Agreement Of Intraocular Pressure Obtained By Goldmann Applanation Tonometer And Corvis St In Patients With Keratoconus

To assess the agreement among various types of intraocular pressure (IOP) measurements: IOP obtained by Goldmann applanation tonometer (GAT), the non-corrected IOP obtained by the Corneal Visualization Scheimpflug Technology (Corvis ST), pachymetry-based IOP corrected formulas (Ehlers, Shah, Dresden and Spoerl) and biomechanically corrected IOP obtained by the Corvis ST in eyes with keratoconus and with no history of previous surgical interventions (namely keratoplasty or intracorneal ring segments implantation).

A cross-sectional study was conducted on patients with clinical keratoconus attending the Contactology appointments at the Ophthalmology Department of Centro Hospitalar Universitário de Santo António (CHUdSA) between 2022 and 2024.

Each patient underwent IOP measurement through GAT (IOP-GAT) and Corvis ST imaging on the same day. The IOP-GAT was assessed by the same experienced ophthalmologist. From the Corvis ST, we collected for each patient formulas adjusted for biomechanic: non-corrected IOP (ncIOP) and biomechanically corrected IOP (bIOP); and adjusted for pachymetry: according to different formulas (Ehlers, Shah, Dresden and Spoerl). The difference in mean in IOP readings was assessed by paired samples t-test. Statistical significance was defined as p <0.05.

Thirty-two eyes of 17 patients with a mean±SD age of 39.9 ± 11.7 years were enrolled. Mean±SD pachymetry, k1, k2 and mean k were 483.2±53.1µm, 45.7±4.3 diopter (D), 48.9±4.6D and 47.4±4.5D, respectievly. IOP given by Ehlers formula showed the highest values (16.4±3.5 mmHg), followed by the Shah formula (15.7±3.4mmHg), Dresden (14.7±2.2 mmHg), bIOP (13.2±1.6mmHg), Spoerl (12.7±1.7 mmHg), IOP-GAT (12.1±2.6 mmHg) and ncIOP (12.0±1.7 mmHg). There were statistically significant differences in IOP measurements among all pairwise comparisons using the t-test except between IOP-GAT and ncIOP (p = 0.78), as well as between IOP-GAT and Spoerl formula IOP (p = 0.19). 

In patients with keratoconus, the Corvis-derived IOPs, except for the non-corrected IOP measure, show higher measurements compared to GAT. In this study, we found inconsistency between the measurements given by Corvis, with only the non-corrected IOP and IOP adjusted for pachymetry according to the Spoerl formula exhibiting agreement with the values obtained with Goldmann applanation tonometry. These findings may indicate the superiority of the latest mentioned methods in this group of patients, especially after corneal surgeries such as intracorneal ring segments implantation. Suggestions for the future include incorporating a larger number of participants and stratifying the results based on the severity level of keratoconus.