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Agreement of Corvis ST Biomechanical IOP Measurements With Pachymetry-Based Correction Formulas in Patients With
Fahri Onur Aydın1, Ali Ceylan, Yusuf Berk Akbaş
1Department of Ophthalmology (F.O.A., Y.B.A., İ.Ç.A., S.E., B.K.Y.), University of Health Sciences, Basaksehir Cam and Sakura City Hospital, Istanbul, Turkey; and Department of Ophthalmology (A.C.), Akdeniz University, Antalya, Turkey.
Objectives:
To evaluate the agreement of intraocular pressure (IOP) measurements obtained by Corvis ST using biomechanically corrected IOP (bIOP), noncontact tonometry (NCT), and pachymetry-based correction formulas (Ehlers, Shah, Dresden, Spoerl) across different keratoconus severity levels and in comparison with healthy control eyes.
Methods:
This prospective, cross-sectional study included 77 patients with keratoconus (77 eyes), categorized by severity using Pentacam-based Topographic Keratoconus Classification and 54 healthy control patients. Patients underwent corneal tomography, and IOP was measured with Corvis ST. Uncorrected NCT-IOP (IOPnct), biomechanically corrected IOP (bIOP), and pachymetry-based corrected IOP (Ehlers, Shah, Dresden, Spoerl) were compared. Agreement among methods was assessed using Bland-Altman analysis.
Results:
The mean patient age was 26.4±7.3 years (59.7% male). NCT consistently underestimated IOP compared with bIOP, while Ehlers (+2.6 mm Hg), Shah (+1.4 mm Hg), and Dresden (+0.7 mm Hg) formulas significantly overestimated ( P <0.001). The Spoerl formula (-0.9 mm Hg) showed no significant difference compared with bIOP ( P =0.209). Bland-Altman analysis confirmed that the Spoerl formula had the smallest bias and narrowest limits of agreement across all keratoconus stages. Differences between methods became more pronounced with increasing severity, particularly evident for Ehlers and Shah formulas. In healthy control eyes, the different IOP estimation methods showed relatively small bias relative to bIOP.
Conclusions:
Noncontact tonometry significantly underestimates, and common pachymetry-based corrections (Ehlers, Shah, Dresden) tend to overestimate IOP in keratoconic eyes. The Spoerl formula provides IOP values most closely aligned with the Corvis ST bIOP across all keratoconus stages and was the most consistent pachymetric correction method relative to this biomechanically corrected comparator. Agreement between methods was closer in healthy corneas than in keratoconic eyes, supporting the influence of altered corneal biomechanics on tonometric measurements.
