. Knowing ancillary information leads to biased interpretations,†he said.

Rigid definitions of glaucoma based on specific features are another pitfall in diagnosing the condition, Dr Spaeth noted. He added that the digital information provided by technology such as OCT and HRT provide a deconstructive approach to diagnosis, but do not provide qualitative information. As a result, the confident interpretation of their measurements is generally only possible in about half of cases. “We can deconstruct the glaucomatous optic nerve into its individual components or we can look at a nerve and say, what does that nerve look like? We have marginalised that information, we have forgotten that it can be valid,†he added.