GLAUCOMA IMAGING

Images of the anatomy of trabecular pathway structures including Schlemm’s Canal can now be visualised and objectively measured using high- resolution spectral domain OCT, a technique which may help to shed light on the role of the aqueous outflow system in primary open angle glaucoma (POAG), according to Alfredo R Castillejos MD. Addressing delegates attending the World Ophthalmology Congress, Dr Castillejos, associate professor of ophthalmology at Nuestra Senora de la Luz Hospital in Mexico City and director of the Pan-American Eye Centre, Mexico City, said that advances in imaging technology are bringing researchers closer to answering some fundamental questions concerning aqueous outflow of the human eye. “Many of these are key questions that have remained unanswered for almost 100 years. At this point in time we actually do not know how the aqueous humour drains, nor do we know what routes the aqueous humour follows in normal and pathological conditions. We also have much to learn about the morphological and physiological changes that occur with age as well as the anatomical changes related to POAG,†he said.

Main outflow route

Dr Castillejos noted that the trabecular pathway represents the main outflow route of the eye, accounting for 50 per cent to 90 per cent of the total aqueous humour flow depending on the age of the individual. “The main site of resistance in this pressure-dependent pathway is found between the juxta-canalicular meshwork and the inner wall of Schlemm’s Canal. The question of whether the changes in Schlemm’s Canal morphology related to glaucoma differ from those solely occurring from ageing is a long-standing controversy,†he said. Dr Castillejos added that most of the data currently available on the anatomy and physiology of the outflow pathways is based on animal models or mathematical calculations. “The anatomy and physiology of the collector channels and the intrascleral vascular plexus remain incompletely understood, partly because we do not have a reliable imaging method able to accurately evaluate this portion of the eye. Recent evidence from animal models suggests the possibility of other areas of resistance along this pathway,†he said. Dr Castillejos said that much of the research of his group has been derived from an enhanced imaging technique based on spectral domain anterior segment OCT. He described a technique to improve OCT imaging of the microstructures comprising the outflow pathways during his training years at the New York Eye and Ear Infirmary.

“The OCT Enhanced Imaging Method (EIM) approach is used to capture high-resolution images of Schlemm’s Canal and collector channel junctions. The selected eye is imaged in adduction fixating on a target located at 15cm with the temporal limbus positioned perpendicular to the OCT beam. We then perform radial and tangential scans of the same collector channel and Schlemm’s Canal junctions after which an automated software is used to reconstruct the images of the structures along their entire course,†he said. “We know that there is variability in the dimensions of the trabecular pathway structures even in age-matched subjects with normal eyes and normal IOP. However, it is not known if individuals with a functioning, yet smaller Schlemm’s Canal, may reach the endpoint in which trabecular flow is severely restricted in a shorter period of time as compared with those with normal or above normal-sized structures,†he said. In terms of the risk factors for POAG, Dr Castillejos said his team carried out a study to establish whether reduced Schlemm’s Canal dimensions might be associated with decreased aqueous outflow.

“In the near future, we will be able to evaluate and detect specific sites of aqueous outflow resistance and hopefully we will be able to perform individualised glaucoma surgery by selecting the most beneficial procedure for each patient,†he said.