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Accuracy in pachymetry

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IMPORTANCE PACHYMETRY

Applanation tonometry is actually a lot less accurate than previously recognized! The measurement results obtained with an applanation tonometer (contact or non contact) are also influenced by corneal properties such as rigidity and thickness. When Goldmann designed applanation tonometry, he assumed that most eyes had a corneal thickness of around 500 micron. Actually there is significant variation in corneal thickness and, as a result, big differences with the IOP value measured with Goldmann tonometry.

Information about the actual corneal thickness is important because without it can mask accurate readings of IOP and delay diagnosis of glaucoma. Unless taken into account, thicker corneas contribute to overestimation of IOP values and thinner corneas to underestimation.

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GROWING IMPORTANCE FOR COMPENSATED IOP

All persons that have undergone the popular refractive surgery (LASIK) will have a thinner central corneal thickness and hence whenne measurement with just a tononmeter is done, the result will be an under-estimated.

CENTRAL CORNEAL THICKNESS

The TX-20P will measure the central corneal thickness , based on a cross-sectional quantised image of the slit image of the central cornea.

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COMPENSATED IOP

Based on the central corneal thickness , the true Intra ocular pressure can be calculated : the compensated IOP.

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  1. corrected IOP = measured IOP – (CCT – 554) * 0.045
    Burvenich H, et al. Bull Soc Belge Ophthalmol, 276, 15-18, 2000

  2. corrected IOP = measured IOP – (CCT – 550) * 0.05 Shah S et al.
    Ophthalmology, 106, 2154-2160, 1999

  3. corrected IOP = measured IOP – (CCT – 575) * 0.0725
    Stodtmeister R, et al. Acta Ophthalmol Scand, 76, 319-324, 1998

PARAMETER SETTING FOR COMPENSATED IOP

The TX-20P offers a choice 3 different compensation formulas, with adjustable parameters, to automatically calculate the compensated IOP based on the CCT value.