Slit-lamp illumination can confound goniometry

Lee and his co-investigators in the Department of Ophthalmology, University of Toronto, used anterior segment optical coherence tomography (AS-OCT, Visante; Carl Zeiss Meditec) to evaluate the effect of illumination from the slit-lamp on angle anatomy and grading. Their study enrolled 32 patients of mixed ethnic backgrounds who had been referred for narrow angles.Each participant underwent three AS-OCT scans in different light settings-one with no illumination, a second exam with a light shined over the surface of the iris, and a third exam with the same light shined over the surface of the pupil.

A Shaffer angle grade was determined from each scan based on OCT-derived measurements, and the gradings were compared between the scan obtained in darkness and under the different light exposures. The results showed that, in this population of patients with narrow angles, the gonioscopic grade increased in 38% of eyes as soon as the light contacted the iris, whereas 72% of the eyes had a different gonioscopic grade when the light was shined over the pupil or the iris compared with when the examination was performed in darkness.

Ike K. Ahmed, MD, was the senior author of the study and is an assistant professor in the Department of Ophthalmology at the University of Toronto.

Inclusion criteria

Patients were included in the study if a Shaffer angle grade of 2 or less had been diagnosed in at least two quadrants and if they experienced no alterations of angle or iris anatomy.

Simulation of the slit-lamp illumination during the OCT exam was achieved using a handheld ophthalmoscope as an external light source. The ophthalmoscope had a light intensity of 5.5 eV, which was similar to that of a standard slit-lamp based on measurements performed with a light meter.

For each OCT scan, two objective measurements were calculated for each angle: 1) AOD500, which represents the angle opening distance (the perpendicular distance between the iris and trabecular meshwork) 500 μm away from the scleral spur, and 2) the trabecular iris angle (TIA), which is defined as the angle between the iris and trabecular meshwork at a point 500 μm away from the scleral spur. Both of these parameters previously had been described by study co-investigator Charles Pavlin, MD, using ultrasound biomicroscopy to image the anterior segment.

Next, the AOD500 and TIA measurements were converted to a Shaffer gonioscopy grade using information from a published study of anterior segment OCT study conducted by Wirbelauer et al. Any change in the Shaffer-correlated AOD500 value or Shaffer-correlated TIA value between examinations performed with and without light was used to define patients with a light-induced change in angle grade.

Additional analyses

Additional analyses showed that ethnic background, iris color, and iris thickness were not predictive of light-induced changes in angle grade. A trend was observed for a greater change in angle grade with increased sensitivity to light exposure, however.