Introduction and Overview
Glaucoma is a term used to describe a condition usually characterized by an increase in intraocular pressure (IOP) which damages the tissues in the eye.
Basic Principles of Glaucoma
The eye is filled with fluids which have the function of keeping the structures inside it, the iris, crystalline lens, and retina, in place and nourished. The largest part of the eye, called the posterior chamber, is filled with the vitreous body, also known as the vitreous humour; it is located behind the crystalline lens and fills the back part of the eye. The vitreous isn’t really liquid, but a fairly stiff gelatin.
The aqueous humour fills the front part of the eye, bathing the lens, iris and the back of the cornea in nutrients, called the anterior chamber.
Figure 1 shows a diagram that illustrates the basic structures of the normal eye. The angle formed between the cornea (light blue, farthermost left in the diagram) and the iris (in dark pink) is where fluid drains from the eye. If the angle formed here is too narrow, there may be problems with keeping the pressure within the eye low enough.
The pressure inside the eye must be maintained at a certain level, otherwise the eye would collapse in on itself, but the pressure must not be too high or it will damage the tissues it is meant to nourish. The measurement of pressure within the eye is part of a comprehensive vision examination. The units of measure are the same as that for atmospheric pressure, millimeters of mercury. A normal IOP within the eye would be somewhere between about ten and twenty mmHg. (Pressure measurements use the same method as meteorologists use to describe barometric pressure in the atmosphere.)
If one thinks of the eye as being a sink where the faucet is always running and the drain is always open, glaucoma would be the result of slow or stopped drainage. While this would be wasteful in our kitchens, it is necessary in the eye so that the aqueous humour is always being renewed and, once depleted of nutrients, is drained away.
The aqueous is manufactured in the ciliary body, (Shown in Figure 1 in dark pink) which is located behind the iris and is anatomically part of it; it is a muscular, vascularized gland that is not only responsible for making aqueous but of changing the shape of the lens to allow for flexible focusing.
Once the aqueous has circulated over the lens and filtered between the lens and the iris on into the space between the iris and the back of the cornea, it drains out of the eye via a system of tiny canals located in the angle formed between the back of the cornea and the iris. This network of canals is called the trabecular meshwork.
Primary Open-Angle Glaucoma (POAG)
If the trabecular meshwork loses its efficiency, the drainage slows down and the result is a gradual increase in pressure within the eye over time. This type is called Primary Open-Angle Glaucoma, (POAG) and is the most common type of glaucoma; 90% of glaucoma patients have POAG, not Angle-Closure glaucoma, which is usually considered to be a separate condition entirely. This article is concerned with Angle-Closure Glaucoma, but more information on POAG is available here.
Angle-Closure Glaucoma (ACG) is also known as Acute Angle-Closure Glaucoma or Narrow-Angle Glaucoma. It occurs as the result of a drastic rise in internal pressure from a sudden blockage of the circulation and drainage of the aqueous from the eye.
Symptoms and Signs
Unlike POAG, angle-closure glaucoma causes significant symptoms which appear within hours, and may include pain, nausea, vomiting, blurred vision, redness of the eye and haloes around lights. The pressure may reach 60 to 90 mmHg in an episode of angle-closure glaucoma.
Angle-closure glaucoma is a sight-threatening emergency. If symptoms like this occur, the patient must see an eyecare specialist as soon as possible. Measures can be taken to lower the pressure, including medications and conventional or laser surgery. The pressure must be reduced quickly or the eye will be permanently damaged.
Only about one in 1,000 people might have ACG. It is difficult to predict who may experience this type of pressure increase, but some of the risk factors are hyperopia (far-sightedness), and a smaller than normal anterior chamber, the space between the cornea and iris. The majority of cases of this type of glaucoma are in people of Asian descent, indicating that there is a genetic component so the family history is important. (POAG also tends to run in families.) Also, as we age, the crystalline lens within the eye becomes thicker and may push the iris forward enough to make the patient susceptible to ACG attack.
There are some tests that can be done on those patients who may be susceptible to Angle-Closure Glaucoma. These tests attempt to cause an attack of ACG, either by using drops to dilate the patient’s pupils, as would be done during a routine comprehensive vision examination. This has some risk, as the attack does not usually occur in the office, but several hours later, when the patient is at home and the pupil dilation is wearing off. Because of this, eyecare practitioners doing this may schedule a second visit later in the day to evaluate pressures at that time.
An even simpler way to test for the possibility of an acute attack is to have the patient go into a dark room and allow the pupils to dilate on their own. The physician then can re-examine the angle and re-test the IOP.
There is a chance that patients who are susceptible to ACG will not experience any significant symptoms, but may have episodes of mild pain, slightly blurred vision or see haloes from lights. These subacute attacks usually spontaneously resolve as the angle reopens. Still, these patients are at much greater risk to suffer a full-blown attack of ACG.
Treatment and Prevention
The first task in an acute attack of ACG is to decrease the pressure medically if possible, with the use of pressure-lowering drops or oral medications. Drops to constrict the pupil may be used, to help pull the iris out away from the trabecular meshwork and allow some of the aqueous to flow out.
Recently, with the advent of more types of surgical lasers, patients may have a laser peripheral iridotomy (LPI), which involves using the laser to make a hole through the peripheral iris so aqueous may flow more freely into the drainage system. Laser iridotomy is used to treat those patients who may be subject to future attacks, or if any of the provocative tests mentioned above result in a rise in IOP.
Because once a person has had an attack of ACG in one eye, the chances are quite high (about 50%) that the fellow eye will also suffer one, so most specialists in glaucoma treatment advise LPI of the fellow eye after an angle-closure event.
Laser peripheral iridotomy is quite effective in controlling ACG and preventing recurrence. It is a relatively easy procedure for the patient, usually done in the office or a small surgical center. It does not require general anesthesia and recovery time is relatively short. While some people may experience a mild sharp pain sensation in the eye during the procedure, most will have no pain after the surgery at all.
It is important to continue to see an eyecare practitioner on a continuing basis, even if you’ve had LPI, for monitoring IOP and other factors.
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