Purpose: Among other corneal biomechanical properties, Goldmann applanation tonometry (GAT) has been shown to depend on corneal edema. New tonometry devices have been designed, such as the Tono-Pen XL, iCare, and ocular response analyzer (ORA), to measure the intraocular pressure (IOP) accurately. This study aims to investigate the influence of corneal edema on the accuracy of these IOP-measuring devices in an in vitro model.
Methods: A model of an artificial anterior chamber was developed using a guided trephination system. Eight donor corneas not suitable for keratoplasty were clamped into this artificial anterior chamber. All corneas showed signs of stromal edema. Intracameral pressure (ICP) was adjusted manometrically to 10, 20, 30, 40, and 50 mm Hg. The central corneal thickness (CCT) was determined by ultrasonic pachymetry. For each manometrically defined ICP, tonometry was performed using the iCare, Tono-Pen XL, GAT, and ORA.
Results: The mean CCT increased from 616.1±29.6 µm to 626.9±36.1 µm. At 10 mm Hg, GAT yielded a higher ICP than those manometrically adjusted (10.4±3.3 mm Hg); at all other ICP levels, GAT yielded lower ICP levels than those adjusted. The Tono-Pen XL and iCare showed the greatest difference at 10 mm Hg, with the Tono-Pen XL yielding a value of 14.0±4.0 mm Hg and the iCare yielding a value of 12.5±2.6 mm Hg. All other results of the 2 devices fell within a range of ±2 mm Hg from the adjusted ICP. The ORA provided accurate results only at "physiological" ICP levels with a maximum difference of 2.6 mm Hg at 30 mm Hg. At higher ICP levels, corneal hysteresis decreased significantly with increasing ICP. None of the measurement devices revealed a statistically relevant dependence on CCT in this experimental setting.
Conclusions: The Tono-Pen XL and the iCare yielded the most accurate ICP values across all the adjusted ICP values. This may be because of their relatively small contact area with the cornea and, consequently, greater independence from corneal biomechanical properties. The ORA yielded accurate measurement results only at physiological ICP levels. As anticipated, GAT underestimated ICP. The Tono-Pen XL and the iCare should therefore be used to determine IOP in patients suffering from corneal edema, such as bullous keratopathy or Fuchs endothelial dystrophy.