Glaucoma
Glaucoma is the second most common cause of blindness in the US. There are four major types of glaucoma:
• Open-angle (chronic) glaucoma
• Glaucoma secondary to other diseases or from drugs
• Angle-closure (acute or chronic) glaucoma
• Congenital glaucoma
All but a few types of glaucoma are characterized by increased pressure within the eyeball, sufficient to cause progressive damage to the optic nerve.
Glaucoma affects many parts of the eye, but the major and most serious effect is on the optic nerve. This is because, as the pressure goes up in the eye, it pushes out the most compliant part of the eye wall—and that’s where the optic nerve, with its blood vessels and fibers, enters the eye.
How is pressure in the eye produced?
The front part of the eye is filled with a clear fluid called the aqueous humor, which carries the nutrition of the lens and the cornea as well as many other metabolic necessities. This fluid is constantly made by the ciliary body, just behind the iris. It flows around the lens, forward through the pupil, and after circulating in the anterior chamber, leaves the eye through channels located between the front of the iris and the cornea.
These channels that drain the aqueous humor are in an area called the anterior chamber angle, or simply the angle, through a complex structure called the trabecular meshwork, or just meshwork. The aqueous eventually drains into the bloodstream.
Open-angle glaucoma (OAG) generally is a problem associated with age and family history (first degree relatives), and Hispanic and African descent. It is by far the most common type of glaucoma.
Genetic markers have been localized that predict the development of the disease in some people. The cause for slowing of the aqueous flow through the angle is a loss of the cells that actively transport the aqueous and maintain the meshwork.
Variations of OAG, where the meshwork is “clogged” by particles of pigment (pigment dispersion glaucoma), protein (exfoliation glaucoma), or blood (for example, ghost cell glaucoma) have a different course, treatment and prognosis (outlook) than the typical stealthy and long term OAG.
Unless there is another problem ongoing, OAG is silent, in most cases. There are no symptoms unless the pressure in the eye is very high, or very late in the disease, when loss of contrast and then loss of the peripheral vision creeps in. The detection of damage from OAG is detailed below.
There are instances where the meshwork does not function due to inflammation, that is, the presence of white blood cells, from various causes. Inflammation of any sort is called “-itis”, but the reason for the inflammation may or may not be known. So, a “trabeculitis” may cause the meshwork to malfunction, and the pressure of the eye to increase despite an open angle, due to inflammation from infection (for example, herpes), after surgery, from autoimmune diseases, or unknown causes (Posner-Schlossman syndrome).
Steroid-induced glaucoma is a special case of reversible elevation of pressure in the eye. Steroids, either by eye drops or by mouth, have the potential to cause glaucoma by stabilizing cell membranes of the cells in the meshwork that have to “catch and release” the aqueous, so their function is slowed. People who have a tendency for elevated eye pressure with short or minimal exposure to steroids are termed “steroid responders”, and some studies indicated that they might be at greater risk for OAG as they age.
Narrow angle (angle-closure) glaucoma (NAG) is caused by a narrowing of the access to the meshwork lying in the angle between the iris and the cornea. It can happen intermittently, bouncing the pressure in the eye up and down (chronic narrow angle glaucoma, or CNAG); or it can occur in the form of a sudden, painful attack of severely elevated pressure.
People who are at risk for NAG can have a short eye, with a narrower angle, or a developing cataract, which pushes the iris forward and narrows the angle, or a thick iris, which is more common in the Asian populations.
There are rare reactions to medications that either dilate the iris of a susceptible person (dilating drops, allergy medications, certain antidepressants) or cause a shift in the musculature of the ciliary body (Topamax) that can cause a narrow angle attack.
An attack of narrow angle glaucoma causes a quick, severe, and painful rise in the intraocular pressure (IOP). Its symptoms are pain, redness, colored haloes around lights, and sometimes nausea and vomiting. Angle-closure glaucoma is an emergency. You should not wait to contact your ophthalmologist if this occurs, even if the symptoms clear up.
If you have had acute glaucoma in one eye, you are at risk for an attack in the second eye, and your doctor is likely to recommend preventive treatment (see below).
Congenital glaucoma may occur in families (hereditary). It is present at birth, and is the result of the abnormal development of the fluid outflow channels in the eye. Other structures in the eye may also be affected, such as the cornea, the iris and the lens. The hallmark of congenital glaucoma is a cloudy cornea, due to the aqueous being forced into it by the elevated pressure in the eye. The infant may also have tearing, redness, and appear to be sensitive to light
Exams and Tests
An examination of the eye may be used to diagnose glaucoma. However, checking the intraocular pressure alone (tonometry) is not enough because eye pressure changes. The doctor will need to examine the inside of the eye by looking through the pupil, often while the pupil is dilated. Additional tests are used to detect glaucoma.
