What is glaucoma?
Glaucoma is a progressive optic neuropathy, that is, a disease with progressive damage to the optic nerve. Well, actually, it's not a single disease but a group of progressive optic neuropathies. These diseases have in common the fact that there is damage and degeneration of the nerve fibres leaving the eye to carry information to the brain – the retinal ganglion cells. Consequent to this degeneration of retinal nerve fibres, there are also changes in the optic nerve head, the point at the back of the eye where the axons of these retinal nerve cells leave the eye. (Incidentally, this optic nerve head does not have any photoreceptors to capture light rays and is also known as the blind spot in our vision).
The loss of the retinal ganglion cells will lead to blindness.
What causes the different types of glaucoma?
Glaucomas are popularly thought to be due to increased fluid pressure in the eye (increased intraocular pressure). However, while this is a major risk factors, advanced age, African ancestry and a family history of glaucoma are also risk factors. Other factors indicated to be risk factors, at least in some populations, are male gender, myopia greater than 1 diopter, and pseudo-exfoliation.
As noted by the Victorian Government's Better Health Channel (betterhealth.vic.gov.au/health/conditionsandtreatments/eyes-glaucoma), the different types of glaucoma include:
- Open-angle glaucoma (OAG) - the most common type of glaucoma in Caucasian populations.
- Angle-closure glaucoma (ACG) - the most common form of glaucoma in some Asian populations.
- Glaucoma without high eye pressure (sometimes called normal tension glaucoma).
- Secondary glaucoma
- Congenital glaucoma - a rare form of glaucoma present at birth or that develops in infants.
As you see above, some forms of glaucoma are more common in particular population groups.
The factors that cause these different forms of glaucoma vary:
- Open-angle glaucoma occurs when the fluid in the front part of the inside of the eye, in front of the lens of the eye, does not drain properly, causing the pressure in the eye to rise and eventually damage the optic nerve.
- Angle-closure glaucoma occurs when the peripheral part of the iris (the coloured part of the eye) blocks the outflow pathways.
- Glaucoma without high eye pressure occurs when there is progressive optic nerve damage and loss of peripheral vision, despite the eye pressures being within normal range for the population (or even below normal).
- Secondary glaucoma can develop as a result of other conditions, such as eye injuries, cataracts, diabetes and inflammation of the eye, or the use of certain medications (particularly those containing steroids).
- Congenital glaucoma is caused by the improper development of the baby's drainage channels and can lead to the eyes expanding in size, with excessive eye watering and light intolerance.
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The most common form of glaucoma, open-angle glaucoma, starts with slight loss of side vision (peripheral vision) with no pain or discomfort. The lack of symptoms makes early detection difficult. As open-angle glaucoma progresses, symptoms may include loss of peripheral vision and difficulty adjusting to low light.
A study by Hu and colleagues in 2014 (Hu et al., Am J Medicine) asked 99 patients clinically diagnosed with various types and stages of glaucoma to report what they saw. Most of the patients had primary open-angle glaucoma. As the authors reported:
"The most common symptoms reported by all patients, including patients with early or moderate glaucoma, were needing more light and blurry vision. Patients with a greater amount of field loss were more likely to report difficulty seeing objects to one or both sides, as if looking through dirty glasses and trouble differentiating boundaries and colors."
We have tried to simulate some of these complex defects here:
How one might expect glaucoma to look
How it may look with the brain 'compensating' for missing information*
Level of glaucoma:
*while certain parts of the simulation may appear black, someone who cannot see doesn't perceive "black", but a lack of perception at all.