What is a glaucoma?

Glaucoma is a group of diseases characterised by a build up of pressure within the eye such that nerve cells in the back of the eye slowly die. It can affect people of all ages but is most common after age 40. Untreated glaucoma can cause blindness but thanks to modern eyedrop preparations most cases of glaucoma are easily treated and controlled.

Eye pressure is determined by a balance between the secretion of aqueous fluid from a gland called the ciliary body and drainage of the aqueous fluid out of the eye via the channel known as Schlemm's canal. For normal pressure the rate of production would slightly exceed the rate of drainage. High pressure can result most commonly from obstruction of drainage. Low pressure can occur through a variety of mechanisms, but usually by excessive drainage.

Eye pressure can be measured in a variety of ways but the most accurate is the Goldmann tonometer (pictured right). This allows the eye doctor to balance a known force over a known surface area on the eye to measure the unknown force within the eye. The doctor would put yellow drops into your eye for this test and vision is not affected.

Classifications of Glaucoma

Glaucoma is classified as follows:

1. Open angle glaucoma - the most common and usually not with any symptoms
   
2. Narrow angle glaucoma - often associated with pain in the dark or seeing haloes around lights at night
   
3. Acute closed angle glaucoma - acute, painful, and rapidly blinding
   
4. Normal pressure glaucoma
   
5. Congenital (at birth) or juvenile (childhood) glaucoma - uncommon but often devastating.

Open angle glaucoma

The aqueous is unable to enter Schlemm's canal due to a microscopic alteration in the trabecular meshwork (drain grate) - like leaves blocking a downpipe. This is the most common type and is usually painless and without symptoms.

Narrow angle glaucoma

The aqueous fluid is partly or intermittently prevented from entering Schlemm's canal due to the iris pressing up against the trabecular meshwork. This type may be associated with eye pain in the dark and seeing haloes at night.

Closed angle glaucoma

The trabecular meshwork is completely blocked by the iris and eye pressure rises suddenly and to very high levels. If left untreated blindness usually results within 24 hours. This condition is a medical emergency and urgent attention from an eye doctor is required. This glaucoma causes severe eye pain, loss of vision, vomiting. It may be preceded by symptoms of narrow angle glaucoma.

Congenital glaucoma

A baby may be born with glaucoma. Symptoms include excessive tearing, sensitivity to light, irritability, large-appearing eyes, cloudiness of the eyes (seen in this photo). This is an emergency as blindness may result quite quickly. Seek urgent attention from an eye doctor. Young children can also develop glaucoma and may have similar symptoms as above.

Diagnosis of glaucoma

This is done by your eye doctor (ophthalmologist). After taking a complete history, the doctor will measure vision, examine the eyes, measure the eye pressure, inspect the drainage channels, and look at the optic nerves. These nerves carry eyesight from the eyes to the brain and are selectively damaged in glaucoma.

A test of the side or peripheral vision is often done and is called the "visual fields". In open angle glaucoma it is usually the side vision that is affected first and visual fields allow for early detection and monitoring of glaucoma.

Early or subtle cases may be difficult to diagnose and repeated examinations may be necessary. Some people are labelled "glaucoma suspects" when a firm diagnosis has not been made despite repeated tests. Examining the optic nerves is critical and the eye doctor may be able to immediately asses whether glaucoma damage has occurred.

Above left, is a stereo pair image of the optic nerve showing a central white "cup" and a surrounding pink "rim", stare "through" it to get the stereo effect. The doctor assesses the size of the cup as a measure of glaucoma damage. The above right photo shows an optic nerve with an enlarged cup. This eye has glaucoma and side vision would be affected.

Occasionally other tests may be used such as measuring pressure after opening the pupils, drinking water, or measuring pressure over an entire day.

Symptoms of glaucoma

Chronic (primary open-angle) glaucoma is the commonest type. It has no symptoms until eye sight is lost at a later stage. Damage progresses very slowly and destroys vision gradually, starting with the side vision. One eye covers for the other, and the person remains unaware of any problem until a majority of nerve fibres have been destroyed, and a large part of vision has been destroyed. This damage is irreversible. It is progressive and usually relentless. Treatment cannot recover what has been lost. But it can arrest, or at least, slow down the damage process. That is why it is so important to detect the problem as early as possible, to be able to start treatment with as little damage to the vision as possible.

Can Glaucoma be treated?

Glaucoma is treated in a variety of ways. All treatment methods aim to reduce the rate of formation of aqueous fluid or increase the ease or rate at which it can drain out of the eye through Schlemm's canal. This results in reduction of eye pressure. Generally most cases of glaucoma are effectively treated with various types of eye drops. Eye drops can work by reducing formation of aqueous, increasing outflow from the eye or opening up narrow angles to increase drainage. All eye drops have a risk of side effects. Mostly side effects are confined to the eye. Some eye drops are known to precipitate asthma and have other effects on the body. Ask your doctor about side effects.

Laser treatment is required in open angle glaucoma if pressure is not satisfactorily controlled with eye drops. A technique called "laser trabeculoplasty" is used in this setting. This form of treatment usually does not obviate the need for eye drops.

People who have narrow angle glaucoma or have an acute episode of acute angle closure glaucoma frequently need an opening created in the iris to allow the angle to open up and drain aqueous fluid more effectively. Such an opening is mostly done with laser and is called an "iridotomy" and is indicated by the arrows in the right hand picture.

A surgical procedure called "iridectomy" is also used occasionally for narrow angle or acute angle closure glaucoma. It creates an opening in the iris as seen in the left hand picture above and schematically here.
Surgery is also used in severe cases of open angle glaucoma when all other treatments have proved inadequate for eye pressure control. The aim of surgery is to create an artifical drainage channel, effectively bypassing Schlemm's canal. The operation, known as "trabeculectomy", essentially creates a valve-like mechanism to regulate flow out of the eye.

All eye surgery is associated with potential risks, complications, and adverse outcomes. You should discuss these issues with your eye doctor.

Other types of procedures for glaucoma include cyclophotocoagulation, cyclocryotherapy, Molteno valve implants. Discussion of these is beyond the scope of this overview of glaucoma.