WHAT IS GLAUCOMA
Glaucoma is a relatively common condition which has a genetic component meaning that it can therefore be inherited.
Essentially Glaucoma is optic nerve damage which if left untreated can lead to blindness. The mechanism of damage is not clear but it is known that the pressure within the eye certainly plays a major role.
The eye is fluid filled and has an internal pressure. It also has a pressure regulating mechanisms. A good analogy is of a car tyre which is filled with air which results in a tyre pressure that can be measured with a tyre gauge. Within the eye there is a constant fluid production which flows throughout the eye and which eventually leaves via a drain. This drain is called the trabecular meshwork. If there is any obstruction to fluid leaving via this drain then pressure within the eye can rise.
The obstruction to fluid outflow can be mechanical, such as occurs in narrow angle glaucoma or due to abnormalities within the drain itself.
MANY TYPES OF GLAUCOMA
There are many different types of glaucoma and the classification is quite complex. It is important to realise that pressure is not always elevated and indeed a high pressure is not required for the diagnosis to be made.
Open angle glaucoma is the most common type of glaucoma seen in Australia.
The defining feature is that the trabecular meshwork (drain) is not obstructed by anything. In this type of glaucoma, the pressure within the eye can be high or it can be normal. High pressure open angle glaucoma, referred to as Primary Open Angle Glaucoma (POAG) is easier to understand in that the high pressure does damage. It is not well understood why the pressure is high but most likely due to increase outflow resistance in the trabecular meshwork. Low pressure of open angle glaucoma, called Normal Tension Glaucoma (NTG) is more complex in that the mechanism of damage is less well understood. It is thought that a number of other factors play a role in making the optic nerve susceptible to damage from even normal pressure.
Closed angle glaucoma is more common in Asia but still occurs in Australia. The next section covers it in more detail.
Glaucoma can also be due to obstruction of the trabecular meshwork by pigment (pigmentary glaucoma), pseudoexfoliative material (PXF glaucoma), red blood cells (Ghost cell glaucoma). Mechanical obstruction of the trabecular meshwork can also lead to pressure rises and therefore glaucoma such as by an large lens (phacomorphic glaucoma) or due to obstruction of the meshwork by leaking lens proteins (phacolytic glaucoma).
Additional causes include trauma, aphakic glaucoma, certain congenital conditions such as Iridocorneal endothelial syndrome (ICE).
DIAGNOSIS OF GLAUCOMA
Whatever the cause, the requirements for a diagnosis of glaucoma are optic nerve damage and visual field loss not due to another cause. An elevated intraocular pressure is not required for the diagnosis as seen in the entity of normal pressure glaucoma.
SYMPTOMS OF GLAUCOMA
Unless the intraocular pressure is extremely high, there are no symptoms of glaucoma. That is why it is often referred to as “The sneak thief of sight”. Vision starts to be lost in the peripheral visual field first where it can not be detected but with time it ultimately takes away central vision. Patients judge their glaucoma by their vision but unfortunately this is not a good way to tell as central vision is preserved until the end.
The only way to detect glaucoma is via an eye exam. Most commonly patients are pick up on a routine eye exam. Rarely patients notice that they are bumping into things and seek medical help or occasionally they develop pain and loss of vision from a high pressure which picks it up.
Please see next section.