Glaucoma

Glaucoma is an eye condition where the nerve at the back of your eye (the optic nerve) is damaged. This can lead to loss of vision. In most cases, the damage to the optic nerve is due to an increased pressure within the eye.
Structure (anatomy) of the eye

When one looks at an object, light from the object passes through the cornea of the eye, then the lens, and then it hits the retina at the back of the eye. The cornea and the lens both help to focus the light on to the retina. Nerve messages pass from the ‘seeing cells’ (rods and cones) in the retina, down nerve fibres in the optic nerve to the brain. The messages are interpreted by the brain, which enables one to see.
The pupil (the black area in the middle of the eye) helps to regulate the amount of light that gets into the eye. The muscles of the iris (which gives colour to the eye) control the size of the pupil. Iris muscles can cause the pupil to enlarge (dilate) or get smaller (constrict). When the pupil is dilated, more light can get into the eye; when it is constricted, less light can get in.
The eye also needs to keep its shape so that it can work properly and so that light rays are focused accurately on to the retina. So, most of the eye is filled with a substance a bit like jelly called the vitreous humour (humour meaning fluid). The front of the eye is filled with a clear fluid called aqueous humour, which is more watery. The part of the eye behind the lens, that is filled with vitreous humour, is called the posterior chamber. The part of the eye in front of the lens, that is filled with aqueous humour, is called the anterior chamber.
The aqueous humour is made continuously by the cells of the ciliary body. The aqueous humour fluid passes through the pupil to the front part of the eye, and then drains away through a sieve-like area called the trabecular meshwork located near the base of the iris. So, there is constant production and drainage of aqueous humour fluid. This keeps the fluid levels balanced.
GLAUCOMA
There are different types of glaucoma.
Chronic open-angle glaucoma (also called chronic glaucoma or primary open-angle glaucoma) is the most common type. This develops slowly so that any damage to the optic nerve and loss of sight are gradual. The term ‘open-angle’ refers to the angle between the iris and sclera which is normal, in contrast to:
Acute angle-closure glaucoma where the angle is narrowed. This is uncommon. In this condition there is a sudden increase in the pressure within one eye. The eye quickly becomes painful and red.
Secondary glaucoma is caused by various conditions which can cause a rise in eye pressure. For example, it may develop as a complication to some eye injuries.
Congenital glaucoma is where glaucoma is present from birth.
Chronic open-angle glaucoma?
In chronic open-angle glaucoma (just called glaucoma from now on) there is a partial blockage within the trabecular meshwork. This restricts the drainage of aqueous humour. The reason why the trabecular meshwork becomes blocked and does not drain well is not fully understood. The aqueous humour builds up if the drainage is faulty and this increases the pressure within the eye.
The increased pressure in the eye can damage the nerve fibres running from the retina at the point where they converge to become the optic nerve (known as the optic nerve head or optic disc) which is the main nerve of sight. These damaged fibres result in permanent patches of visual loss. In some cases this can eventually lead to total blindness.
Glaucoma can affect both of the eyes. However, it can often progress more quickly in one eye than in the other.
What’s the difference between increased eye pressure and glaucoma?
Glaucoma means that part of the optic nerve is damaged, usually caused by increased eye pressure. Another term for eye pressure is intraocular pressure. However, about 1 in 5 people with glaucoma have eye pressures in the normal range. This is called normal pressure glaucoma. In this condition the optic nerve is damaged by relatively low eye pressures. Other factors, such as a poor blood supply, may make the optic nerve sensitive even to modest pressure.
In contrast, some people have an increased eye pressure with no ill effect to the optic nerve and no visual loss. Raised eye pressure without glaucoma is called ocular hypertension. However, as a rule, if the eye pressure is high, one has ocular hypertension and is at a much increased risk of developing glaucoma and visual loss.
Who gets chronic open-angle glaucoma?
Glaucoma is common. It is unusual in people under the age of 35. It becomes more common with increasing age. Glaucoma can affect anyone, but it is more common in the case of:
• A family history of glaucoma.
• Very short sight.
• Diabetes.
What are the symptoms of glaucoma?
