The influence of central corneal thickness on IOP
Dr Claes Feinbaum investigates emerging evidence that CCT is an important predictor for the development of primary open-angle glaucoma (POAG), and that it may be relevant as a predictor of both glaucoma progression and response to IOP-lowering medication
There has been considerable interest in determining the relevance of central corneal thickness (CCT) in the context of managing glaucoma suspects and those with definite glaucoma. Much of this research has centred on determining the effect that CCT has on applanation tonometry techniques because it is widely accepted that a higher CCT in a structurally normal cornea leads to an erroneously high measurement of intraocular pressure (IOP).1 There is emerging evidence, however, that CCT is also an important predictor for the development of primary open-angle glaucoma (POAG), and it may be relevant as a predictor of both glaucoma progression and response to IOP-lowering medication.
The purpose of this study is to evaluate the influence of CCT on IOP on different locations of the cornea without the influence of topical anaesthetics.
In guidelines established by the American Academy of Ophthalmology (AAO), CCT was recommended and rated as part of the initial examination for POAG and the glaucoma suspect.2 Interestingly, IOP, gonioscopy and evaluation of the optic nerve head, retinal nerve fibre layer (RNFL), and visual field were all rated lower in importance.3,4
The mechanical rigidity of the normal cornea is provided principally by the lamellae of collagen fibrils within the stroma, which constitute 70% of the corneal dry weight and 90% of the thickness of the cornea. CCT measurements are indicative of the structural composition as well as the hydration and metabolic status of the cornea and can be thought of as a correlate of corneal rigidity. Thus, if a cornea is not of average thickness, or has abnormal rigidity or hydration, then IOP measurements will become less accurate or reliable.7 Consequently, a structurally thick cornea would lead to an artifactually elevated IOP reading, while a thin cornea would lead to an artifactually reduced IOP reading. Nevertheless, CCT as a correlate is only applicable if the cornea is structurally normal. Hydration, for example, although producing a thicker cornea, results in a falsely low IOP reading.8
