Glaucomatous damage of the macula

Abstract

There is a growing body of evidence that early glaucomatous damage involves the macula. The anatomical basis of this damage can be studied using frequency domain optical coherence tomography (fdOCT), by which the local thickness of the retinal nerve fiber layer (RNFL) and local retinal ganglion cell plus inner plexiform (RGC+) layer can be measured. Based upon averaged fdOCT results from healthy controls and patients, we show that: 1. For healthy controls, the average RGC+ layer thickness closely matches human histological data; 2. For glaucoma patients and suspects, the average RGC+ layer shows greater glaucomatous thinning in the inferior retina (superior visual field (VF)); and 3. The central test points of the 6° VF grid (24-2 test pattern) miss the region of greatest RGC+ thinning. Based upon fdOCT results from individual patients, we have learned that: 1. Local RGC+ loss is associated with local VF sensitivity loss as long as the displacement of RGCs from the foveal center is taken into consideration; and 2. Macular damage is typically arcuate in nature and often associated with local RNFL thinning in a narrow region of the disc, which we call the macular vulnerability zone (MVZ). According to our schematic model of macular damage, most of the inferior region of the macula projects to the MVZ, which is located largely in the inferior quadrant of the disc, a region that is particularly susceptible to glaucomatous damage. A small (cecocentral) region of the inferior macula, and all of the superior macula (inferior VF), project to the temporal quadrant, a region that is less susceptible to damage. The overall message is clear; clinicians need to be aware that glaucomatous damage to the macula is common, can occur early in the disease, and can be missed and/or underestimated with standard VF tests that use a 6° grid, such as the 24-2 VF test.

Patterns of functional vision loss in glaucoma determined with archetypal analysis

“…Furthermore, although our VF measurements focus on locations outside the macula, our plots verify two glaucoma-related macular findings [33]: the nerve fibre trajectories of the upper central archetype ( figure 12), which resemble a pattern previously termed 'arrow shaft' by Goldbaum et al [19], is restricted to the socalled macular vulnerability zone (see fig. 9 in [33]). This zone is related to early central upper hemifield damage which is typically hard to detect by the coarse spatial resolution of the 24-2 protocol.…”

Automatic identification of the temporal retinal nerve fiber raphe from macular cube data

“…Both thickness [2,8,9] and intensity [11][12][13][14] of the RNFL have been used by other groups to estimate the orientation of the temporal raphe, and both are reasonable indicators since the apparent "missing" fibers at the raphe should reduce both thickness and intensity recorded by OCT at the level of the RNFL.…”

“…Since axons originating in the temporal retina follow a path to the optic nerve head that avoids the fovea, such maps need to incorporate the location of the temporal raphe: the boundary between axons that follow a superior course and those that follow an inferior course [1]. The location of the temporal raphe is particularly important for glaucoma because characteristic visual field losses, such as nasal steps and arcuate shaped patterns, occur in this region [2].…”

“…Some efforts have been made towards measuring or inferring the orientation of the raphe with currently available clinical tools. Hood et al located the axis of minimum RNFL thickness from 128 healthy eyes, reporting approximately horizontal orientation of the raphe despite a large physiological variation in the relative positions of the optic disc and fovea [2]. In glaucomatous eyes, Le et al, Amini et al and Kim et al have exploited characteristic patterns of glaucomatous damage coupled with measured visual field loss to infer the axis between superior and inferior fields, again relying on changes in thickness of particular layers measured with OCT [8][9][10].…”

Strategies to improve early diagnosis in glaucoma

“…Glaucomatous changes in the macula are also likely to have particularly serious consequences for quality of life given the importance of central vision. As the macula is largely devoid of large vessels and has a readily identifiable center (the fovea), assessment of this region may also overcome some limitations of circumpapillary measurements, such as interference from retinal and optic nerve head vasculature, parapapillary atrophy, and variable placement of the measurement circle around the disc (Hood et al, 2013).…”