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Abstract #19924 Published in IGR 9-4
Novel pressure-to-cornea index in glaucoma
Iliev ME; Meyenberg A; Buerki E; Shafranov G; Shields MBBritish Journal of Ophthalmology 2007; 91: 1364-1368
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-mmHg 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 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 mmHg, 140 when untreated pressure ≥22 mmHg. 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.
Dr. M.E. Iliev, Department of Ophthalmology, University of Bern, Inselspital, 3010 Bern, Switzerland. Milko.iliev@insel.ch
Classification:
2.2 Cornea (Part of: 2 Anatomical structures in glaucoma)
9.2.2 Other risk factors for glaucoma (Part of: 9 Clinical forms of glaucomas > 9.2 Primary open angle glaucomas)
6.1.3 Factors affecting IOP (Part of: 6 Clinical examination methods > 6.1 Intraocular pressure measurement; factors affecting IOP)
