GLAUCOMA IN DOGS: DIAGNOSIS,TREATMENT , CARE & MANAGEMENT
Glaucoma is an eye disease in which the pressure within the eye increases. Forty percent of dogs affected with glaucoma end up blind in the affected eye within the first year, regardless of medical treatment or surgery.
What is glaucoma in dogs?
Glaucoma is a condition that causes a build-up of fluid (referred to as aqueous fluid) and pressure in the eye. If too much fluid is made or too little fluid is drained, the pressure of the eye, called intraocular pressure (IOP), increases and causes damage to the retina and the optic nerve. This can ultimately lead to blindness.
What is intraocular pressure and how is it maintained?
Intraocular pressure is the fluid pressure inside the eye that is able to be maintained if the absorption or drainage and production of the aqueous humor fluid are equal. When the eye is unable to drain enough of the aqueous humor fluid it can lead to glaucoma in dogs.
Why is an increase in intraocular pressure a problem?
When intraocular pressure is high it can lead to damage or degeneration of the optic nerve and retina. This is an issue because the retina’s function is to convert images into nerve signals and the optic nerve carries these signals from the retina to the brain to produce vision. Thus, an increase in intraocular pressure will almost always result in blindness in the affected eye because of the damage to the retina or optic nerve.
Signs and symptoms of glaucoma in dogs
Glaucoma causes pain that’s more severe in dogs than in humans. However, dogs do not show pain in the way humans can, so it may be difficult to detect when they are actually hurting. Clinical signs to look out for include:
- Eye pain: Rubbing up against the floor or another object or with the paw
- Redness of the eye
- Cloudy cornea
- Squinting
- Tearing
- Avoidance of light
- Weak blink response
- Fluttering eyelid
- Pupils appear different in size
- Appearance of vessels in the white of the eye
- Bulging swollen eye
- Vision problems: bumping into objects, difficulty finding things, walking cautiously
As a dog ages, the clinical signs may occur in one eye and then the other. Rarely will it occur simultaneously in both eyes or with equal pressure. It can also be years before the second eye becomes affected.
If you notice any signs and symptoms of glaucoma, have your dog checked out immediately. Time is of the essence when it comes to this condition. Not only is it painful, it will lead to irreversible blindness if left untreated.
Types
There are two forms of glaucoma in dogs:
- Primary Glaucoma – The fluid flow rate into or out of the eye is abnormal resulting in an increase in eye pressure.
- Secondary Glaucoma – A separate condition causes intraocular fluid drainage to be slowed or blocked, resulting in increased pressure. Twice as common as primary glaucoma.
Intraocular pressure is measured using a tonometer:
- Normal Intraocular Pressure – 15-25mmHg
- Primary Glaucoma – 25-30mmHg
- Secondary Glaucoma – 10-30mmHg
What causes glaucoma in dogs?
Primary glaucoma is caused by increased IOP in an otherwise healthy eye. It is often due to genetic, inherited abnormalities of the eye’s drainage angle.
- Goniodysgenesisis an inherited risk factor for closed-angle glaucoma and puts affected dogs at a higher risk of glaucoma in the future. It can be diagnosed through genetic testing, and dogs with this gene should have regular eye exams to check IOPs.
- Primary open-angle glaucomacauses a more gradual increase in IOP, and vision loss is very slow. Genetic testing is available for this condition.
Secondary glaucoma is the most common type in dogs, and is caused by increased IOP due to injury or a medical condition of the eye.
- Lens luxationoccurs when the lens which usually sits behind the iris, dislocates in front of the iris, and blocks the drainage angle or the pupil, thus trapping fluid behind it.
- Uveitis(inflammation of the interior components of the eye) causes inflammation, debris, and/or scar tissue that blocks the drainage angle, causing fluid accumulation.
- Cataractsaffect the lens of the eye and can cause inflammation or debris that blocks the drainage angle, leading to fluid accumulation.
- Tumorscan lead to physical obstruction and/or inflammation, causing fluid accumulation and a lack of drainage.
- Bleedingin or around the eye from trauma, retinal detachment, etc., can prevent fluid drainage from the iridocorneal angle.
Diagnosing glaucoma in dogs
Time is a critical factor for treating glaucoma, which is why it’s vital that a dog showing signs sees a veterinarian immediately. Along with gathering as much medical history as possible, the veterinarian will conduct an ophthalmologic exam. In some cases, your veterinarian may refer you to an ophthalmologist for a more detailed examination of both eyes.
The veterinarian will test the intraocular pressure (IOP) using a tonometer, a tool for checking the pressure exerted by the fluid inside the eye. There are a few types of tonometers — those that touch the eyeball directly, one that only touches the eyelid, and another that doesn’t touch the eye at all.
