THE INTERFACE BETWEEN THE RETINA
AND GLAUCOMA
Like Gaul , the eye consists of many parts, but in total its minions (the cells) number about six million. Each part has its own governor; in the case of the eye, the governor is the specialist. How fortunate for the Group that the specialist at the October 16 lecture was Harry Engel, MD, a wise and talented retinologist who clearly defined the role the vitreo-retinal interface plays in the diagnosis and management of glaucoma.
ANATOMY OF THE EYE
THE VITREOUS: The vitreous is a structure located in the back of the eye. It consists of about 97% water and 3% protein and sugar molecules and it is totally transparent. With age the vitreous becomes more watery developing watery pockets in the center. The protein in the vitreous is arranged in long stringy fibers. These fibers crisscross, and they are arrayed like a jungle gym leaving spaces in which the sugar molecules nestle. Protein fragments, (condensed protein) float through the middle of the eye casting a shadow on the back of the eye. They remain in the eye and we experience them as floaters. Everyone has them and the condition begins to appear during teenage and also with people who are highly near-sighted. A floater is most visible in homogeneous backgrounds such as a blue sky or a white sheet of paper. Generally, floaters are less apparent or not even visible when looking at different colors and patterns such as a patterned carpet.
Vitreous Separation: Probably 1 in 3 seniors experience a phenomenon, called a posterior vitreous detachment, (in medical parlance PVD) where the vitreous separates from the retina. This is not a retinal detachment but a separation of the vitreous from the retina that just pulls right off. Spontaneous separation can occur in a denser part of the vitreous that’s adhered to the retina following injury or surgery. A detached vitreous can lead to problems. Among them are retinal tears, retinal detachment, vitreous floaters.
Retinal Detachment or tears: These occur when the vitreous exerts traction on the retina. Symptoms are absent in the presence of a tear but a person may experience flashes like sparks or lightning that seem to occur spontaneously. You may be walking down the street, cooking a meal, watching television, checking your IPad and suddenly you see sparks. Generally, these symptoms are a sign of a vitreous detachment not necessarily a retinal detachment, for about 1 in 2,000 individuals experience a retinal tear but probably 1 in 3 or 1 in 4 seniors a vitreous detachment. When the retina rips it releases many new floaters in the eye because it rips across blood vessels releasing blood into the eye resulting in a spray of black dots. Even if there’s no damage related to a tear, you are, unfortunately, still left with a lot of new floaters, and if the floaters are right down the middle of the eye, they cast a shadow on the macula causing a transient haze or a cloud. This is a common occurrence.
With a retinal detachment, the fluid seeps through the hole and beneath the retina separating it in the process. There is dramatic evidence of a detached retina for the patient gets a dark, nearly black shadow or curtain obliterating sight. This is a permanent situation. The message here is that if you experience sudden flashes or floaters, have it checked out immediately. Dr. Engel did say that if the situation occurs on a weekend and you want to see your own doctor at the beginning of the week, this time lag is not crucial.
Fixing the Problem: Tears can be reattached using a laser treatment and the retina can be attached during a procedure called a buckle. The vitreous, however, cannot be attached for in adulthood it serves very little purpose but in utero development, it helps support a network of blood vessels.
The vitreous may cause problems such a wrinkling of the retina. If the vitreous detachment is partial, it can pull on the retina or the macula producing a vitreo-macular traction syndrome causing a systemic retinal swelling. This can be surgically corrected by removing the vitreous and thus releasing the traction. Patients who have a detachment in one eye may well experience a detachment in the other eye.
An occasional spontaneous result of a vitreous detachment or tear may result in a wrinkled retina--imagine a crumpled up piece of cellophane. Scar tissue has grown spontaneously on the surface of the retina causing the wrinkle. Your doctor might describe it as a wrinkle or a pucker in the retina, or in medical terminology an epi-retinal membrane disorder. The bottom line, however, is that the interface is distorted on the surface of the retina.
A Macular hole occurs when the vitreous separates, which can then tug on the surface of the retina and rip and distort it. In this case a vitrectomy is performed using some of the following procedures. A physician will put the instruments right through wall (white part) of the eye. Multiple instruments are used for this procedure. One resembles a flashlight. The other instruments cut and remove the vitreous. In some cases, the doctor may use tweezers, or forceps to peel the vitreous away from structures of the eye. Then a bubble of air is placed on the eye that presses against the retina. Though not a cure, it helps in a lot of cases.
Reasons for a vitrectomy range from removal of vitreous floaters, vitreous blood, or release of traction on the retina especially in cases of retinal detachment, diabetic conditions, bleeding in the eye (where the vitrectomy instruments are used to evacuate the blood).
