Novel pressure-to-cornea index in glaucoma

Abstract

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, a new pressure-to-cornea index (PCI) is proposed. Methods: PCI (IOP/CCT 3 ) was defined as the ratio between untreated IOP and CCT 3 in mm (ultrasound pachymetry). PCI distribution in 220 normal controls, 53 patients with normal-tension glaucoma (NTG), 76 with ocular hypertension (OHT), and 89 with primary open-angle glaucoma (POAG) was investigated. PCI's ability to discriminate between glaucoma (NTG+POAG) and non-glaucoma (controls+OHT) was compared with that of three published formulae for correcting IOP for CCT. Receiver operating characteristic (ROC) curves were built. Results: Mean PCI values were: Controls 92.0 (SD 24.8), NTG 129.1 (SD 25.8), OHT 134.0 (SD 26.5), POAG 173.6 (SD 40.9). To minimise IOP bias, eyes within the same 2 mm Hg range between 16 and 29 mm [18][19] etc) were separately compared: control and NTG eyes as well as OHT and POAG eyes differed significantly. PCI demonstrated a 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 cutoff value 133.8. Conclusions: A 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 mm Hg, 140 when untreated pressure >22 mm Hg. 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 and intraocular pressure in the Cameroonian nonglaucomatous population

“…The IOP/CCT3 index came close to that worked out by Iliev et al,13 which was 92.0 ± 24.8 for nonglaucomatous subjects. At the end of their work, they fixed the norm of this index between 120 and 140; the cutoff point for the normal subject being 120, which is similar to our results.…”

Normal Pressure Glaucoma

“…41 They proposed the "pressure-to-cornea index" (PCI = IOP/CCT 3 , with IOP being the highest recorded IOP without treatment in mmHg and CCT measured in mm to integrate the combined risks of IOP and CCT for developing glaucoma). They found the PCI of normal eyes = 92, NTG eyes = 129, OHT eyes = 134, and HTG eyes = 174.…”

Pressure-cornea-vascular index (PCVI) for predicting disease progression in normal tension glaucoma

“…A unified parameter that measures the relative effects of these two factors is desirable. This has been attempted using the pressure-to-cornea index (PCI) 6. PCI was defined as maximum IOP/CCT 3 (CCT in mm), and was used to distinguish between non-glaucoma individuals (normal and ocular hypertensive (OHT)) versus glaucoma (POAG or NTG).…”