How do we describe the anatomy of the eye?
The eye is roughly globe shaped and is a complex structure with multiple components that work in unison to form an image on the retina that is perceived as sight by the brain.
The eye can be thought of as analagous to a camera:
- The cornea, which is the transparent covering on the front of the eye, acts much like a lens cover and a focussing element by bending entering light rays through the central ‘black hole’, the pupil, which combined with the coloured iris, the shutter, acts like the aperture of a camera.
- The lens is the next component in line and acts like the lens in a camera by its ability to focus light onto the retina, which acts very much like the film of the camera.
- The retina contains many light sensitive nerve cells called photoreceptors which transmit the light rays into electrical impulses which are delivered to the brain via the optic nerve where an image is perceived.
Similarly to a camera, if the film is of poor quality or malfunctioning, no matter how effective the rest of the components are, the final picture will be poor.
The macula is a small area (5mm) located in the very central part of the retina. There is a greater concentration of photoreceptors at the macula than the rest of the retina, which subsequently provides us with sharp central vision as well as recognition of colours. The macula can be thought of as analagous to the CBD of the retina.
The eyeball cavity itself is filled with a gel-like substance with a similar consistency as uncooked egg white, as is known as the vitreous. The vitreous is normally adherent (in close contact) to the surface of the retina. Around the age of 50-60, the vitreous contracts and separates from the retina. This physiological phenomenon is known as a posterior vitreous detachment (PVD). For more information on PVD, please refer to the section about Floaters, Posterior Vitreous Detachment & Retinal Tears.
What is an Epiretinal Membrane?
In certain people, a thin layer of cells settles on the surface of the macula, commonly after a posterior vitreous detachment. With time these cells contract similarly to scar tissue. This is known as an epiretinal membrane. As the scar tissue continues to contract, it can pull and distort the macula changing its unique shape and affecting its function, resulting in blurry and ‘wavy’ central vision. The macula may also become swollen. Epiretinal membranes will affect ‘straight-ahead’ activities, such as reading and driving.
Who is at risk of developing an Epiretinal Membrane?
Epiretinal membranes typically affect people over the age of 50, following a posterior vitreous detachment. It is not known why some people are more susceptible than others. Epiretinal membranes may also be associated with other eye conditions such as eye inflammation (uveitis), diabetic eye disease, trauma and retinal detachment. Epiretinal membranes do not represent or increase the risk of age-related macular degeneration.
What are the symptoms of an Epiretinal Membrane?
Many people are completely unaware of the development of an epiretinal membrane and are often an incidental finding during a regular eye check. Mild epiretinal membranes may not cause any symptoms but more advanced membranes can result in:
- Increasing blur of printed words
- Difficulty reading
- Distorted vision (straight lines look wavy)
- Reduced intensity or brightness of colours
- Difficulty recognizing faces
Epiretinal membrane progression is highly variable. Most mild epiretinal membranes will remain stable or change very slowly over time. Membranes that have a history of recent and/or rapid growth are more likely to continue to grow and reduce vision.
How are Epiretinal Membranes Detected?
Your eye care practitioner will dilate your pupil with eye drops to obtain a detailed view of the back of your eye. A special lens and slit lamp microscope are used to view the retina and macula in detail. These drops will blur your vision for 2-3 hours and it is recommended that you do not drive on the day of your eye exam.
Optical Coherence Tomography (OCT):
Optical coherence tomography (OCT) allows non-invasive imaging of the macula in cross-section. The OCT "takes a picture" of the back of the eye with a particular focus on the macula. This can confirm if an epiretinal membrane is present and highlights areas of associated thickening or swelling. OCT is often used to monitor membrane progression.
In fluorescein angiography, a small needle is inserted into a vein on the hand or upper limb and a small volume of a vegetable based fluorescein dye is injected which travels through the circulatory system into the blood vessels within the retina. A series of photographs are taken as the dye passes through the blood vessels in the retina. The camera combined with the dye provides a detail view of the function of the blood vessels in the retina. Fluorescein angiography can be used to evaluate the extent of macular damage from an epiretinal membrane, as well as detect and/or exclude other possible macular conditions.
What treatments are available for Epiretinal Membranes?
Unfortunately, there are currently no medications, eye drops or laser treatments that can prevent an epiretinal membrane from developing or halting progression of an already established epiretinal membrane.
A mild epiretinal membrane causes mild blurriness and distortion, and does not usually require treatment. Your retinal specialist will recommend regular reviews to monitor the membrane. You may also be given an Amsler Grid for self-monitoring purposes. This grid is highly sensitive to central distortion.
If an epiretinal membrane progresses to cause significant vision loss, your retinal specialist may recommend vitrectomy surgery to remove the membrane. Vitrectomy surgery is performed under local anaesthesia and copious sedation. Three small instrument ports are inserted into the white of the eye (the sclera) at the front of the eye. Very fine surgical instruments are inserted via these ports and the vitreous is removed under microscopic guidance. A dye is injected into the eye cavity via one of these ports during the procedure to identify the scar tissue layer. The epiretinal membrane is delicately peeled away from the underlying macula under magnification. At the completion of the surgery, a thorough examination of the peripheral retina occurs with treatment applied to any potential weak areas or tears. The eye is filled with filtered air which serves to seal the port sites, maintain the structure of the eye and assist in recovery of the macular. Patients are required to 'posture' after the surgical procedure, which involves positioning the head such that the filtered air in the eye floats to the macula and acts as an internal support structure. The filtered air will be absorbed by the eye over approximately 7-10 days and the eye will produce a liquid known as Aqueous to replace the filtered air. As the filtered air is absorbed, vision will begin to return, however the process of restoring vision occurs over 6-18 months.
Vitrectomy will significantly reduce or eliminate visual distortion and prevent further vision lost. In 80% of cases, it can even restore part of the vision that has been lost. Approximately 20% of patients have no improvement in vision following vitrectomy but the surgery will have prevented further vision loss. In a small number of patients (2%), the vision may be mildly worse following surgery. Patients who prolong surgery and experience greater loss of vision often do not re-gain those lines of vision and can be left with permanently distored central vision.
What can I expect after Epiretinal Membrane surgery?
There is typically no pain following vitrectomy surgery, although you may experience temporary mild-to-moderate redness and grittiness, and swelling and drooping of the eyelid.
Vision is generally limited in the first week or so. Most patients return to their preoperative level of vision one month after surgery. Complete vision improvement usually takes 6 to 18 months, and approximately half of the vision lost is regained. Some mild central distortion may persist. If you have not yet had cataract surgery, a vitrectomy will accelerate the growth of cataract and the cataract will require earlier surgical removal.
If an air bubble is inserted into the eye by your retinal surgeon at the end of the vitrectomy, you will not be able to fly for at least a week. It is safe to return to light physical activity 24 hours after surgery, and moderate activity by 2 weeks. Heavy physical exercise should be delayed for at least 6-8 weeks. You may shower and bathe normally following surgery but take care to avoid getting water into the eye during the initial month after surgery.