Study of regional deformation of the optic nerve head using scanning laser tomography

Previous studies have suggested that IOP-induced deformation of the optic nerve head (ONH) at the level of the lamina cribrosa may contribute to axonal damage in glaucomatous optic neuropathy. Our purpose was to introduce a novel enucleated eye model for characterizing acute IOP-induced changes in ONH topography, and to develop improved analytical methods for detection of regional topographic change in the ONH.

Using a specially designed experimental apparatus, enucleated human eyes were progressively pressurized to 5, 15, 30, and 50 mmHg. Seven topographic images of the optic disc were taken at each pressure by a scanning laser tomographer (Heidelberg Retina Tomograph-HRT). The dependence of ONH topography on IOP was quantified for the entire nerve using standard HRT indices of ONH topographic change. The supero-inferior and nasal-temporal hemifields were also analyzed. A new method of analysis was developed which computes the location of the point of maximum slope within a 10 degrees sector of the ONH, as well as the magnitude of this slope. This method, termed "Inflection Point Analysis," was designed to be robust to the potential artefacts of image translation, reference plane location, and the subjective determination of ONH limits.

The results of three eyes are presented to illustrate the techniques. In our enucleated eye model, average ONH depth progressively increased with IOP, showing a maximum average posterior displacement of 36 microm as IOP was changed from 5 to 50 mmHg. Significant regional variability in ONH displacement was observed, which both Inflection Point Analysis and standard HRT parameters were able to detect. Inflection point analysis showed several advantages over standard HRT parameters: it was insensitive to artefacts due to tilt, was able to objectively delineate the boundary between the optic cup and neuroretinal rim, and was able to sensitively track changes in the location of this margin.

Scanning laser tomography is capable of detecting regional variation in the deformation of the ONH in response to acute changes in IOP. Our enucleated eye model and Inflection Point Analysis are promising tools for basic studies of ONH deformation in response to IOP. More extensive studies of both enucleated and in vivo eyes are required to determine the potential of Inflection Point Analysis for studying and tracking the progression of glaucomatous optic neuropathy.