The Internal Cirulation of the Eye

Some time before birth, the lens of the eye ceases to have a blood supply and depends on the lens muscle to make a special watery fluid. This fluid provides the lens’ cells with nutrients and an oxygen substitute. The million or so lens cells are contained in a very thin membrane much like a bag of cellophane or "handiwrap". This fluid flows to and around the lens giving its cells nourishment and then flows forward through the pupil toward the cornea. It leaves the eye through a microscopic drain where the cornea and iris meet to form an angle.

Glaucoma

One of the most frequently asked questions in an eye doctor's office is "What is glaucoma?" Glaucoma is a group of conditions in which the drain for this watery fluid (see above) is blocked by any of various means, resulting in a higher than normal pressure inside the eye. Thus Glaucoma is a group of conditions in which tissues of the eye become damaged by this higher than normal pressure. This results further in damage to the optic nerve and the surface nerves of the retina. Finally, the width and depth of one's vision is affected, resulting in visual field defects or constriction.

Glaucoma results in vision loss, which can occur with or without elevated intraocular pressure. Most types of glaucoma do not give a patient any clue to their condition because they are painless and without symptoms. One usually does not notice any loss of vision until very late in the disease when a large portion of visual field has already been lost. This is because the central sharp vision is the last to be affected. Since the sharp vision is what we most depend on, we generally don’t notice the loss of peripheral vision readily. This is especially true when the other eye helps cover for the most affected eye by seeing an area of visual field missed by the more severely damaged eye. The visual loss is due to damage to the optic nerve and is irreversible. Whatever vision is lost cannot be recovered by treatment.

When examining the eye, the anterior chamber angle, formed by the iris and the cornea contains the trabecular meshwork, where the watery fluid made by the lens muscle drains from the eye. Glaucoma can be classified into three groups of disorders: those with open angles; those with closed angles; and those with growth of blood vessels in the angle - which produces an irreverible scar.

In open-angle glaucoma, the mechanism of raised intraocular pressure is microscopically impaired outflow of aqueous humor within the drainage system; the drain itself is open. However, not all forms of open-angle glaucoma have elevated intraocular pressure. In low-tension glaucoma, intraocular pressure plays a small role in the progression of the disease, and the pressure may actually be below normal. In the open-angle and low-tension glaucomas, the patient usually has no symptoms until damage to the optic nerve is marked. The vision loss is slowly progressive and usually painless.

Primary open-angle glaucoma is the most common form of glaucoma in America and Europe. It is open-angle glaucoma wherein no particular reason for blockage of the drainage angle can be found. The risk factors for developing primary open-angle glaucoma are advanced age, family history of glaucoma, elevated intraocular pressure, highly pigmented skin, myopia (nearsightedness), and systemic diseases such as diabetes, hypertension, and cardiovascular disease. Secondary forms of open-angle glaucoma exist as well in which the drainage system becomes clogged with various substances that can be identified, such as pigment (dispersion) and pseudoexfoliation.

In acute primary angle-closure glaucoma, the flow of aqueous humor out of the eye is physically blocked by the iris. The iris, which rests abnormally close to the cornea, becomes adherent to the trabecular meshwork (drain). This results in elevated intraocular pressure, and the patient typically has excruciating pain, redness, blurred vision, and in some cases, nausea. Halos may be seen around lights. Blindness can occur within a few hours to a couple of days. There are also several forms of secondary angle-closure glaucoma, where iris tissue blocks the angle as a consequence of another preexisting ocular disease.

Elevated intraocular pressure is still considered a key feature of glaucoma even though it is not considered essential to its diagnosis. Intraocular pressure remains the one risk factor that can be modified, and glaucoma treatment is aimed at lowering the intraocular pressure. This can be accomplished through medications, laser treatment, or surgery. Medications work to decrease the intraocular pressure by either decreasing the amount of aqueous humor that is produced inside the eye or by increasing the ability of the aqueous humor to flow out of the eye. Laser can be used to treat both the open-angle and angle-closure forms of glaucoma. In open-angle glaucoma, laser spots are directed at the trabecular meshwork to facilitate flow through the drainage system. Laser can be used to treat angle-closure glaucoma by making a hole in the iris. This enables the pressure to equalize on both sides of the iris and allows the iris to move away from the trabecular meshwork, opening the angle. Surgery to bypass the drainage system is recommended in cases that do not respond to medical treatment.

A careful history and examination are important in the detection of glaucoma. There are generally no reported symptoms, and an elevated intraocular pressure or suspicious-appearing optic nerve may be found on routine eye examination. Further testing can be performed to detect any loss of peripheral vision or defects in the nerve fiber layer of the retina.

The management of glaucoma is best left to the ophthalmologist, but the importance of detection calls for the cooperation and assistance of all medical personnel. Careful periodic evaluation of the optic nerve and measurement of intraocular pressure are of great importance in the follow-up of glaucoma patients. Proper identification and management of glaucoma would lead to fewer patients losing vision as a result of this blinding disease.

Glaucoma Treatments

Patient cooperation is required to keep follow-up appointments and testing, as well as to assure that all prescribed anti-glaucomatous drops and oral glaucoma medication are taken and refilled without fail. When a patient does not take their glaucoma medication or skips a dose, they cannot depend on getting any feedback from their eye in the way of pain or visual loss to remind them to take the medicine. Furthermore, when doses of medication are missed, slow imperceptible damage to the optic nerve and loss of visual field occurs without giving the patient any warning.

