What is Glaucoma?

Glaucoma is a term that describes a group of eye conditions that affect vision. Glaucoma often affects both eyes, usually in varying degrees. One eye may develop glaucoma quicker than the other. If left untreated, glaucoma can cause blindness. But if it is diagnosed and treated early enough, further damage to vision can be prevented.

Glaucoma occurs when the drainage tubes (trabecular meshwork) within the eye become slightly blocked. This prevents eye fluid (aqueous humour) from draining properly. When the fluid cannot drain properly, pressure builds up. This is called intraocular pressure. This can damage the optic nerve, which connects the eye to the brain, and the nerve fibres from the retina (the light-sensitive nerve tissue that lines the back of the eye).

[From the NHS website]

How Do We See?

From the optical viewpoint, the eye is rather like a camera. Light rays from an object being viewed are focussed by the transparent “window” in front of the eye (the cornea) and by the lens situated behind the iris diaphragm (the coloured part of the eye).

The light rays are focussed on the retina producing an image, just like the image produced on the film in your camera. This image causes complex changes in certain light-sensitive cells in the retina so that the light energy is transformed to electrical nervous impulses or messages. These messages travel up the optic nerve to the brain, where they are interpreted, and the image is perceived.

Why is the Optic Nerve Important?

The optic nerve, like any other nerve, can only function if it is supplied with the proper amount of blood, carrying all the nutritious substances to the sensitive nerve cells.

The commencement, or head, of the optic nerve, just within the eyeball, is known as the optic disc. The tiny blood vessels nourishing the optic disc are particularly important, as we shall see.

The front part of the eye is filled with a watery fluid, which nourishes the cornea, lens and other structures. This fluid is constantly being produced, and is constantly drained away, so that a certain pressure is normally maintained within the eye (the intra-ocular pressure).

If the fluid is not drained away as fast as it is produced, then the intra-ocular pressure will rise.

We are now in a position to see what goes wrong with the functioning of the eye in glaucoma.

Types of Glaucoma

Why is high pressure dangerous?

We will focus our attention on those tiny blood vessels supplying blood to the optic disc. In glaucoma, the flow of blood in these vessels is reduced, so that the optic disc is starved of blood, and is therefore damaged. The damaged optic nerve does not conduct the electrical messages properly, and so vision becomes impaired.

In glaucoma, the flow of blood in the vessels of the optic nerve head is reduced if either the feeding blood vessels are narrowed by disease, or if the pressure in the eye is raised, thus compressing the vessels, or both (usually both).

Glaucoma is divided into different types depending on the ways in which the intra-ocular pressure becomes raised, i.e. the ways in which the drainage of fluid from the front part of the eye is obstructed.

Types of Glaucoma

In the rare congenital glaucoma, affecting infants, there is a faulty development of the drainage channels.

Because the infant’s eye is distensible (can swell and stretch), it tends to become larger in the face of increasing pressure. Tiny splits therefore occur in the inner surface of the cornea, allowing fluid to enter its substance so that the cornea becomes cloudy.

In acute glaucoma, the eye is so constructed that the lens and iris are very close to the cornea. Under certain conditions there is a sudden and severe obstruction to the outflow of fluid from the front part of the eye, and therefore the intra-ocular pressure rises suddenly and markedly. This causes the eye to become very red and painful, and the vision becomes blurred.

In chronic simple glaucoma, by far the commonest kind, there is a slow, progressive development of obstruction to the drainage of fluid from the eye, accompanied by a slowly progressive deficiency in blood supply to the head of the optic nerve.

This kind of glaucoma usually affects middle-aged or elderly people, but may affect younger people, particularly if other members of the family are affected.

In secondary glaucoma, the rise in pressure within the eye is due to another disease, commonly inflammation.

Whatever the cause of the raised intra-ocular pressure, the end result, if the condition is untreated, is the same; damage to the optic nerve head due to diminished flow in its tiny blood vessels.

What To Look For?

We have already described the symptoms of an acute attack of glaucoma, and if you should develop a red, painful eye with blurred vision, you should consult your doctor, or attend your local eye hospital at once.

Sometimes, before the acute attack, you may notice episodes of misty vision with the appearance of a rainbow-coloured halo when looking at a light. You should seek advice about this too, as these symptoms can also be caused by chlorine.

It is more difficult to know whether you might be developing chronic simple glaucoma, because the early symptoms are virtually non-existent, although you may notice slight blurring of the vision and slight aching.

Prevention is better than cure

It is wise to attend your optometrist with symptoms such as these, and indeed for a routine sight test every two years or so after the age of 45. They will be able to decide whether there might be a possibility of glaucoma (or other eye disease), and will refer you to a consultant ophthalmic surgeon if necessary.

It is important for blood relatives of patients with chronic simple glaucoma to be checked for this disease in the thirties and onwards, since there is some tendency for it to run in families.

How Do We Check for Glaucoma?

There are six routine tests:

1.  Your vision will be recorded in the usual way by asking you to read letters on a distant chart.

2.  Your eyes will be examined with particular reference to the optic disc, which, as we have seen, is damaged in glaucoma. The degree of damage can be assessed by direct observation, using a special instrument that directs a beam of light into your eye, and any change in the appearance since your last attendance is noted.

3.  Your visual fields will be measured. The visual field is the range of vision you have on each side of the object on which your eye is fixed. It is “what you see out of the corner of your eye.”

4.  Your intra-ocular pressure will be measured. We have seen that one of the causes of damage to the optic disc in glaucoma is that the pressure inside the eye is too high. It is important that this pressure is measured each time you visit.

5.  OCT (Ocular Coherence Tomography).  This is a more in-depth assessment of the optic disc and nerve fibre layer on the retina.  The OCT is similar to an MRI scan except it uses light waves and not electro magnetic waves.  Research indicates that it can potentially identify risk factors associated with glaucoma up to five years earlier than the standard test techniques.

6.  Frequency Doubling Assessment.  The Frequency Doubling instrument is a form of visual fields which targets areas which are especially vulnerable to glaucoma.