Glaucoma: Treat to restore vision and comfort in horses - DVM
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Glaucoma: Treat to restore vision and comfort in horses


DVM360 MAGAZINE


Assess the situation Treatment of glaucoma depends on the athletic needs and use of the individual, together with its age, pre-existing conditions, degree of vision loss, personality, and the client's expectations and desires. The severity of glaucoma at the time of diagnosis strongly dictates what is possible. Vision may be a very attainable goal if there is a strong commitment to therapy, and it may permit a prolonged athletic career in individuals who must be visual to compete.

Initially, medical management is begun to attempt immediate relief from discomfort, and attenuate retinal damage from elevated IOP. Timolol 0.5% is a beta-blocker that mildly reduces IOP. Dorzolamide 2% (Trusopt(r)) is a carbonic anhydrase inhibitor (CAI), and is more potent. Preferably, combination therapy is administered BID-TID and is available pre-mixed (Cosopt(r)). Ophthalmic solutions are dosed at 0.15-0.2 mL per eye in horses. This combination reduces IOP modestly in normal horses and mild/moderate glaucoma cases. Uncommonly, oral acetazolamide (3-4mg/kg PO TID) may be used, but it is poorly bioavailable and does have systemic effects. In other species, combining oral and topical CAIs is not significantly different than one route alone in most individuals. Control of uveitis is evidently a critical step to controlling glaucoma. Topical corticosteroids and oral NSAIDs are aimed at reducing active inflammation.


Photo 5: Laser cyclophotocoagulation results in a rise in IOP, being reduced by aqueouscentesis. Centesis is not recommended for medical control of glaucoma in horses.
Atropine has been advocated to treat glaucoma in horses, but its effects on IOP are unpredictable. It is most useful when uveitis is active concurrently. Atropine frequency may be fine-tuned while monitoring IOP to balance uveitis control and uveoscleral outflow. Xalatan is a highly effective ocular hypotensive in many species but is not recommended in horses. The Ohio State University (OSU) reported an IOP reduction of 5-17 percent in normal horses (more in mares than geldings), but with a side-effect profile that largely negated its value. It may potentiate miosis and uveitis, and worsen ocular pain. It still can be useful in selected individuals who respond insufficiently to other therapy. If medications do not control IOP, or athletic competition rules preclude chronic administration, surgical intervention may be highly effective.

Surgical treatment Surgical control of glaucoma and IOP may be attempted by reducing aqueous production (partial ciliary ablation) or by increasing outflow (implanting a drainage device). Gonioimplants are performed rarely in horses to date. Controlled ciliary ablation is best attained with a laser, a procedure termed trans-scleral cyclophotocoagulation (TSCPC). Recent work from OSU detailed the equine ciliary body anatomy that permits highly accurate lasering to maximize ablation of fluid-producing cells with minimal peripheral damage. Preferred sites are 4-6mm posterior to the limbus avoiding the nasal quadrants and the ciliary arteries. Immediately post-surgical, the IOP may elevate and require centesis (photo 5, p. 4E), and this may recur for several days because of increased uveitis. IOP begins to drop as inflammation subsides and aqueous-production decreases. The University of Florida retrospectively analyzed 23 eyes of 16 horses treated by Nd:YAG laser. Preoperative IOP was 51 (+17) mmHg, and declined post surgery into the low 20s (+9) until beyond five months. Vision was present in 60 percent of eyes compared to 52 percent at presentation. Achieving stable IOPs <30 mmHg declined from 93 percent at 24 hours to 70 percent at five months.


Options for glaucoma control in horses
Diode lasers are now more commonly used, and OSU reported 64 percent of 27 horses retained vision at an average three years, although 90 percent required concurrent medication. Laser TSCPC may be performed under general anesthesia or after sedation and retrobulbar blockade. Morbidity, surgery duration and cost are all reduced as a result, making the procedure conveniently possible as outpatient hospital therapy, with follow-up performed at the barn. Post-operative therapy is aimed at uveitis control and IOP maintenance in the short term, and may be weaned subsequently. Laser TSCPC may need to be repeated periodically in some individuals if the ciliary body regenerates.

Summary Glaucoma results in corneal opacification, reduced vision and discomfort and is now recognized more commonly than ever. IOP measurement is easier and more accurate with the Tonopen, and therapeutic success is improved with better monitoring. Early medication maintains vision and limits complications from primary inflammation. Control of uveitis may eliminate the need for specific IOP medications. Laser surgery has very good success for individuals with vision and minor complications, and still may restore partial vision in more advanced cases. Laser TSCPC may be performed standing with retrobulbar blockade and typically has a short recovery time of two to three weeks.


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Source: DVM360 MAGAZINE,
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