Learn more about Ophthalmic diagnosis, care, and treatments with this comprehensive encyclopedia.
Rods and ConesThe two types of light-sensitive cells of the retina are rods and cones. Named because of their cylindrical shape, rods are interspersed throughout the retina. They are highly sensitive to low light and shadow and allow for at least some degree of nighttime vision.
Cones are present throughout the retina also, but are very highly concentrated in the fovea of the macula. Different cones are sensitive to different wavelengths and allow one to see vivid colors and sharp detail in conditions of bright light. There are 20 times more rods than cones.
Retinal Venous OcclusionAn occlusion is a blockage that occurs in a blood vessel as a result of debris in the bloodstream, inflammation, or high blood pressure. Within the veins of the retina, occlusions most often occur due to high blood pressure or high intra-ocular pressure and at a point where arteries and veins cross over each other. A blockage may occur in the main vein of the retina or in any of the smaller veins that branch off from it.
When an occlusion occurs, the veins of the retina cannot drain properly. This prevents the retinal cells in the area from receiving a fresh supply of oxygen- and nutrient-rich blood. Deprived of oxygen and nutrients, the affected cells cannot function properly and may even die. Perhaps more destructive is the hemorrhage of vessels that occurs because the backed-up blood has nowhere to go. The surrounding retinal tissue then succumbs to edema and swelling.
Occlusions occur suddenly, causing decreased vision in one eye that will vary from mild to severe. In most cases, complete vision cannot be restored, but lasers can treat any persisting edema and the related bleeding from burst blood vessels. Laser treatment can improve vision significantly. In addition, the FDA is expected to approve Lucentis, an anti-vegf injectable medication used for the treatment of wet AMD, for use in treating branch retinal vein occlusions sometime in late 2010. Anti-vegf agents inhibit the growth of new blood vessels from damaged tissue and play a role in preventing vessels from leaking.
Rhegmatogenous Retinal DetachmentA rhegmatogenous retinal detachment (RRD) occurs when the retina breaks, tears, or develops a hole. Fluid from the vitreous will then seep in through the tear or hole and cause the retina to detach from its pigment epithelial layer.
RRD’s are more common than serous pigment epithelial detachments. They most often occur in the elderly, nearsighted, those who have an advanced stage of retinopathy or other retinal disease, those who have had cataract surgery, or those who have experienced trauma. Sudden vision changes such as floaters, blurry or distorted central vision, flashes of light, or “shadows” that sweep inward from the periphery might indicate a rhegmatogenous retinal detachment.
Retinal detachments require surgery. Left untreated, the retina can detach completely and can cause blindness. However, there are several surgical options for retinal detachments, and all are highly successful. Laser photocoagulation is used to seal off tears or holes. Surgical procedures to re-attach the retina include scleral buckling, vitrectomy, and pneumatic retinopexy. It is important to know that even after the retina is successfully re-attached, vision may not be perfectly restored.
RetinopathyAny change in the retina due to disease or damage is called a retinopathy. Some retinopathies are the result of underlying disease, such as diabetes or hypertension. Other retinopathies occur because of exposure to a toxin or an adverse reaction to something in the environment. Certain drugs, extreme bright sunlight, and even high altitude can cause retinopathy.
Retinopathy may result in blurred vision or vision loss, due to the abnormal growth of capillaries in the retina and any leaking or hemorrhaging that occurs. In many cases, treatment of the underlying disease improves vision.
Retinitis PigmentosaRetinitis pigmentosa is one in a group of diseases classified as hereditary chorioretinal dystrophies. There are multiple forms of retinitis pigmentosa, each occurring as a result of a specific abnormal gene. The disease is inherited in a number of different ways. Genetic testing and counseling are very helpful in determining the form of the disease, how it has been inherited, and what treatments are available or are being researched. Retinitis pigmentosa is usually diagnosed during young adulthood and can be difficult to accept. Knowledge is power, and information about retinitis pigmentosa can empower a young adult to learn to live with this condition.
In contrast to other chorioretinal dystrophies, retinitis pigmentosa results in the loss of night vision and peripheral vision first and will eventually affect central vision as well. During the progression of the disease, cellular malfunctions cause the destruction of rods, which are the photoreceptors responsible for vision in low light conditions. Rods are interspersed throughout the retina, which is why peripheral, or side, vision loss occurs. Vision loss will sweep inward, until only a small, central area of vision remains. Most people with retinitis pigmentosa are legally blind by 40 years of age.
There is no cure for retinitis pigmentosa, but it is an area of promising research. Technology continues to improve upon low-vision aids. Gene therapies, delivery methods of agents to rods and cones, and the effects of vitamin A and DHA on the progression of the disease are currently being studied.