In the management of glaucoma, we aim to preserve our patient's quality of life, yet we sometimes take our attention away from the region of the eye that provides the highest quality of vision: the macula. For example, the standard 24-2 visual field test samples the macular area quite sparsely, using a grid that is separated by 6 degrees and can easily miss central visual defects. A patient with 20/400 visual acuity and a macular hole will have a normal field of vision if the foveal sensitivity monitor is not engaged. Considering that the macula occupies less than 5% of the retinal surface area but contains almost a third of the retinal ganglion cells, the potential importance of monitoring this tissue in glaucoma becomes apparent.
Recently, spectral-domain optical coherence tomography (OCT) commercial platforms have provided opportunities to evaluate for glaucoma within the macula, either by segmenting out the ganglion cell layer, the ganglion cell layer plus the retinal nerve fiber layer (RNFL), or the entire macular thickness. All three approaches have been shown to successfully identify glaucoma.
Evaluation for glaucoma in the region responsible for central vision makes sense for variety of reasons. Regardless of its prevalence, glaucomatous damage within the macula will be functionally impactful. But glaucoma does affect central vision, even early in the disease.1 In moderate glaucoma, up to one half of eyes may have glaucoma within the central 3 degrees of fixation.2
Our standard technique of imaging the optic nerve and peripapillary retina has drawbacks. Optic nerve anatomy and topography demonstrate significant inter-individual variability. Indeed, much of what makes one patient more or less suspicious of glaucoma may simply be related to physiological variability's optic nerve appearance. Imaging the optic nerve for a patient with a suspicious appearing optic nerve may lead to confirmation bias: comparing suspicious optic nerves to a normative database of patients with normal appearing optic nerves may only identify statistical outliers with “atypical” optic nerve anatomy, when we seek instead to identify glaucoma. Because patients with unique optic nerve anatomy will not a priori have an abnormal macular retinal nerve fiber layer, macular scanning for the glaucoma patient could provide an independent (higher specificity) confirmation of glaucomatous damage. Additionally, macular thinning that respects the temporal raphe is akin to visual field damage that respects the horizontal meridian in glaucoma. (Figure 1)
One area in my practice where macular imaging in glaucoma suspects has fallen short has been myopic eyes. Unfortunately, these patients may have obliquely inserted optic nerves along with atypical macular anatomy (including posterior staphyloma). Furthermore, long axial lengths are associated with both lower OCT signal quality and with lower retinal thickness values and the normative databases typically exclude myopic eyes, limiting the conclusions that can be drawn from these studies.
Finally, scanning both the nerve and macula in our glaucoma patients, many of whom are elderly, will also allow us to confirm that central visual field defects present in these patients are not caused by other age-related macular pathologies. In conclusion, including macular OCT analysis at the time of optic nerve head imaging for glaucoma patients may allow us to keep our patient's visual function at the center of our attention.
Figure 1. An OCT of a patient with low-risk ocular hypertension with a normal visual field. The optic disc analysis shows a normal average RNFL thickness without any focal retinal nerve bundle defects. However, the ganglion cell analysis overlaying the macula revealed a linear demarcation of retinal ganglion cell loss that respected the temporal raphe, suggesting damage originating at the optic disc, or glaucoma.
1. Hood DC, Raza AS, de Moraes CG, Liebmann JM, Ritch R. Glaucomatous damage of the macula. Prog Retin Eye Res. 2013 Jan;32:1-21.
2. Schiefer U, Papageorgiou E, Sample PA, Pascual JP, Selig B, Krapp E, Paetzold J. Spatial pattern of glaucomatous visual field loss obtained with regionally condensed stimulus arrangements. Invest. Ophthalmol. Vis. Sci. 2010;51:5685–5689.
Nathan Radcliffe, MD, a glaucoma specialist, is an assistant professor of ophthalmology and director of the Glaucoma Service at Weill Cornell Medical College and New York-Presbyterian Hospital in New York.