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Editors Selection IGR 23-2

Anatomical Structures: OCT Pitfalls: When Glaucoma coexists with AMD

Comment by Lucy Shen & Melissa Yuan on:

104446 Measurement of the Inner Macular Layers for Monitoring of Glaucoma: Confounding Effects of Age-Related Macular Degeneration, Chew L; Mohammadzadeh V; Mohammadi M et al., Ophthalmology. Glaucoma, 2023; 6: 68-77

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The utility of macular OCT for the monitoring of glaucoma, particularly advanced glaucoma, is becoming well recognized,^1,2 with measurement of the inner macular thickness playing an important role. In this retrospective study, Chew et al. examined how nonexudative age-related macular degeneration (AMD) affects the inner macular layer measurement on Heidelberg Spectralis OCT. The 65 patients included in this study had non-exudative AMD and were excluded if they had any other retinal pathology affecting the macula, diagnosis of glaucoma or glaucoma suspect, or any other optic nerve disorders. Patients were also excluded for poor image quality. The important takeaway is that the automated segmentation by the Glaucoma Module Premium Edition software incorrectly identified the boundaries of the ganglion cell complex (GCC) in 9.5% of the images and is likely to fail with larger drusen (with heights above 185 µm), but was generally accurate with small to intermediate sized drusen. Specifically, segmentation was accurate 78% of the time with drusen height between 145 and 185 µm, and 64% of the time with drusen height above 185 µm. When drusen height was normalized with total retinal thickness, incorrect segmentation was found 36% of the time with drusen to total retinal thickness ratios of 0.45 or higher. Furthermore, larger drusen caused displacement of the inner macular layer, especially with drusen heights above 176 µm. Artifactual inner retinal displacement has the potential to cause a decreased GCC thickness that may be misinterpreted as glaucomatous damage. Additionally, geographic atrophy caused incorrect segmentation in 87% of images.

With these findings, physicians will be aware of which clinical and OCT findings are most likely to cause artifact ‐ namely, larger drusen and geographic atrophy ‐ and that small drusen are not likely to impact inner macular thickness on OCT

The strength of this study is that it provides preliminary guidelines in interpreting inner macular measurements in the presence of dry AMD. With these findings, physicians will be aware of which clinical and OCT findings are most likely to cause artifact ‐ namely, larger drusen and geographic atrophy ‐ and that small drusen are not likely to impact inner macular thickness on OCT. One important limitation is the use of the Heidelberg Spectralis OCT with a specific scanning and segmentation protocol. The protocol used in this study involved taking macular cube scans centered on the fovea, with each B-scan repeated nine to 11 times to decrease the noise and improve resolution. The GCC segmentation was performed using the Glaucoma Module Premium Edition software, which is a built-in Spectralis application but requires a separate purchase. The scanning protocol and segmentation software described in this paper are different from those of other OCT machines, such as the Cirrus (Zeiss) or the RTVue-100 (Optovue), and may limit the generalizability of the authors' findings. While this study is also limited due to its exclusive focus on AMD, its findings are likely reflective of how outer retinal diseases may affect inner macular OCT segmentation. It will be interesting for future research to explore how macular diseases and coexisting glaucoma interact when macular OCT is used for glaucoma monitoring.

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