What is glaucoma?
The human eye is connected to the brain through a long nerve known as the optic nerve. The optic nerve is more like a conduit delivering approximately 1.4 million nerve fibres from the inside of the eye to the brain. Along the optic nerve all the information related to what the eye sees is conveyed to the brain to give us the picture that we see. The optic nerve, like many other structures, does slowly become weaker and thinner with years, but even in advanced age, it is rarely more than 5% or 10% less capable than when we are young. It is extremely difficult to measure age-related abnormalities of this nerve.
In the condition of glaucoma, this precious optic nerve becomes rapidly weak and rapidly less capable of delivering vision signal to the brain. It does so in a progressive manner, for some rapidly, but for most relatively slowly over many decades. Thus, it is best to understand glaucoma as being a progressive reduction in the ability of the optic nerve to function properly, with the threat of blindness.
Glaucoma is therefore separated from most conditions which cause us sudden loss of vision related to optic nerve disease. These would include strokes of the nerve or trauma to the nerve, and even rare conditions such as cancer or inflammation.
What causes glaucoma?
The three most common causes for glaucoma are:
Age is overwhelmingly one of the greatest risk factors we have for the formation of glaucoma and obviously one that at this moment we have no control over. Thankfully age as such is only of a minor influence on the rate at which glaucoma advances.
Hereditary glaucoma is also poorly predicted. Having a family history does not necessarily predispose one to develop glaucoma, although it does increase the risk quite markedly, particularly if the family members include brothers and sisters. More often than not, the family history is exerted on the eye as an elevation of pressure, but some people can have glaucoma that advances with family history without pressure being elevated, although this is a rare group.
At this time there is little that can be done about influencing the genetic cause of glaucoma.
It has been known for many years that elevating the pressure of the eye above normal can trigger glaucoma and can cause it to be more rapidly progressive than if the pressure was normal. Studies in recent years have confirmed this without doubt. Complicating the matter is that for some people glaucoma is influenced by not having particularly elevated pressure; if anything, pressure in the normal range, and sometimes glaucoma can advance even in the face of lowish pressure. We call this form of glaucoma 'normal pressure glaucoma'. The human eye has pressures that range between 12 and 21 and, if the pressure is elevated above 21 consistently, we call this ocular hypertension, and ocular hypertension is a known risk factor for the development of glaucoma. If the pressure is in the normal range at the time of diagnosis of glaucoma, then we call this normal pressure glaucoma.
How is pressure made?
The human eye produces fluid internally and the production of fluid is minimally affected by age. Therefore, when the pressure of the eye goes up, it usually goes up not because the eye makes more liquid, but rather the eye drains less. If the eye drains less, the pressure is elevated. At this time, the majority of treatments available to reduce pressure are aimed at increasing the amount of fluid that drains out of the eye, thus reducing the pressure of the eye. Most of the eye drops are targeted in this area, although some of the eye drops also do a relatively good job at reducing the amount of fluid that the eye makes. The only tablet available for pressure control indeed works through reducing the amount of fluid that the eye makes. This tablet is known as Diamox (acetazolamide). Laser treatment, known as selective laser trabeculoplasty, also works through increasing fluid drainage out of the eye. Conventional glaucoma surgery, known as a glaucoma filter, sometimes known as a trabeculoplasty, also works through increasing fluid egress from the eye. For more desperate glaucoma, occasionally the fluid production zone can be knowingly damaged by laser or freeze therapy to reduce the damage from glaucoma.
How is glaucoma diagnosed?
As indicated earlier in this section, glaucoma is a disease of progressive optic nerve damage and therefore the diagnosis is based on the detection of progressive weakening of the optic nerve. The most accurate way we have of determining this is through nerve analysis system such as nerve photography and OCT, but also through functional tests such as the field analysis. Humphrey field analysis and other forms of field analysis, however, unfortunately do not become acceptable means of determining glaucoma until the optic nerve has significant disease already. It is said that the optic nerve needs to be weakened by 30-50% before abnormality in the visual field becomes evident, so field analysis is not a particularly attractive means of determining early glaucoma. The early onset of glaucoma is determined, therefore, by witnessing progressive disease of the optic nerve with nerve photography or OCT. Often a person who is at risk of developing glaucoma will have serial nerve analyses performed, looking for evidence of progressive damage.
Once glaucoma is well and truly established and the visual field does become abnormal, then indeed field analysis becomes a very legitimate means of tracking the course of glaucoma. Indeed, it becomes the most preferred in highly advanced disease.
Frequently, visual field and nerve assessments with photography and OCT are performed together.
As indicated, the treatment of glaucoma is aimed at reducing pressure, either from high to normal or from normal to low normal. The hierarchy of therapy at this time is for SLT to be performed followed by drops, or the introduction of drops and SLT together. When these have both been performed and there is evidence of still significant progression of glaucoma, a glaucoma filtering procedure is performed.
Narrow Angle Glaucoma
The above description is relevant for those forms of glaucoma when the overall anatomy of the eye is normal. That is, the eye is able to make fluid and the eye is able to at least provide fluid to the drain mesh of the eye known as the trabecular meshwork. When the front chamber where drainage occurs becomes severely narrowed, we call this state 'narrow angle' and, if it becomes critically narrow, we call the raised pressure that occurs 'angle-closure glaucoma'. This is a particularly aggressive and often painful event that can occur literally overnight. If a person has an eye that is at risk of angle closure, we would often prevent this from occurring by performing a peripheral iridotomy or sometimes cataract surgery, as the formation of cataract can often be what tips an eye into a state of angle-closure glaucoma. Your specialist will talk more about this if this becomes important.