Treatment of equine glaucoma by transscleral neodymium:yttrium aluminum garnet laser cyclophotocoagulation: a retrospective study of 23 eyes of 16 horses

MicroPulse™ transscleral cyclophotocoagulation in the equine patient: A case series of four horses

OBJECTIVE

To describe the clinical application and outcome of MicroPulse™ transscleral cyclophotocoagulation (MP-TSCPC) treatment in horses with glaucoma.

ANIMALS STUDIED

Four client-owned horses with primary (n = 2) or secondary (n = 2) glaucoma.

METHODS

Horses were treated with MP-TSCPC under standing sedation with a minimum of 30 days of follow-up (range 30-1241 days). Affected eyes were treated with a 31.3% duty cycle and 3000 mW laser power for a total of 180 s. Data collected included signalment, pre- and post-procedure intraocular pressures (IOPs), laser settings, medications, complications, and repeat therapy.

RESULTS

Four horses (5 eyes) received at least one treatment with MP-TSCPC. Mean preoperative IOP was 44 mmHg (range 33-49 mmHg). The immediate mean postoperative IOP was 34 mmHg (4 eyes; range 19-55 mmHg). At 1 week, IOP was 38 mmHg (5 eyes; range 21-80 mmHg), at 2 weeks was 40 mmHg (3 eyes, range 17-80 mmHg), at 1 month was 35 mmHg (5 eyes; range 20-50 mmHg), at 3 months was 18 mmHg (2 eyes; range 14-21 mmHg), at 6 months was 35 mmHg (2 eyes; range 30-39 mmHg), and at >300 days was 24 mmHg (3 eyes; range 18-29 mmHg). Complications included corneal ulceration (n = 1 eye), uncontrolled IOP (n = 3 eyes), and need for repeat treatment (n = 2 eyes).

CONCLUSIONS

MP-TSCPC used with the above-described settings was unsuccessful in treating the majority of cases. Future studies should be targeted at primary glaucoma cases and with use of alternative laser settings.

A review of glaucoma surgical therapy

Glaucoma is a disorder of all species due to a rise of intraocular pressure (IOP) beyond which is compatible with ganglion cell and axon function, often resulting in optic nerve degeneration and irreversible blindness. Glaucoma treatment with surgical intervention aimed at either reducing aqueous production, or increasing or altering aqueous outflow has evolved over preceding decades, but there remains no cure. The present article is intended to provide a concise review of glaucoma surgical therapies in veterinary ophthalmology.

The location of sites and effect of semiconductor diode trans-scleral cyclophotocoagulation on the buphthalmic equine globe

PURPOSE

To determine appropriate location and energy settings for trans-scleral cyclophotocoagulation (TSCPC) for buphthalmic equine globes.

ANIMALS

Eleven horses with a buphthalmic eye blinded by glaucoma presented for enucleation.

METHODS

Globe and corneal dimensions were measured via ultrasonography and calipers, and TSCPC was performed under general anesthesia immediately prior to enucleation. Part 1: In nine globes, sixty sites were lasered 4 mm posterior to the limbus in the dorsotemporal and ventrotemporal quadrants at settings of 1500 milliwatts and 1500 ms. Globes were processed and sectioned sagitally over the temporal aspect in two blocks, each with five histologic sections examined by light microscopy. A digital imaging system was used to determine the location and length of the pars plicata on one slide from each block. Part 2: Based on results in Part 1, two globes were measured and lasered using the same time and energy settings at the following distances posterior to the limbus: 8 mm dorsally, 6 mm dorsotemporally, 5 mm ventrotemporally, and 5 mm ventrally.

RESULTS

Globe and corneal dimensions exceeded normal values in all globes. Part 1: In all nine globes, photocoagulation affected the anterior ciliary processes and iris base and in 8/9 coagulation of the pectinate ligaments was noted. Part 2: In both globes, coagulation was confined to the pars plicata.

CONCLUSIONS

The previously recommended TSCPC sites are located too far anteriorly for a buphthalmic globe. Buphthalmic equine globes should have TSCPC performed at the following distances posterior to the limbus: 6-8 mm dorsally, 5-6 mm dorsotemporally, 4-5 mm ventrotemporally, and 4-5 mm ventrally.