One type of tonometer blows a puff of air onto the eye to measure the indentation. Another type presses a small plastic disk against the eyeball to measure pressure. A drop of anesthetic is often applied first to keep the dog comfortable during the exam.
In the event of an eye abscess, injury, or tumor, an X-ray or ultrasound may be done to get a closer look at the space around the eye.
With sudden glaucoma, the pupil has a sluggish response to light, the blink response is weak or nonexistent, the cornea is swollen or cloudy, the eye is red, inflamed, tearing, and the dog may be squinting.
In chronic glaucoma, the pupil has no response to light, and the blink response is absent. The cornea is cloudy, the eye is red and inflamed, and often enlarged. Tearing is possible, and vessels are seen on the cornea.
Treatment for glaucoma in dogs
MEDICAL THERAPY
Drug therapy for glaucoma can help on a short- or long-term basis and may delay the onset of glaucoma in the contralateral eye.
Osmotic agents
Osmotic agents are commonly used in the emergency management of glaucoma because of rapid efficacy. They are administered systemically and distributed to extracellular fluids, increasing plasma osmolality. When plasma osmolality exceeds that of the intraocular fluid, water diffuses from the aqueous and vitreous humor down-gradient to plasma, essentially dehydrating the vitreal and aqueous chambers. Common osmotic agents used in veterinary glaucoma patients include mannitol, glycerin and isosorbide.
Mannitol is administered intravenously at dosages ranging from 1 to 2 g/kg over 30 minutes. The reduction in intraocular pressure (IOP) generally begins within 30 minutes to one hour, with effects lasting six to 10 hours. Mannitol is not metabolized but can elevate blood glucose concentrations and should not be administered to diabetic patients. It should be administered through a filter because of its propensity to form crystals.
Glycerin is easy to administer, inexpensive and does not require intravenous access or special storage. It is administered orally at a dosage of 1 to 2 g/kg. A reduction in IOP should be observed within an hour of administration and can last as long as 10 hours. Administration may result in vomiting. Glycerin should not be administered to diabetic patients, as it metabolizes to glucose and will result in hyperglycemia.
Isosorbide can be administered orally like glycerin, but unlike glycerin, it will not result in hyperglycemia. The recommended dosage in dogs is 1 to 1.5 g/kg; efficacy in one report was similar to that of glycerin.1
The use of hyperosmotic agents is contraindicated in uveitic eyes because of the increased permeability of inflamed eyes. They should not be administered with fluids, and water should be withheld for about two hours after administration. Because of the expected increase in intravascular volume associated with these agents, hyperosmotics should not be administered in patients with significant cardiovascular disease.
Carbonic anhydrase inhibitors
Carbonic anhydrase inhibitors (CAIs) inhibit the formation of bicarbonate in the ciliary body that is necessary for aqueous humor production. Commonly used topical agents include dorzolamide and brinzolamide (Azopt-Alcon). Dorzolamide is available as a generic, is fairly cost-effective and is available as a combination drug with timolol, which is now available as a generic. Oral CAIs include methazolamide and acetazolamide.
Topical CAIs can be administered two to three times a day. Maximum efficacy may take four to five days to achieve, but decreased aqueous humor production occurs within 30 minutes to a few hours of dosing. Topical CAIs are effective in both dogs and cats. They can be used in all types of glaucoma, have no effect on pupil size and do not appear to contribute to intraocular inflammation. The efficacy of topical CAIs has been shown to be of equal efficacy to that of systemic CAIs.2
Methazolamide is dosed at 2 to 5 mg/kg orally twice a day. The recommended dosage for acetazolamide is 4 to 8 mg/kg orally two to three times a day, although this medication is rarely used currently because of the potential side effects. Commonly reported side effects of systemic CAIs include polyuria/polydipsia, gastrointestinal upset and panting (to compensate for metabolic acidosis). Systemic CAIs should not be used in patients with respiratory compromise. Cats appear to be more susceptible to metabolic acidosis, so systemic CAIs should be used with extreme caution in this species.
Beta-blockers
Beta-blockers are very effective for the reduction of IOP in people and are the most commonly prescribed class of drugs for the treatment of glaucoma in people. Timolol is the most widely used of these medications in both human and veterinary medicine, but other topical beta-blockers are available, including levobunolol, betaxolol, metipranolol and carteolol.
Beta-blockers reduce IOP by decreasing aqueous humor production, but the exact mechanism of this effect is not completely understood. In dogs and cats, their efficacy is considered relatively poor, and bradycardia and mild (bilateral) miosis associated with their use has been documented.3 Their use in equine glaucoma appears more promising. In veterinary medicine, timolol 0.5% is administered topically twice daily. Use with caution in patients with cardiovascular disease.
