Glaucoma is when the intraocular pressure is elevated, compromising vision either partially or completely.
What causes Glaucoma to develop?
In a patient with healthy eyes the aqueous humor produced within the Ciliary body located behind the Iris flows through the Pupil and drains through a sieve structure found in the corner of the eye known as the Iridocorneal Cleft. The fluid is produced and then drained at a steady rate resulting a stable intraocular pressure (IOP) of 15-20mmHg.
In the unhealthy eye, there is inadequate outflow of aqueous humor through the Iridocorneal Cleft resulting in the build up of fluid within the eye. This causes the IOP to increase, the more this increases the more damage is done to the Optic nerve which blocks nerve impulses causes blindness.
Does it cause permanent blindness?
The longer the IOP is increased the more damage is done to the Optic nerve, once this is permanently damaged vision can not be restored. Therefore, early surgical intervention is strongly recommended.
Is there only one type of Glaucoma?
No, there are two types of Glaucoma. These are:
Primary Glaucoma - This is thought to be the inherited type and is seen mainly in purebred dogs. It is caused by either:
Open Angle Glaucoma - Where the point where the Iris meets the Corneal is open at the correct angle but the Iridocorneal Cleft becomes clogged over time resulting in a slow loss of peripheral vision until the whole eye is effected. This type of Glaucoma has little warning signs and is seen most commonly in Beagles and Norwegian Elkhounds.
Narrow Angle Glaucoma - This type of Glaucoma occurs suddenly when the Iris is pushed or pulled forward blocking the drainage angle. Commonly seen in Cocker Spaniels and is a medical emergency causing pain, redness of the eye, dilated pupils, nausea and vomiting.
Gondiodysgenesis - This is a developmental abnormality of the actual drainage angle causing decrease fluid outflow when the eye becomes inflamed. It is commonly seen in Basset Hounds.
Secondary Glaucoma - This is often the result of pre-existing ocular conditions such as Uveitis, Lens dislocation, Intraocular tumours and trauma to eye interfering the natural flow of ocular fluid.
The clinical signs of Glaucoma include:
Excessive tear production
Yellow/Green Ocular discharge
Behavioural changes due to pain and loss of vision
Enlarged Pupil that doesn’t respond to light
How is Glaucoma diagnosed?
Diagnosis is made by evaluation of clinical signs and taking a detailed history from the client. In addition to this two diagnostic techniques are used, these are:
Tonometry - The measurement of IOP with a Tonopen.
Gonioscopy - Evaluation of the drainage angle, done by placing anaesthetic drops into the eye and then installing a dome shaped lens onto the corneal surface. The front of the eye can then be examined with a slit lamp.
What treatment is available?
Glaucoma in animals is much more difficult to treat than when it is present in human eyes. Mannitol is the intravenous drug of choice used to decrease the IOP, while eye drops such as Pilocarpine are used to increase the outflow of Aqueous humor. Once the IOP is stable, surgical options become available.
Laser Cyclophotocoagulation - A laser is used to burn through the white outer layer of the eye and selectively destroy small areas of the ciliary body to reduce the production of eye fluid. Occasionally more than one surgery is needed to achieve a positive outcome from this treatment.
Cyclocryothermy - A small probe is placed on the outside of the eye and small areas of ciliary body are frozen to decrease the amount of intraocular fluid being produced.
Anterior Chamber Shunts - A small valve is implanted under the white of the eye through a small incision acting as a new drainage pathway for the fluid to leave the eye.
Evisceration and Implantation of Intrascleral Silicon Prosthesis – A silicone implant is implanted within the eye. This procedure involves shelling out the eye leaving the fibrous sclera and cornea, the shape of the eye is maintained with a sterile silicone sphere and the eye is pain free for the patient. Complications include corneal ulceration.
Ciliary Ablation by Intravitreal Injection of Gentamycin – Gentamycin (a antibiotic) is injected into the eye in high concentrations, the ciliary body is killed resulting in the cessation or reduction of aqueous humor production. A GA is needed and complications can include: shrinking of the eye, return of glaucoma and chronic pain.
Enucleation – Removal of the eye.
