Macular edema is a leading cause of treatable vision loss in patients with diabetic retinopathy [1,2]. The high longitudinal resolution of OCT is effective in quantifying macular thickness due to the well-defined differences in optical reflectivity at the anterior and posterior boundaries of the neurosensory retina. Measurements of central foveal thickness with OCT correlate with visual acuity and can provide an indication of the relative contribution of macular edema, as opposed to other factors such as macular capillary non-perfusion, to vision loss . Retinal thickness has been shown to be better correlated with visual acuity than fluorescein leakage in patients with macular edema [4,5], Sequential OCT tomograms allow the ophthalmologist to accurately track increases or decreases in retinal thickness with more sensitivity than slit-lamp biomicroscopy . Alternatively, multiple OCT tomograms may be obtained at a series of radial or linear sections through the macula to screen for early retinal thickening throughout the macula 13}.
The morphological changes in macular edema visible in diabetic retinopathy include a decreased intraretinal reflectivity corresponding to cystic changes and fluid accumulation. The reduced back-scattering is usually most evident in the outer retinal layers, and tomograms of cystoid macular edema closely correspond to classically described histopathologic findings. Chronic cystoid macular edema may lead to the development of a lamellar macular hole, which is evident on OCT as a partial-thickness loss of retinal tissue and an abnormal retinal contour suggesting cystic rupture. Hard exudate appears as a focal area of high intraretinal backscatter which completely shadows the reflection from the neurosensory retina and choroid below the lesion. Hemorrhage also blocks the reflections returning from the deeper retinal layers due to the high scattering and associated high attenuation of light propagating through blood.
Proliferative diabetic retinopathy often leads to neovascularization and preretinal fibrosis. Preretinal membranes are visible in cross-section as thin, reflective bands anterior to the retina. Retinal traction and detachment are often present, and their extent can be directly measured from the OCT tomograms. The distinction between preretinal fibrosis and a detached posterior vitreous is made on the basis of reflectivity. The posterior hyaloid tvpicallv has a lower reflectivity l J J IT J J
than a preretinal membrane due to the optical transparency of the vitreous. Cotton wool spots due to retinal ischemia appear in OCT cross-section as regions of increased reflectivity of the retinal nerve fiber laver j J
and inner neurosensory retina.
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