Relationship between corneal thickness and measured intraocular pressure in a general ophthalmology clinic

Objective: To assess whether central corneal thickness (CCT) is a confoundi ng factor in the classification of patients attending for glaucoma assessme nt in a district general hospital. Design: Cross-sectional study by a single observer. Participants: Patients attending a general ophthalmic clinic: 235 clinicall y normal eyes, 52 eyes with normal-tension glaucoma (NTG), 335 eyes with pr imary open-angle glaucoma (POAG), 12 eyes with pseudoexfoliative glaucoma ( PXE), 42 eyes with chronic angle closure glaucoma (CACG), and 232 glaucoma suspect (GS) eyes. Intervention: Central corneal thickness was measured using ultrasonic pachy metry. Main Outcome Measure: Correlation of CCT and diagnosis. Results: Mean CCT was 553.9 mu m (95% confidence intervals [Cl] for the mea n, 549.0-558.8 mu m) in the clinically normal eyes, 550.1 mu m (95% Cl, 546 .6-553.7 mu m) in the POAG eyes, 514.0 mu m (95% Cl, 504.8-523.3 mu m) in t he NTG eyes, 530.7 mu m (95% Cl, 511.2-550.1 mu m) in the PXE eyes, 559.9 m u m (95% Cl, 546.8-573.0 mu m) in the CACG eyes, and 579.5 mu m (95% Cl, 57 4.8-584.1 mu m) in the GS eyes. The differences of mean CCT between the gro ups were highly significant (P < 0.001 analysis of variance). Eighty-five p ercent of eyes with NTG and only 36% of eyes with POAG had a mean CCT of 54 0 mu m or less. Thirteen percent of eyes with POAG and 42% of GS eyes had a mean CCT greater than 585 mu m. Conclusions: The CCT measurement is desirable in patients attending for gla ucoma assessment in a district general hospital to avoid misclassification resulting from the relationship between CCT and tonometric pressure. Centra l corneal thickness alone is not an accurate predictor for the clinical dia gnosis in this group of eyes. However, many eyes diagnosed as having NTG ha ve thin corneas, which would tend to lower the tonometrically recorded intr aocular pressure (IOP), so the finding of a less-than-normal thickness corn ea introduces some doubt as to the diagnosis of NTG, For the GS eyes, most eyes had thick corneas, which would tend to increase the tonometrically rec orded IOP, Thus, GS eyes with modest elevation of IOP and a thick cornea ma y be at low risk of progressing to POAG, Thus, many patients with "high IOP s" and a thick CCT do not necessarily have high IOPs and may not need to be followed as GS eyes.