Aim: To investigate which tonometer is practical and accurate to use in routine clinical practise for established keratoconus (KC).
Methods: A prospective study of 118 normal and 76 keratoconic eyes where intraocular pressure (IOP) was measured in random order with the Goldman applanation tonometer (GAT), Pascal® dynamic contour tonometer (DCT), Reichert® ocular response analyser (ORA) and Tono-Pen® XL. Corneal hysteresis (CH) and corneal resistance factor (CRF), as calculated by the ORA, were recorded. Central corneal thickness (CCT) was measured using an ultrasound pachymeter.
Results: The difference in IOP values between instruments was highly significant in both study groups (p<0.001). All other IOP measures were significantly higher than GAT except the Goldmann corrected IOP, IOPg, as measured by ORA, in normals and CH corrected IOP, IOPcc measures in KC. CCT, CH and CRF was significantly less in KC (p<0.001). Apart from the DCT, all techniques tended to measure IOP higher in eyes with thicker corneas.
Conclusion: The DCT and the ORA's IOPcc are currently the most appropriate tonometers to use in KC. Corneal factors such as CH and CRT may be of more importance than CCT in causing inaccuracies in applanation tonometry techniques.