Evaluation of diode endoscopic cyclophotocoagulation in bovine cadaver eyes

OBJECTIVE

To evaluate the anterior chamber approach and energy levels for endoscopic cyclophotocoagulation (ECPC) and assess ECPC-induced tissue damage in phakic eyes of bovine cadavers.

SAMPLE

12 bovine cadaver eyes.

PROCEDURES

Angle of reach was measured in 6 eyes following placement of a curved endoscopic probe through multiple corneal incisions. In another 6 eyes, each ocular quadrant underwent ECPC at 1 of 3 energy levels (0.75, 0.90, and 1.05 J) or remained untreated. Visible effects on tissues (whitening and contraction of ciliary processes) were scored (scale of 0 [no effects] to 6 [severe effects]), and severity and extent of histologic damage to the pigmented and nonpigmented ciliary epithelium and fibromuscular stroma were each scored (scale of 0 [no effect] to 3 [severe effect]) and summed for each quadrant. Overall mean scores for 6 quadrants/treatment were calculated.

RESULTS

Mean ± SD combined angle of reach was 148 ± 24° (range, 123 ± 23° [ventromedial] to 174 ± 11° [dorsolateral]). At the 0.75-, 0.90-, and 1.05-J levels, mean visible tissue effect scores were 3.12 ± 0.47, 3.86 ± 0.35, and 4.68 ± 0.58, respectively; mean histologic damage scores were 4.79 ± 1.38 (mild damage), 6.82 ± 1.47 (moderate damage), and 9.37 ± 1.42 (severe damage), respectively. Occasional popping noises (venting of vaporized interstitial water) were heard at the 1.05-J level.

CONCLUSIONS AND CLINICAL RELEVANCE

Multiple incisions were necessary to facilitate 360° ECPC treatment in bovine eyes. For ECPC in vivo, the 0.75- and 0.90-J energy levels had the potential to effectively treat the ciliary epithelium.

Animal models of glaucoma

Glaucoma is a heterogeneous group of disorders that progressively lead to blindness due to loss of retinal ganglion cells and damage to the optic nerve. It is a leading cause of blindness and visual impairment worldwide. Although research in the field of glaucoma is substantial, the pathophysiologic mechanisms causing the disease are not completely understood. A wide variety of animal models have been used to study glaucoma. These include monkeys, dogs, cats, rodents, and several other species. Although these models have provided valuable information about the disease, there is still no ideal model for studying glaucoma due to its complexity. In this paper we present a summary of most of the animal models that have been developed and used for the study of the different types of glaucoma, the strengths and limitations associated with each species use, and some potential criteria to develop a suitable model.

Histologic effect of semiconductor diode laser transscleral cyclophotocoagulation on the normal equine eye

OBJECTIVE

To determine the acute histologic effects of semiconductor diode laser transscleral cyclophotocoagulation (TSCP) on the normal equine eye.

ANIMAL STUDIED

Part 1: eight eyes of four horses. Part 2: 10 eyes of five horses.

MATERIALS AND METHODS

Part 1: TSCP was performed on four eyes at 4 mm and four eyes at 6 mm posterior to the limbus with 15 sites treated in four quadrants at 1800 mW for 1500 ms. The globes were sectioned transversely or sagitally to examine all quadrants and histologic sections were taken every 1 mm for the entire globe. Part 2: Based on the results from Part 1, TSCP was performed at 20 sites 4 mm posterior to the dorsotemporal limbus with a constant energy varying from 0.75 to 4 J/site. Histologic sections were taken every 1 mm for a total of 10 sections per eye and 20 sections per energy level group.

