Understanding glaucoma: what is the aqueous humour?
Glaucoma is linked to a build-up of pressure in the eye caused by an imbalance in the production and drainage of a fluid called the aqueous humour.
The increased pressure can damage the optic nerve, which can create blind spots in the visual field (small areas of your vision where there is no image detection). If left untreated, glaucoma can lead to irreversible vision loss.1 But in order to understand the condition and how it can progress, it’s important to know what causes it, and the important role aqueous humour plays in this.
What is the function of aqueous humour?
Aqueous humour is a thin, watery fluid produced by the eye and, in a healthy eye, it’s in a constant cycle of production and drainage. The aqueous humour is 99.9% water and 0.1% nutrients such as glucose, proteins and vitamins. These nutrients nourish the cornea and lens and keep the eye healthy, as well a protect the cornea against the wind, pollen, dust, and pathogens.
This fluid fills the spaces between the cornea and iris (anterior chamber) and the iris and lens (posterior chamber), which gives the eye its round shape and controls intraocular pressure (IOP).2 It’s secreted by a structure called the ciliary body which is an extension of the iris (the coloured portion of the eye).
How does aqueous humour control IOP?
In a healthy eye, a certain amount of aqueous humour flows into the eye and the same amount drains out. This process keeps the IOP within ‘normal limits’.
However, variations in the production or outflow of aqueous humour can lead to significant changes in intraocular pressure. In most forms of glaucoma, vision loss occurs due to an increase in IOP caused by an imbalance in the production or outflow of the aqueous humour, and it’s this build-up of pressure which can cause permanent damage to the optic nerve.3
How is aqueous humour produced and drained?
Aqueous humour is actively secreted into the eye by the ciliary body (the ciliary body also controls the shape of the lens). This is the part of the eye that connects the cornea to the choroid (the vascular layer of the eye). Active secretion is the most important part of aqueous humour production and this is controlled by enzymes and proteins deep inside the ciliary body.4
The fluid leaves the eye through two principle routes: the trabecular meshwork (an area of tissue at the base of the cornea) and the uveoscleral pathway.
The trabecular meshwork is responsible for 75% of the resistance to aqueous humour outflow. If this site becomes blocked by a deposit of proteins or other substances, for instance, an imbalance is caused in the production and drainage cycle — leading to an increase in IOP.
The second pathway for aqueous humour drainage, the uveoscleral pathway, is relatively independent of IOP, but effective drainage via the uveoscleral pathway decreases with age.4 Age, potentially as a result, is also one of the key risk factors for glaucoma development.
How does aqueous humour help inform glaucoma treatment?
The role of the aqueous humour in the development of glaucoma is well known, but the type of glaucoma will usually depend on which part of the production or drainage system has been affected. Current treatments for glaucoma focus on lowering the IOP by increasing the outflow of aqueous humour or suppressing its production.
Eye drops are the most common form of glaucoma treatment and can be tailored to glaucoma type and need. For example, eye drops that contain carbonic anhydrase inhibitors (e.g. dorzolamide) and beta-blockers (e.g. timolol) decrease the production of aqueous humour by the ciliary body. Prostaglandins (e.g. latanoprost), on the other hand, increase the outflow of aqueous humour through the uveoscleral pathway and trabecular meshwork. While eye drops containing alpha-adrenergic agonists (e.g. brimonidine) perform both functions.
Surgical treatments for glaucoma can also be used to lower IOP by increasing aqueous humour drainage or reducing its formation.
Some laser treatments target the part of the ciliary body responsible for aqueous humour production to lower IOP. Other procedures such as trabeculectomy or laser trabeculoplasty modify the trabecular meshwork to decrease outflow resistance.
Glaucoma shunts are sometimes recommended for some patients to bypass the trabecular meshwork and improve drainage. Another option is canaloplasty — a procedure that opens up the uveoscleral pathway and increases aqueous humour outflow through existing pathways.4
If you have any concerns and queries regarding glaucoma treatment, please don’t hesitate to ask your optometrist. They will be able to advise you effectively, as well as explain more about the function of aqueous humour.
If you found this information about aqueous humour helpful, however, then do browse our dedicated glaucoma resources. There’s a wealth of expert-led content explaining the causes of glaucoma, glaucoma diagnosis and glaucoma treatment.
1. University of Washington. (no date). Eye Diseases – Glaucoma. [Online]. Available at: https://faculty.washington.edu/chudler/glaucoma.html [Accessed 6 August 2019].
2. Vision Eye Institute. (no date). The Aqueous Humour. [Online]. Available at: https://visioneyeinstitute.com.au/eyematters/aqueous-humour/ [Accessed 6 August 2019].
3. American Academy of Ophthalmology. (no date). Causes of Glaucoma. [Online]. Available at: https://www.aao.org/eye-health/diseases/glaucoma-causes [Accessed 6 August 2019].
4. Goel, M., Picciani, R. G., Lee, R. K., & Bhattacharya, S. K. (2010). Aqueous humor dynamics: a review. The open ophthalmology journal, 4, 52–59. [Online]. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3032230/ [Accessed 6 August 2019].
DOptom BSc(Hons) MCOptom
Josie is an optometrist registered with the UK General Optical Council and New Zealand Board of Optometrists and Dispensing Opticians. She graduated from Cardiff… Read more