Glaucoma is an eye disease in which the optic nerve at the back of the eye is damaged in a characteristic way. This can initially result in a patchy loss of peripheral vision, rarely noticed by the affected individual. If the condition is undetected and allowed to progress the "side" vision gradually deteriorates and in rare circumstances can eventually result in "tunnel" vision (only a small central island of vision remaining). Glaucoma is the second commonest cause of blindness worldwide behind cataracts (see CATARACT section).
What causes glaucoma?
The exact cause of glaucoma is usually unknown but the most important risk factor in most cases is the intra-ocular pressure (IOP). This is the pressure of the fluid inside the eyeball. The IOP is easily measured using tonometry and in 95% of the normal population is between 10-21 mm of Hg (mercury). An elevated IOP above 21mm of Hg can be the first sign of glaucoma developing. The IOP tends to rise if the fluid produced inside the eye cannot efficiently drain away from the eye into the blood stream. The commonest type of problem is when the tiny drainage channels, for reasons unknown, become clogged up = Primary open angle glaucoma (POAG).
Other risk factors for developing POAG include:
- Age: glaucoma rarely affects individuals below the age of 40yrs. The chance of developing glaucoma above 40yrs old is approximately 1%, but this figure rises to 5% for those over 65yrs of age.
- Race: the incidence of glaucoma is higher in Africans and people from Eastern Asia.
- Genetics: those people with a family history of a first degree relative (ie sibling or parent) with glaucoma have a higher risk of developing the disease.
- Short-sightedness (myopia)
- Diabetes: probably due to a reduced blood supply to the optic nerve.
A full detailed eye examination is required to diagnose glaucoma including:
- 1. Tonometry
- Measurement of eye pressure (IOP) which is sometimes done at different times of the day in order to detect any peaks and troughs.
- 2. Corneal pachymetry
- This is an ultrasound measurement of the corneal thickness (window at the front of the eye) which, once taken into account, refines the IOP reading to increase accuracy.
- 3. Gonioscopy
- A contact lens mirror device that allows the drainage angle to be visualised, assessed and graded.
- 4. Ophthalmoscopy
- An instrument which is used to directly view the optic nerve and detect structural damage typical of glaucoma.
- 5. Perimetry
- A test of peripheral visual fields to look for abnormal defects (or blind spots) in the side vision eg: Humphrey visual fields.
- 6. Hiedelberg Retinal Tomography (HRT 3)
- Detailed optic nerve analysis using a laser scanning photograph which can demonstrate and document early glaucomatous damage.
What is Ocular Hypertension (OHT)?
Ocular hypertension is where there is a constantly elevated IOP but no optic nerve damage and therefore no loss of peripheral vision. However, OHT can be a precursor for developing glaucoma and in some cases it may be recommended to reduce the IOP, usually with specific pressure lowering eye drops. Overall the chance of OHT leading onto POAG is approximately 15% over a 5yr period (OHT's study).
Other rarer types of Glaucoma
The term glaucoma really refers to a group of conditions which can damage the optic nerve in a typical pattern. Although POAG is by far the commonest, there are many other types of glaucoma including:
There are several possible causes eg: blunt trauma to the eye (angle recession), oral and topical steroids, or iritis (see IRITIS section).
Angle Closure Glaucoma (ACG)
This relatively uncommon condition results from a sudden block to the fluid draining out of the eye. Unlike POAG, which is painless, this acute rise in IOP results in a very painful, red eye with blurred vision. This condition is an ophthalmic emergency and requires urgent intervention to prevent permanent sight loss. ACG occurs in people who have an unusual anatomical situation whereby the peripheral iris (surrounding the pupil) comes into contact with the cornea. This results in blocking the drainage route for fluid leaving the eye.
Normal Tension Glaucoma (NTG)
Normal tension glaucoma (also known as low pressure glaucoma) occurs in a sub-group of people who initially appear to have optic nerve changes secondary to POAG, but their IOP is not elevated above the range in the normal population. Factors other than eye pressure seem to be important in NTG, such as the blood supply to the optic nerve. People who suffer from conditions which may reduce blood flow to the optic nerve are thought to be more vulnerable to developing NTG eg: migraine which can result in vasospasm (blood vessel contraction).
How is Glaucoma Treated?
There are many different types of glaucoma but, whatever the cause, the goal of treatment is the same - to prevent loss of vision. Any sight loss due to glaucoma is irreversible and therefore it is extremely important that the condition is detected early. There are several possible management options including:
There are many different types of eye drops which can effectively lower the IOP and the most appropriate is selected depending on the type of glaucoma and other factors, such as current or previous health problems.
Rarely used for long term management, as can be associated with systemic side effects, but can be an extremely helpful interim measure in some circumstances.
The vast majority of patients can be successfully managed medically, usually with regular daily eye drops, and if picked up early most people with glaucoma do not go on to lose their sight. There are occasions, however, when additional laser or surgical intervention is required:
The most common procedure for open angle glaucoma is trabeculoplasty, whereby a laser is used to increase the drainage of fluid out of the eye.
For closed angle glaucoma the laser is used to make a hole through the iris - peripheral iridotomy, which improves the flow of aqueous into the front chamber of the eye. This procedure can be helpful in the prevention or treatment of acute angle closure glaucoma.
A relatively small number of patients, with open or closed angle glaucoma, require the eye pressure to be lowered using surgery. A trabeculectomy involves fashioning a new drainage route through the sclera (white of the eye). The fluid collects under the lining of the eye surface and forms a small "blister-like" elevation which as known as a "bleb". To reduce the chance of the small drainage channel healing most surgeons use additional anti-scarring agents eg: 5-fluorouracil.
A trabeculectomy is usually performed under local anaesthetic as a day procedure. Several post-operative visits are required to ensure that the new channel is working correctly and that the eye pressure is optimal.
A minority of patients require more specialist glaucoma surgery such as using a Drainage Implant. There are many different designs but they all involve the placement of a tube which allows fluid to drain out of the eye to lower the IOP. Traditionally this type of surgery has been reserved for cases when a previous trabeculectomy has scarred up and failed.
There are many charities and groups who offer support to people with glaucoma, such as the International Glaucoma Association (IGA) www.glaucoma-association.com