RESULTS

Part 1: At 4 mm posterior to the limbus, coagulation of the nonpigmented epithelium (NPE) of the pars plicata was observed in the temporal (14%) and dorsal quadrants (12%). Retinal detachment was observed in the nasal quadrant (12%). Hemorrhage was common in the nasal (19%) and temporal (12%) quadrants. At 6 mm posterior to the limbus, coagulation of the NPE of the pars plicata was observed in the dorsal (14%), ventral (16%), nasal (2%), and temporal (2%) quadrants. Retinal detachment was observed in the dorsal (8%), ventral (18%), nasal (20%) and temporal (2%) quadrants. Part 2: Settings of 0.75 J/site were ineffective; 1.5, 2.25 and 3 J/site damaged the pars plicata without disruption of anatomy; and 4 J/site caused disruption of normal architecture.

CONCLUSIONS

The most appropriate site for equine TSCP appears to be 4 mm posterior to the dorso- and ventrotemporal limbus avoiding the 3 and 9 o'clock positions and using an initial energy setting of 2.25 J/site. This results in effective damage to the pars plicata while minimizing surgical complications such as retinal detachment and hemorrhage.

Equine glaucoma

Glaucoma is a diverse group of vision-impairing disorders that have as a common bond an elevation of intraocular pressure(IOP) to a level incompatible with the health of the eye. Glaucoma can be congenital, primary, or secondary. Congenital equine glaucoma is associated with developmental abnormalities of the iridocorneal angle or, in many cases, with the more severe anterior segment dysgenesis.

Flash electroretinography in standing horses using the DTL microfiber electrode

PURPOSE

The goal of our study was the evaluation of a practical method for the recording of flash electroretinograms (ERGs) in sedated, standing horses with the DTL microfiber electrode.

METHODS

The horses were sedated intravenously with detomidine hydrochloride (0.015 mg/kg). The pupil was dilated and the auriculopalpebral nerve was blocked. The ERGs were recorded with the active electrode on the cornea (DTL), the reference electrode near the lateral canthus, and the ground electrode over the occipital bone. The light intensities of the white strobe light were 0.03 cd x s/m2 (scotopic) and 3 cd x s/m2 (scotopic and photopic). Photopic and scotopic single flash and flicker responses to Ganzfeld stimulation were recorded. During the 20-min dark adaptation period the retina was stimulated every 5 min with the 0.03 cd x s/m2 single flash.

RESULTS

The median b-wave amplitudes and implicit times were 38 microV and 33 ms (photopic cone-dominated response), 43 microV and 63 ms (5-min dark adaptation), 72 microV and 89 ms (10 min), 147 microV and 103 ms (15 min), 188 microV and 109 ms (20 min, 0.03 cd x s/m2, rod response), and 186 microV and 77 ms (20 min, 3 cd x s/m2, maximal combined rod-cone response). A steady increase in amplitude and implicit time was noted during dark adaptation. No oscillatory potentials could be isolated.

CONCLUSIONS

The use of detomidine hydrochloride sedation and the DTL microfiber electrode allowed the recording of good quality ERGs. This protocol should permit the detection of functional problems in the retina without the risk involved with general anesthesia.

Ophthalmic findings in the geriatric horse

Ophthalmic conditions of the older horse are becoming increasingly common as geriatric care advances. The close relation between the eye and other structures of the head exposes the globe to direct expansion of clinical disease and raises management issues. Integrating an ophthalmic and fundic examination into the annual evaluation should reward the investment, permit enhanced patient care, and potentially provide early identification of systemic conditions. Surgical and medical management of neoplastic conditions is most successful with early identification and therapy. It is hoped that this article prompts the further identification and description of ophthalmic conditions as equine geriatric care increases in importance.

Lasers in ophthalmology

Laser technology continues to progress with the addition of new lasers, new delivery systems, and new applications. The introduction of lasers to veterinary ophthalmology has radically changed the level of care that we can provide to our patients. The development of the diode laser has particularly had an impact on veterinary ophthalmology. The diode's affordability, portability, and broad applications for veterinary patients have allowed laser surgery to become a routine part of veterinary ophthalmology practice. Educating the public and veterinary community in available laser techniques will generate improved ophthalmic care and provide more data on which to build future applications.

