CME — What It Is, and Why People with Retinal Diseases Should Know About It
Dr. Gerald Fishman discusses cystoid macular edema (CME), a potentially damaging accumulation of fluid in the retina affecting up to 32 percent of people with retinitis pigmentosa (RP).
In this Q&A, Gerald Fishman, M.D., a world-renowned clinical researcher and longstanding member of the Foundation's Scientific Advisory Board, discusses cystoid macular edema (CME), a potentially damaging accumulation of fluid in the retina affecting up to 32 percent of people with retinitis pigmentosa (RP) and related conditions. It’s a complication that can make matters worse for retinas already fragile from degenerative disease. While diagnosis isn’t always straightforward, CME can be treated to minimize vision loss.
What exactly is CME?
It’s the buildup of fluid, usually in the central region of the retina called the macula, which gives us the ability to read, perceive colors and see in lighted conditions. CME is analogous to having a sponge that’s wet all the time. It eventually deteriorates. If the retina sustains and leaks fluid for a long period of time, it can lead to permanent breakdown of the retinal structure.
A supportive layer of cells called the retinal pigment epithelium (RPE) normally pumps the fluid out. But with CME, the RPE can’t keep up.
How is CME diagnosed?
CME often causes reduced or blurry vision, but there isn’t always a correlation early on between the fluid buildup in the macula and vision loss. This can be surprising. Regardless, patients should always see their doctors with a significant change in vision to address any potential problems, CME or otherwise.
Ultimately, only a doctor can make the diagnosis of CME. If there’s enough fluid in the macula, we’ll often see a honeycomb or four-leaf clover when looking at the back of the eye with an ophthalmoscope during an exam. But we can be fooled; the fluid is there, but we may not see these signs.
To make a definitive diagnosis of CME, we capture an image of the retina’s layers using optical coherence tomography (OCT), a technique employing infrared light. If the fluid’s there, the OCT will detect it.
Even if I don’t see evidence of fluid when looking at the back of the eye, I like to do a baseline OCT for all my RP patients to see if they have CME. OCT will also tell us something about the anatomy in and around the macula.
What retinal diseases can lead to CME?
CME most often occurs in people with night-blinding retinal diseases such as RP, Usher syndrome, choroideremia, gyrate atrophy and enhanced S-cone syndrome. Also, people with X-linked retinoschisis can develop cystic lesions, which are somewhat different from CME; they’re accumulations of fluid, which are treatable, but they don’t leak. We don’t see CME in juvenile macular dystrophies, such as Stargardt disease. It’s also rarely observed in cone-rod dystrophy.
What are the treatments?
My first line of therapy is a topical carbonic anhydrase inhibitor (CAI), such as dorzolamide, which helps the RPE pump the fluid out faster. Some physicians prefer oral CAIs first, because they’re stronger. But those can have unpleasant side effects, such as fatigue and tingling in the fingers and toes.
Other treatment options include steroids and non-steroidal anti-inflammatory drugs. Also, the age-related macular degeneration therapies — Lucentis, Avastin and Eylea — may be considered, because they reduce fluid accumulation.
Not every patient responds to treatment for CME, but this is true for many other drugs and conditions. In some cases, people may not show improvement for as long as four months. I like to see some movement by two months to justify continuing treatment.
In some cases, a patient responds for a while, but the CME recurs. So I’ll pull them off the drug for a few months and then re-start it. Sometimes, they show a positive response again.
Treating CME is an art, and with each case, I try to get better at practicing it.
One other important point: I always make sure the patient is getting the eye drops in their eyes. So I teach them how to do it. If the patient is a youngster, I instruct the parents.