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Electrodiagnostic tests involve very sensitive instruments that can detect and measure the electrical nerve impulses which are generated in the eye and travel through the optic nerve to the brain. Similar in operation to an electrocardiogram, an electroretinogram (ERG) can provide practical information about the functioning of the retina in patients with acquired or inherited retinal disorders. The more comprehensive visual evoked response (VER) test will measure the electrical activity along the visual pathway all the way to the visual cortex of the brain. Electrodes are attached to the specific spots on the head, but there is no discomfort. The VER makes it possible to differentiate the vision in each eye, assess the potential visual acuity, and detect amblyopia objectively. It can be extremely useful with young children or retarded adults whose subjective responses could be unreliable or difficult to obtain.

Electroretinography (ERG) is the study of the function of the retina when stimulated by light. An electroretinogram is a test often performed to diagnose retinitis pigmentosa (RP) and other eye diseases. RP is a group of progressive, hereditary diseases that cause retinal dystrophy or degeneration. First the rods and then the cones of the retina stop functioning. Early symptoms include night blindness and progressive loss of peripheral vision. An electroretinogram may be necessary to confirm the diagnosis of RP. The test measures the electrical activity of the retina when exposed to light stimulus. Each eye is tested separately. First, one eye is patched to exclude all light. Next, drops are administered to dilate the pupil of the other eye. Anesthetic drops are then administered and a contact lens is attached to electrodes placed on the cornea of the eye. The chin is placed in a chin rest, and the patient looks into the test machine. Lights that stimulate the retina are flashed into the eye in both dark and light environmental conditions. The electrodes on the contact lens record the responses of the retina and produce a graph that is interpreted by the ophthalmologist.

Endothelial cell count is a measurement of the number of endothelial cells remaining in the cornea. It is one of the best ways to determine the health of the cornea and to predict how well it will stand up to cataract surgery. The instrument used for the cell count is similar to a slit lamp, called an endothelial cell camera, which helps the ophthalmologist count, fairly accurately, the number of cells present in the endothelium. The endothelium is a single layer of cells lining the inside of the cornea, which is bathed in the aqueous fluid in the anterior chamber (the front part of the eye). These cells prevent the aqueous fluid from penetrating the cornea. Everyone is born with a certain number of endothelial cells but during the aging process or cataract surgery, some of the cells die and are lost. The cornea has a very limited ability to make new cells and repair itself. A reduction of endothelial cells could allow seepage into the cornea resulting in swelling, loss of transparency, and loss of vision. A certain number of cells are needed to maintain a clear cornea; and the cornea requires enough of these cells prior to surgery to allow for cell loss during the procedure. Approximately 1,000 endothelial cells are the minimum required before surgery in order to function well afterwards. Around 500 suggests the need for corneal transplant surgery and may require a triple procedure that combines the transplanting of the cornea and intraocular lens along with the cataract surgery. As many as 5-10% of the epithelial cells are normally lost during cataract and implant surgery. Therefore, if a person has only 50% of these cells left when cataract surgery is needed and loses another 15 or 20 percent during the surgery, the person may have too few cells left to ensure a healthy cornea. Although the operation may be successful, the vision could be hampered by a cloudy cornea, resulting in permanent eyesight loss and a cornea transplant may be needed. However, most people about to undergo cataract surgery do not need an endothelial cell count as certain signs detected with the slit lamp will alert the doctor if the count is needed.

