February 2009




Better detection of macular edema

by Matt Young EyeWorld Contributing Editor


Study uses OCT and baseline central point thickness to analyze edema

This coronal OCT scan depicts the corneal graft and placement of a glaucoma implant tube; OCT may also be useful for detecting macular edema Source: Julian P. S. Garcia Jr., M.D.

Although optical coherence tomography (OCT) has become an important modality in detecting macular edema (ME), until now there has been no standardized method for detection. That is changing. A study published in the June 2008 issue of Retina, suggests that a 40% or greater increase in baseline center point thickness (CPT) as determined by OCT is a reliable and valid means of reporting clinically relevant post-cataract ME. “Currently in the literature, there is no validated or universally accepted method for reporting postcataract ME,” according to lead study investigator Stephen J. Kim, M.D., Department of Ophthalmology, Emory University School of Medicine, Atlanta. “Standardized reporting of post-cataract ME would allow for uniform accounting of disease incidence and objective assessment of treatment outcomes. More importantly, such a definition would enable direct comparison of results among different studies.”

Dr. Kim’s study provides a compelling argument for why the percent increase in baseline CPT method is more reliable than others as a standardized testing method.

Impressive data

Dr. Kim analyzed 130 eyes from two cataract surgery cohorts—one group of diabetes patients and one group of uveitis patients and healthy controls. Fifty eyes (38.5%) had a history of diabetes, 30 eyes (23%) had a history of uveitis, and 50 eyes (38.5%) were healthy controls.

Eyes underwent OCT analysis within four weeks before surgery and at one month and three months after surgery.

At one month, 18 eyes developed cystoid abnormalities (and ME) as determined by OCT. No non-cystoid or diffuse ME surfaced, and no new ME cases appeared at three months. “The development of post-cataract ME in this study appeared to be an all or none event,” Dr. Kim reported. “The lowest baseline increase in CPT for eyes with ME was 81 microns, with several eyes having increases of more than 300 microns. In contrast, the highest baseline increase in CPT for eyes without ME was 72 microns (one eye only), with the remaining eyes tightly clustering around the mean baseline increase of 8 microns. Therefore, a baseline increase of greater than 80 microns might appear to be a valid cutoff for defining ME, but such a definition would underestimate ME in patients at the lower end of normal baseline CPT and conversely overestimate ME in patients at the higher end.”

Dr. Kim gave the example of one eye having a baseline CPT of 120 microns (low normal) and a post-op CPT of 192 microns at one month.

“This 62-micron increase would not meet the greater than 80 micron cutoff mentioned above yet was associated with a two-line loss of vision.,” Dr. Kim noted. Percent increase in baseline CPT, in Dr. Kim’s view, has more advantages as a standardized method to define ME. “The average percent increase in baseline CPT for eyes with and without ME was 115 ± 67% and 6 ± 11%, respectively,” Dr. Kim wrote. “Therefore, 3 SDs above the 6% mean for eyes without ME is roughly 40%. Using a greater than 40% increase in baseline CPT as a cutoff for ME accurately categorized all 18 eyes with ME (100%) and 111 (99%) of 112 eyes without ME.”

There was one eye mistakenly thought to have ME. This eye had a 46% increase in baseline CPT, but 20/20 vision at one month with now cystoid changes observed by OCT. “Because the coefficient of variation of OCT is as high as 10%, it seems plausible that an underestimation of baseline CPT in combination with an overestimation of post-operative CPT could result in rare false-positive results for sufficiently large sample sizes,” Dr. Kim noted. Dr. Kim suggested percent increase in baseline CPT has the following advantages: 1) It calculates the percent change using the patient’s own baseline and post-op CPT readings and therefore helps eliminate interpatient and interinstitution variability. 2) It automatically adjusts for extreme normal ranges of baseline CPT. 3) It can be adaptable for future OCT generations. and 4) It’s simple. “Our conclusions may not be valid for instruments other than OCT 3 [OCT 3, Carl Zeiss Meditec, Dublin, Calif.],” Dr. Kim cautioned. “Nevertheless, we are confident that our methodology can be readily validated for future OCT versions and offers the best overall means of defining ME.”

OCT is also a better method of finding ME than other methods, said Mark Packer, M.D., clinical associate professor of ophthalmology, Casey Eye Institute, Oregon Health & Science University, Portland, Ore. “There’s no question OCT is very sensitive to finding macular thickening,” Dr. Packer said. “It’s probably better than florescein angiography. It’s much less invasive than that.”

Still, for most patients that have normal vision after cataract surgery, ophthalmologists can assume they have a normal macula, Dr. Packer said. The OCT is particularly useful before cataract surgery to see if someone who is scheduled for surgery has a normal macula, Dr. Packer said. That helps to set expectations, because even when the cataract is removed, some patients could still experience decreased vision resulting from ME, he said. In a world where patients are asking for multifocal IOLs for optimal vision, setting realistic expectations is as important as ever.

Editors’ note: Dr. Kim has no financial interests related to this study. Dr. Packer has no financial interests related to his comments.

Contact information

Kim: Skim30@gmail.com
Packer: 541-687-2110, mpacker@finemd.com

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