Imaging Glaucomatous Damage Across the Temporal Raphe.

Glaucoma is a group of blinding optic neuropathies that represent a major irreversible cause of vision loss in the elderly. The disease is characterized by damage to the ganglion cells and their axons and a resulting thinning of the retinal nerve fiber layer (RNFL) and cupping of the optic disc. The damage leads to a related loss in visual function. Because the vision loss resulting from glaucoma is irreversible, it is important to detect glaucoma in its early stages and to carefully monitor status as the disease progresses. In practice, this requires both functional and structural measurements. The current “gold” standard for testing visual function is visual field testing performed using static automated perimetry (SAP). For a given eye, visual sensitivity is measured at discrete retinal locations, and compared with normative values. As glaucoma progresses, the visual defects often appear in certain distinct locations; for instance, arcuate defects, paracentral scotomas, and nasal steps. A major disadvantage of visual field testing is that there is large individual variability,1–3 and this variability, because it lowers the sensitivity for detecting glaucoma, has led to an increased emphasis on measurements of changes to retinal structure as a complement to visual field testing. Traditionally, structural measures of glaucomatous damage relied on fundus cameras and confocal scanning laser ophthalmoscopy (SLO) for evaluating the optic disc and imaging RNFL defects,4–10 and scanning laser polarimetry for evaluation of the peripapillary RNFL.11 Recently, optical coherence tomography (OCT) has been increasingly used to detect and monitor glaucomatous damage to the RNFL,12 because of its high axial resolution and ability to quantify changes in the thickness of individual retinal layers. Another technique that was brought to the retinal imaging field is adaptive optics (AO) imaging, which allows for high-resolution retinal imaging without degradation by ocular aberrations.13 Adaptive optics retinal imaging has been used for investigation of glaucomatous damage of nerve fiber bundles in human eyes.14,15 However, most of current imaging-based measures were applied near the optic disc, where the RNFL is the thickest. Imaging at this location has the advantage that all ganglion cell axons must exit the eye at the optic disc, so the analysis can concentrate on this area and provide a summed measure of the axons. However, it also has the disadvantage that although there are general spatial relations between locations in the visual field and the thickness of the RNFL at particular portions of the optic disc and peripapillary region, there is large individual variability in this relation and so relating a particular structural measurement to a particular region of the visual field can be challenging.16–23 An apparent solution is to look at structural changes to the RNFL closer to the locations of the measured visual field defects. One portion of the retina where this becomes possible is near the temporal raphe in the temporal retina. In this region, the ganglion cell axons are first incorporated into axon bundles, and then course across the retina toward the optic disc. For eccentricities approximately 9° temporal to the fovea, the ganglion cells have relatively small displacement from the position of their cone inner segments, and thus there is a close spatial relation between the local sensitivity and the local RNFL.24,25 This region also has an advantage for studying the relation of early structural and functional loss because the raphe region is believed to be one of the major sites where early visual function abnormalities appear.26–29 Unfortunately most clinical techniques cannot measure the RNFL reliably near the raphe because the RNFL is relatively thin and has low contrast. In the past few years, efforts have been made to image and measure the RNFL in this region using high-resolution imaging techniques. In particular, the AO scanning laser ophthalmoscope (AOSLO) has demonstrated the ability to image the temporal raphe in healthy young subjects.30,31 Similarly, the latest OCT techniques have been demonstrated to image the temporal raphe.32 The current study used an AOSLO to measure en face structure of the RNFL near the raphe and a commercial OCT to measure the thickness of the RNFL and ganglion cell complex (GCC) in the same region, and related these measurements to visual sensitivity as measured using SAP in patients with glaucoma and healthy control subjects.