Background: Several conversion tables and formulas have been suggested to correct applanation intraocular pressure (IOP) for central corneal thickness (CCT). CCT is also thought to represent an independent glaucoma risk factor. In an attempt to integrate IOP and CCT into a unified risk factor and avoid uncertain correction for tonometric inaccuracy, we propose a new pressure-to-cornea index (PCI).
Methods: PCI (IOP/CCT3) was defined as the ratio between untreated IOP and CCT3 in mm (ultrasound pachymetry). PCI distribution in 220 normal controls, 53 normal-tension glaucoma (NTG), 76 ocular hypertension (OHT), and 89 primary open-angle glaucoma (POAG) patients was investigated. PCI's ability to discriminate between glaucoma (NTG+POAG) and non-glaucoma (controls+OHT) was compared with that of three published formulas for correcting IOP for CCT. Receiver Operating Characteristic curves (ROC) were built.
Results: Mean PCI values were: Controls 92.0±24.8, NTG 129.1±25.8, OHT 134.0±26.5, POAG 173.6±40.9. To minimize IOP bias, eyes within the same 2mmHg-range between 16 and 29 mmHg (16-17,18-19,etc) were separately compared: control and NTG eyes as well as OHT and POAG eyes differed significantly. PCI demonstrated larger area under the ROC curve (AUC) and significantly higher sensitivity at fixed 80% and 90% specificities compared with each of the correction formulas; optimum PCI cut-off value 133.8.
Conclusions: PCI range of 120-140 is proposed as the upper limit of "normality", 120 being the cut-off value for eyes with untreated pressures ≤21, 140 when untreated pressure ≥22. PCI may reflect individual susceptibility to a given IOP-level, and thus represent a glaucoma risk factor. Longitudinal studies are needed to prove its prognostic value.
- central corneal thickness
- intraocular pressure
- ocular hypertension
- risk factors