Prostaglandin analogues
The prostaglandin analogues appear to be the most effective drugs in the treatment of canine glaucoma. These drugs increase aqueous outflow. The mechanism of action is mediated through binding to prostanoid FP receptors. In dogs and people, activation of the prostanoid FP receptor results in increased uveoscleral outflow (i.e. unconventional outflow) through remodeling of the ciliary body musculature. Increased conventional outflow also occurs through morphological changes in the trabecular meshwork.
The most commonly prescribed prostaglandin analogue in veterinary medicine is latanoprost, which is now available as a generic. Other available prostaglandin analogues include bimatoprost and travoprost. In cats, latanoprost and other prostaglandin analogues are considered less effective because activation of prostanoid EP receptors is required for similar effects in this species. The prostaglandin analogues are generally administered topically twice a day in dogs. In dogs, prostaglandin analogues result in marked miosis. Because they work through activation of inflammatory mediators, they should be used with caution in cases of secondary glaucoma caused by anterior lens luxation or uveitis.
Other drug classes
Other, less commonly used anti-glaucoma drug classes include the cholinergic agonists (pilocarpine, carbachol, demecarium bromide, echothiophate iodide), adrenergic agonists (dipivefrin) and alpha2-adrenergic agonists (apraclonidine, brimonidine). Because of associated local side effects, lack of availability and systemic side effects, they are seldom used.
SURGICAL TREATMENT OPTIONS
If your patient is receiving anti-glaucoma drugs but the IOP is still > 25 mm Hg, it’s time to discuss to a surgical option. Recommended surgical options often depend on whether vision is considered salvageable. Surgical options for the visual eye include laser ciliary body cyclophotocoagulation and gonioimplants. Eyes that have lost vision are most commonly treated with enucleation, cyclocryotherapy, chemical ciliary body ablation or intrascleral prosthesis surgery to improve the quality of life by controlling pain related to elevated IOP. Consultation with a veterinary ophthalmologist is often helpful when determining which surgery is right for your patient.
Endolaser cyclophotocoagulation (ECP)
Endolaser cyclophotocoagulation (ECP) is a relatively new surgical option. A diode laser is used to target the pigmented tissue of the ciliary body, thereby photocoagulating nonpigmented epithelium. The advantage of this procedure is that the laser is built into an endoscopic probe, allowing for localization and direct treatment of the ciliary processes with little damage to surrounding tissue. Because the laser energy is likely to induce cataract development, the process is often combined with phacoemulsification. No studies evaluating the long-term efficacy of this surgery have been published, but preliminary results are promising.
Transscleral cyclophotocoagulation (TSCPC)
Transscleral cyclophotocoagulation (TSCPC) involves laser ablation of the ciliary body, similar to ECP. Both Nd:YAG and diode lasers have been used. The main difference between TSCPC and ECP is that TSCPC does not involve direct visualization of the ciliary body. Rather, external landmarks are used to position the laser probe over the external surface of the eye, and the laser energy is directed through the sclera.
Possible postoperative complications include excessive intraocular inflammation (with the potential to induce retinal detachment), cataract development, hyphema, and, as with any glaucoma surgery short of enucleation or evisceration, uncontrolled IOP. Immediate postoperative spikes in IOP are common and can result in postoperative vision loss.
Gonioimplants and valves
Gonioimplants facilitate drainage of aqueous humor through a tubing system implanted directly into the anterior chamber. Both valved and nonvalved implants exist. Valved systems allow for egress of aqueous humor at IOPs > 12 mm Hg, while nonvalved systems are limited only by resistance of the tubing itself. Although these shunt systems are generally efficacious in the short term, postoperative uveitis with fibrin development and valve occlusion, requiring intracameral tissue plasminogen activator administration, is relatively common.
The long-term efficacy of shunts is limited by avascular bleb development, in which an avascular fibrotic capsule develops around the base of the device. This fibrosis is observed so commonly that gonioimplant failure is generally considered inevitable, and implant placement is often combined with a cyclodestructive procedure to provide more long-term control of IOP.
Enucleation
Enucleation is probably the most commonly performed surgery of comfort for glaucoma. Possible postoperative complications include hemorrhage, postoperative fistulas or mucocele development caused by incomplete removal of conjunctival, caruncular or third eyelid glandular tissue and, rarely, orbital emphysema. In cats, the optic nerve is relatively short, and excessive traction on the enucleated globe must be avoided during surgery to prevent damage to the optic chiasm and contralateral blindness.
Cyclocryotherapy
Cyclocryotherapy allows for reduction in IOP without loss of the globe. This procedure is generally reserved for dogs. Nitrous oxide gas can be used. The cryotherapy probe is applied by using external landmarks to estimate the site of probe placement. The cryodose applied depends on the preoperative IOP. The biggest advantage is that the surgery is noninvasive; postoperative therapy generally involves anti-inflammatories (often continued for life). Disadvantages include the potential for persistently elevated IOP, cataract development, hyphema, retinal detachment and globe phthisis.
