EXTENDED REPORT

Novel pressure-to-cornea index in glaucoma

Milko E Iliev¹, Alexander Meyenberg¹, Ernst Buerki², George Shafranov³ and M Bruce Shields³

¹ Department of Ophthalmology, University of Bern, Inselspital, Bern, Switzerland
² Private practice, Ophthalmologists Association Thun, Thun, Switzerland
³ Department of Ophthalmology & Visual Science, Yale University School of Medicine, New Haven, CT, USA

Correspondence to:
Milko E Iliev, MD, Department of Ophthalmology, University of Bern, Inselspital, 3010 Bern, Switzerland; milko.iliev{at}insel.ch

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/CCT³) was defined as the ratio between untreated IOP and CCT³ 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 Hg (16–17, 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 cut-off 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.

Abbreviations: CCT, central corneal thickness; IOP, intraocular pressure; NTG, normal-tension glaucoma; OHT, ocular hypertension; PCI, pressure-to-cornea index; POAG, primary open-angle glaucoma; ROC, receiver operating characteristic

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