Not long ago, a colleague asked me if I performed Goldmann tonometry on all of my glaucoma patients. Without hesitation I said, “No.” When asked why not, I simply answered that not all of my patients are physically able to have the test performed on them. For various reasons, some of our patients just can’t get positioned in the slit lamp.
This is the same reason why I do not perform threshold visual field studies on all of my glaucoma patients since some patients are either physically or cognitively (or both) unable to complete the test, let alone with any sense of tangible reliability. For example, I am reminded of a patient who comes in twice a year on a stretcher for me to check her diabetic retinopathy. I simply complete the entire examination with her lying down with a visual acuity chart, a penlight, an Icare tonometer, a binocular indirect ophthalmoscope, and a 20 D condensing lens.
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Tonometry—the standard of care
Goldmann applanation tonometry has long been the standard of care for measuring intraocular pressure (IOP) in patients with glaucoma (or glaucoma suspects). However, it is not without its limits (some more antiquated than others). Based on the research performed by Goldmann and Schmidt,1 a presumed central corneal thickness of 520 µm was decided upon for the Goldmann tonometer.
We know, of course, that central corneal thickness can have a clinically significant effect on “true” IOP in patients. I use the word “true” in quotation marks because direct IOP measurement involves invasion into the globe. Nonetheless, the notion that patients with thicker central corneas may tend to have lower IOPs as measured by Goldmann tonometry (and vice versa) prompted the circulation and use of correction tables in an attempt to calculate “true” IOP based on a linear relationship between Goldmann readings and central corneal thickness.
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The science behind this attempt to cope with the fact that the central cornea is almost never 520 µm thick, however, breaks down when one takes into account corneal hysteresis, notably the “squish factor.” The hallmark example of such would be a patient with Fuch’s endothelial corneal dystrophy, in which central corneal thickness could be up around 650 µm but in such a “squishy” cornea that IOP could end up being underestimated.