David J Browning, MD, PhD
This talk emphasized the superiority of optical coherence tomography (OCT) to detect diabetic macular edema (DME) over clinical exam and fluorescein angiography (FA).
The Early Treatment Diabetic Retinopathy Study (ETDRS) criteria for clinically significant DME (CSME) underdiagnose DME. In the Diabetic Retinopathy Clinical Research network (DRCR.net) MMG laser trial currently in press, 55% of eyes had baseline thickening <300 microns. Clinical examination misses this degree of thickening in about 25% of eyes. Since focal laser treatment usually stabilizes, rather than improves, visual acuity, using ETDRS criteria for CSME implies that many patients will be treated late. In 2006, OCT is the de facto standard for CSME diagnosis and treatment decisions, not the ETDRS clinical exam criteria.
CSME is diagnosed on the basis of macular thickening, but the key patient variable is visual acuity (VA). VA for a given macular thickness can vary greatly. For example a 400-micron thick retina may have a VA ranging from 20/20 to 20/125.
The OCT scanner automatically computes 3 variables: centerpoint thickness, central subfield mean thickness, and the total macular volume. The centerpoint thickness might be expected to correlate most closely with VA, but some OCT scanners compute this value from just 6 radial scans, whereas the central subfield mean thickness is computed from 512 scans, so it is more robust to artifact. The DRCR.net MMG study found that the correlation of VA with central subfield macular thickness was 0.52. (Diabetic Retinopathy Clinical Research Network. Relationship between optical coherence tomography-measured central retinal thickness and visual acuity in diabetic macular edema. Ophthalmology. 2006; In press. DOI: 10.1016/j.ophtha.2006/06/052.)
Using OCT to make treatment and retreatment decisions requires that the ophthalmologist know the repeatability of the test. This is especially important given the normal spontaneous variability of DME. The 95% confidence value for a change in central subfield macular thickness is 11%. With current OCT scanners, it is difficult to integrate and analyze longitudinal patient data, so we have developed an Excel spreadsheet template to track central subfield macular thickness, total macular volume, visual acuity, and intraocular pressure. The spreadsheet and instructions can be downloaded from http://www.theretinaexchange.com/topic.php?forumtopic= doct</A>.
Even before the advent of clinical OCT, the utility of FA for treatment planning was in question. FA does show leakage, and the early ETDRS required it, so it was considered a necessary treatment planning tool during the 1990s. However, the publication of ETDRS report number 19 (Arch Ophthalmol. 1995;113:1144-1155.) showed that no aspect of FA was predictive of the outcome of focal laser treatment for DME. Another study showed that retina fellows, but not experienced retina specialists, benefitted from FA for treatment planning (Kylstra JA, Brown JC, Jaffe GJ, et al. The importance of fluorescein angiography in planning laser treatment of diabetic macular edema. Ophthalmology. 1999;106:2068-2073.). FA is necessary if there is a chance that ischemia is causing decreased visual acuity, but this is rare (fewer than 1% of patients in DRCR series). Clinically, ischemia may be suspected if the patient’s visual acuity is much worse than would be expected from the degree of DME observed on clinical exam. Ischemia is unlikely in patients with less than severe nonproliferative diabetic retinopathy.
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