Understanding glaucoma - Silent thief of sight

What is glaucoma?
Glaucoma is the name for a group of eye conditions in which the optic nerve is damaged at the point where it leaves the eye. This nerve carries information from the light sensitive layer in your eye, the retina, to the brain where it is perceived as a picture.

Your eye needs a certain amount of pressure to keep the eyeball in shape so that it can work properly. In some people, the glaucoma damage is caused by raised eye pressure. Others may have an eye pressure within normal limits but damage occurs because there is a weakness in the optic nerve. In most cases, both factors of high pressure and weakness in the optic nerve are involved, but to a varying extent.

Eye pressure is largely independent of blood pressure.
What controls pressure in the eye?
A layer of cells behind the iris (the coloured part of the eye) produces a watery fluid, called aqueous. The fluid passes through a hole in the centre of the iris (called the pupil) to leave the eye through tiny drainage channels. These are in the angle between the front of the eye (the cornea) and the iris and return the fluid to the blood stream. Normally the fluid produced is balanced by the fluid draining out, but if it cannot escape, or too much is produced, then your eye pressure will rise. (The aqueous fluid has nothing to do with tears).
Why can increased eye pressure be serious?
If the optic nerve comes under too much pressure then it can be injured. How much damage there is will depend on how much pressure there is and how long it has lasted, and whether there is a poor blood supply or other weakness of the optic nerve. A really high pressure will damage the optic nerve immediately. A lower level of pressure can cause damage more slowly, and then you would gradually lose your sight if it is not treated.
There are four main types.

Chronic glaucoma
The most common is chronic glaucoma (chronic = slow) in which the aqueous fluid can get to the drainage channels (open angle) but they slowly become blocked over many years (see Figure 1). The eye pressure rises very slowly and there is no pain to show there is a problem, but the field of vision gradually becomes impaired.
Acute glaucoma
Acute glaucoma (acute = sudden) is much less common in western countries. This happens when there is a sudden and more complete blockage to the flow of aqueous fluid to the eye. This is because a narrow “angle” closes to prevent fluid ever getting to the drainage channels (see Figure 2). This can be quite painful and will cause permanent damage to your sight if not treated promptly.
Secondary and developmental glaucoma
There are two other main types of glaucoma. When a rise in eye pressure is caused by another eye condition this is called secondary glaucoma. There is also a rare but potentially serious condition in babies called developmental or congenital glaucoma which is caused by malformation in the eye. This document is about chronic and acute glaucoma.
The most common types of glaucoma — primary open-angle glaucoma and acute angle-closure glaucoma — have completely different symptoms.
Primary open-angle glaucoma signs and symptoms include:
• Gradual loss of peripheral vision, usually in both eyes
• Tunnel vision in the advanced stages
Acute angle-closure glaucoma signs and symptoms include:
• Severe eye pain
• Nausea and vomiting (accompanying the severe eye pain)
• Sudden onset of visual disturbance, often in low light
• Blurred vision
• Halos around lights
• Reddening of the eye
Both open-angle and angle-closure glaucoma can be primary or secondary conditions. They're called primary when the cause is unknown and secondary when the condition can be traced to a known cause, such as eye injury, inflammation, tumor, or advanced cataract or diabetes. In secondary glaucoma, the signs and symptoms can include those of the primary condition as well as typical glaucoma symptoms.

Movement of fluid in the eye

Aqueous humor continuously circulates from behind the iris into the anterior chamber at the front of the eye. It exits where the iris and the cornea meet. The fluid filters through a spongy tissue containing microscopic channels (trabecular meshwork) before passing into a larger channel called Schlemm's canal, where it eventually merges with the bloodstream.

Open-angle glaucoma

The trabecular meshwork regulates the outflow of the aqueous humor so that intraocular pressure remains steady. Blockage of the trabecular meshwork slows fluid drainage, leading to a slow increase in intraocular pressure. Rising pressure can result in gradual vision loss.
Angle-closure glaucoma

The angle formed by the cornea and the iris closes, preventing the aqueous humor from draining normally out of the eye. This can lead to a rapid increase in intraocular pressure, a serious medical condition.

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