A common cause of vision loss in patients with diabetic retinopathy is macular edema. The loss of visual acuity may be correlated with the amount of actual retinal thickening at the center of the fovea. Current treatment of macular edema from diabetic retinopathy is the application of focal and/or grid laser as defined by the ETDRS. Using ETDRS guidelines, laser treatment is applied when patients meet the criteria of clinically significant macular edema. Despite laser treatment, a number of patients progress to moderate visual loss (doubling of the visual angle, eg. from 20/40 to 20/80 or worse) or even severe visual loss (visual acuity less than 5/200) as a result of diabetic macular edema.
A number of new treatments for diabetic macular edema are being investigated. These include a variety of surgical approaches as well as a number of pharmacologic agents. As such, quantifying the amount of macular edema, both in terms of total area of involvement as well as maximal foveal thickness (height) would be helpful in assessing the efficacy of these treatments.
The current standard of assessing macular edema in the setting of diabetic retinopathy is the use of stereoscopic fundus photography as was performed in the ETDRS. Optical coherence tomography, as described above, has been cited in the literature as possibly efficacious in the evaluation of retinal edema (4). The use of both cross-sectional images as well as topographic maps described above may assist with these evaluations. Optical coherence tomography in the setting of diabetic macular edema may be useful in the evaluation of the following:
1. Presence and area of retinal thickening.
2.
3. Amount of retinal thickening at the center (quantified).
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5. Proximity of retinal thickening to the center.
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7. Presence or absence of cyst formation.
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Presence or absence of clinically significant macular edema (ETDRS definition).
Accurate longitudinal comparisons of serial OCT scans depends upon reproducibly locating the central fovea. In patients with central fixation each OCT scan is centered on the patient's fixation such that the OCT scan passes through the central fovea. In patients with eccentric or imperfect fixation, the location of the fovea can be estimated from each OCT scan using a computer algorithm that searches for a focal minimum in total intraretinal reflectivity which typically coincides with the central foveal depression (4). Longitudinal evaluation of topographic maps will utilize the following two methods:
1. Comparison of interpolated retinal thickness within the central circle of 500 micron radius.
2.
Comparison of interpolated retinal thickness within the eight remaining inner and outer subfields.
Should the interpolated data within each of the nine regions prove to be insufficient, each topographic map may be broken down into raw data plots whereby each of the 600 A-scans that comprise the topographic map will be presented in raw data format displaying each individual measurement.
Cystoid macular edema (CME) is a cause of vision loss in a variety of disorders including uveitis, following cataract extraction and venous occlusions. The exact cause of CME remains unknown but may be the result of an increase in prostaglandins resulting in perifoveal capillary leakage or may be due to mechanical stress on perifoveal capillaries from vitreous traction as in the case of cystoid macular edema following cataract extraction.
Ophthalmoscopically, CME appears as elevation or thickening of the central macula. Intraretinal cyst formation is often present. The area of retinal elevation often has ill defined borders both on ophthalmoscopy and clinical examination. The presence of media opacity and/or a small pupil, as is common in uveitic patients, may make determination of the presence and area of CME difficult.
The use of stereoscopic fluorescein angiography to assess degree of macular thickening and area of angiographic leakage in patients with CME has been demonstrated in a few small studies (5,6). The use of stereoscopic fundus photography may also be used to assess degree of macular thickening but like clinical ophthalmoscopy, the area of retinal elevation often has ill defined borders and the presence of media opacity or small pupil may make the determination of both the area of CME as well as the height of CME difficult.
The use of optical coherence tomography for the measurement of cystoid macular edema may be useful (7). Longitudinal measurement of either axial scans and/or topographic images as described above for diabetic macular edema can be utilized. Additionally, the amount of media opacity and pupillary miosis in patients with uveitis will not likely interfere significantly with the images obtained by optical coherence tomography.
Age-Related Macular Degeneration:
Age-related macular degeneration is the leading cause of blindness in patients over the age of 65 in the developed world. Most of the vision loss in this disease is the result of choroidal neovascular membrane formation. Choroidal neovascularization typically appears as either classic choroidal neovascularization (well delineated) or occult neovascularization (less well delineated). The Macular Photocoagulation Study demonstrated that some patients with classic choroidal neovascularization may benefit from laser photocoagulation. The Treatment of Age-Related Macular Degeneration with Photodynamic Therapy Study (TAP) demonstrated that patients with predominantly classic lesions (greater than or equal to 50% of total lesion area) benefited from treatment with photodynamic therapy.
A number of new surgical approaches and pharmacologic approaches are being applied to macular degeneration. Vision loss in age-related macular degeneration typically results when choroidal neovascular tissue with or without concomitant hemorrhage and exudation into the fovea occur. The presence of hemorrhage, subretinal fluid or hard exudate under the fovea are usually detrimental to vision. The presence of choroidal neovascularization itself underneath the central fovea may likewise be detrimental to vision.
Optical coherence tomography, because of its high resolution capability, is able to image subretinal fluid, intraretinal thickening and sometimes choroidal neovascularization (8,9). As a result these capabilities, OCT may have utility in the assessment of new treatment modalities for age-related macular degeneration. Similar to the protocol described above for both diabetic macular edema and cystoid macular edema, assessment of the structure of the fovea on a longitudinal basis can be assessed.
Fundus Photograph Reading Center
Department of Ophthalmology and Visual Sciences
University of Wisconsin - Madison
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