GLAUCOMA SUSPECTS AND THE VITREOUS: There are a group of patients where a glaucoma diagnosis is uncertain. These patients’ nerves look suspicious and their pressure may be high. They are called glaucoma suspects and they need to be followed. Dr. Engel related a study demonstrating the interface between glaucoma and the retina. This study occurred some years back when a team at Albert Einstein Hospital
Bear in mind that this test does not provide a diagnosis, but it does infer that the test results for glaucoma patients may be influenced by the status of the vitreous. Similarly, other aspects of glaucoma evaluation require the judgment of whether pressure is too high or too low, the size of the disk, the extent of cupping, and whether the visual defects on the perimetry are actually defects. Expert interpretation is the key that governs medical decisions. Whether the OCT is useful for glaucoma diagnosis is not generally accepted. Doctors still prefer the standard visual field test.
The macula and fovea: These bodies in the back of the eye contain the highest concentration of neural cells. Neural connections conduct the critical business of the eye. Six million optic fibers carry the information via the optic nerve from the retina to the brain. This mass of information is possible because the majority of these nerve fibers arise from the macular region transferring visual information into a detailed picture for visual processing.
The retina acts like the film in a camera, the film being the neuronal cells. A good functioning macula produces good vision—ability to read the chart and see both far and near, but when the macular is impaired and the eye chart cannot be seen, vision may drop to 20/200 or worse. It is remarkable that this extremely important area in the eye is only about ¼ inch in diameter. The entire peripheral part outside the macula involves motion and movement and is not responsible for fine detail.
The most critical area is the fovea. Macular disease occurs in this central zone, the most specialized area of the retina. The fovea possesses a contour dip in the very center. The nerve fibers and cells are very highly concentrated on either side of this depression. The retina depending upon the need for detail is classified into 8 to 14 layers. The critical part for glaucoma is the inner part that is closest to the vitreous cavity that includes the ganglion cell layer, the nerve fiber layer, and the internal membrane. This is the zone the impacts on glaucoma, for when nerve fiber and ganglion cells are lost glaucoma worsens.
A pathologist examining specimen eyeballs for disease, for example, will examine the ganglion cell layer and the nerve fiber layer and if that layer is very thin, diagnose glaucoma. This in a nutshell is what glaucoma is about—loss of nerve fiber and ganglion cells in the inner retina.
The particular area relevant to glaucoma is called the internal limiting membrane, the surface connective tissue on the retina. This is the interface between the vitreous cavity and the retina. It’s a layer of very loose connective tissue where the structural cells of the retina connect to the protein that makes up the vitreous cavity.
With ocular Coherence Tomography (OCT which many or most ophthalmologists have on site now, a picture of the retina can be taken and its condition evaluated. This is an easy test to take. The patient sits on a stool, and places the head against a head rest. The patient is told to look at a flashing light. The procedure takes about 3 seconds. There is no discomfort. In the space of a few minutes, the OCT using a laser evaluates the health of the retina and macula. It does so under a very low level of light that does not damage the eye. As a diagnostic tool it has the ability to show the perfect contour of the retina, the laminations, the retinal layers, and a little orangey zone that is the fovea. The inner retina, the nerve fibers of the ganglion cells are all towards the surface. The readout is in microns, millions of a meter, thousands of a millimeter. The physician now can check the printout that provides a contour of the retina and a measure of its thickness for glaucoma damage. If the nerve fiber layer is thin glaucoma damage has occurred. If it is thick, the patient is on safer ground. It’s possible that the nerve fiber layer is thin because of near-sightedness, but now the doctor has tools to rule out a number of possibilities or disease states, clearly delineating even a microscopic detachment. The image may show that the retina has spaces in it, is swollen. When your physician, therefore, tells you that your retina is swollen, s/he has found these spaces. The swelling or cystic condition results from extra fluid in the retina. These are microscopic changes that until the availability of the imaging machines were not readily discerned. This technique has supplanted almost all of the retinal investigational techniques for clinical purposes over the last two years saving time for both ophthalmologist and patient.
Vein occlusion: The retina contains dual circulation-- the arteries and vein that serve the inner half of the retina, towards the vitreous. Another set of blood vessels form underneath the retina that supply the outer retina-- rods and the cones. The retina needs this rich system of dual blood supply for nutrition and maintenance. A vein occlusion occurs in the blood vessels that surround the macula and the veins that are draining or controlling the blood supply to the macula. The main problem occurs when there is an obstruction of blood flow and the retina swells impairing vision because the macula gets affected.
There is a spectrum of change – mild, medium, and severe that may occur during a vein occlusion. A vein can close completely, or partially. During a complete obstruction, the blood does not get exchanged, and the retina becoming damaged and losing its blood supply, dies. Generally, the doctor does not use these scary terms in talking with the patients but that’s the reality of a loss of blood supply-- ischemia, tissue death, loss of nerve fibers, loss of ganglion cells, loss of function, retinal swelling.
