Glaucoma

Glaucoma is a condition which affects the optic nerve, which, with its 1.2 million nerve fibers, transmits all visual information to the brain. The problem in glaucoma is that the pressure in the eye is higher than normal. This leads to pressure being exerted on the nerve as it enters the back of the eye. As a result of this pressure, nerve fibers are damaged.

Typically, particular areas of the nerve are affected early on in the disease, leading to characteristic changes to the field of vision. Ultimately, if left untreated, all the fibers are lost and profound visual impairment will ensue. The incidence of glaucoma increases with age. Less than 1% of the population under 40 are affected, rising to as high as 15% in the ninety age group. There are genetic factors at play; if you have a first degree relative (mother, father, brother, sister) who is affected, you have about an 8% risk of developing the disease. Glaucoma can also be associated with diabetes and cataract.

© 2009 American Academy of Ophthalmology

Left image: The effects of raised pressure are felt here at the optic nerve head where the nerve leaves the eye.
Right image: This nerve is completely cupped, there will be almost
total loss of visual field and poor vision.

There are two main types of glaucoma:

• Open angle
• Closed angle (narrow angle)

The angle refers to the area inside the eye (in front of the pupil) where aqueous fluid drains away from the eye, thus relieving pressure.

© 2009 American Academy of Ophthalmology ; Courtesy Dr Benelli

The diagrams show the relationship between the angle and the drainage canal in the eye. In open angle glaucoma the problem lies in the drainage canal itself. The picture on the right shows the appearance of narrow angles as seen with the slit-lamp microscope

There are some less common variations of the two types described above including:

• Low Tension or Normal Tension glaucoma – the pressure is “normal” despite optic nerve head changes and field loss.
• Pigmentary glaucoma, where pigment from the iris is chafed off and obstructs the drainage angle, raising the eye pressure.
• Pseudexfoliative glaucoma, where an abnormal protein derived from the lens obstructs the drainage angle raising the eye pressure.

© 2009 American Academy of Ophthalmology ; Courtesy Matt Poe

In Pseudexfoliative glaucoma (left) material seen on the lens blocks the drainage canal. Similarly in pigmentary glaucoma (right) pigment is shed from the back of the iris and blocks the drainage canal.

There is a condition called ocular hypertension, where the pressures in the eye are higher than normal, but there is no damage to the optic nerve. These cases may require treatment with drops if the pressure gets too high in order to prevent retinal artery or vein occlusions. Patients with this condition do need to be monitored as some of them do subsequently go on to develop glaucoma.

Narrow angles – No glaucoma

Some people, often those who are longsighted (hyperopic), have narrow angles but no glaucoma. There is a risk that they will develop the acute form of angle closure glaucoma. A laser iridotomy (see below), may be all that is needed to prevent them form running into trouble.

The large majority of glaucomas (90%) are of the open angle type, and most of these are called Primary Open Angle Glaucoma. Closed angle glaucoma is more common in Southeast Asians, those who are long-sighted and those with a family history.

Unfortunately there are rarely any symptoms until late on in the disease. Usually there is no pain or redness and the vision remains good. It is only in advanced cases – where a large amount of field is lost or vision is reduced – that you will notice anything amiss. Sometimes it is only after the first eye has lost sight that the disease is picked up.

In the case of acute closed angle glaucoma there are symptoms. These may consist of episodes of visual loss (blurring, like looking through frosted glass), associated with a dull throbbing pain. These can sometimes go on for several days or may rapidly deteriorate to unremitting pain and throbbing in the eye. At this stage there is usually profound loss of vision. The eye becomes red, and the pupil may become dilated. Sometimes the pain can lead to vomiting. With these symptoms you should seek the attention of an ophthalmologist urgently.

Since most types of glaucoma are asymptomatic, screening is the only way to detect the disease. All people over the age of 40 should have their pressures checked. The optometrist can do this when you reach the age of 40, which ties in conveniently with when you may be considering reading glasses. Anyone with a family history of glaucoma should have the eye pressures checked. The optic nerve should be examined with a microscope or photographically. A field-test and scan of the nerve head may be included by the optometrist. If there are any parameters that the optometrist is not happy about, you will be referred to an ophthalmologist.

© 2009 American Academy of Ophthalmology

The field test on the left shows the characteristic arcuate scotoma. The scan on the right shows the development of field defects over time.

Medical therapy:

The aim is to get the pressure in the eye down to normal levels. This can either be done with medications (usually drops) or surgery if drops fail to control the spressure. In most cases drops suffice and less than 10 % of cases require surgery. The groups of pressure controlling drops are:

• Prostaglandin analogues – Xalatan, Travatan, etc
• Beta blockers – Timolol, Betaxolol
• Alpha-2-agonist – Alphagan
• Topical carbonic anhydrase inhibitors – Trusopt, Azopt
• Cholinergic drugs – Pilocarpine
• Diamox tablets

There are also many multi-combination drugs, which feature a mixture of the two classes of drugs for added potency and convenience.

The drops work by either reducing the production of aqueous fluid in the eye, or by allowing more fluid to drain out of the eye. Drops are generally well tolerated. The evidence suggests that by controlling pressures, the progression of the disease be can be slowed down so that it will not have a major impact upon vision.

The most common problem with management is non-compliance with the drops, particularly amongst those who have not detected a change in the vision. It is so easy to lose awareness of the importance of an asymptomatic condition.

Laser Therapy:

Laser trabeculoplasty, is a painless treatment that alters the drainage angling such that more aqueous fluid passes through it. It is effective in many cases and is an alternative to drops or can reduce the number of drops required. It can also be repeated. It is a useful treatment when there is allergy to drops, compliance problems or intolerance of drops.

Laser iridotomy is used to make a hole in the iris when the angles are narrow and the vision is threatened.

Surgery

Surgery is usually a last resort as attendant risks need to be considered. This is either in the form of a trabeculectomy, where there is a new passage created in the wall of the eye, allowing the fluid to drain. Alternatively a tube can be inserted in the eye, allowing the fluid to drain.

We do supply an information booklet that details the risks and benefits of laser and surgical procedures.

© 2009 American Academy of Ophthalmology

A tiny hole is made in the eye and the fluid drains out through the hole under the conjunctiva – this is effective at lowering pressure when drugs fail.

For more information on glaucoma try:

www.glaucoma.org.au
http://www.glaucoma-association.com
http://www.rcophth.ac.uk/page.asp?section=365&sectionTitle=Information+Booklets

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