Choroidal Neovascular Membrane
In the eye, between the retina and the sclera (what is known as the “white of the eye”) is a structure called the choroid. The choroid is highly vascular, meaning that it has many blood vessels, which have the function of bringing nutrients and oxygen to the nerve cells of the retina, particularly those near the macula. Separating the choroid and the retina is a thin layer of tissue known as Bruch’s membrane, which has the job of keeping the choroid from breaking through into the outer layer of the retina.
Breaks in Bruch’s membrane allow blood vessels to begin to grow from the choroid just beneath the retina, the underlying pathology of a choroidal neovascular membrane. New blood vessels are always extremely fragile and break easily, sometimes from things we take for granted, like coughing, for example. When blood and fluid begin to accumulate beneath the retina or grow into it, damage to the delicate tissues result. Sometimes, the damage becomes severe enough that it can eventually cause scarring, especially if not treated; it is possible for the result to be severe, permanent loss of vision in the central part of the visual field.
The new blood vessels tend to form a mat of blood vessels, like a membrane, which is where it gets its name. This diagram shows an artist’s conception of new blood vessels branching out from existing ones, forming a web-like structure.
Causes not Always Known
Choroidal neovascular membranes have been associated with AMD, age-related macular degeneration, particularly the “wet” form, because it is characterized by leakage of serum and fluids by new blood vessels into the macular area of the retina.
However, choroidal neovascularization has also be associated with other diseases, including ocular histoplasmosis and other retinal degenerations or trauma. A definitive cause in many cases remains unknown.
People over 65 years of age are at higher risk to develop a choroidal neovascular membrane or AMD. Family history of any of these conditions is also an indicator of the need for regular testing.
The structures most affected are responsible for central vision, which can be devastating, making it impossible to read, drive or even recognize faces. Peripheral vision is usually not affected, however, so total blindness is not usually a result. Symptoms associated with a choroidal neovascular membrane include:
- Sudden distortion or blurring, especially in central vision
- Central light flashes
- Subtle changes in colour vision perception
- Painless loss of vision in one eye
The severity of these symptoms are largely dependent on the size of the membrane and how close it is to the macula. If the damage begins in the non-dominate eye, it is possible to develop severe vision loss without symptoms until each eye is tested separately.
Time is of the essence to intervene as the sooner treatment begins the better the ultimate result on progressive vision loss. Preventing vision loss depends to some extent on early detection during a vision examination. If a patient’s history includes macular degeneration or presumed ocular histoplasmosis, he or she should be getting regular checkups from an eyecare practitioner in any case, and using an Amsler Grid for home-monitoring of vision.
The first line of treatment is the use of relatively new drugs known as anti-VEGFs, which are injected into the back of the eye to halt the progression normally seen over time. VEGF stands for Vascular Endothelial Growth Factor, a substance that signals the formation of new blood vessels in the area affected. Anti-VEGFs slow this process down and can stop it completely. There are currently three medications available for this type of treatment.
Injection of ranibizumab, one of three anti-VEGFs was approved by the FDA in the US for treatment of all types of neovascularization in age-related macular degeneration. Anti-VEGFs are the only treatments that have been shown to improve visual clarity up to 40% of the time.
Anti-VEGFs are delivered via an injection into the vitreous body, the gelatinous substance that fills the back of the eye and helps hold the retina in place. The main limitation of anti-VEGF therapy is that it usually requires repeated injections, sometimes with tachyphylaxis, a loss of effectiveness following repetitive use. In this case, a short “drug holiday” will usually restore the lost effectiveness. Also, in rare cases, tolerance to the drug, which develops more slowly can occur, but increasing dosages or shortening the interval between injections will improve its performance and effectiveness.
Follow-up care is extremely important for effective treatment of choroidal neovascular membranes; repeated injections may be required to maintain the regained vision at the previous levels. If less-frequent follow-up results in fewer injections, the initial gain may be lost.
Previously, the best treatment available for a choroidal neovascular membrane was photodynamic therapy (PDT), a two-stage process that begins with an injection of light-sensitive dye called Visudyne into the blood vessels that supply the eyes. Visudyne is designed to collect in the abnormal blood vessels; then, a surgeon can use a laser shining into the eye to activate the dye, which destroys them by sealing them off from the source.
Currently, PDT may be used as part of a protocol of treatment of a neovascular membrane along with anti-VEGFs, especially in cases with larger areas of neovascularization that are located further away from the macular area, as part of a protocol of treatment involving both types of treatment.
Unfortunately, rates of recurrence of neovascularization are high; there is also the possibility of side effects, adverse events and complications resulting from any therapy, but treatment is nearly always preferable to doing nothing.
Preserving good vision is always a worthy goal. Like many other eye diseases, choroidal neovascular membranes don’t always have symptoms that are apparent, resulting in vision loss before individuals become aware of them.
The best way to prevent this and other causes of vision loss is to have regular examinations by an eyecare practitioner, both for early diagnosis and for regular follow-up care.
Patients over the age of 60 should be seen for a comprehensive vision examination each year and more often if necessary to follow and treat conditions such as this.