If you have high blood pressure your arteries can be damaged and this can lead to retinal vascular problems for the entire circulation can be affected. Long-term high blood pressure is associated with vein occlusions. If the arteries in the retina get thick, they press on the veins, and it’s at these pressure points that people develop occlusions. This can happen at any blood pressure level but the higher the blood pressure, the more likely arterial damage and thickening has occurred causing pressure on the veins. The blood pressure can affect large blood vessels. There are also questions about small blood vessel supply. There are some areas that are called watershed areas where if the blood pressure drops too much, there’s not enough blood supply to the area. There are two areas that are of main concern. One area is the optic nerve. The other area is in the back of the head. Take an individual with everything under control where if the blood pressure is dropped, let’s say during a surgical procedure if her blood pressure is lowered too much, problems in the eye may occur. Either end of the spectrum can cause problems.. In the eye blood pressure can be the cause of veneous occlusive disease such as central vein occlusion and branch retinal vein occlusion. It predisposes the patient to circulatory impairment on the arterial side – arterial obstruction, arterial emboli. There’s a vast literature on this subject. Although there is no magic number, Dr. Engel suggested that closer the blood pressure is to 120/80, the less likely that a problem will occur.
Drusen is generally considered a possible sign of macular degeneration. There are a number of forms but generally the material is tiny white or yellow bits of matter that, for the most part, do not interfere with vision. A layer called the retinal pigment epithelium (RPE) lies underneath the retina. The cells in this area supply the retina with nutritional support. When the RPE cells get a little sick or die they secrete lumpy products, nodules that develop underneath the retina that causes some impairment in function. This can change the contour a little bit but it’s in the spectrum of age-related macular degeneration which is the number one cause of visual loss related to macular disease in the western hemisphere. It runs in families. There’s a genetic profile for macular degeneration that was discovered about two years ago. It has to do with the temperature of your clotting and inflammatory system in the body. There are specific genetic markers for those people who are more predisposed for macular degeneration.
Glaucoma is a loss of nerve fiber, loss of ganglion cells. That precious optic nerve is only one millimeter and a half thick. When the doctor examines the optic nerve, s/he checks for evacuation in the center and its color. If the nerve is pale it may signify loss of cells. In general the deeper the cup, the greater the loss, but the doctor cannot tell how many fibers are gone except by examining the visual field test and evaluating loss of function. Unfortunately, about a third of the cells are lost before the loss is recorded in the visual field. Basically, a loss of about 2 million nerve fibers occur before the eye doctor can evaluate the effects of glaucoma. The OCT, an objective test provides a better picture of the thickness of the nerve fiber layer. The doctor doesn’t possess objective data to say you have glaucoma or not although family history is a strong predictor. Three parameters help the doctor decide if glaucoma exists—observation of the optic nerve, the visual field test and pressure, which on average is about 13. Two standard deviations above is about 22. A substantial number of people are perfectly healthy and walk around with 18. With glaucoma there is a pressure associated entity, not that pressure is unimportant, but for some people 18 is too high. Some patients have a pressure range in the low 20’s, say between 20 and 25. With these people the doctor’s not sure whether glaucoma exists. Treatment may be with drops or to simply prescribe additional visual field tests. Nevertheless, these parameters may not be sufficient for a diagnosis and physicians are searching for answers. At the present time, genetics does not provide a definitive answer.
The Sclera: There are some disease states where the sclera is blue, which may be caused by a little extra calcium. In some syndromes, however, the sclera is as blue as a blue sky and this association is linked to connective tissue and bone problems. Also, a thin sclera may be a result of being very near-sighted that causes the eye to lengthen stretching the sclera. Sometimes there are conditions where the sclera is so thin that the underlying tissues are coming through. In this case the doctor can patch it using an eye bank sclera. With multiple glaucoma operations, the sclera may become very thin and the doctor will either use eye bank sclera or preserved connective tissue patches to reinforce it.
NUTRITION; Blueberries are high in antioxidants and are good for you but will not improve the vision. The macula lutea is usually shortened to just the macula. In latin it means yellow spot, and it’s yellow because it contains more zeaxanthin pigment. Zeaxanthin is a nutrient that can be found in yellow and green fruits and vegetables.
Artificial Retinas: Three groups are looking at putting chips or artificial retinas into the eye. One group is at UCLA. The techniques they’re exploring all involve putting a number of sensora underneath the retina which then are activated to transmit images to the brain. The retina possesses 120 million rods and cones, each of these constituting individual sensors in the retina. In contrast the experimental retina contains perhaps some one-hundred microchips. Witness the magnitudes here of what the eye can do and what these very rudimentary structures can do. The science of artificial vision is just at the very beginning of studying whether some devices can be built to restore lost vision. Currently, the science is limited to patients who still possess a functioning nerve. Even patients with very limited vision might benefit from some of the studies underway. But it will be years before artificial vision makes the cut into any level of regular use.
We want to thank Dr. Harry Engel for his thoughtful, concise and pertinent delivery and for taking the time from his busy schedule to meet with us. He expanded the knowledge about the eye to those present offering a greater understanding of the function of the eye and the relationship of the retina to glaucoma conditions.
