IOP: Central Corneal Thickness

Core Messages

››Variations in central corneal thickness (CCT) influence the accuracy of all tonometry techniques to some degree.

››The Ocular Hypertension Treatment Study demonstrated that CCT is an independent predictive factor for the later development of glaucoma among ocular hypertensives, with thinner CCT conferring increased glaucoma risk; this finding was externally verified in the European Glaucoma Prevention Study.

››The evidence that CCT is an independent risk factor for progression in established glaucoma is weaker than for glaucoma conversion in ocular hypertension, but ongoing studies will likely clarify this relationship.

››CCT is an inherited ocular characteristic and appears to change little during adulthood in otherwise healthy eyes. On average, CCT decreases by a few microns per decade of life.

››Nomograms for “correcting” IOP measurements with CCT are not valid in individual patients and should not be used clinically. The influence of other factors such as corneal hydration and viscoelasticity probably dwarf the impact of CCT on IOP measurements.

J. D. Brandt

Department of Ophthalmology & Vision Science, University of California, Davis, 4860 Y Street Suite 2400, Sacramento, CA 95917-2307, USA

e-mail: jdbrandt@ucdavis.edu

››On average, measured IOP drops following all forms of keratorefractive surgery, but there are substantial numbers of patients in whom IOP rises, so the use of a fixed nomogram based on CCT, refractive correction, or laser ablation should not be used.

10.1  Why Has Central Corneal Thickness

(CCT) Become So Important?

10.1.1  Goldmann Tonometry

Ever since the recognition that glaucoma was associated in many patients with a firm eye, ophthalmologists have been attempting to measure intraocular pressure (IOP) clinically. Prior to the introduction of Goldmann Applanation Tonometry (GAT) in the 1950s, tonometry techniques were inconvenient and unreliable. Professor Goldmann’s tonometer rapidly gained widespread acceptance following its introduction – it was reasonably-priced, based on easily-under- stood physical principles, fitted seamlessly into the workflow of the slit-lamp exam, and appeared to provide accurate, reproducible measurements. GAT’s status as a tonometry “Gold standard” went largely unchallenged for 50 years, even though Professor Goldmann himself drew attention to various potential sources of error for the device in his first description of his tonometer [1]. In particular, Goldmann and Schmidt acknowledged that their design assumptions were based on a central corneal thickness (CCT) of 0.5 mm (500 µm) and that the accuracy of their device would vary if CCT deviated from this value – “Under conditions