By Eyecare NGOverview | Types | Causes | Signs and Symptoms
Glaucoma is a group of eye diseases that progressively damage the optic nerve and can lead to permanent loss of vision if not detected and treated early. It is often associated with increased eye pressure but there are also cases of glaucoma under normal or low eye pressure.
Glaucoma is a leading cause of vision loss and blindness. A person who has glaucoma usually does not experience symptoms until significant loss of vision has occurred. Hence, it is called the silent thief of sight. Vision loss in glaucoma starts from the periphery and eventually gets to the central vision if it is not treated.
Glaucoma is not curable and any optic nerve damage and vision loss due to the disease are irreversible. The condition, however, can be managed to ensure no further damage to the optic nerve and vision occurs.
Causes of Glaucoma
Increased eye pressure
The major cause of glaucoma is increased eye pressure due to a build-up fluid in the eye. This build-up usually results from an imbalance between the production of the fluid and its drainage from the eye. This fluid is known as the aqueous humour. It is produced by the eye’s ciliary body and drained through the trabecular meshwork located at the angle between the cornea and the iris (known as filtration angle) into the circulatory system.
The aqueous humour is important for maintaining a level of eye pressure that supports the structures of the eye and prevents them from collapsing. However, if a lot of it is produced and/or its flow through the drainage system in the eye is impeded, it builds-up in the eye and increases eye pressure to the levels that are dangerous to the structures of the eye, particularly the optic nerve. The increased pressure in the eye pushes against the optic nerve and gradually damages the nerve fibres that transmit visual information from the eye to the brain resulting in glaucoma.
Unusually sensitive optic nerve
Glaucoma can also occur under normal eye pressure which ranges from 10 to 21 mm Hg. This is known as normal tension glaucoma. Unusually sensitive or fragile optic nerve that is damaged by normal pressure levels in the eye is believed a cause of this type of glaucoma.
Reduced blood flow to the optic nerve
Reduced blood flow to the optic nerve is believed to be another cause of glaucoma under normal eye pressure. It causes an inadequate supply of oxygen to the optic nerve and this result to the progressive death of the nerve fibres. Reduced blood flow to the optic nerve is usually associated with vascular-related disorders such as low blood pressure, migraine headaches and diseases that affect the ability of the blood to clot.
Types of glaucoma
Glaucoma can be broadly classified into primary glaucoma and secondary glaucoma depending on whether it is caused by another eye condition or not. Primary glaucoma is glaucoma that is not caused by other eye conditions while secondary glaucoma is glaucoma that occurs as a result of other eye conditions.
The major types of glaucoma are open angle glaucoma, closed angle glaucoma, and normal or low tension glaucoma.
Open angle glaucoma
This is glaucoma that occurs in the presence of an open (filtration) angle between the cornea and the iris. It is the most common type of glaucoma. The build-up of fluids in the eye despite an open angle is thought to be due to some abnormalities that cause the drainage channels to gradually get clogged; thereby, reducing the outflow of aqueous humour.
Open angle glaucoma is a chronic condition and the risk of developing it increases with age. This type of glaucoma affects Blacks more and rarely causes symptoms at the early stages because the pressure increase in the eye is gradual.
Closed angle glaucoma
This is a type of glaucoma caused by narrowing or blockage of the angle between the cornea and the iris (filtration angle) with a sudden rise in intraocular pressure and consequent damage to the optic nerve. It is more common in Asians and usually has noticeable signs and symptoms such as headaches, severe pain, cloudy and reduced vision, red eyes, nausea/vomiting, halos and stone-hard eyeballs. Closed angle glaucoma requires immediate treatment to prevent vision loss
Normal or low tension glaucoma
This is glaucoma that occurs in the presence of normal or low eye pressures. It is believed to be caused by the poor blood supply to the optic nerve or by an unusually pressure-sensitive optic nerve.
Other types of glaucoma
There are other types of glaucoma which are less common. These include:
- Congenital glaucoma – This is a type of glaucoma that occurs in children. It is inherited and present at birth. It occurs due to a defect in the structures responsible for the drainage of fluid in the eye. Learn more about glaucoma in children.
- Neovascular glaucoma: This is secondary glaucoma that occurs due to the development of blood vessels in the iris and the clogging of the drainage channels by these blood vessels; thereby obstructing the outflow of aqueous humour. Neovascular glaucoma is usually seen in diabetic patients and patients with ischaemic conditions (conditions that restrict blood supply to tissues). Treatment is targeted at stopping the new vessels from growing and controlling pressure with eye drops.
- Pigmentary glaucoma: This is a rare type of glaucoma that occurs in people with pigment dispersion syndrome, a condition in which granules of pigments fall off the iris and float around in the aqueous humour. The pigment floating in the aqueous humour accumulates over time and eventually blocks the filtration angle resulting in an increase in the intraocular pressure.
Signs and symptoms of glaucoma
People with open angle glaucoma usually do not experience symptoms in the early stages of the disease. In the later stages, they may experience loss of peripheral vision. In some cases, the eyes may be red and depending on how high the eye pressure is, there may be haloes around light, slight headaches and eye pain.
Signs and symptoms in closed angle glaucoma include red eyes, nausea, pains, cloudy vision, haloes around light and headaches.