Enophthalmos is an inward displacement of the eye within the orbit. The condition appears as a sunken or deep-seated eyeball. The opposite condition is exophthalmos or proptosis (bulging eyes). Enophthalmos may be unilateral or bilateral. It may be caused by injuries, tumors, or ageing. Injuries may cause orbital fractures that result in enophthalmos. Aging may cause the condition as fatty tissue within the orbit is absorbed and fails to support the eyeball at the former level. In cases that involve continuous diplopia (double vision) and cosmetic deformity, surgery may be required. Tumors that cause enophthalmos are usually metastatic (malignant) tumors that originate in other places in the body but spread to the orbit through the blood stream. Metastatic scirrhous carcinomas are a common cause of enophthalmos. These fibrous tumors often originate from breast carcinoma in women and bronchogenic carcinoma in men. Such tumors may be treated with radiation or chemotherapy. Degrees of enophthalmos may be measured with an exophthalmometer. As the instruement is held up to the eyes, mirrors on the device superimpose a millimeter scale over a side view of the eye. This enables the examiner to measure the distance from the lateral orbital rims to the corneal apices. Measurement differences between the eyes are often given most importance since general measurements of exophthalmos vary greatly because of individual anatomy. A variance of two millimeters between the eyes is considered serious and worthy of further investigation.

Entropion is a disorder of the eyelid in which the lid turns inward, allowing the skin of the eyelid and the eyelashes to rub against the conjunctiva. In addition to irritation, entropion can cause excessive tearing, discharge, crusting of the eyelid, and a feeling of something being lodged in the eye. In severe cases, the turned-in lashes may scratch the cornea, resulting in an infection or scarring, which impairs vision. Most often, this condition develops when the tissues of the eyelid relax as a result of aging. One of the first signs is irritation of the eyes in the morning, clearing up later in the day. As the disorder advances, the irritation may become more frequent, even constant. Although generally associated with aging, it may also occur as the result of an injury, burn, conjunctival scarring, or trachoma. Entropion usually affects only the lower lid and is most common in those over fifty. The disorder may also be seen in newborns, but often rectifies itself without treatment during the first few months. Entropion can be temporary or chronic. In temporary cases, it usually begins in response to something else, as itchy eyes caused by an allergy. The sufferer blinks hard to get rid of whatever is creating the irritation, causing it to become worse. The condition sometimes responds to a treatment as simple as a piece of adhesive tape placed on the skin of the lower eyelid to prevent it from turning in. Although artificial tears or lubricating ointments help, the primary means of treatment is surgery to reposition the eyelid. Eyelid surgery may be performed on an outpatient basis with a local anesthetic where the doctor adjusts muscles or tendons of the eyelid. The procedure may also involve the removal of the sections of the lid or excision of scar tissue with accompanying skin grafts. A tarsorrhaphy, the placement of stitches to keep the lid in a permanent position, may be performed to hold the lid in an outward position. After surgery, an eye patch will be worn overnight and an antibiotic ointment is used for about a week.

Enucleation is the surgical removal of the eyeball. Enucleation is performed when the eye contains a malignant tumor as a melanoma, when the eye is blind and causes pain, or when the eye is nearly blind and sympathetic ophthalmia (an inflammation that occurs in both eyes as a result of an injury to one eye) is a risk. Enucleation is performed under general anesthetic. An incision is made in the limbus (where the sclera meets the cornea) and the conjunctiva is opened. The eye is severed and removed from the optic nerve and the six extrinsic muscles that hold it in place. The vessels are cauterized to stop internal bleeding. The membrane and muscles are tied together and the conjunctiva is stitched closed. After a period of two or more days, the conjunctiva-lined socket of the eye is fitted with a plastic shell in preparation for an artificial eye or prosthesis. A prosthetic eye is fitted three to four weeks later and is generally undetectable from a natural eye.

Esotropia is a condition of strabismus, or misaligned eyes, in which one eye turns inward. Often termed “squint” or “crossed-eyes”, strabismus may cause visual problems or loss of vision. Esophoria is a type of convergent strabismus in which the eyes tend to turn toward each other. As in all strabismus, the eyes view two differing subjects. The brain received the two images and double vision results. Often, in an effort to reconcile the two pictures, the brain may suppress the message received from one eye, reducing the vision of that eye. This results in a condition called amblyopia. Treatment may include prescription eyedrops, eyeglasses or bifocals, or exercises. Exercises or eye patching may be prescribed if amblyopia is present. Surgery may be necessary to completely align the eyes.

