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Accuracy in pachymetry

Applanation tonometry is far 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 the applanation tonometry test, he assumed that most eyes had a corneal thickness of around 500 microns. In actual fact, there is significant variation in the corneal thickness and, as a result, there are substantial differences with the intraocular pressure (IOP) value measured with the Goldmann tonometry.

Information about the actual corneal thickness is important. Without it, accurate readings of IOP can be masked and diagnosis of glaucoma delayed. Unless taken into account, thicker corneas contribute to overestimation of IOP values and thinner corneas to underestimation.

Growing importance for compensated IOP

Individuals who have undergone the popular refractive surgery (LASIK) will have a thinner central corneal thickness and hence, when measured with just a tononmeter, the result will be under-estimated.

Central corneal thickness

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

Compensated IOP

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

  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 of three different compensation formulas with adjustable parameters to automatically calculate the compensated IOP, based on the CCT value.