Which Pachymetry-Based Intraocular Pressure Correction Formula Best Reflects Biomechanically Corrected Intraocular Pressure In Keratoconus Patients? A Stage-Based Corvis St Analysis

Accurate intraocular pressure (IOP) assessment in keratoconus is clinically challenging since conventional applanation-based techniques are highly dependent on corneal thickness and biomechanics, which are both profoundly altered in ectatic corneas. Biomechanically corrected IOP (bIOP) provided by Corvis ST incorporates corneal deformation parameters and may better approximate true IOP in ectatic eyes. This study aimed to compare uncorrected non-contact tonometry (IOPnct), four pachymetry-based correction formulas (Ehlers, Shah, Dresden, Spoerl), and bIOP in keratoconus patients, and to determine which correction method most closely reflects biomechanically adjusted IOP across increasing disease severity.

Department of Ophthalmology, Basaksehir Cam and Sakura City Hospital, a tertiary referral center, Istanbul, Turkey.

This prospective cross-sectional study included 77 eyes of 77 keratoconus patients classified according to Pentacam-based Topographic Keratoconus Classification (TKC). All IOP measurements were performed with Corvis ST under standardized conditions. IOPnct, bIOP, and pachymetry-based corrected IOP values (Ehlers, Shah, Dresden, and Spoerl) were obtained for each eye. Statistical differences were evaluated using repeated-measures ANOVA with post hoc pairwise testing, and agreement with bIOP was assessed using Bland–Altman analysis.

The mean age was 26.4±7.3 years, and 59.7% were male. IOP values remained stable among keratoconus stages when assessed by bIOP, whereas clear inter-method differences were observed. IOPnct consistently underestimated IOP relative to bIOP across all stages. In contrast, Ehlers (+2.6mmHg), Shah (+1.4mmHg), and Dresden (+0.7mmHg) corrections significantly overestimated IOP compared to bIOP (all p<0.001). Spoerl correction (−0.9mmHg) showed no significant difference from bIOP (p=0.209). Bland–Altman analysis demonstrated that the Spoerl formula had the smallest mean bias and the narrowest limits of agreement across all keratoconus stages, while overestimation and variability with Ehlers and Shah increased with advancing disease severity. 

In keratoconus, uncorrected non-contact-tonometry underestimates IOP, while pachymetry-based correction formulas (Ehlers, Shah, and Dresden) tend to overestimate, particularly in advanced stages. Among the evaluated methods, the Spoerl formula demonstrated the closest agreement with bIOP across all stages, without significant bias. When biomechanical assessment is unavailable, Spoerl correction may represent the most reliable pachymetric alternative for IOP estimation in keratoconic eyes.