Exophoria is a condition of strabismus, or misaligned eyes. It is a type of latent strabismus in which the eyes tend to turn outward, away from each other rather than the “cross-eyed” or “squint” condition of esophoria. Exophoria occurs when binocular vision is lost due to patching of one eye or when the individual is tired.

Exophthalmos, or proptosis refers to protruding eyeballs. The condition is usually accompanied by a retraction of the eyelids and infrequent blinking. The condition may be unilateral (involving one eye) or bilateral (affecting both eyes). Unilateral exophthalmos may be the result of muscle palsy, a vascular condition, tumors, cysts, edema, or trauma and its accompanying hemorrhage and infection. Bilateral exophthalmos is usually caused by hyperthyroidism, or Graves’ disease, a disorder of the autoimmune system. It causes the thyroid to overproduce hormones that overstimulate the body’s metabolism. Over half of those who develop this disease develop Graves’ ophthalmopathy, a condition in which the tissues and muscles of the eye are affected. Pseudoproptosis is the false appearance of exophthalmos, occurring when the other eye has become sunken because of a fracture or trauma. The normal eye appears to bulge forward in comparison. An overly large eye, as in the case of unilateral high myopia, may also appear to be exophthalmic. The amount of exophthalmos can be measured with an exophthalmometer. This instrument determines the degree of protrusion and symmetry between both eyes. Readings beyond the normal range of 12-20 millimeters may indicate exophthalmos. Other tests may determine the underlying cause. X-rays may illuminate a tumor, fracture, or erosion within the orbit. A CAT scan or biopsy may detect a morphology within the eye or orbit, and a culture of any discharge may identify an infection. Treatment may include therapy for underlying causes, as well as antibiotics, steroids, radiation, chemotherapy, radiation iodine treatments, or surgery such as tarsorrhaphy, a stitching of the eyelid, or removal of the mass causing proptosis.

Exotropia is a type of strabismus where there is a permanent turning of the eye outward. Exotropia is also called “divergent strabismus” and “walleye”. This condition results in diplopia.

External eye exam is a quick check of the eyes with no special instruments other than a light. The doctor is checking for such things as the position and movment of the eyes, eyelids, eyelashes, and the pupils to see if they respond normally, and the cornea and iris for clarity and shininess.

Eye banks are organizations that acquire, evaluate, and distribute eyes from eye donors. The eyes are used for corneal transplants, medical research, and educational purposes. Donated eyes are necessary for corneal transplantations, or keratoplasty. According to the Eye Bank Association of America, more than 46,000 corneas were replaced by transplants in the year 2000, 90% of which successfully restored vision. Eyes are donated anonymously from people who die. Donors may make known their intention to donate their eyes before death by using a donor card or filling out the section about organ donations on some state’s or provincial driver’s licenses. The Uniform Anatomical Gift Act allows relatives of the deceased to donate the eyes if the donor has not provided written intention. Eye donors are not limited by age or degree of eyesight and pay no fee to donate their eyes. The eye is enucleated (removed) from the donor as soon as possible after death. It is usually stored in special solutions or fixatives, or in some cases, quickly frozen and stored at low temperatures. Tissue deterioration may occur if the eye is not treated within six to eight hours after death. The eye banks evaluate a donor’s general health, medical history, and the health of the eyes according to strict medical standards. After this evaluation, the cornea is distributed to the corneal surgeon who makes the final determination as to whether the cornea is suitable for transplantation. Tissue and blood typing and matching are not usually necessary in corneal transplantations, except in rare instances, as in cases of major chemical burns or graft rejection. Corneas that qualify for transplantation are distributed to recipients according to fair, just, and equitable standards. Distribution is made without regard to age, sex, religion, race, creed, color, or national origin. If the corneas are not suitable for transplantation, or the sight-restoring procedure of epiketophakia, the eyes are used for research and education. Because many sight disorders cannot be simulated, donated eyes are valuable in the research of glaucoma, diabetes, retinal diseases, and other eye disorders and conditions.

