Cornea and IOP

CORNEA characteristics can affect the accuracy of IOP measurements, but their influence also varies depending on the tonometry technology that is used, Jesper Hjortdal MD, PhD, told attendees at the 9th EuCornea Congress. Dr Hjortdal said that central corneal thickness has a large effect on Goldmann applanation, air-puff and rebound tonometry. The IOP reading obtained with these technologies may be artificially low in eyes with a thin cornea and overestimated if the cornea thickness is greater than the normal range, he explained. Dr Hjortdal noted that corneal curvature might also be predicted to affect IOP measurements considering that a steeper cornea would be more difficult to applanate or indent than a flatter cornea. Available studies, however, indicate that at least when using Goldmann applanation tonometry, corneal curvature appears to have a minimal effect on the IOP readings. Corneal stiffness would also be expected to influence corneal applanation, and in contrast to curvature, corneal stiffness has been shown to affect IOP readings. Dr Hjortdal noted that IOP will be underestimated in eyes with keratoconus and overestimated in eyes with conditions associated with increased corneal stiffness, including corneal scarring and mucopolysaccharidosis. Of the available devices for measuring IOP, digital contour tonometer readings are possibly least influenced by corneal factors, said Dr Hjortdal, Department of Ophthalmology, Aarhus University Hospital, Aarhus, Denmark. “However, this technology is cumbersome, time-consuming, and more expensive to use,” he concluded.