MEASURING IOP

While the Goldmann applanation tonometer is still considered the current gold standard method in measuring intraocular pressure (IOP), its measurements are significantly affected by the changes of corneal properties after LASIK and should ideally be supplemented with additional measurements using a device such as the Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments), according to a study presented at the French Implant and Refractive Surgery Association (SAFIR) annual meeting.

“Corneal refractive surgery is becoming more popular all the time, with LASIK still the most popular technique. LASIK photoablation results in a reduction of the corneal thickness, changes the corneal curvature and also alters the biomechanical characteristics of the cornea, with a reduction of the corneal hysteresis. These three parameters come into play and have an impact on the IOP measurements carried out using the standard Goldmann tonometry method, meaning that these measurements are not completely reliable,†Alice Grise- Dulac MD told delegates.

Dr Grise-Dulac, Fondation Ophtalmologique Adolphe de Rothschild, Paris, France, said that this knowledge prompted her to compare IOP measurements obtained using either Goldmann or ORA, and to compare the biomechanical parameters – corneal hysteresis, corneal resistance and corneal curvature – on three groups of patients before LASIK, one month after LASIK and several years after LASIK. The three groups were comprised of 86 eyes of 43 patients in the pre-LASIK group, 86 eyes of 43 patients for the one-month post-LASIK group and 34 eyes of 18 patients who were examined several years after LASIK (average 4.56 years).

“The reduced numbers in the latter group underscores one of the key problems of refractive surgery where the patients are usually very happy with their vision and it can be difficult to get them back for followup visits,†said Dr Grise-Dulac. Only myopic LASIK patients were included in the study and patients who had undergone previous LASIK or corneal surgery were also excluded. All patients underwent IOP measurement with Goldmann tonometry and central corneal thickness was measured using Orbscan (Bausch + Lomb). The ORA exam measured IOP using a non-contact air puff system, and allowed for in vivo measurement of the IOP and biomechanical properties of the cornea.

These included corneal hysteresis (CH) values, corneal resistance factor (CRF) and two subsequent measures: the IOPg, which is the Goldmann Correlated IOP, and the IOPcc (Corneal Compensated IOP), which takes account of certain biomechanical properties of the cornea. Corneal curvature measurements, based on parameters developed by David Luce, were also taken by Dr Grise-Dulac’s group. Dr Grise-Dulac said that the measurements taken by Goldmann tonometry confirmed the findings of several other studies in the scientific literature.

“The ORA corneal compensated IOP measurements seemed to provide the most stable and viable measurement of the IOP. The GAT IOP was found to be significantly reduced after LASIK by an average of 2.0 mmHg. The measurement of IOP with ORA seemed closer to the reality, at least for the IOPcc. Interestingly, we found no statistically significant difference between the preoperative IOP values and the values four years after LASIK using corneal compensated IOP measurements,†she said.