Cystoid Macular Edema ("CME")
Description of Cystoid Macular Edema CME
Cystoid Macular Edema CME refers to fluid accumulation and swelling inside the retina in the macular area. The macula is the small area of the retina responsible for our central vision and it is the area where the cone cells are most concentrated. The macula provides the most functional vision.
The retina is made up of 10 different layers and there is a strong blood-retinal barrier to prevent any influx of fluid. Certain things can cause a breakdown of this barrier and resultant fluid accumulation and swelling inside the retina. If you think of the eye like a camera then the retina would be the film in the camera on which the image is produced. If the retinal integrity is disturbed then the image will not be seen properly, especially if the disturbance is in the macula.
There are inner and outer blood barriers in the retina and edema (swelling) may be due to disruption of either. If the inner barrier is compromised the cells inside the retina first begin to swell then burst and fluid accumulates in the extracellular space. The retinal pigment epithelium (RPE) is a layer of pigmented cells with fluid pumps, which maintain the integrity of the outer barrier. If this layer is disturbed for any reason extracellular edema will also result.
CME is difficult to detect through routine ocular examination. It may be noticed as a loss of the foveal light reflex but this is not helpful since loss of this reflex is common in older patients. When CME is suspected based on visual acuity, symptoms or surrounding circumstances it can be diagnosed in one of two ways. The first is a fluorescein angiography (IVFA) which is the old standard of care. An IVFA is a test which requires dye be injected into a vein in the arm and as the dye passes through the retinal blood vessels pictures are taken with UV light. If CME is present the macula will show fluid leakage in a petalloid pattern. The problem with this test is that some people are sensitive to this dye and the injection could be fatal for them. The second diagnostic test which is becoming much more popular is called optical coherence tomography (OCT). The OCT takes a quick cross sectional picture of the retina by a technique known as interferometry. Light beams are directed into the eye at certain angles and the amount of interference is measured creating a very detailed image of the entire depth of the retina. The new OCT machines are spectral domain and have 200% better resolution than an MRI. They give a much more detailed image (5-10 microns) than the older time domain OCT machines. The advantage to an OCT picture is that it is much less invasive, completely safe and much faster than a fluorescein angiography. It takes a couple seconds to get an image just like taking a picture with a regular camera.
The images below were taken with an IVue OCT and demonstrate a normal macula.
Causes of Cystoid Macular Edema CME
Some of the causes of Cystoid Macular Edema CME are listed below:
Ocular diseases such as CMV retinitis, retinitis pigmentosa, progressive pigmentary degeneration, etc
Vitreo-macular traction or epiretinal membrane traction
Eye surgery, including cataract surgery, repair of a detached retina or macular hole
Retinal vaso-occlusive disease
Severe carotid or ophthalmic artery disease
Inflammation of the eye
Injury of the eye
Side effect of oral and topical medications
Ocular diseases or inflammatory conditions that compromise the retinal pigment epithelium could lead to CME due to a breakdown of the blood barrier.
Anything that pulls at the macula has the potential to create an influx of fluid and the resultant cystic spaces of CME. Vitreomacular traction and epiretinal membranes both have this potential. The vitreous is a gel-like material inside the eye which naturally degenerates over time. It is attached to the retina in several places and as it degenerates it detaches from the retina. There are instances when the vitreous begins this process of pulling away from the retina but remains attached at the macula. The macular surface becomes pulled with the vitreous and CME may occur. An epiretinal membrane is an abnormal layer of cells which grows across the macula and begins to distort the tissue. This creates the same effect of pulling and possible CME. In the image below the arrow labeled “evolving PVD” (posterior vitreous detachment) is pointing to the vitreous pulling away from the retina. The arrow to the left of that is showing the vitreous pulling on the surface of the macula.
CME that occurs as a result of cataract extraction is referred to as Irvine-Gass syndrome. It usually starts about six to ten weeks after the procedure and is transient in most cases. The incidence has been studied and reported to occur in about 19% of people after cataract extraction and lens implantation but some believe the incidence may be related to surgical skill. There is no age, race or sexual predilection.
Certain topical ocular medications can precipitate CME. Epinephrine and propine are two examples that used to be prescribed for glaucoma but are not commonly used anymore. Prostaglandin analogs (Lumigan, Travatan Z, Xalatan), which are topical medications commonly prescribed for glaucoma, have been implicated in the start of CME especially when the posterior lens capsule has been compromised for some reason. The oral medication Niacin is used for elevated cholesterol levels. It may cause CME without the fluid leakage typically seen on fluorescein angiography and is therefore referred to as pseudocystoid macular edema. Nicotinic acid which is present in certain nutritional supplements could also contribute to CME.
The OCT image below is an example of CME. The arrow pointing to cystic edema is showing a dark fluid-filled cyst inside the retina, below the surface of the macula. This mild edema may not be visible with routine ophthalmoscopy performed at a vision exam.
The OCT image below shows a severe case of CME. You can see the dark holes in the tissue where fluid has collected and the macula is very edematous (swollen).
Symptoms of Cystoid Macular Edema
Should macular edema become severe enough, it will start to affect vision. It may be noticed as decreased visual acuity, metamorphopsia (distortion of images), color vision change, loss of contrast sensitivity and prolonged recovery from bright lights.
Treatment/Prognosis of Cystoid Macular Edema
The prognosis for untreated CME after cataract surgery (Irvine-Gass syndrome) is very good. About half of the cases will regain normal vision in about 6 months. A smaller percentage (up to 20%) will persist for up to 5 years. Any medications that could cause CME should be discontinued immediately and a comprehensive eye exam should be done to ensure there are no other causative factors. Many studies indicate treatment with prostaglandin inhibitors such as topical indomethacin 1% four times a day for one to four months. This treatment coupled with topical steroids four times a day for three weeks and then tapered may be very effective. For prevention of CME, most cataract surgeons will pretreat patients with a topical non-steroidal anti-inflammatory (NSAID) medication a few days before surgery and for a few weeks after surgery. Topical NSAIDs are also effective when CME occurs 6 weeks or more post-operatively. Systemic NSAIDs and oral indomethacin may also be given.
If the CME is a result of RPE leakage due to conditions such as Retinitis Pigmentosa or chronic inflammation, then oral carbonic anhydrase inhibitors (CAI) such as Diamox (acetazolamide) have been shown to be most effective. Acetazolamide (Diamox) 500mg per day over a month with possible tapering depending on the response is the typical treatment. Acetazolamide works by increasing the fluid resorption through the RPE.
If CME is the primary condition with no known cause then several treatments may be attempted such as prostaglandin inhibitors, oral corticosteriods, steroid injections and topical steroids. Intravenous methylprednisolone 1000mg daily for three days may be attempted for recurrent cases. A vitrectomy (removing the vitreous) may be successful in cases of vitreomacular traction and even in certain cases that are unresponsive to other treatments though no vitreous disturbance is present.
The patient should be seen regularly for follow-ups until the CME is completely resolved.