The Concurrent Pressure to Cornea Index Classifies Glaucoma Risk in Early Normal Tension Glaucoma

Abstract

Purpose: : To investigate the utility of an age-adjusted concurrent pressure to cornea index (CPCI) in discriminating between glaucoma and non-glaucoma.

Methods: : CPCI was defined as the age-adjusted ratio between untreated intraocular pressure (IOP) and central corneal thickness³ (CCT³) in mm (IOP/CCT³ +4*age), measured within 2 hours of each other. In a population-based cross-sectional study, the distribution of CPCI in 294 normal controls (with normal visual fields), 124 with normal-tension glaucoma (NTG), 11 with ocular hypertension (OHT with normal visual fields), and 14 with primary open-angle glaucoma (POAG) was determined. CPCI’s ability to discriminate between glaucoma (NTG+POAG) and non-glaucoma (normal+OHT) and between early stage NTG and normal was tested. In order to simulate the earlier stages of NTG, the better eye of NTG subjects (i.e. with smaller vertical cup:disc ratio, VCDR) was compared to the worst eye of controls (i.e. with larger VCDR) and only eyes with VCDR of less than 0.6 and IOP less than 22 mm Hg were included. Area under the Receiver operating characteristic (ROC) curve was calculated to compare CPCI against IOP, CCT, and retinal nerve fibre layer thickness (RNFL) measurements (HRT II; Heidelberg Engineering).

Results: : Mean (± SD) CPCI values were 303.13 (± 42.12) in controls, 357.17 (± 56.02) in NTG, 372.84 (± 46.62) in OHT, and 441.04 (± 62.98) in POAG. CPCI demonstrated larger AUC and significantly higher sensitivity at fixed 80% and 90% specificities (AUC=0.785, p<0.001) than other indices (IOP; AUC=0.534, p=0.257, CCT; AUC=0.577, p=0.01, mean RNFL; AUC=0.655, p<0.001). When eyes were controlled for IOP (<22 mmHg), VCDR (< 0.6) and subjected to conditions simulating early glaucoma, the performance of CPCI (AUC=0.849, p<0.001) improved further against IOP (AUC=0.557, p=0.251), CCT (AUC=0.615, p=0.020), and HRT II measured RNFL thickness (mean RNFL, AUC=0.557; p=0.298). Partitioning CPCI into three tertiles based on its mean and standard deviation ([T1] < 311, [T2]: 311- 359, [T3] > 359), compared to the lowest tertile (<311), the middle (311-359) and upper tertile (>359) of CPCI was associated with an odds ratio of 6.5 and 35.2 for NTG, respectively. A cut-off value of 310 as the upper limit of normality (or as having low risk for NTG), 311-359 as moderate risk, and 360 and above as high risk of NTG is recommended.

Conclusions: : The CPCI has better discriminatory ability for glaucoma than IOP and CCT alone, and may be a useful summary indicator of glaucoma risk. Longitudinal studies are needed to prove its prognostic value.

Keywords: clinical (human) or epidemiologic studies: risk factor assessment • intraocular pressure • optic nerve