While performing qualitative analysis one should simultaneously perform morphological examination (changes in retinal profile - surface and posterior layers, and presence of abnormal structures) and reflectivity examination (hyper-reflectivity, hypo-reflectivity, and shadowing effects) (Brancato & Lumbroso, 2004). Pathological changes in retinal surface contour may represent disappearance of the normal foveal depression (in macular edema). Steepening of the foveal contour may be associated with epiretinal membranes, macular pseudoholes or lamellar holes. OCT can distinguish between lamellar holes, pseudoholes or various stages of full thickness macular holes. Pathological changes in posterior layers may be RPE detachments (form steep angles with the choriocapillaris) and neurosensory retinal detachments (form shallow angles with the RPE and protrude less). Retinal drusen produce wavy undulations of the pigment epithelium line. Abnormal intraretinal structures may be cotton wool spots (superficial hyper-reflective nodules with indistinct margins in the NFL), hard exudates (round numerous or plaque-like hyper-reflective spots usually in the inner layers, shadowing the deeper structures), choroidal neovascular membranes (nodular or rounded fusiform hyper-reflective structures in front of the RPE, or sometimes visualized as localized thickening of the RPE, choriocapillaris and OS, usually associated with edema or serous retinal detachment), fibrous scars (hyper-reflective structures in the outer retina that deform reduced in thickness retinal layers).
Retinal pathological features can be associated with changes in optical properties of the tissue and thus be detected on the OCT scan as changes in reflectivity. While performing this reflectivity analysis one should always remember that the reflectivity displayed on the scan is a result from the tissue reflectivity, the amount of light absorbed by overlying structures, and the amount of light that reaches the sensor after it has been further attenuated by interposing tissues. Thus care is required in interpreting OCT images when media opacities, poor alignment of the OCT instrument while imaging, high astigmatism or poorly centered intraocular implants are present, as these may reduce signal intensity.
Pathological features that can be hyper-reflective are: epiretinal and thick vitreal membranes, cotton wool spots, hard exudates, thick hemorrhages, retinal fibrosis, RPE hyperplasia or pigmented choroidal nevi, neovascular membranes, atrophy of the retina and RPE (the later cause increased reflectivity of the underlying choroid). Reduced reflectivity (hypo-reflectivity) is most often caused by fluid accumulation: intraretinal edema (it may be associated with formation of optically non-reflective cystoid spaces), or subretinal edema (serous neuroepithelial retinal detachment, serous pigment epithelial detachment). Hypo-reflectivity may also be present in retinal and RPE atrophy or RPE hypopigmentation, where along with tissue hypo-reflectivity there is increase of the reflectivity of the underlying choroid.
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