الفهرس 
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Abstract
The macular region contains a high concentration of more than 50% of retinal ganglion cells (RGCs), which can be quantified relatively easily compared to peripheral RGCs that may be too thin for OCT to measure reliably. In addition, the macular region is the primary location of glaucomatous damage in the disease’s early stage. So it does make sense to measure RGCs in the macular region. On the other hand, unlike the circumpapillary retinal nerve fiber layer (cpRNFL) that covers the entire of RGC axons, current ganglion cell analysis ignores about 50% of RGCs outside the macular region, representing the strategy’s major weak point.
Many studies have investigated GCC and/or GCIPL performance in glaucoma assessment compared with the cpRNFL. They all agreed the usefulness of both in the diagnosis of glaucoma and the assessment of its progression. While some studies took another perspective and combined ganglion cell analysis with conventional cpRNFL and optic disc analysis instead of comparing their performance. These researchers found that combining structural measurements improved glaucoma assessment performances more than using them separately. Currently, no definitive index illustrates the magnitude of glaucomatous damage that is automatically calculated based on the available structural measurements. Actually the assessment of the role of GCL and cpRNFL depends more or less on a good OCT image acquisition technique.
The macular region is the easiest location to perform OCT imaging, because the optical axis of the eye is naturally aligned to the foveola. Conventional cpRNFL requires an ONH scan, which needs an optical path shifted from the optimal central path. In addition, patients are instructed to look at a fixation target that deviates from the natural center, which places additional tension and strain on the eye. All of these affect OCT signal quality. It was found that there is a consistent trend of better OCT signal quality with macular scans than ONH scans, especially in elderly and diseased eyes. So it is ideal to perform both macular and ONH scans for glaucoma assessment when possible. However, clinicians should prioritize macular scans, because the ganglion cell analysis provides equivalent glaucoma assessment performance, more or less, as the cpRNFL. Therefore, all glaucoma patients undergo macular scans first, then ONH scans.
Two new OCT parameters for glaucoma assessment have been introduced: ganglion cell inner-plexiform layer (GCIPL) and ganglion cell complex (GCC) thickness. Neither is a pure ganglion cell layer analysis, however, because it is difficult to segment the border between ganglion cell and inner-plexiform layers. So these two layers are thus combined together as GCIPL to reduce the inaccuracy of automated layer segmentation. GCC goes one step further by including macular RNFL on top of the GCIPL. Because all inner retinal layer borders are generally harder to segment in a precise and reproducible way than inner limiting membrane and retinal pigment epithelium, combining multiple layers improves the stability (or reproducibility) of the segmentation performance; at the same time, It may also reduce the sensitivity to glaucomatous damage by including a structure that is not the primary site of glaucomatous damage which is the inner-plexiform layer.
With a much larger dataset, it is now possible to make more accurate correlation maps and to scan larger regions of the posterior pole with newer models of FD-OCT that have higher scan speed. More complex modeling is to apply the relationship between structure and function which allows to predict function from structure. In addition, using the regional relationship between GCC, NFL, and VF may improve the results of indices for staging and detecting glaucomatous damage that currently use global parameters, such as the combined index of structure and function (CSFI).
Our study found that both GCC and pRNFL thickness are highly significant when comparing patient and control groups. Our results also showed better diagnostic capability of GCC over pRNFL to detect early glaucomatous changes. We also found that total GCIPL is the best macular parameter with AUC 0.840 while the total pRNFL AUC 0.791.
Our results also showed a low correlation of structural damage with functional damage and this is in agreement with other studies that proved RGC dysfunction may precede axonal numerical loss in the presence of normal structures.
To conclude, the assessment of GCC parameters plays an important role in diagnosis and monitoring of glaucoma. Our study showed that SDOCT is a useful ancillary diagnostic tool for evaluation of early macular and circumpapillary structural changes in glaucomatous eyes