There are usually no symptoms at first. There is no pain or redness in the eye. Most people with glaucoma do not notice problems until quite a bit of visual loss has occurred. This is because the first part of the vision to go is the outer (peripheral) field of vision. Central vision, used to focus on an object such as when reading, is spared until relatively late in the disease. Also, although glaucoma usually affects both eyes, it may not affect them equally. The better eye may fill in for a while if the other eye starts to lose patches of visual field.
Some elderly people with glaucoma put their gradually failing vision down to “just getting old”. They might not have had their eyes checked for many years and may needlessly lose their sight. Untreated glaucoma is one of the world’s leading causes of blindness. But, blindness can be prevented if glaucoma is diagnosed and treated early enough.
Because there are usually no symptoms at first but glaucoma then gradually causes blindness, screening for glaucoma is very important.
Who should be tested for glaucoma?
Everyone aged over 35 to 40 should have an eye check by an optician at least every five years. A check every two to three years is advised if you are aged over 50. Eye checks are particularly important if one is in any of the at-risk groups listed above. The eye check will detect early signs of glaucoma before any significant vision loss occurs. Most people with glaucoma have it detected at a routine eye check.
What does an eye test for glaucoma involve?
The eye test usually involves examining the eyes in detail using a special light and magnifier called a slit lamp. In particular, the back of the eye where the optic nerve leaves your eye (known as the optic disc) will be examined. There are specific changes that can be seen in this area in someone with glaucoma.
The pressure in the eyes (intraocular pressure) will also be measured. The thickness of the cornea may also be measured. This is because the thickness of the cornea can affect the intraocular pressure reading. A special lens may also be used to examine the drainage area (or trabecular meshwork area) of the eye. This examination is called gonioscopy. The field of vision may also be tested. This is essentially how much one can see whilst looking forward. As mentioned above, in glaucoma, it is usually the periphery (outside) of the field of vision that is affected first.
What is the treatment for chronic open-angle glaucoma?
The aim of treatment is to lower eye pressure. If the eye pressure is lowered, further damage to the optic nerve is likely to be prevented or delayed. However, unfortunately, treatment cannot restore any sight that has already been lost. The eye pressure to aim for varies from case to case. It partly depends on how high the original pressure is. The eye specialist will advise. Eye pressure can be lowered in various ways.
Eye drops
A variety of eye drops can lower eye pressure. They work either to:
• Reduce the amount of aqueous humour made
• Or, increase the drainage of aqueous humour
It is vital to use the drops exactly as instructed. An eye specialist should be seen to keep a regular check on eye pressures, optic nerves, and field of vision. How often a follow up is needed depends on a patient’s particular situation. However, it is important to attend follow-up appointments.
Tablets
Tablets work by reducing the amount of aqueous humour made. However, side-effects can be troublesome and so tablets are not commonly used these days.
Laser treatments
If eye drops are not helping to lower eye pressure enough, laser treatment may be suggested. A laser can burn the trabecular meshwork which improves the drainage of the aqueous humour. This treatment only takes a few minutes. A special contact lens is put on the eye to help the specialist focus the laser beam. A pricking sensation is felt and some flashing lights are noticed but the procedure is usually well tolerated.
Another technique is to use a laser to destroy parts of the ciliary body. This reduces the amount of aqueous humour that is made. Note: sometimes eye drops are still needed after laser surgery.
Surgery
If other treatments are not effective, an operation called trabeculectomy is an option. This involves creating a channel from just inside the front of the eye to just under the conjunctiva. So, the aqueous humour can bypass the blocked trabecular meshwork. Surgery may be advised if a trial of eye drops has failed to achieve target eye pressures, especially in younger people, or in the case of very high eye pressures.
Like with all operations, there is a small risk of complications. Also, the operation may have to be repeated in some cases. This is usually because some scar tissue forms at the site of the channel and prevents it working to drain the aqueous humour.
What is the outlook (prognosis)?
It is important to realise that most people treated for glaucoma will not go blind. However, in order to preserve sight, it is very important to follow the treatment plan outlined by the doctor.
Driving and glaucoma
Many people can drive after glaucoma is diagnosed. Even if vision is reduced in one eye, one may still be allowed to drive if the vision is good enough in the other eye. However, advice from the eye specialist is needed.

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