The macula is composed of multiple layers organized from innermost to outermost: inner limiting, nerve fiber, RGC, inner plexiform, inner nuclear, outer plexiform, outer nuclear, and the retinal pigment epithelial (RPE) layers. Glaucoma primarily affects the axon and body of RGCs, which constitutes the inner retina.
Differentiation of individual retinal layers by ocular imaging requires a high level of resolution and an advanced segmentation algorithm. Ishikawa et al. developed a customized segmentation algorithm for macular segmentation using TD-OCT images.16
They demonstrated comparable glaucoma diagnostic capability of the inner retinal layers with circumpapillary RNFL. Moreover, inner retinal layer thickness was statistically significantly better in discriminating between healthy and glaucomatous eyes than total macular thickness. Wang et al. measured the RGC layer thickness in glaucomatous eyes with an SD-OCT device, using a computer-aided manual segmentation procedure.20
They reported that it was feasible to obtain local measurements of RGC thickness that corresponded to functional findings.
Manufacturers of SD-OCT devices incorporated automatic macular segmentation analysis into their operating system. Ganglion cell complex (GCC) was designated for thickness measurements from the internal limiting membrane to the inner nuclear layer, which is composed of RGCs, along with their axons and dendrites. Other manufacturers segmented the inner retina into the ganglion cell inner plexiform layer (GCIPL). Cho et al. examined the relationship between VF mean sensitivity (MS) and macular GCC thickness, and reported a similar level of correlation as with circumpapillary RNFL thickness.21
Rao et al. also demonstrated that the strongest structure-function association among the various macular measurements using SD-OCT was found to be the inner retinal thickness.22
Assessing the diagnostic performance of GCC thickness, Seong et al. reported that GCC thickness was comparable to circumpapillary RNFL thickness in terms of glaucoma diagnostic capability.23
Sakamoto et al. compared macular RNFL images obtained by 3D SD-OCT with those obtained by color and red free fundus photography. They found that more macular RNFL defects were detected on 3D SD-OCT images than on color fundus photographs.24
Kim et al. classified glaucoma into three disease severity groups based on VF’s mean deviation. They reported that the macular GCC and circumpapillary RNFL thicknesses showed similar diagnostic performances in detecting early, moderate, and severe glaucoma.25
Kotera et al. showed that the mean macular inner retinal thickness was significantly thinner in suspected glaucoma and preperimetric glaucomatous eyes than in the healthy eyes, while mean total retinal and macular NFL thicknesses were not.26
This report might point out the utility of macular inner retinal measurements as an early indicator of glaucomatous change.
Assessing covariability associated with macular inner retinal thickness, Kim et al. showed that thin GCC thickness correlated with older age and longer axial length.27
Similarly, Mwanza et al. reported that thinner GCIPL was associated with thinner RNFL, older age, longer ocular axial length, and male sex.28
Taken together, published studies indicated that the segmented macular inner retinal layer thickness performs better than the total macular thickness and similar to (but not better than) the circumpapillary RNFL thickness in glaucoma diagnosis.