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Numbers help tell the story of glaucoma diagnosis, treatment
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In general, if we can put a number on something, it is easier to understand and interpret. Advanced diagnostics along with older well-established techniques are providing us more numbers, and especially more reliable numbers, to better diagnose and treat glaucoma.
IOP is a number we have followed for more than a century. Yet, we need better numbers even for this old reliable marker. Ideally, we need to know the patient’s IOP at least every hour 24 hours a day, 7 days a week, 365 days a year to fully understand an individual’s disease, response to therapy and adequacy of treatment. In addition, it would be extremely helpful to know at least the patient’s blood pressure, systolic and diastolic, 24/7, as they say. Even better, an exact measurement of blood flow at the optic nerve, along with the pO2 to measure for things such as sleep apnea, would be great.
We are on our way to accessing these diagnostics from many sources, including perhaps the exciting collaboration between Google and Alcon/Novartis to develop electronic contact and IOLs with diagnostic capability. Visual fields provide us with pattern recognition opportunities such as Seidel and Bjerrum scotomas and nasal steps. But as we go deeper, measures such as mean standard deviation and pattern standard deviation to evaluate decibel sensitivity are powerful tools.
As our visual field instruments advance, we can hope for an ability to measure the threshold sensitivity of any spot on the retina to any desired color and intensity of light stimulus. Perhaps the variability in patient cooperation and fatigue can be circumvented some day with potentially objective tests such as an ERG.
In regard to imaging, we have advanced from GDx, which gave us useful numbers, to ever more sensitive OCT. The most advanced OCT can now resolve individual cells and provide 3-D reconstructions of ocular tissues. Vertical cup, horizontal cup, area and even volume are useful parameters. Retinal nerve fiber layer thickness can be measured objectively and followed for change. We might even imagine the possibility of counting the number of ganglion cells in an area and looking for change over time. It is also possible to measure metabolic activity of cells and look for stress.
The ultimate goal is to measure both structure and function at a cellular or subcellular level, looking at organelles such as mitochondria. The more I can put a number on things and also understand the sensitivity and specificity of that number in helping me make a diagnosis of glaucoma or evaluate the efficacy of my treatment, the happier I will be. We will still need to be good observers and seasoned clinicians because a number will never recognize a disc hemorrhage or subtle notch in a cup. Clinician judgment and a full understanding of each patient as an individual will never be reduced to a number or equation. Still, the numbers we have to evaluate when diagnosing and treating glaucoma, as well as their reliability, sensitivity and specificity, are advancing rapidly to the great benefit of clinicians and patients.