Glaucoma

Glaucoma is one of the most challenging ophthalmic diseases we face as veterinarians. Glaucoma is painful and often results in permanent vision loss. The elevation in intraocular pressure occurs due to impaired outflow of aqueous humor through the iridocorneal angle. Glaucoma may occur secondary to many conditions including lens luxation and uveitis but most cases are primary and inherited, usually affecting dogs between 3-6 years of age. The typical case of Glaucoma presents with a history of redness and cloudiness of variable duration. When only one eye is affected, the symptoms may be overlooked for some time until globe enlargement is finally noted. This generally indicates that the pressure has been in excess of 40-50 mmHg (upper limit of normal is 25 mmHg) for weeks and that irreversible vision loss has resulted. When the second eye becomes affected (as is usually the case with inherited glaucoma), the patient then presents with a history of acute blindness. When only one eye is affected, examination of the "normal" eye by gonioscopy (use of a contact lens and a slit lamp to directly examine the iridocorneal angle) can help in predicting whether this eye is likely to become affected. In some cases topical medication may be used prophylactically.

Breeds Predisposed to Inherited Glaucoma

• Alaskan Malamute
• American Cocker
• Spaniel
• Basset Hound
• Beagle
• Bouvier des Flandres
• Bullmastiff
• Cairn Terrier
• Chow Chow
• Dalmatian
• Fox Terrier
• Great Dane
• Keeshound
• Norwegian Elkhound
• Poodle
• Samoyed
• Shar Pei
• Siberian Husky

Symptoms of Glaucoma Symptom Cause

Scleral injection	elevated IOP results in impaired venous return
Corneal edema	direct pressure effect on corneal endothelium
Pupillary dilation	direct pressure effect on pupillary sphincter muscle, indicates pressure in excess of 40 mmHg
Afferent vision deficit (may be reversible if < 72 hours)	pressure effect on retina and optic nerve
Blindness
Globe enlargement	effect of chronic pressure elevation, indicates permanent vision loss
Lens luxation	may be primary (usually anterior luxation) or secondary (posterior luxation/subluxation) due to globe enlargement and breakdown of zonule fibers

Primary glaucoma with corneal edema, scleral injection, a mid-size and fixed pupil.

Glaucoma secondary to anterior lens luxation

Treatment of Glaucoma

Treatment for glaucoma is directed at restoring both vision and comfort. This requires an accurate assessment of the extent and duration of the pressure elevation. In eyes which have the potential for return of functional vision (no globe enlargement, duration less than one week), immediate medical therapy is used to initially reduce the pressure. This is followed by a choice of surgical options to attempt to permanently normalize the pressure. Medical management is rarely successful long-term due to recurrent pressure elevation and drug toxicity.

The diode laser is used to treat glaucoma through photocoagulation of the ciliary body. A 1 mm probe is applied to the sclera 2-2.5 mm posterior to the limbus to deliver 2.5-3.5 joules (2.5 mwatts for 1000-1400 msec) of energy to the ciliary body over 20-30 sites. This results in localized destruction of the secretory epithelium of the ciliary body thus reducing aqueous humor production. There is also some increase in extrascleral outflow of aqueous after this procedure further reducing intraocular pressure. We are fortunate that such a laser has been made available to us by IRIDEX in Mountain View. This procedure has been used in both blind and visual eyes with encouraging results. Postoperatively, the eyes are comfortable with minimal inflammation. The laser procedure can be repeated if necessary.
 

Medical Therapy for Glaucoma
Drug	Dose	Comments
Mannitol	20% 5 ml/lb BW IV	Evaluate BUN first, withhold water but monitor for dehydration
Glycerine	0.25 ml/lb BW PO	Available over the counter; can be repeated q 5 hours for 2-3 doses
Methazolamide	1-2 mg/lb PO BID-TID	Can induce hypokalemia and GI upset. Often not tolerated long-term
Xalatan (latanoprost)	topical SID-BID	Available at all major pharmacies.
Azopt (brinzolamide)	topical BID-QID	Available at all major pharmacies.

 

Surgical options for visual eyes
Procedure	Method of action	Advantages/Disadvantages
Laser cyclophotocoagulation   Following laser COC, the treatment sites can be seen through the thinned sclera.	decreases aqueous production	specialized equipment required variable success 70% in dogs, less in cats
Posterior sclerectomy- cyclodialysis-transcleral iridencleisis	increase aqueous outflow	variable success: 75% short-term, 50% long-term
Gonio-implant	increase aqueous outflow	extremely variable success expense of implant ($250)

Veterinary Vision acknowledges the support of Iridex Iris Medical, Inc. in providing the equipment and technical support which has made possible the development of many of the techniques described in this article.

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