Do Non-Ionic Contrast Media Temporarily Improve Corneal Transparency?

Dear Editor, Phacoemulsification in eyes with scarred corneas is challenging and due to the poor surgical view, carries an increased risk of complications. 1 In healthy eyes, the cornea is transparent due to the small diameter, and relatively regular size and spacing of stromal collagen fibrils, in addition to similarity of the refractive indices of collagen fibrils (1.411) and the extrafibrillar matrix (1.365). Corneal scars have higher refractive index and reduce transparency as they disrupt collagen fibril organization and increase light scattering. It has long been recognized that tissues can be made transparent by impregnation with high refractive index solutions. 4 Previously, Maurice described a series of studies investigating the possible use of high refractive index radiological contrast media as corneal clearing agents. 5 The rationale was that since corneal scars are of high refractive index, 5 if the cornea is impregnated with a contrast solution that matches the refractive index of the scar, light scattering could be reduced and corneal transparency should be improved. Although improvement in transparency would not be permanent, the cornea may clear enough to improve the view during cataract surgery. In Maurice's experiments, central scars were created in rabbit corneas of such severity that no details of the retina could be seen with an indirect ophthalmoscope. The cornea was then treated with the contrast media metrizamide or iohexol (GE Healthcare, Bucks, UK). The treatment process involved debridement of the corneal epithelium followed by application of the contrast agent for 20 minutes. Treatment resulted in sufficient corneal clearing to allow full details of the fundus to be observed " as in the normal eye ". 5 The cornea remained clear for 10 to 20 minutes before reverting to its previous state. Despite promising animal studies, contrast media have not been tested as corneal clearing agents in humans. We therefore conducted a pilot study to evaluate the use of the contrast medium iohexol as a corneal clearing agent for the first time in humans. Iohexol (refractive index of 1.46) is a contrast agent licensed for oral, intravenous, intrathecal or body cavity use (GE Healthcare, Omnipaque prescribing information), which has previously been used in ophthalmology for dacryocystography and computer tomography guided localization of anesthetics following retrobulbar injection. 6 The study was approved by the UK Medical Healthcare Products Regulatory Agency (MHRA) and local Institutional Review Board (IRB). Informed consent was obtained from all patients and the research adhered …