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

Abstract

Objective To assess whether central corneal thickness (CCT) is a confounding factor in the classification of patients attending for glaucoma assessment in a district general hospital. Design Cross-sectional study by a single observer. Participants Patients attending a general ophthalmic clinic: 235 clinically normal eyes, 52 eyes with normal-tension glaucoma (NTG), 335 eyes with primary 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 pachymetry. Main outcome measure Correlation of CCT and diagnosis. Results Mean CCT was 553.9 μm (95% confidence intervals [CI] for the mean, 549.0–558.8 μm) in the clinically normal eyes, 550.1 μm (95% CI, 546.6–553.7 μm) in the POAG eyes, 514.0 μm (95% CI, 504.8–523.3 μm) in the NTG eyes, 530.7 μm (95% CI, 511.2–550.1 μm) in the PXE eyes, 559.9 μm (95% CI, 546.8–573.0 μm) in the CACG eyes, and 579.5 μm (95% CI, 574.8–584.1 μm) in the GS eyes. The differences of mean CCT between the groups were highly significant ( P < 0.001 analysis of variance). Eighty-five percent of eyes with NTG and only 36% of eyes with POAG had a mean CCT of 540 μm or less. Thirteen percent of eyes with POAG and 42% of GS eyes had a mean CCT greater than 585 μm. Conclusions The CCT measurement is desirable in patients attending for glaucoma assessment in a district general hospital to avoid misclassification resulting from the relationship between CCT and tonometric pressure. Central corneal thickness alone is not an accurate predictor for the clinical diagnosis in this group of eyes. However, many eyes diagnosed as having NTG have thin corneas, which would tend to lower the tonometrically recorded intraocular pressure (IOP), so the finding of a less-than-normal thickness cornea 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 recorded IOP. Thus, GS eyes with modest elevation of IOP and a thick cornea may be at low risk of progressing to POAG. Thus, many patients with “high IOPs” and a thick CCT do not necessarily have high IOPs and may not need to be followed as GS eyes.