If pressure increases it may begin to affect the Optic Nerve Head (the eye end of the nerve which is visible inside the eye), either by directly squashing the nerve fibres, or pressing closed the tiny blood vessels supplying them.
This causes an atrophy (withering) of the Optic Nerve Head, and characteristic reductions in sensitivity of peripheral (round about) vision. Unfortunately, in open angle glaucoma this damage can progress quite unnoticed to the individual until it is at a fairly advanced stage. Left untreated, Glaucoma causes a constriction of peripheral vision- losses spreading outwards and inwards from above and below fixation (the centre of vision). Later these losses coalesce, resulting in “tunnel vision”, where only straight ahead vision remains, but this too will eventually be extinguished, resulting in total blindness.
One would imagine that these changes would be obvious, but we all have a large blind spot at the moment, and we hardly ever notice it- this is normal, as the optic nerve head has no photoreceptors. If you look at the centre of the flower with your right eye, and close the left, if you move towards or perhaps away from the screen, at some point the star should completely vanish, but when you close one eye you are normally unaware of this large blank zone- the brain fills in the details, and continues to do so if more blind spots occur. But vision is still being lost!
Glaucoma is relatively common- affecting approximately 0.5% of the over 40’s, rising to about 5% of those over 65 years. Those with an immediate family member with the condition, highly shortsighted people and people from certain ethnic backgrounds have an increased risk.
It cannot be cured by medicine, but (like high blood pressure) it can be controlled, most commonly with eye drops- these reduce the pressure to a safe level at which progression of the disease can be halted.
Regular eye examinations will not prevent someone developing Glaucoma, but it does ensure that it will be detected as early as possible, and therefore treated at a stage where the individual is unaware of a problem.
Optometrists check everyone for Glaucoma, with more detailed checks for those over 40 or with an increased risk. For those they will check the pressure inside the eye using a machine which puffs air into the eye, or perhaps another which uses a probe to press on the eye. Pressure alone cannot always determine those who are normal, because normal pressures can vary from one individual to another, and it is useful to have readings from previous examinations to compare- to check for a rise in pressure. Glaucoma can occur at normal or even lower than average pressures, so although IOP checks are useful it is not, as commonly termed “The Glaucoma Test”.
More important than the pressure is a detailed examination of the Optic Nerve Head, looking for early signs of the changes characteristic to Glaucoma- again previous exam records are helpful to pickup changes. Next the Optometrist may use a Visual Field Screener to map the sensitivity of the retina and examine the front of the eye with a Slit Lamp- basically a high powered microscope to check for abnormalities.
These pieces of information, along with previous records, Patient’s general health and family history, and any other known risk factors are used to decide who is normal, who may need to be monitored and who needs further Ophthalmological assessment.