Open-angle glaucoma is generally able to be treated with anti-glaucoma eye-drop medications. When this does not achieve appropriate lowering of eye pressure and prevention of optic nerve damage and visual loss, other steps can be considered, including applying a laser treatment to the trabecular meshwork drain or to perform an operation called trabeulectomy. This procedure creates a small narrow hole in the angle where the cornea and iris meet to drain the watery fluid inside the eye into the scleral tissue and out underneath the conjunctiva (the filmy covering of the white of the eye where blood vessels get red in "pink eye").

Closed-angle glaucoma generally involves not only a blockage of the trabecular meshwork drain by iris tissue moving forward in an eye with an already-narrow drainage angle. It also usually involves a build-up of pressure behind the iris due to abnormally circulating watery fluid going behind the lens and pushing it forward, closing off the pupil by pushing the lens forward up against the central edges of the iris. Thus one of the main treatments considered for closed-angle glaucoma is use of a laser to make a new tiny hole in the peripheral iris. This is called a YAG laser peripheral iridotomy. In eyes with little or no fluid flow through the pupil, this procedure will allow the fluid behind the iris to flow forward by a new second route. This may reduce the pressure and in some cases relieve the angle-closure as well. When this laser procedure is insufficient to relieve the pressure rise, anti-glaucoma medications and trabeculectomy surgery may be of help in controlling the pressure. Many patients who experience an attack of acute closed-angle glaucoma suffer some degree of permanent visual loss and need on-going eye pressure management.

Narrow-angle glaucoma suspects are patients who are recognized as having narrow drainage angles, but have not yet experienced an attack of acute closed-angle glaucoma. These patients can often be detected before such an attack and may then be eligible to receive the same YAG laser peripheral iridotomy as a patient who suffers an attack of acute angle-closure glaucoma. However, in the narrow-angle suspect, the laser iridotomy is almost always successful in preventing a future attack of angle-closure, as opposed to waiting for such an attack to occur.

Pterygia

Pterygia are growths of tissue extending from the conjunctiva onto the edge of the cornea. They are usually on the nasal side of the eye and are precipitated by exposure of the eye to ultraviolet light, either from the sun, a tanning bed, or by welding without proper protection. They usually grow slowly and may rarely interfere with vision. When pterygia extend close to the visual axis, it is appropriate to consider removing them. However, it is well known that when removed, they tend to grow back, frequently more quickly and bothersome than before.

In recent years, new methods of removal and treatment have been shown to be more effective in discouraging regrowth of pterygia. These are done by same-day surgery and may involve replacing the tissue removed from the conjunctiva with a conjunctival graft, or better yet a graft of sterile amniotic membrane. In addition, other methods may occasionally be considered to decrease the risk of recurring pterygia.

Ectropion

An ectropion is the condition of an eyelid (usually lower eyelid) turning outward, allowing the inner surface to dry out and scar. This in turn allows the surface of the eyeball, including the clear cornea in front of the iris, to dry out and get infected or scarred. The underlying problem is that the eyelid has loosened with aging.

An eyelid which is not taut can fall outward away from the eyeball, forming an ectropion. This is repaired under local anesthesia during same-day surgery by detaching and shortening the eyelid, reattaching it at its outer corner tighter than before. This almost always fixes the problem, improving both the appearance and function of the eyelid, and protecting the cornea from drying out, infection and scarring. Occasionally other methods are necessary, especially when significant scarring of the eyelid in an turned-out position has already occurred.

Entropion

An entropion is the condition of an eyelid (usually lower eyelid) turning inward, allowing the eyelashes to scrape the surface of the eye, causing irritation, abrasion, scarring or infection of the cornea. As with ectropion, the underlying problem is that the eyelid has loosened with aging.

An eyelid which is not taut can fall inward toward the eyeball, forming an entropion. This is repaired under local anesthesia during same-day surgery by detaching and shortening the eyelid, reattaching it at its outer corner tighter than before. In addition, it is necessary to remove a little eyelid-closure muscle (orbicularis) from the lower lid. This prevents the muscle from riding upward during eyelid closure, pushing the lashes onto the eyeball. This almost always fixes the problem, preventing the eyelashes from rubbing the cornea, which could produce irritation, infection, or scarring. It also improves both the appearance and function of the eyelid. Occasionally other methods are necessary, especially when the eyelid is scarred into a fixed turned-in position. Upper eyelid entropion is more unusual and requires different approaches to surgical correction.

Ptosis

Ptosis is the problem of an eyelid drooping in a manner which can be cosmetically bothersome or may even block some vision. It has several possible causes including injury, eyelid tumors, myasthenia gravis, Grave’s or aging. The latter occurs due to separation of the muscle which lifts the eyelid from the eyelid’s cartilage-like plate. When this occurs, surgical reattachment of the muscle of the eyelid to the plate of the eyelid usually produces correction to improve vision. Sometimes the muscle of the eyelid is still well-attached to the plate of the eyelid, however the weight of the excess skin accumulated from continual stretching of the skin over the years has produced ptosis. In this case, a simple trimming of excess skin and fat from the eyelid may be sufficient to repair the sagging of the lid.

When ptosis occurs in children younger than seven, it is usually congenital (present to at least some degree at birth) and in this age group it can cause amblyopia - decreased vision due to insufficient exposure of the brain to sharp images of one's surroundings by one or both eyes at a young age. This inexposure to sharp images prevents the brain's nerves in the visual pathway from making the connections needed for it to learn to see. Congenital ptosis has different surgical considerations than adult-onset ptosis.