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.
Intraocular Pressure Measurement:
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Normal intraocular pressure for most mammal eyes is approximately 15 mm Hg to 25 mm Hg.
Since individuals vary, comparing IOP between the two eyes can be useful.
However, acute glaucoma in dogs typically results in sudden and dramatic IOP elevation rather than a subtle increase (40 mm Hg or higher rather than 28 mm Hg).
Breeds Predisposed to Inherited Glaucoma
- Alaskan Malamute
- American Cocker Spaniel
- Basset Hound
- Bouvier des Flandres
- Cairn Terrier
- Chow Chow
- Fox Terrier
- Great Dane
- Norwegian Elkhound
- Shar Pei
- Siberian Husky
Symptoms of Glaucoma & Their Sources
|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|
|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|
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|
|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|
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|
|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.