Review paper: relating optical coherence tomography to visual fields in glaucoma

Modern management of glaucoma relies on the combination of information from imaging of the retina with measurements of visual performance (visual field assessment typically). This review article describes in detail recent research directed at customising the combination of this information on an individual basis. We review current mapping schema that are used to relate information between structure and function, and illustrate how such maps can be customised for individual eyes. We also discuss how such structure-function mapping might be used to relate localised retinal damage measured with optical coherence tomography (OCT) to localised visual field sensitivity. Future promising applications include selecting visual field test locations or algorithms using OCT information, and improvements to statistical analysis procedures when combining information across platforms.

The citation for the paper is: Denniss J, Turpin A, McKendrick AM. Relating optical coherence tomography to visual fields in glaucoma: structure–function mapping, limitations and future applications. Clinical Experimental Optometry. 2019: 102: 291-299.

This is a special issue of Clinical Experimental Optometry directed to Optical Coherence Tomography.

To download the full article visit here.

An illustration of high resolution features that can be measured in individual eyes that impact on the mapping between structure and function for an individual eye (the position of the optic nerve relative to the fovea, and the position of the temporal raphe).

An illustration of how different positions of the temporal raphe and optic nerve head may result in different maps between ONH position (left hand side colour wheel) and visual field locations (right hand side maps).

Imaging of the eye shows individual differences in the shape of the macula

One of our major research areas is in the field of glaucoma, an eye disease that affects the retinal ganglion cells (nerve cells). These cells are most densely located within the central 10° of the retina from the very centre, the region which is called the macula. Recently there has been an increased interest in the study of the morphology (shape) and functionality of the macula in people suffering from glaucoma, because we expect that this is where the initial pathological changes would appear.

Many researchers have tried to connect functional changes with morphological changes within the central 10° of the retina in people with glaucoma by creating a ‘structure-function map’. In particular, some researchers have included individual anatomical information to improve these maps. For example, at the very centre (foveola), one photoreceptor (the light-sensitive cell of the retina) connects to approximately one retinal ganglion cell, but the connection is slightly displaced spatially. Correcting for this factor would be unnecessary, however, if the maculae of all eyes were equal.

In this paper we explored whether some macular parameters such as the maximum thickness, the minimum thickness and the horizontal distance from the two previous values (we call it “radius”) of high-resolution optical coherence tomography (OCT) scans centered at the foveola are different between individuals. The figure below shows where the scans are centred.

Retinal photo showing where the scans are taken (at the very centre of the retina, the fovea)

The three parameters compared: central thickness, maximum thickness (height) and radius.

We found that the measured foveal shape parameters are different between different people. We also found that the foveal shape is not symmetrical between superior and inferior parts of the retina, which is important because the damage is often asymmetric between the superior and inferior hemiretinae in glaucoma.

This work formed part of Juan Sepulveda’s Masters project. You can read the full published article in Investigative Ophthalmology and Visual Science here.

New paper: Automatic identification of the temporal raphe from standard retinal scans

Glaucoma is a disease of the optic nerve. The nerve fibres that travel along the optic nerve to the brain are spread out across the retina, and can be visualised using an imaging technique called  optical coherence tomography (OCT). Individual nerve fibre bundles cannot be seen with standard clinical tests. It is typically assumed that if a nerve fibre bundle is lost in one part of the retina, then a person will have corresponding visual loss in that part of their field of view (i.e. visual field). Recently it has been recognised that different people can have quite different distributions of retinal nerve fibres, which means that the relationship between the location of loss of visual field and the location of nerve fibre damage differs between individuals and can be hard to predict.

Our lab has recently developed sophisticated models that overcome this problem to predict just how a particular nerve fibre pattern should map onto an individual patient’s visual field. These models depend critically upon a single measured parameter from the patient’s eye, namely the axis of symmetry between fibres that travel along the upper (superior) retina and those that travel along the lower (inferior) retina. This axis is known as the temporal nerve fibre raphe or temporal raphe for short (shown in the figure below as a red line).

Figure 1. The temporal raphe (red), where the nerve fibres travelling from the upper and lower retina meet, can be identified using retinal scans

The location of the temporal raphe is typically difficult to appreciate with standard imaging techniques. However, it has been shown recently that by collecting OCT data with scans of very high spatial density, the raphe can be visualised in exquisite detail and its orientation hence measured very precisely. Unfortunately, collection of such data requires a lot of time, good image quality, and particularly steady fixation, which are luxuries that we do not have in a clinical setting.

In this paper we set out to determine just how well the temporal raphe can be determined from standard scans acquired routinely in the clinic (‘macular cube’ scans). By acquiring additional high density scan data on the same eyes, we were able to measure how accurately several automated algorithms can estimate the orientation of the temporal raphe. Our results show that the best performing algorithm gave a mean error of 1.5°. The ability to quantify the orientation of the raphe with this level of precision paves the way for future work that uses these measurements to accurately map retinal structure (by OCT) to retinal function (by visual field), in order to more accurately assess the health of the retina and optic nerve in glaucoma.

This paper has been published in Biomedical Optics Express and is available to read here.