Glaucoma evaluation and treatment
Glaucoma is the name given to a group of conditions in which the optic nerve suffers damage at the back of the eye and which is often associated with a raised level of pressure within the eye. Glaucoma symptoms vary in severity from patient to patient, but consist of damage that causes patchy loss of vision which usually gets worse over the course of many months or several years.
The loss of vision in glaucoma is permanent. However, early glaucoma treatment can minimise the damage. Appropriate examinations are essential to detect glaucoma early and prevent significant sight loss, as most patients with glaucoma are not aware of problems with their vision - the glaucoma symptoms are not immediately apparent. This is because the central vision (for reading and recognising people) is only affected when glaucoma has advanced to a late stage. Even when central vision is still good, glaucoma may affect the vision needed for driving and getting about (for instance, seeing steps). Although blindness from glaucoma is rare it is still the leading cause of preventable blindness in the UK.
Glaucoma risk factors
Glaucoma is found in about 2% of the population over the age of 40. It can also affect children and young adults, although much less frequently. The great majority of those with glaucoma have a chronic form of the condition, primary open angle glaucoma (POAG), and studies demonstrate that half of all cases remain undiagnosed - symptoms of glaucoma often remain initially undetected. Groups at risk include:
- Anyone over the age of 40
- Afro-Caribbeans over 35
- Individuals with a family history of glaucoma
- Diabetics and cardiovascular patients
- Individuals with raised eye pressure but no apparent visual field defect.
- Individuals with hypertension
- Long-term users of steroids
- People who are myopic or short-sighted
- Anyone who has suffered a traumatic eye injury
Close blood relatives of patients with POAG have a four-fold increased risk, while people of African-Caribbean origin are far more at risk than Caucasians. People from families in which a member has glaucoma should be tested from the age of 35 onwards and those with other risk factors in addition to the family history should be tested from an even earlier age.
What causes glaucoma?
Pressure is needed within the eye to keep its shape. This intraocular pressure (IOP) is maintained by the flow of a fluid (aqueous humour) within the eye. Most cases of glaucoma occur because the outward flow becomes restricted and the IOP rises. This causes damage to the optic nerve.
In some eyes with glaucoma, the pressure is not high. In these cases it is thought that a poor blood supply or a weakness in the optic nerve fibre structures may make the nerve susceptible to damage by normal eye pressure. This is known as normal tension glaucoma. Some eyes can have moderately raised eye pressure but no sign of loss of vision or optic nerve damage and this is known as ocular hypertension.
Most treatments for POAG are intended to lower the IOP and there is an extensive range of eye drops available.
Methods of evaluating glaucoma
Visual Field Assessment: The Humphrey Field Analyzer II-HFA II
The Humphrey Field Analyzer is a wheelchair accessible, computerized perimeter used to examine the patient's visual field by testing their response to a visual stimulus. It has become an industry standard machine for the diagnosis and monitoring of glaucoma and other ocular and neurological diseases, and looks for defects in the sensitivity of the eye. As a good eye is able to compensate for poor vision in the other eye, the effects of visual field damage are often unnoticed by the patient until there is significant damage to the vision. The Humphrey test is designed to pick up the earliest changes to the function of the eye.
Intra ocular Pressure Measurement: Goldman Applanation Tonometry
The pressure in the eye is measured using a probe which touches lightly on the front surface of the eye. Drops are used to numb the eyes for several minutes during the readings. The pressure readings only take a couple of minutes each time. As there may be a particular time of day when the pressure increases, the consultant may take readings throughout the day to identify any fluctuations.
Retinal Nerve Fibre Layer Thickness: The GDx Nerve Fiber Analyzer
The GDx is a sophisticated new machine used to build up a 3-D image of the optic nerve fibres at the back of the eye and, by analysing the information obtained, detect health changes within them. The machine scans the eye with a bright red light for a couple of seconds and uses a computer to analyse the results. The test usually takes less than 5 minutes, is non-invasive and painless. The equipment is fully wheelchair accessible.
It is estimated that up to 50% of fibres may be lost before a visual field defect is detectable. The advantage of the GDx is that it can help consultants detect earlier and more sensitive, objective changes in individuals who may have glaucoma even in individuals who experience no vision loss.