What is primary open angle glaucoma (POAG)?
This is an open angle type of glaucoma, i.e. there is no physical obstruction of the drainage angle of the eye. As its name suggests, this is a primary glaucoma which means that there is no known cause relating to another conditon.
What is the cause of primary open angle glaucoma (POAG)?
In this form of glaucoma the drainage angle is 'open' – there is a clear view of the drainage angle when it is examined. In most eyes, the pressure in the eye is higher than normal because there is a restriction to the flow of aqueous humour through the trabecular meshwork.
This high pressure causes damage to nerve fibres at the optic nerve head.
In about one third of eyes with glaucoma, the pressure is not high and the condition is called normal tension glaucoma or low tension glaucoma. In these cases, it is thought that a poor blood supply or a weakness in the optic nerve structure may make the nerve susceptible to damage by eye pressures which are not abnormally high.
The cause of high pressure in primary open angle glaucoma.
Who is at risk of primary open angle glaucoma (POAG)?
Age: POAG becomes much more common with increasing age. It is uncommon below the age of 40 but affects one per cent of people of European origin over this age and about four per cent of those over 80.
Race: If you are of African-Caribbean origin you are about four times more at risk of POAG than a person of European origin, and it may also come on at a younger age and be more severe.
Family History: If you have POAG, your close relatives have at least a four-fold increased chance of developing the condition. You should advise members of your family to have regular eye tests by your optometrist (optician).
Short sight: People with a high degree of short sight (myopia) are more prone to POAG.
Vascular factors: If you suffer from migraine or cold hands and feet, then you may be more at risk of normal (low) tension glaucoma.
Diabetes: People with diabetes may have an increased risk of developing POAG.
What are the symptoms of primary open angle glaucoma (POAG)?
In the early stages of POAG, there are usually no symptoms – your vision may seem perfectly normal and there is no pain. Because of this, the condition is usually first picked up when your eyes are examined by your optometrist (optician). For this reason, regular visits every year or two are essential if glaucoma is to be detected early. On rare occasions, a glaucoma patient may seek advice because (s)he notices that the sight is less good in one eye than the other.
In the later stages of glaucoma, when a considerable amount of the field of vision has been lost, getting about and using stairs becomes difficult, although reading vision and vision for recognising people is still good. However, if the glaucoma is left untreated, even the centre of the field of vision may be damaged so that reading vision becomes affected and sight may be lost.
Detection and diagnosis
Everyone over 40 years of age should ensure that they have regular sight tests every two years to discover if their eyesight has deteriorated. These tests are a good opportunity to check for any signs of glaucoma. During these visits, make sure you ask the optometrist (optician) to carry out the three glaucoma tests.
The three glaucoma tests are:
1. Eye pressure (tonometry):- Tonometry is the measurement of intraocular pressure (the pressure inside the eye) and can be carried out with several different instruments. In general, Goldmann applanation tonometry (with the blue light) is used in hospitals and non-contact (air puff) tonometry is used in optometric (optician) practices.
Goldmann applanation tonometry involves local anaesthetic drops (with yellow dye called fluorescein) being given to allow the instrument to touch the front of the eye.
The drops sting a little, but the procedure itself is completely painless. It is generally considered to be the most accurate method of measuring the pressure.
In non-contact tonometry, a puff of air is blown at the eye from the instrument. This does not require any form of anaesthetic and is generally considered to be somewhat less accurate.
2. Optic disc appearance (ophthalmoscopy):- The appearance of the optic disc can be examined using an ophthalmoscope (a special sort of torch) or by the use of a slit lamp. This allows the examiner to assess the degree of cupping of the optic disc and the health of the retina.
3. Visual field (perimetry):- Perimetry is the technique of mapping out the blank or less sensitive areas in the field of vision, so that the presence of optic nerve damage can be assessed.
These three tests together increase the likelihood of glaucoma detection by four times when compared with ophthalmoscopy alone.
The three tests are quick and painless and, depending on the instruments used, should only take around 15 minutes in addition to the normal eye examination.
Treatment of Primary Open Angle Glaucoma (POAG)
The loss of vision in glaucoma is permanent, but with early treatment, the damage to vision can be minimised. In some patients, the loss of vision occurs very slowly and treatment may not be necessary. However, most patients do require treatment. This prevents or slows further loss of vision in most patients. Taking the treatment is important, even though the vision may seem normal, because the loss of vision may get worse without the patient noticing.
All treatment for glaucoma is designed to reduce the level of intraocular pressure to a point at which further damage to the optic nerve is prevented. This reduction in the level of intraocular pressure is achieved by a variety of means including eye drops, tablets, laser treatments for glaucoma and surgery for glaucoma.
Each has advantages and disadvantages and the choice of the glaucoma management regime best suited to an individual patient should be made as a collaborative decision involving the patient and the ophthalmologist.
Eye drops usually form the first stage of treatment for glaucoma. There are now a wide variety of eye drops which work in a number of different ways. Most commonly the drops act to reduce the amount of aqueous humour being produced by the ciliary body, some increase the outflow of aqueous humour from the eye either by the conventional outflow pathway through the trabecular meshwork, or by the non-conventional outflow pathway (also known as the uveoscleral outflow pathway), and some do some of each.
Lasers are used to treat both open and closed angle glaucomas with different lasers and different techniques used according to need.
There are also a variety of types of surgery for glaucoma depending on the individual needs of the patient.
Treatment for glaucoma is effective and in the vast majority of cases useful sight can be retained for life providing the treatments are used properly and the agreed management regime followed.