techniques, including slit-lamp biomicroscopy, stereoscopic fundus photography, and optical coherence tomography. On clinical examination, a contact lens is particularly helpful for detecting subtle fluid changes in the macula. Stereoscopic fundus photography is useful, but requires a skilled photographer. Optical coherence tomography is an imaging modality that uses near-infrared light beams to create high-resolution cross-sections of the vitreoretinal interface, retina, and subretinal space. Essentially, it provides an in vivo histological section that can demonstrate the cystic spaces that occur in macular edema (Figs. 1–3). Optical coherence tomography can also detect any traction contributing to the macular edema and provide quantitative measurements of the macular fluid.

Clinically, hard exudates often accompany macular edema and appear as well-defined, yellowish white intraretinal deposits. They generally occur in the posterior pole at the border of edematous and nonedematous retinas. These hard exudates are lipid deposits that presumably accumulate in association with lipoprotein leakage caused by breakdown of endothelial tight junctions in microaneurysms or retinal capillaries.

Edema fluid may wax and wane within the retina without visual consequence in some cases, but can result in permanent visual loss if the fluid chronically disrupts the delicate macular architecture. Lipid deposits, especially when under the center of the macula, are a poor prognostic sign as they are often associated with permanent visual loss (35, 36). Increased serum lipid is significant because it correlates with the degree of retinal lipid deposits (37, 38).

Capillary Closure

One of the most serious consequences of diabetic retinopathy is the obliteration of the retinal capillaries. When patches of acellular capillaries, seen early in the course of diabetic retinopathy, increase and become confluent, the terminal arterioles that supply these capillaries often become occluded. Adjacent to these areas of nonperfused retina, clusters of microaneurysms and tortuous, hypercellular vessels often develop. It is difficult to determine whether these vessels are dilated preexisting capillaries or

Fig. 3. The right eye of a 65-year-old woman with a history of nonperfusion and severe macular edema. (A) Red-free photograph shows a ring of hard exudates with marked retinal thickening and edema involving the center of the macula, although not visible in this nonstereoscopic photograph. Several microaneurysms are also visible within the thickened areas. Best-corrected visual acuity was 20/200. Panretinal photocoagulation scars initiated for severe nonperfusion are also evident in this photograph. (B) Fluorescein angiogram at 28 s shows patches of nonperfusion temporal to the fovea and some perifoveal leakage. (C) Optical coherence tomography illustrates the loss of foveal contour in the setting of extensive cystic edema.