Abstract

SA.16.08

Correction factors for central corneal thickness in Goldmann applanation tonometry

Kutschan A., Wiegand W.
Abteilung für Augenheilkunde, Asklepios Klinik Nord Heidberg, Hamburg

Objective: For many years it has been discussed, which corneal parameters are able to falsify the measurement of intraocular pressure (IOP). As substantial parameter the central corneal thickness (CCT) is accepted, whereby different measuring methods (GAT=Goldmann applanation tonometry, DCT=Dynamic Contour-Tonometry, IOPcc=corneal compensated pressure measured with the Ocular Response Analyzer) may lead to a completely different dependence on corneal thickness.
Methods: In 67 eyes of cataract patients and 77 eyes of glaucoma patients with strongly different CCT (measured with Orbscan pachymetry)in the context of a study approved by the ethics commission before surgery (cataract-op or trabeculectomy) the anterior chamber was cannulized and the intraocular IOP-values (IOPio) were measured simultaneously with a pressure absorber and with GAT (Perkins Tonometer)at different pressure values. (In each case average values from several individual measurings were obtained)
Results: The individual measurings exhibit an extraordinarily high deviation. In case of the cataract patients weak correlations of the difference between GAT and IOPio with correction factors of 0,61mmHg/100µm CCT at pressure level 20mmHg, 1,5mmHg/100µm CCT at pressure level 30mmHg and 1,3mmHg/100µm CCT at pressure level 40mmHg respectively. In glaucoma patients the differences between GAT and IOPio exhibit almost no correlation with CCT and correction factors for all pressure levels were below 1,0mmHg/100µm CCT.
Conclusions: The determination of CCT-dependend correction factors for GAT is not permissible and in view of the high deviation of the individual measurings not meaningful in glaucoma patients. This is contrary to some earlier and a recently published study.

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