Home > Encyclopedia of Nursing & Allied Health > Glaucoma

Definition

Glaucoma is a condition where the optic nerve is subject to damage—usually, but not always, because of excessively high intraocular pressure (pressure within the eye, also called IOP). If untreated, the optic nerve damage results in progressive, permanent vision loss, starting with unnoticeable blind spots in the field of vision, progressing to tunnel vision, and then to blindness.

Description

More than 2 million people in the United States have glaucoma, and 80,000 of them are legally blind as a result. It is the leading cause of preventable blindness in the United States and the most frequent cause of blindness in African-Americans, whose glaucoma risk is three times higher than the rest of the population. The risk of glaucoma increases with age, but it can strike any age group, even newborns and fetuses.

Glaucoma is a class of diseases. There are at least 20 different forms that can be divided into two categories: open-angle glaucoma and narrow-angle glaucoma. To understand glaucoma, it is useful to understand eye structure.

The eyes are spherical. A tough, non-leaky protective sheath (the sclera) covers the eye with the exception of the clear cornea at the front and the optic nerve at the back. Light comes into the eye through the cornea, then passes through the lens, which focuses it onto the retina (the innermost surface at the back of the eye). The rods and cones of the retina transform the light energy into electrical messages, which are transmitted to the brain by the optic nerve.

The iris is located between the dome-shaped cornea and the lens. It controls the amount of light that enters the

eye by opening and closing the pupil. The iris, cornea, and lens are bathed in a liquid called the aqueous humor, which is similar to plasma. This liquid is continually produced by the nearby ciliary body and moved out of the eye into the bloodstream by a system of drainage canals (the trabecular meshwork). The drainage area is located in front of the iris, in the angle formed between the iris and the cornea.

Glaucoma occurs if the aqueous humor is not removed rapidly enough or if it is made too rapidly, causing pressure to build up. This high pressure distorts the shape of the optic nerve and destroys nerve cells. The destruction of nerve cells results in blind spots—spots where the image from the retina is not transmitted to the brain.

Open-angle glaucoma accounts for over 90% of all cases. It is called "open-angle" because the angle between the iris and the cornea is open, allowing drainage of the aqueous humor. It is usually chronic and progresses slowly. In narrow-angle glaucoma, the angle where aqueous fluid drainage occurs is narrower than normal, thus causing the fluid to drain more slowly and increasing the risk that the flow may be blocked. When the drainage area is blocked, a closed-angle glaucoma attack results. This can occur, for example, if the iris and lens suddenly adhere to each other and the iris is pushed forward. In patients with very narrow angles, this can occur when the eyes dilate (e.g., when entering a dark room or when taking certain medications).

One rare form of open-angle glaucoma is different. People with normal-tension glaucoma have optic nerve damage in the presence of normal IOP. As of 2001, the mechanism of this disease is unknown.

Glaucoma is also a secondary condition of over 60 widely diverse diseases and can result from injury as well.

Causes and symptoms

The cause of vision loss in all forms of glaucoma is optic nerve damage. There are many underlying causes and forms of glaucoma. Most causes are not known, but it is evident that different processes are involved, and a malfunction in any one of them could cause glaucoma. For example, eye trauma may result in the angle becoming blocked, or, as a person ages, the lens may become larger and push the iris forward. The cause of optic nerve damage in normal-tension glaucoma is also unknown, but there is speculation that the optic nerves of these patients are susceptible to damage at lower pressures than what is usually considered to be abnormally high. It is probable that most glaucoma is inherited. At least 10 defective genes that cause glaucoma have been identified.

Initially, chronic open-angle glaucoma has no noticeable symptoms. The pressure build-up is gradual and there is no discomfort. Moreover, the vision loss is gradual and one eye fills-in the image where its partner has a blind spot. However, left untreated, vision loss becomes evident, and the condition can be painful.

Acute closed-angle glaucoma is obvious from the beginning of an attack. The symptoms are blurred vision, severe pain, sensitivity to light, nausea, and halos around lights. The normally clear cornea may be hazy. This is an ocular emergency and needs to be treated immediately. Similarly, congenital glaucoma is evident at birth. Symptoms include bulging eyes, cloudy corneas, excessive tearing, and sensitivity to light.

Diagnosis

The initial glaucoma diagnosis is made through an eye examination by an optometrist (O.D.) or ophthalmologist (M.D.). The examination begins with an ophthalmic assistant, technician, or scribe gathering patient information, including any family history of glaucoma. Then the ophthalmic assistant takes a reading of the patient's intraocular pressure (IOP). IOP is measured with an instrument called a tonometer, using a technique called applanation tonometry. The test is performed after anesthetic drops are administered to the eye. The anesthetic allows the examiner to touch the patient's eyeball without causing discomfort for the patient. Another type of tonometry called noncontact applanation shoots a puff of air into the patient's eye. This is slightly less accurate than applanation tonometry.

