The size and shape of the normal eye is maintained by intraocular pressure (IOP). This is regulated by the amount of fluid within the eyeball. The fluid, called aqueous humor, is constantly produced by a structure called the ciliary body and it contains nutrients and oxygen which are used by the structures within the eye. The excess fluid is constantly drained from the eye between the cornea and the iris. This area is called the drainage angle, or iridocorneal angle.
The pressure remains more or less constant since the production and absorption of aqueous humor is equal.
What is glaucoma?
Glaucoma is an increase in IOP i.e a raised pressure within the eye. This is measured using an instrument called a tonometer. The cause of glaucoma is always related to inadequate drainage of aqueous humor rather than its over production. Glaucoma can be primary or secondary.
"The cause of glaucoma is always related to inadequate drainage of aqueous humor rather than its over production."
Primary glaucoma results in increased intraocular pressure in an otherwise healthy eye. Some breeds are more prone to this than others. It occurs due to anatomical abnormalities in the drainage angle which have been present from birth. For example in narrow angle glaucoma there is a shallow anterior chamber which causes the iris to block the angle which in turn interferes with the drainage of aqueous fluid. Abnormalities within the drainage angle itself (goniodysgenesis) can lead to a raised IOP in middle age and this is the most frequent form of primary glaucoma encountered in dogs.
Secondary glaucoma results in increased intraocular pressure due to disease or injury to the eye. This is the most common type in both dogs and cats. Causes include:
- Uveitis (inflammation), resulting in debris and scar tissue blocking the drainage angle, and severe intraocular infections.
- Anterior dislocation of the lens. This falls forward and physically blocks the drainage angle or pupil such that fluid is trapped behind the dislocated lens.
- Tumours which cause physical blockage due to their size.
- Intraocular haemorrhage. The blood clot can physically prevent aqueous drainage.
- Damage to the lens. Lens proteins leaking into the eye as a result of a ruptured lens can cause a reaction which results in blockage of the drainage angle.
What are the signs of glaucoma?
The commonest signs noted by owners are:
- Ocular pain, your dog may keep the eye partially closed and initially start to rub at it. He may shy away as you go to stroke this side of his head.
- As the pressure increases a watery discharge will be noted and the dog becomes depressed.
- Physical swelling of the eyeball becomes obvious. The white of the eye looks red and engorged
- The cornea or clear part of the eye may become cloudy (usually blue-ish in colour)
- Blindness. Since the increase in pressure will compromise the delicate blood vessels and nerves within the eye, blindness can occur very quickly.
All these signs can occur very suddenly with acute glaucoma. In chronic glaucoma they develop more slowly. They may have been present for some time before your pet shows any signs of discomfort or alteration of appearance or habits.
How is glaucoma diagnosed?
Veterinary diagnosis depends upon accurate pressure measurements, (tonometry) and the internal examination of the eye using special instruments. Careful examination of both eyes is required.
Acute glaucoma is an emergency. Sometimes specialist help and advice is necessary.
Each case has to be treated on its merits. Initially it is important to reduce the IOP as quickly as possible and for this special drugs will be prescribed.
It is also very important that we examine your pet fully and treat any underlying disease which may be responsible for the glaucoma (such as an intraocular tumour or a dislocated lens for example).
Medical treatment might not be effective long term and often sometimes has to be combined with surgery. Analgesics are prescribed to control the pain and discomfort associated with the condition.
"Medical treatment might not be effective long term and often sometimes has to be combined with surgery. Analgesics are prescribed to control the pain and discomfort associated with the condition."
Veterinary ophthalmologists employ various surgical techniques to reduce intraocular pressure. Should such a procedure be necessary, this will be carefully discussed with you. In the meantime it is important that medication is carefully and conscientiously administered. If you do have any difficulties, please call us without delay.
Unfortunately glaucoma is a very serious ocular disease. It can result in blindness, and some patients require removal of the eye if it is permanently blinded by the raised pressure and if the pain that this causes cannot be controlled. However if the condition is diagnosed early and treated appropriately, many dogs will maintain some useful vision for long periods, but lifelong treatment is frequently necessary.
Adapted by Philip H Brain, BVSc, CMAVA, FACVSc (small animal medicine), FAVA © Copyright 2016 LifeLearn Inc. Used and/or modified with permission under license.