As a diagnostic modality, OCT is particularly well suited to image the pathologic changes in age-related macular degeneration (AMD) (8). The retinal pigment epithelium (RPE), the site of many pathologic changes in AMD, is well delineated on OCT. Small elevations of the well defined RPE/choriocapillaris reflection on OCT images indicate soft drusen and are consistent with the accumulation of material within or beneath Bruch's membrane (std. case 6). Confluent soft drusen may present as larger areas of elevation and may be distinguished from small pigment epithelial detachments by the lack of shadowing beneath the area of elevation. Elevation of the RPE/choriocapillaris reflection due to retinal pigment epithelial detachment typically exhibits shadowing of structures beneath the area of elevation. This may be the result of changes in the retinal pigment epithelium that do not occur with drusen.
Geographic atrophy is also well imaged on OCT (std. case 7). OCT images demonstrate absence of the minimally reflective band corresponding to the retinal photoreceptor layer. Because of increased penetration of the OCT probe light through atrophic neurosensory retina and RPE the choroid exhibits a well-defined area of increased optical reflectivity. Images taken through an atrophic fovea will lack the normal contour of the foveal depression.
As in non-exudative AMD, the pathologic changes characteristic of exudative AMD are well detailed on OCT (9). Retinal pigment epithelial detachments are identified by focal elevations of the reflective band corresponding to the RPE.
1. Serous RPE detachment (std. case 8a): These appear as focal elevations of the reflective RPE band over an optically clear space. The detached RPE is slightly more reflective than normal, perhaps as a result of morphologic changes in detached RPE. The increased reflectivity of the RPE band severely shadows reflections from the underlying choroid. The angle of the edge of detachment is typically acute, probably because of the tight adherence of RPE cells to Bruch's membrane at the edge of the detachment.
2. Hemorrhagic RPE detachments (std. case 8b): These are distinguished by a moderately reflective layer directly beneath the detached RPE. The blood directly underneath the RPE is only moderately reflective because of attenuation of the OCT probe light through detached RPE. Penetration through the hemorrhage is usually less than 100 microns. Reflections from deeper portions of the hemorrhage and the choroid are severely attenuated.
3.Fibrovascular RPE detachment (std. case 8c): Fibrovascular RPE detachments demonstrate moderate reflectivity throughout the entire sub-RPE space. The lower scattering coefficient of the fibrovascular proliferation compared with blood allows for penetration of the OCT probe light through the entire lesion down to the level of the choroid, where attenuation of the choroidal reflection is typically noted.
no evidence
questionable
definite
can't grade
center involvement
no evidence
questionable
definite
type
serous
hemorrhagic
fibrovascular
can't grade
b. Choroidal neovascularization
Choroidal neovascular membranes typically have 1 of 3 presentations on OCT:
1. Fibrovascular RPE detachment (see above).
2.
3. Well-defined (std. case 9a).
4.
5. Poorly-defined (std. case 9b).
6.
A well-defined choroidal neovascular membrane (CNV) on OCT images appears as a fusiform thickening of the reflective band corresponding to the RPE/choriocapillaris layer. The thickening extends anteriorly on the image creating an elevation in the normally smooth contour of the RPE/choriocapillaris reflection. A poorly defined choroidal neovascular membrane on OCT exhibits an ill-defined, diffuse area of increased choroidal reflectivity consistent with the neovascular membrane. The area of enhanced reflectivity typically blends into the normal contour of the RPE band and a distinct boundary cannot be ascertained.
Optical coherence tomography images can, in some cases, distinguish a Type II choroidal neovascular membrane where the majority of the neovascular complex is anterior to the RPE (std. case 9c), from a Type I choroidal neovascular membrane where the majority of the neovascular complex is below the RPE band (std. case 9d). In a Type II choroidal neovascular membrane, the OCT images may demonstrate an area of increased reflection consistent with the choroidal neovascular membrane that penetrates through the RPE/choriocapillaris band to lie primarily in the subretinal space. In contrast, a Type I choroidal neovascular membrane is located predominantly in the sub-RPE space and basically represents a fibrovascular retinal pigment epithelial detachment which is described above.
no evidence
questionable
definite
can't grade
center involvement
no evidence
questionable
definite
appearance
fibrovascular RPE detachment
well-defined
poorly-defined
can't grade
type
I
II
mixed
can't grade
c. Retinal pigment epithelial tear
Tears of the RPE can be demonstrated by OCT images. The OCT images correspond to the expected histopathology of these lesions. The thick, elevated area of high reflectivity corresponds to the folded double layer of RPE. Choroidal reflection beneath this area is completely shadowed because of decreased penetration of the probe light through the double layer of RPE. The adjacent area of high reflection from the choroid is due to increased penetration of the probe light into the choroid as a result of the absence of overlying RPE.
no evidence
questionable
< std. case 10
>= std. case 10
can't grade
Center involvement
no evidence
questionable
definite
3. Secondary retinal changes
Neurosensory retinal detachments and intraretinal edema may result from the presence of choroidal neovascularization. These 2 entities on OCT imaging do not differ in appearance from that described in the section on retinal pathology. Neurosensory detachments due to exudative AMD appear as elevations in the moderately reflective band just above the more reflective RPE/choriocapillaris band. Intraretinal edema as well as secondary intraretinal cysts can be detected and measured by optical coherence tomography.
Fundus Photograph Reading Center
Department of Ophthalmology and Visual Sciences
University of Wisconsin - Madison
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