8.3Optical Coherence Tomography and the Retinal Thickness Analyzer

OCT allows the quantitation of retinal thickening and assessment of morphologic changes. Some of the latter include cysts, presence of subretinal ßuid,

and preservation or absence of the IS/OS junction, which may have prognostic signiÞcance.50,75

There are many OCT instruments in use, and they use different segmentation algorithms for measuring thicknesses in the retina. The inner retinal boundary is the same Ð the internal limiting

membrane of the retina. However, the outer boundaries differ. The outer boundary for the Stratus OCT is the IS/OS junction. For the Cirrus OCT, the outer boundary is the apical surface of the RPE cell layer. For the Spectralis OCT, the outer boundary is the basal surface of the RPE cell layer.

Artifacts can be a problem in OCTs obtained in eyes with RVO (Fig. 8.6). There is some evidence that the rate of artifacts in OCT measurements is higher in RVO than in diabetic macular edema, for instance. Manual measurement of CPT by the Wisconsin Reading Center was performed in 29% of SCORE OCTs but only 19% of OCTs from a clinical trial of diabetic macular edema.26 Possible reasons include the effects of intraretinal and subretinal blood and of subretinal ßuid in RVO.26

OCT-measured macular thickness correlates modestly with visual acuity in nonischemic BRVO with macular edema with the correlation coefÞcient ranging between 0.232 and 0.591.7,45,73 OCT-measured macular thickness is routinely used as a primary and secondary outcome measure in studies of BRVO and as an important index to follow in the clinical care of patients with BRVO.94 Changes in OCT-measured macular thickness and changes in visual acuity correlate poorly in BRVO. In the SCORE BRVO study, the Pearson correlation coefÞcients between visual acuity changes and OCT CPT changes over 12 months were −0.19, −0.30, and −0.10 for the grid laser, 1, and 4 mg groups, respectively.94

Although some studies use center point thickness from the OCT as the outcome variable, most use CSMT because of its greater reproducibility. There are many outcome variables of interest including macular thickness, absolute and relative macular thickening, and logOCT. For eyes with marked macular thickening as is frequently the case with RVO, the best outcome variable is relative change in macular thickening.

Certain morphological characteristics of the OCT have predictive importance for Þnal visual outcome in RVO with macular edema. In branch retinal vein occlusion associated with macular edema persistent for at least 6 months, the Þnal visual acuity was not correlated with total foveal thickness (r = 0.336, P = 0.092). However, it did correlate with the thickness measured from the posterior surface of macular cysts and the RPE (termed the photoreceptor layer thickness) (r = −0.571, P < 0.0001) and with the integrity of the inner segmentÐouter segment junction. Eyes with a continuous IS/OS junction had signiÞcantly better Þnal visual acuity than eyes with an interrupted IS/OS junction (P < 0.0001).71,76,77 In cases where the foveal IS/OS junction cannot be seen at baseline, presumably due to swelling and misalignment of photoreceptors secondary to leakage from inner retinal capillaries, the parafoveal IS/ OS junction has also been found to predict Þnal visual outcome.76 The confounding effects of pretreatment visual acuity need to be controlled before IS/OS junction continuity is accepted as an important predictor of Þnal visual outcome in

Fig. 8.5 A 69-year-old woman with hypertension, hypercholesterolemia, and obstructive sleep apnea syndrome developed a left superotemporal BRVO. (a) Montage color fundus photograph shows the extent of the involved retina. Cotton wool spots are evident and the temporal fovea is impinged by the blood. (b) MagniÞed view of the involved arteriovenous crossing. The artery crosses the vein at the black arrow. The blue arrow denotes thrombus with white Þbrin marbling downstream of the involved arteriovenous crossing. The red circle encompasses a cotton wool spot. (c) A frame from the early-phase ßuorescein angiogram shows delayed Þlling of the superotemporal retinal vein. (d) Frame from the mid-phase ßuorescein angiogram. The thrombus downstream of the crossing is

hyperßuorescent due to pooling of stagnant ßuorescein trickling through the occlusion (blue arrow). The orange arrow shows speckled hyperßuorescence in a dilated upstream section of the obstructed vein, a sign of sluggish blood ßow. The green arrow denotes stained venules in the involved retina, a sign of ischemia. The yellow oval encircles a zone of poor capillary perfusion. The orange oval encircles the area of the cotton wool spot (compare (b)), which in the ßuorescein angiogram blocks the capillary detail. (e) Frame from the late-phase ßuorescein angiogram. Most of the involved retina shows extreme hypermeability of microvasculature (hyperßuorescence), but the nonperfused areas are relatively hyperßuorescent (yellow oval)

Fig. 8.6 Fundus images of a 75-year-old woman with an old hemicentral retinal vein occlusion of the right eye developed sudden blurring of the left eye from an acute superotemporal branch retinal vein occlusion. (a) Glaucomatous cupping of the right optic disk is seen. The inferotemporal retinal veins are sheathed and the inferotemporal retinal arteries are narrowed. (b) Glaucomatous cupping of the left optic disk is seen along with fresh hemorrhages of the superotemporal retina. (c) Frame from the mid-phase ßuorescein angiogram of the right eye. Areas of capillary nonperfusion are present inferior to the macula. (d) Frame from the mid-phase ßuorescein angiogram of the left eye. Capillary perfusion is good. (e) Optical coherence tomography of the left eye 9 months before the left branch retinal vein occlusion. An artifact of the segmentation algorithm renders the nasal false-color map artifactitious (red ovals). (f) OCT scan at the time of the acute BRVO of the left eye. No artifacts are present. Note the marked increase in the superior inner and outer paracentral sectors compared to the scan from December 15, 2010 (compare purple and blue arrows from (e) and (f). The variation in thickness of

the inferior paracentral sectors (blue arrows, 219 vs. 222 m) exempliÞes the intersession agreement of OCT measurement because nothing has changed in this sector. The variation in thickness in the superior paracentral sectors (purple arrows, 229 vs. 255 m) represents a real change, in excess of the intersession variability of OCT measurements. (g) OCT of the right eye from December 15, 2010, shows a segmentation artifact (red oval). In this display, without the segmentation lines shown, only the false-color maps indicate that an artifact has occurred. (h) OCT of the right eye from August 11, 2011, showing a different segmentation artifact from the December 15, 2010 study. In this case, the display shows the segmentation lines, and it is clear that the algorithm mistook the posterior hyaloid face for the internal limiting membrane (red oval). The artifact has not compromised the central subÞeld measurement (compare yellow arrows from (g) and (h) both 197 m), but the difference in the outer paracentral subÞeld measurements in (g) and (h) (279 vs. 215 m) is artifactitious (compare blue arrows from (g) and (h))

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