Aqueous humour is a transparent fluid located inside the eye, specifically within the anterior and posterior chambers. It has several key functions, and plays an important role in disease states such as glaucoma.
In this article, we will discuss the function, composition and production of aqueous humour, before considering some clinical applications.
The functions of aqueous humour are as follows:
- Maintaining the intraocular pressure and the shape of the globe.
- Providing nutrients and oxygen for ocular tissue including the posterior cornea, trabecular meshwork, and lens.
- Removal of metabolic by-products from intraocular cells.
- Facilitating the passage of light to the retina.
The composition of aqueous humour directly relates to its function within the eye.
Ultrafiltration of the aqueous humour within the ciliary body produces nearly protein-free liquid to provide an optically clear medium for vision.
The high level of lactate is due to anaerobic glycolysis by cells of the anterior eye segment e.g. lens epithelium. The high level of ascorbic acid protects against UV-radiation that can cause the formation of free radicals.
Table 1: Composition of aqueous humour relative to plasma
Movement of Aqueous Humour
The iris, ciliary body and choroid are part of the uvea. This is a layer of vascular tissue that delivers blood supply to the ocular tissues. The ciliary body, specifically the pars plicata (in the form of ciliary processes), is responsible for the production of aqueous humour.
The aqueous humour is constantly produced into the posterior chamber and then flows through the pupil into the anterior chamber (AC).
Production of Aqueous Humour
On average, the ciliary body produces around 2.5 μl of aqueous humour per minute. The 3 processes that play a part in this production include:
- Active secretion.
While diffusion and ultrafiltration are passive processes, they help to collect ultrafiltrated plasma within the ciliary body’s stroma. The ciliary body epithelium then actively secretes the aqueous fluid into the posterior chamber. This active process involves an Na+/K+ ATPase that hydrolyses ATP for energy.
As the Na+/K+ ATPase actively transports Na+ ions into the posterior chamber, the water from the stroma of the ciliary body follows.
It is worth noting that the sympathetic nervous system affects the aqueous humour secretion. Beta-2 adrenoceptor and alpha-2 adrenoceptor activation increases and decreases the production of aqueous humour, respectively.
Drainage of Aqeuous Humour
From the AC, the aqueous fluid is drained through the trabecular meshwork into the canal of Schlemm, which then drains to episcleral veins. Schlemm’s canal (SC) is a circular structure, similar to a lymphatic vessel, that is located in the scleral sulcus right behind the corneoscleral junction. While the inner wall of SC communicates with the AC, the external portion communicates with the episcleral veins.
While the majority of aqueous humour is drained via the trabecular meshwork, approximately 10% exits through the uveoscleral route. The aqueous humour flows across the iris and anterior side of the ciliary muscle through the sclera into the suprachoroidal space, an area between the sclera and choroid.
Glaucoma is at term used to describe a group of diseases characterised by a loss of retinal ganglion cells and changes in the optic disc. As it has been shown that there is a link between increasing intraocular pressure (IOP) and worsening degeneration of retinal ganglion cells, most treatments aim to lower the IOP to slow or prevent the progression of glaucoma. To assess for glaucoma you must:
- Perform fundoscopy: Look at the optic nerve, assessing for an increased cup to disc ratio, deep or asymmetrical cupping, notching, or bayonetting of the blood vessels (kinking of vessels as they pass the cup).
- Measure intraocular pressure: Tonometry is used to estimate the pressure in the anterior chamber (normal range 10-21mmHg). This estimation must be corrected for with a measurement of the central corneal thickness; a thin cornea leads to an underestimate of pressure which can lead to high pressures being missed
- Assess visual fields: An arcuate (arc-shaped) field loss may indicate early glaucoma. Treatment aims to preserve the visual fields so this is an important part of monitoring.
- Perform gonioscopy – Gonioscopy assesses the angle of drainage between the iris and cornea. This helps classify the type of glaucoma as well as informing the management plan.
Primary Open Angle Glaucoma
Primary Open Angle Glaucoma (POAG) is one type of glaucoma. It is characterised by a wide aqueous humour drainage angle, visual field loss and optic nerve changes in the absence of a primary cause. In this type of glaucoma, the IOP may be normal or elevated.
The aim of treatment is to reduce the pressure with topical drugs such as prostaglandin analogues or carbonic anhydrase inhibitors. In drug-resistant forms of the disease, surgery management may be indicated.
Primary Open Angle Glaucoma
Primary Angle-closure Glaucoma (PACG) is another type of glaucoma where the drainage angle is so narrow that the iris is in contact with the trabecular meshwork, blocking the drainage of aqueous humour.
Relative pupillary block is one of the common mechanisms behind PACG. In relative pupillary block, the reduction of aqueous movement through the pupil contributes to a pressure difference between the anterior and posterior chambers. This causes anterior bowing of the iris, thus preventing the drainage of aqueous humour.
These patients may present more acutely with a red eye, headaches, nausea and seeing haloes around lights. On examination it is classical to see a stony, firm red eye (due to the very high IOP), corneal oedema and a mid-dilated pupil.
These patients need emergency treatment with acetazolamide, pilocarpine and timolol before having surgery to drain aqueous humour between the anterior and posterior chambers by making a small break in the iris with a laser (iridotomy).