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Abstract #15979 Published in IGR 2-3
Accurate intraocular pressure measurement-the myth of modern ophthalmology? (editorial)
Shah SOphthalmology 2000; 107: 1805-7
In two editorials guest editors of Ophthalmology discuss the accuracy of intra-ocular pressure measurement and the necessity of such accuracy. The first author starts by explaining the Imbert-Fick law which is the basis for applanation tonometry. Goldmann tonometry uses a tonometer head of 3.06 mm and is valid for normal central corneas of 500 micrometers thickness. The debate is whether a corneal thickness that deviates from this 500 micrometers will have a significant effect on the measured IOP. It turns out that different population studies come up with a different mean central corneal thickness (CCT). The range of CCT's is certainly different from that assumed by Goldmann. Everyone involved in this subject has been thinking of an IOP correction factor for a deviation of CCT from the average. This calculation can be based on population studies, on manometric studies, or on findings after refractive surgery. Correction factors as high as one mmHg per 10 micrometer thinning of the cornea have been proposed. Several studies have shown that the CCT in Ocular Hypertension is usually above normal and in Normal Pressure Glaucoma usually below normal. Consequences of these would be that a certain percentage of these diagnosis up to as much of 30% were misclassified. An interesting point is that the CCT of an individual eye may vary by a mean of 7% and this variation may be as high as 14%. This author feels that the large number of eyes could be misclassified based on IOP without a correction for CCT. As a rough guide he proposes that every 10 micrometer difference in CCT means approximately 0.5 mmHg difference in measured IOP. This enthusiastic author feels that we should all do pachymetry in our glaucoma patients.
Classification:
6.1 Intraocular pressure measurement; factors affecting IOP (Part of: 6 Clinical examination methods)
