Glaucoma is a term used to describe a group of diseases that have in common a progressive damage to the optic nerve in the back of the eye.
The visual image is converted to an electrical signal in the retina and this is then relayed via the optic nerve to the occipital cortex, the part of the brain that processes that signal. The optic nerve therefore plays a critical role in normal vision.
In glaucoma the nerve fibres that make up the optic nerve die off and so the information getting to the brain is lost and parts of the image are not seen. An analogy would be a fibre optic cable transmitting a picture and gradually more and more individual fibres breaking. The resulting picture seen would be patchy with parts missing.
The progressive damage occurs at the optic nerve head, just as the nerve leaves the eye. This produces characteristic structural changes in the optic nerve head appearance that allows the diagnosis to be made.
One way to assess the function of the optic nerve is to test the peripheral vision. This is commonly done in glaucoma examinations and is called computerised perimetry. A computer programme randomly tests multiple points in the visual field and produces a map of thresholds of light sensitivity, that is, just how bright or dim each point is to just be able to be seen. Patients with glaucoma will often have abnormalities in these visual fields tests and they may get worse as the glaucoma progresses.
The central vision, what one can see on an acuity chart or detailed reading vision is often preserved until very late in the glaucoma process. Patients with advanced glaucoma still may be able to read, watch television etc despite having lost a large amount of their peripheral vision that may prevent them from driving. The loss of vision which can eventually result in the loss of central vision as well usually occurs slowly without any symptoms. This is why glaucoma is often referred to as the “Sneak Thief” of vision.
It is important to understand that Glaucoma is not one single disease. There are many different subtypes of glaucoma that have in common the same damage to the optic nerve. One way to think about this is the same as considering arthritis. There are many different types of arthritis, common types like osteoarthritis, less common types like Rheumatoid arthritis & arthritis associated with joint trauma or infection, and more rare types. All these forms have in common a damage to joints.
Glaucoma is similar with common types such as Primary Open Angle Glaucoma through to rarer types such as Congenital glaucomas. Each of these subtypes may behave differently in their age at onset or degree of aggressiveness or disablility which is why accurate diagnosis is important