Remember that the hallmark of glaucoma of any type is pressure in the eye sufficient to damage the optic nerve. Usually this means IOP that is elevated above the range of 8-22, but it also can mean that the vascular supply or the supporting structures or the optic nerve make it more susceptible to damage at “normal” IOP.
Usually the doctor will perform a complete examination of the eyes.
Tests may include:
• Visual acuity Visual field measurement. This measures the subjective functioning of the nerve fiber layer and can show characteristic loss of the peripheral vision. It can clearly indicate the presence of glaucoma in a person who is a good test taker (specificity), but it may not detect glaucoma until there is already considerable damage (not sensitive). It is also subject to a number of potential sources of error, like patient reliability and attentiveness, or problems of the eye like cataract or macular degeneration not related to glaucoma.
• Pupillary reflex response, which will not be equal if one eye is more damaged from glaucoma than the other.
• Intraocular pressure (IOP) measurement by tonometry. The normal range is 8-22 mmHg (millimeters of mercury). There are several instruments used to measure the pressure in the eye, and often more than one is used to check the accuracy of the reading. Also, the doctor may check the IOP at different times of day at different visits, or several times over the course of a day (serial tonometry). The IOP can be falsely high if the person is squeezing his eyes shut, or holding his breath, or has a thicker than average cornea.
• Corneal thickness (pachymetry) is measured to determine the effect of thickness and therefore compliance on the measured IOP. This concern arose with the widespread use of refractive surgery, which thins down the cornea. A thinner cornea has greater compliance and might seem to have a lower IOP than what is true. Conversely, a thicker cornea might seem to have a higher IOP than true.
• Gonioscopy uses a special lens to see the outflow channels of the angle between the iris and the cornea, like a dentist’s mirror. The lens touches the eye, so the patient has an anesthetic drop placed in the eye first.
• Slit lamp examination. This instrument is a microscope with a linear beam of light used to evaluate the front of the eye, and, with an additional lens, the magnified and 3 dimensional image of the optic nerve.
• Optic nerve photographs, to document the effect of pressure constantly pushing on the nerve, or “cupping”. The normal cup-to-disc ratio is 20-60%, but there are many variations in the appearance of the optic nerve and its appearance must be evaluated in the context of the person’s history, symptoms and examination.
• Optic nerve fiber analysis (instruments include the OCT, HRT or GDx currently). No matter what the look or the shape of the optic nerve, the numbers of nerve fibers coming into the eye and fanning out under the retina tend to fall within a range for a given age of patient. So, these instruments measure the thickness of the nerve fiber and give a statistical analysis of the patient’s nerve fiber layer vs. their peers. It adds to the information obtained by the visual field test, and does not depend on the patient’s response. It can be followed over time, and some instruments allow a digitalized measurement of the cupping and contour of the optic nerve.
• Retinal examination
Sometimes, the diagnosis of glaucoma is not clear. If someone walks into the exam with an IOP of 40, that’s an easy diagnosis. But if someone has no symptoms, a strong family history of OAG, is over 50 and he has a borderline eye pressure (say, 23) and a large cup-to-disc ratio (C/D), what should the physician do? What is the visual field is not reliable, and a cataract makes the nerve fiber layer analysis less than optimal?
Sometimes, the answer can only be determined over the course of several visits, over months or even years. All the while, the patient may not have any symptoms at all!
When to contact a Medical Professional
Call your health care provider if you have severe eye pain or a sudden loss of vision, especially loss of peripheral vision.
Call for an appointment with your health care provider if you have risk factors for glaucoma and have not been screened for the condition.
Prevention
There is no way to prevent open-angle glaucoma, but you can prevent vision loss from the condition. Early diagnosis and careful management are the keys to preventing vision loss.
Most people with open-angle glaucoma have no symptoms. Everyone over age 40 should have an eye examination at least once every 5 years, and more often if in a high-risk group. Those in high-risk groups include people with a family history of open-angle glaucoma, and people of African or Latino heritage. Studies are equivocal whether the presence of diabetes is a risk factor for glaucoma. Hypertension does not seem to be a risk factor.
People at high risk for acute glaucoma may opt to undergo laser iridotomy before having an attack. Patients who have had an acute episode in the past may have the procedure to prevent a recurrence, and should consider having a preventative iridotomy in the fellow eye. Again, this is a very safe procedure.
