Aqueous humor is a clear, alkaline, watery liquid found at the front part of the eye. It resembles blood plasma and its composition includes 99.9% water while the remaining 0.1% is made up of proteins, vitamins, sugars and so on. Aqueous humor will generally be found in front of the iris and behind the cornea. The iris is the round colored part of the eye. The cornea is the clear, outer, dome-shaped covering of the iris and pupil.
In a normal healthy eye, the eye should produce a reasonable amount of aqueous humor. A similar amount should drain out. Aqueous humor drains out through the trabecular meshwork (TM) in the drainage angle. This angle is that part of the eye at the front where the aqueous humor drains from. The TM is a spongy tissue found near the cornea. The fluid draining out is called the aqueous humor outflow.
It is important that there is a balance between the fluid being produced and the one flowing out. An imbalance destabilizes normal pressure leading to high intraocular pressure (IOP). IOP measures the pressure of the fluid inside the eye. This measurement is similar to the measurement of high blood pressure; the more the pressure, the greater the damage. High IOP is associated with diseases like glaucoma which can lead to vision loss.
Three chambers make up the eye. These are the vitreous chamber, the anterior chamber and the posterior chamber. Of the three, the vitreous chamber is the largest. It is found at the back of the eye. Next to it is the posterior chamber described as the area between the iris and the lens. The lens refers to the clear part of the eye behind the iris that assists in focusing light on the retina.
The anterior chamber occupies the front outermost part of the eye between the iris and cornea.The ciliary body epithelium (part of the eye above the lens) is found in the posterior chamber. It is this body that is constantly producing the aqueous humor.
Aqueous humor occupies both the posterior and anterior chambers. From the posterior chamber, aqueous humor will flow via the pupil to enter the anterior chamber. It will then drain through the TM into Schlemm’s canal and through aqueous veins. Schlemm’s canal refers to a passageway shaped like a ring around the outer angle of the anterior chamber. It is located in front of the iris. Aqueous humor can also exit through the ciliary muscle and other downstream tissues.
Aqueous humor plays a significant role in the health of the eye. It has the following functions:
- Nourishes the cornea and lens
- Keeps the IOP stable
- Supplies nutrients such as amino acids and glucose to the non vascularized tissue of the anterior chamber such as TM, corneal endothelium and lens
- Supplies the front segment of the eye with the antioxidant vitamin C
- Removes waste products
- Indirectly protects the cornea against pathogens, dust, pollen and wind by inflating the cornea
Associated symptoms & disorders
Any interference with the IOP can lead to vision-threatening diseases such as glaucoma. For example, in open-angle glaucoma, the fluid does not flow freely through the TM thus increasing the IOP. Once the IOP increases, the optic nerve is damaged leading to vision loss.
Other diseases associated with IOP include corneal diseases such as keratoconus. The thin cornea in keratoconus may present inaccurate readings of IOP. In ocular hypertension, the IOP reading is high and could be a potential sign of glaucoma. Treating uveitis (inflammation of the uvea) can lead to high IOP.
Diagnosis of associated disoders
To check for IOP and its associated diseases, the following may take place:
- Use of tonometry to measure IOP
- Dilated eye exam and imaging tests to check for optic nerve damage
- Visual field test that checks for areas that may have loss of vision
- Gonioscopy to inspect the drainage angle
- Pachymetry to measure the thickness of the cornea
Treatment of associated disoders
Treatment of aqueous humor-related diseases like glaucoma is aimed at lowering the eye pressure. The therapy increases the outflow of aqueous humor from the eye while decreasing its production. The methods used will depend on the condition being treated. For example, glaucoma is treated by lowering the IOP. A patient may use eye drops such as prostaglandins.
Prostaglandins enhance the outflow of aqueous humor thus decreasing IOP. Beta blockers lessen the production of fluids in the eye. Other eye drops include alpha-adrenergic agonists, carbonic anhydrase inhibitors, rho kinase inhibitor and miotic or cholinergic agents. Oral medications may also be prescribed.
Surgical options which increase the outflow of aqueous humor from the eye include:
- Trabeculoplasty - A type of laser surgery that opens clogged channels in the TM
- Trabeculectomy – An opening is made in the sclera (white of the eye). This action redirects aqueous humor from the anterior chamber to the area beneath the conjunctiva (the clear covering of the sclera)
- Drainage tubes – Excess fluid is drained using a small tube shunt
- Minimally invasive glaucoma surgery - A number of techniques are available including cataract surgery where the procedure lowers the IOP