Next, an O.D., M.D., or skilled ophthalmic assistant uses an ophthalmoscope (a hand-held instrument with a light source) to examine the optic nerve, retina, and back of the eye. Other types of lenses may also be used to examine the back of the eye. A slit lamp (biomicroscope) allows the physician or assistant examine the cornea, iris, and lens.

Visual field tests (perimetry), performed by an O.D.,M.D., or ophthalmic assistant, can detect blind spots in a patient's field of vision before the patient is aware of them. Certain defects may indicate glaucoma.

Another test, gonioscopy, is used to distinguish between narrow-angle and open-angle glaucoma. A gonioscopy lens, which is a hand-held contact lens with a mirror, allows visualization of the angle between the iris and the cornea.

Physicians may also perform a nerve fiber layer assessment which can show early damage to the eye. Fundus photography or stereoscopic photography through a dilated pupil may also be performed by an O.D., M.D., or ophthalmic assistant to document the appearance of the optic nerve so that changes may be detected on subsequent examinations.

Blood pressure also is monitored, as some prescribed treatments may raise pressure and heart rate.

Intraocular pressure can vary throughout the day. For that reason, patients should schedule several return visits to measure the IOP at different times of day. This yields the most accurate diagnosis.

Treatment

The first line of glaucoma treatment is the use of prescription eyedrops. Several classes of medications are effective at lowering IOP and thus preventing optic nerve damage in chronic and neonatal glaucoma. Beta blockers(e.g. timolol), carbonic anhydrase inhibitors (e.g. acetazolamide), and alpha-2 agonists (e.g. brimonidine tartrate) inhibit aqueous humor production. Miotics (e.g. pilocarpine) and prostaglandin analogues (e.g. latanoprost) increase the outflow of aqueous humor.

It is important for patients to inform their doctors of any health conditions they have or any medications they take, including over-the counter drugs. Certain drugs used to treat glaucoma are not prescribed for patients with pre-existing conditions. The drugs prescribed to treat glaucoma all have side effects, so patients taking them should be monitored closely, especially for cardiovascular, pulmonary, and behavioral symptoms. Each medication lowers IOP by a different amount, and a combination of medications may be necessary. To ensure that IOP is lowered sufficiently, it is important that patients take their medications and be monitored regularly. IOP should be measured three to four times per year.

Normal-tension glaucoma is treated by reducing IOP to less-than-normal levels, on the theory that overly susceptible optic nerves are less likely to be damaged at lower pressures. Research underway may point to better treatments for this form of glaucoma.

Attacks of acute closed-angle glaucoma are medical emergencies. IOP is rapidly lowered by successive deployment of acetazolamide, hyperosmotic agents, a topical beta-blocker, and pilocarpine. Epinephrine should not be used because it exacerbates angle closure.

Trabeculectomy, to open the drainage canals or make an opening in the iris, can be effective in increasing the outflow of aqueous humor. This surgery is usually successful, but the effects often last less than one year. Nevertheless, this is an effective treatment for patients whose IOP is not sufficiently lowered by drugs and for those who can't tolerate the drugs.

Laser peripheral iridotomy is a procedure used almost exclusively to treat narrow angle glaucoma. It involves creating a small opening in the peripherial iris that allows aqueous fluid to drain from behind the iris directly to the anterior chamber. This procedure typically result in "opening up" the narrow angle between the iris and the cornea, in essence converting a narrow angle into an open angle.

Argon laser trabeculoplasty is usually recommended when medications have not been able to sufficiently control IOP, although it is increasingly advocated as primary therapy for patients who are not good candidates for the use of glaucoma medications or who cannot use eyedrops. In this procedure, the beam of an argon laser is directed at the trabecular meshwork. Typically about 180° of the trabecular meshwork is treated with laser spots. As a result of this procedure, the drainage of aqueous fluid out of the eye increases, thus lowering IOP.

Gene therapy may also be part of future treatments. A mutation in the gene myocilin is believed to cause most cases of juvenile glaucoma, and 3–4% of adult glaucoma. As of 2001, researchers are investigating drugs that inhibit myocilin production. The drug therapy would not just treat IOP, but also could be used before glaucoma's onset.

Vitamin C, vitamin B₁ (thiamine), chromium, zinc, and rutin may reduce IOP.

Patients using alternative methods to attempt to prevent optic nerve damage should be advised they also need the care of a traditionally trained ophthalmologist or optometrist who is licensed to treat glaucoma, so that IOP and optic nerve damage can be monitored.

Prognosis

About half of the people who have glaucoma are not aware of it. For them, the prognosis is not good, and many of them will become blind. On the other hand, the prognosis for treated glaucoma is excellent.