Chemical ciliary body ablation
Pharmacologic ablation of the ciliary body can be performed with gentamicin or, as more recently described, cidofovir.4 Gentamicin is cytotoxic to both the ciliary body and retina, so it should never be used in a visual eye. Even in blind eyes, however, this procedure should be used with caution since retrospective pathologic studies have reported an increased incidence of intraocular tumor development postoperatively.5,6
The outcome of this procedure is also the least predictable of the cyclodestructive procedures, with published reports citing a success rate of around 65%.7 Other possible postoperative complications include cataract development, retinal detachment, hyphema, chronic uveitis and phthisis. Intravitreal gentamicin should not be administered to patients with renal compromise, as the drug is detectable in plasma postoperatively.8
Evisceration and intrascleral prosthesis
Intrascleral prosthesis surgery involves creating a 180-degree limbal incision at the corneoscleral junction, removing all intraocular contents and replacing these contents with a silicone intraocular prosthetic. The advantages of intrascleral prosthesis include complete resolution of glaucoma and maintenance of a cosmetic globe. Disadvantages include the potential for recurrent corneal ulcerations due to decreased corneal sensitivity (caused both by historical glaucoma and transection of corneal nerves intraoperatively) and the potential for keratoconjunctivitis sicca. Intrascleral prostheses should generally be avoided in eyes with underlying intraocular neoplasia, preoperative keratitis or keratoconjunctivitis sicca.9
PROGNOSIS
Unfortunately, glaucoma remains a disease with no effective “cure,” and although primary glaucoma often presents initially as a unilateral process, most at-risk dogs will develop glaucoma in the contralateral eye within one year of diagnosis. Initiating prophylactic anti-glaucoma and anti-inflammatory treatment in the normotensive eye has been shown to delay the onset of glaucoma in a dog at risk by a median of 324 days.10 Client education and early intervention can delay vision loss and improve the quality of life for your patients.
Is there a cure for glaucoma in dogs?
There is no cure for glaucoma, but there are ways to treat it and in some cases manage the condition. When it comes to secondary glaucoma, there is the possibility for a cure, and in turn one less potential contributor to glaucoma.
What dog breeds are prone to glaucoma?
Glaucoma can affect all dogs, although certain breeds have a genetic predisposition for developing the condition within their lifetime, including:
- Cocker Spaniel
- Chow Chow
- Samoyed
- Poodle
- Beagle
- Jack Russell Terrier
- Siberian Husky
- Dalmatian
- Chihuahua
- Basset Hounds
- Great Dane
Is glaucoma in dogs contagious for humans or other pets?
Glaucoma is not contagious between animals or even between humans. It is hereditary in certain breeds, however.
Recovery and management of glaucoma in dogs
If the eye needs to be removed as the result of vision loss because of glaucoma, your dog will need to wear an Elizabethan collar/cone for protection until the sutures are removed.
The lingering effects of the anesthesia and the possibility of discomfort may be present in the first few days after surgery but can be managed with medication. A checkup is required a few days afterward, but the sutures won’t be removed until after 10-14 days. Complications are rare but still possible, although the majority of dogs who have the surgery make a full recovery.
If glaucoma is detected early, treatments will be monitored, so follow-up appointments will be part of your dog’s management plan. For breeds predisposed to developing glaucoma, checkups twice a year help in early detection.
Regardless of whether management consists of medication and monitoring, or taking things a step further with surgery, the result is the same — to bring your dog relief.
How to prevent glaucoma in dogs
Secondary glaucoma can be prevented by keeping your dog safe, doing your best to avoid injuries and accidents, staying on top of health issues and seeking out treatment for any infection, especially eye related, as soon as possible.
Primary glaucoma, however, is not preventable because it is the result of genetics. But steps can be taken beforehand to try to slow any degenerative changes to your dog’s eyes and reduce their chances of developing glaucoma.
- Antioxidants like beta-carotene, vitamins E and C, as well as nutraceuticals can all be taken to reduce the amount of damage that occurs to the cells of the eye.
- Reducing stressors in your pet’s environment can help manage the oxidative damage that occurs throughout the body, including the eyes.
- Eliminating pressure to your dog’s neck is also critical because we don’t want to increase inter-cerebral or intraocular pressure through any type of tight collar or harness system.
- For aging pets and higher risk breeds, make sure your veterinarian check your dog’s eye pressure during wellness exams.
Regardless of the type of glaucoma your dog has, early detection is the best way to prevent the progression of the condition and the resulting blindness that is most often associated with glaucoma. Identifying those subtle pressure changes in the eye and addressing medically early on, is the best way to prevent further damage.
(Picture Credit: Ирина Мещерякова/Getty Images)
DR. AMIT,CANINE SPECIALIST,PUNE
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