Year : 2018 | Volume
: 11 | Issue : 1 | Page : 1--2
Corneal thickness: It's time we all get rid of the correction factor from the glaucoma equation!
Aisha S Al Busaidi
Department of Ophthalmology, Sultan Qaboos University Hospital, Muscat, Oman
Aisha S Al Busaidi
Sultan Qaboos University Hospital, P.O. Box: 1025, PC 133, Al Khuwair, Muscat
|How to cite this article:|
Al Busaidi AS. Corneal thickness: It's time we all get rid of the correction factor from the glaucoma equation!.Oman J Ophthalmol 2018;11:1-2
|How to cite this URL:|
Al Busaidi AS. Corneal thickness: It's time we all get rid of the correction factor from the glaucoma equation!. Oman J Ophthalmol [serial online] 2018 [cited 2018 Apr 19 ];11:1-2
Available from: http://www.ojoonline.org/text.asp?2018/11/1/1/226342
The measurement of central corneal thickness (CCT) by pachymetry plays an integral part of the contemporary glaucoma workup. It is one of the relatively simple and straightforward ancillary tests performed today in the clinic. Clinicians have embraced this measure since the emergence of understanding its impact on the accuracy of tonometry such as Goldmann applanation tonometry (GAT). GAT, which is the gold standard of tonometry, assumes a central cornea thickness of approximately 520 μ. Any deviation from this assumption will result in an inaccurate intraocular pressure (IOP) measurement by as much as 7 mmHg/100 μ as evident by the experimental studies. We now know that CCT varies greatly among healthy normal individuals. Differences in corneal thicknesses exist with various factors such as age, gender, race, as well as other environmental factors. Therefore, the accuracy of tonometry in daily practice is affected which can lead to erroneous IOP estimates in certain situations with misclassification of patients in terms of glaucoma risk.
To attempt to improve accuracy of IOP measurements, it is still common practice to see ophthalmologists use the variable of CCT with correction factors to translate the measured IOP to an “adjusted” IOP; one that is thought best to reflect intracameral IOP. After documenting this presumed “true” IOP in the patient's records, they are left to wonder what else is there to do with this information? What does this corrected IOP actually mean? Is there a clinical value to it? Unfortunately, it actually means very little in the literal numerical sense.
It is important to realize that there are many factors, besides CCT, that have an impact on the measured IOP. Studies have shown corneal material properties, corneal curvature, astigmatism, inappropriate amount of fluorescein, Valsalva's maneuver, eyelid squeezing, and indirect pressure on the globe all can give rise to an artifact of measure.
Another important thing to consider besides the accuracy of the measured IOP is the variation in clinical IOP readings, which may arise from either true IOP fluctuations or measurement errors. IOP is a measurement with too much variability, much of which we do not understand very well. There are short-term IOP fluctuations that are diurnal or circadian-related and more long-term IOP variations. Imprecision of technique can result from miscalibration of the tonometry device or flawed technique among others. Measurement precision may be quantified by repeatability coefficient or interobserver level of agreement. Typical repeatability coefficients for GAT are 2.2–5.5 mmHg, and for different observers measuring IOP in the same participants, the 95% limits of agreement have been reported to be ± 2.2 to ± 3.9 mm Hg.
To add to the complexity, there are different clinical devices that can be used for either tonometry or pachymetry. Although GAT is widely used clinically as a method of tonometry, there are some other methods, which are largely independent of corneal influence and agree closely with manometric readings of IOP. Unfortunately, these are not widely developed and adopted into clinical practice, and therefore, we still heavily rely on GAT. Furthermore, devices used to measure CCT are many, ranging from ultrasound to optical pachymetry. These devices, if interchangeably used, are not only subject to differences in IOP and thickness data but can also alter the IOPCCT relationship.
Even if one assumes that the measured IOP is precise enough, adjusting for accuracy remains a problem due to lack of an accurate algorithm. Many correction nomograms have been published and exist, but none are adequately validated or universally accepted. Most of the algorithms that these nomograms rely on are based on a presumed linear relationship between CCT and IOP that are only estimates at best. However, mathematical models suggest that the relationship is nonlinear and complex in nature with the influence of CCT being less significant at lower IOPs. In addition to this, the correction factors do not take into account other factors, mentioned above, that are known to influence tonometric readings such as corneal elasticity and viscoelasticity. Therefore, with selecting an IOP correction formula arbitrarily, one runs the risk of introducing further errors into the equation rather than reducing them.
A single-CCT measurement may not be good for life as the cornea tends to slowly thin over time with age, ocular surgical interventions, and topical medications. We should also finally consider that the influence of CCT on glaucoma risk is thought to go far beyond just tonometry artifact, but rather is a surrogate for structural or physical factors involved in the pathogenesis of primary open-angle glaucoma (POAG).,
Instead of focusing on “corrected IOPs” which in itself is flawed, the clinician should incorporate CCT into the thinking process by broadly classifying it as thin, thick, or average (~540–560 μ). Based on this, an ocular hypertensive with a thin cornea has a greater risk of developing POAG, and a glaucoma patient with a thin cornea has more chance of disease progression than that with an average corneal thickness.
In view of the multiple layers of complexity related to CCT in glaucoma, it is important for us to change the way, we incorporate CCT in clinical practice. Clinicians are to use this data wisely, not merely as a correction factor but as a risk factor to aid in the interpretation of IOP measurements, risk stratification, setting of a target IOP, and determining follow-up intervals. After all, IOP is just one aspect of the overall glaucoma equation.
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