Eyesight covers the technicality of “how we see” or how the eye construction functions. Light bouncing off an object will first hit the cornea. The image will now be about 60% focused as the cornea bends (refracts) the rays of light. Light then passes through the pupil and enters the lens, which bends the rays of light another 40%. In the ideal situation, this should result in an image focused to a sharp point on the retina. The light acts like an electric shock through the ten layers of cells in the retina and stimulates them to send their messages to the brain. The bipolar cells receive, organize, and transmit information to ganglion cells which send these signals to the brain through the optic nerve. Most of the light is focused on an area of the retina called the macula, where vision is the sharpest and clearest. It is this “central vision”, supplied by retinal cells called cones, that allows for things to be “seen”. The rest of the retina, populated by cells called rods, gives peripheral, or side, vision. Without peripheral vision, there would only be tunnel vision. The cones, which are mostly active in vision during daylight, also supply color vision. There are specific cones for red, yellow, and blue which enable us to appreciate the hundreds of colors available from only three primary colors. If the rays of light were always focused to a sharp point on the retina, glasses would not be needed. However, when this point of focus falls in front of the retina, myopia or nearsightedness is the result. If the point of focus falls behind the retina, hyperopia or farsightedness happens. In spite of its complexity, the retina merely sends to the brain whatever image it receives, clear or blurred. Glasses or contact lenses correct the refractive error and clear up the blurred image by bending the rays to a point and then bringing this point into sharp focus onto the retina. Eye problems sometimes do not begin in the eye at all but are the result of such systemic diseases as diabetes, hypertension, carotid artery disease, or thyroid disease.

Eyedrops – To use eyedrops correctly, there are two things to remember: First, wash your hands! Secondly, do not touch the tip of the container either with your hands or to your eyes. Tilt your head back, pull the lower lid down, squeeze a drop or two inside the lower eyelid, and close your eyes for a good two minutes. Place your finger at the point where the upper and lower eyelids meet near the nose and hold it there during this time. Try not to blink. By gently pressing this area, the tear drainage ducts will be blocked allowing the drops to penetrate the eye and preventing them from draining into the bloodstream.

Eyeball muscles work individually or together. Each eyeball has six muscles attached to the sclera, which allows for movement of both eyes as well as a shift in the visual field without the head having to move. The extraocular muscles are a medial and lateral rectus (mostly for horizontal movement), superior and inferior rectus and oblique muscles (for vertical and torsional or twisting movements). The brain coordinates these eye movements so the eyes will move in unison when tracking an object – left, right, up, down, or diagonally. The six extra-ocular muscles are divided into three pairs with each set working in opposition to each other. For instance, contraction of the medial rectus and relaxation of the lateral rectus muscles will point the eye inward. The eye movements are so habitual that we are not aware of them. The teamwork among the muscles is precise and accurate as they must also match and coordinate with the movements of the other eye. Since there is no connection between the muscles of the two eyes, this coordination is remarkably done by nerve signals from the brain.

Eyelids are the eyes’ protection from the environment. They shut automatically when an object threatens to enter the eye and provide protection from unwanted light. They blink to produce cleansing tears and have lashes that filter out dust and small debris. Apart from any conscious winking or blinking, the subconscious centers of the brain have the eyelids blink at regular intervals of five to ten time a minute to sweep away any foreign particles. The eyelids are subject to several conditions including blepharitis, a hordeolum, chalazion, ptosis, entropion, ectropion, dermatochalasis, tumors, trachoma, trichiasis, and dacryocystitis. Any one of these conditions may develop from changes in eyelid tissue or a weakening of the eyelid muscles and may progress to a point where it irritates the eye or impairs vision.

Updated October 2012