Health care team roles

Nursing and allied health professionals play an important part in the diagnosis and treatment of glaucoma. Skilled ophthalmic technicians and assistants record the patient history and perform many of the preliminary tests. Depending on skill level, these ophthalmic assistants may perform measurement of visual acuity under both low and high illumination, assessment of ocular motility and binocularity, visual fields, measurement of IOPs with tonometers, evaluation of pupillary responses, and refraction.

Before surgical procedures, nurses and assistants also prepare the operating room (OR). Many ophthalmologists now have their own ambulatory surgery centers where skilled technicians and ophthalmic nurses play a critical role in preparing the OR and patients for the surgery. Ophthalmic nurses also assist the ophthalmologists during surgery and discuss outcomes with patients post-operatively.

Nurses and assistants assist patients by explaining the sometimes difficult regimen of glaucoma medication. In some cases, patients require several doses of a combination of medications. Ophthalmic nurses and assistants show patients the correct technique for inserting eyedrops, and reinforce the physician's instructions for medication compliance.

Patient education

Ophthalmic assistants and nurses help to ensure that patients return to the physician's office in a timely manner so that IOPs can be monitored. Nurses and assistants also emphasize the importance of adhering to the eyedrop schedule to keep IOPs at a lower level, and answer any questions concerning proper eyedrop instillation.

Prevention

Because glaucoma may not initially cause symptoms, the best form of prevention is to have regular eye exams.

Patients with narrow angles should avoid certain medications (including some over-the-counter medications, such as some cold or allergy medications). Patients who are glaucoma-susceptible (i.e. have narrow angles and borderline IOPs) should be advised to read the warning labels on over-the-counter medicines and inform physicians of products they are considering taking. Steroids may also raise IOP, so patients may need to be monitored more frequently if it is necessary for them to use steroids.

Not enough is known about the underlying mechanisms of glaucoma to prevent the disease itself. However, prevention of optic nerve damage from glaucoma is essential and can be accomplished when glaucoma is diagnosed and treated. As more is learned about the genes that cause glaucoma, it may become possible to test DNA and identify potential glaucoma patients, so they can be treated before IOPs become elevated.

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KEY TERMS

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Agonist—A drug that mimics one of the body's own molecules.

Alpha-2 agonist (alpha-2 adrenergic receptor agonist)—A class of drugs that binds to and stimulates alpha-2 adrenergic receptors, causing responses similar to those of adrenaline and noradrenaline. They inhibit aqueous humor production and have a wide variety of side effects, including dry mouth, fatigue, and drowsiness.

Aqueous humor—A transparent liquid, contained in the anterior chamber (between the cornea and lens) of the eye, that is composed of water, sugars, vitamins, proteins, and other nutrients.

Beta blocker (beta-adrenergic blocker)—A class of drugs that binds to beta-adrenergic receptors and thereby decreases the ability of the body's own natural epinephrine to bind to those receptors, leading to inhibition of various processes in the body's sympathetic system. Beta blockers can slow the heart rate, constrict airways in the lungs, lower blood pressure, and reduce aqueous secretion by ciliary tissues in the eye.

Carbonic anhydrase inhibitor—A class of diuretic drugs that inhibits the enzyme carbonic anhydrase, an enzyme involved in producing bicarbonate, which is required for aqueous humor production by the ciliary tissues in the eye. Thus, inhibitors of this enzyme decrease aqueous humor production. Some side effects of these drugs are urinary frequency, kidney stones, loss of the sense of taste, depression, and anemia.

Cornea—Clear, bowl-shaped structure at the front of the eye. It is located in front of the colored part of the eye (iris). The cornea lets light into the eye and partially focuses it.

Gonioscope—An instrument used to examine the trabecular meshwork. It consists of a magnifier and a lens equipped with mirrors, which sits on the patient's cornea.

Hyperosmotic drugs—A class of drugs for glaucoma that increases the osmotic pressure in the blood, which then pulls water from the eye into the blood.

Iris—The colored part of the eye just behind the cornea and in front of the lens that controls the amount of light sent to the retina.

Lens (the crystalline lens)—A transparent structure in the eye that focuses light onto the retina.

Miotic—A drug that causes pupils to contract.

Ophthalmoscope—An instrument, with special lighting, designed to view structures in the back of the eye.

Optic nerve—The nerve that carries visual messages from the retina to the brain.

Prostaglandin—A group of molecules that exerts local effects on a variety of processes including fluid balance, blood flow, and gastrointestinal function.

Prostaglandin analogue—A class of drugs that are similar in structure and function to prostaglandin.

Retina—The inner, light-sensitive layer of the eye containing rods and cones.

Sclera—The tough, fibrous, white outer protective covering that surrounds the eye.

Tonometry—The measurement of pressure.

Trabecular meshwork—A sponge-like tissue located near the cornea and iris that functions to drain the aqueous humor from the eye into the blood.

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Mary Bekker