Effects of topical administration of 0.005% latanoprost solution on eyes of clinically normal horses

OBJECTIVE

To determine the effect of 0.005% latanoprost solution on intraocular pressure (IOP) of eyes of clinically normal horses and establish the frequency of adverse effects of drug administration.

ANIMALS

20 adult clinically normal horses.

PROCEDURE

IOP was recorded (7, 9, and 11 AM; 3, 5, and 7 PM) on days 1 and 2 (baseline), days 3 to 7 (treatment), and days 8 to 9 (follow-up). Latanoprost was administered to 1 randomly assigned eye of each horse every 24 hours during the treatment period, following the 7 AM IOP recording. Pupil size and the presence or absence of conjunctival hyperemia, epiphora, blepharospasm, blepharedema, and aqueous flare were recorded prior to IOP measurement.

RESULTS

IOP was reduced from baseline by a mean value of 1.03 mm Hg (5%) in males and 3.01 mm Hg (17%) in females during the treatment period. Miosis developed in all treated eyes and was moderate to marked in 77% of horses, with the peak effect observed 4 to 8 hours after drug administration. Conjunctival hyperemia, epiphora, blepharospasm, and blepharedema were present in 100, 57, 42, and 12% of treated eyes, respectively, 2 to 24 hours following drug administration. Aqueous flare was not observed at any time point.

CONCLUSIONS AND CLINICAL RELEVANCE

Although IOP was reduced with every 24-hour dosing of latanoprost, the frequency of prostaglandin-induced adverse events was high. Because recurrent uveitis appears to be a risk factor for glaucoma in horses, topical administration of latanoprost may potentiate prostaglandin-mediated inflammatory disease in affected horses.

Description of ciliary body anatomy and identification of sites for transscleral cyclophotocoagulation in the equine eye

OBJECTIVE

To determine the most appropriate anatomical location for diode laser probe placement to accurately photoablate the equine ciliary body using a contact, transscleral approach.

DESIGN

Original research.

PROCEDURES

Forty-two freshly enucleated adult equine eyes were evaluated. The horizontal, medial vertical, central vertical, and lateral vertical lengths of the cornea were measured from limbus to limbus. Needles were inserted perpendicular to the sclera at specific distances posterior to the external limbus at the 1, 2, 4, 5, 6, 7, 8, 10, 11 and 12 o'clock positions. The per cent frequency that needles penetrated internal anatomical regions (lens, lens zonules, pars plicata, anterior, middle, and posterior pars plana, ora ciliaris retinae, or retina), when inserted at 2, 4, 6, and 8 mm posterior to the limbus, were calculated for all eyes combined, for right vs. left, for males vs. females, and for three age groups. The internal distance from the limbus to the anterior pars plana, and from the anterior pars plana to the ora ciliaris retinae were also measured in six of the horses (12 eyes). Mean distances and standard deviations were computed for all parameters.

RESULTS

Average corneal sizes and standard deviations were: 30.24 mm +/- 1.53 (horizontal); 24.69 mm +/- 1.52 (central vertical); 22.79 mm +/- 1.49 (medial vertical); and 19.79 mm +/- 1.55 (lateral vertical). Internal distances of the pars plicatas ranged from 5.33 mm +/- 0.49 to 10.67 mm +/- 1.15. Internal distances of the pars planas ranged from 0.33 mm +/- 0.49 to 3.17 mm +/- 0.39. High probabilities of penetrating the pars plicata correspond to positions 4 mm posterior to the external limbus dorsotemporal 10, 11 o'clock (OD), and 1, 2 o'clock (OS), dorsonasal 1 o'clock (OD) and 11 o'clock (OS), and ventrotemporal 5-7 o'clock (OU). Low probabilities of penetrating the pars plicata correspond to positions ventronasal 4 o'clock (OD), 8 o'clock (OS) and dorsonasal 2 o'clock (OD), 10 o'clock (OS) at 4 and 6 mm posterior to the external limbus as well as ventrotemporal 4 o'clock (OS), 8 o'clock (OD) at 6 mm posterior to the external limbus.

CONCLUSIONS

Transscleral cyclophotocoagulation may be a viable alternative to medical therapy for control of intraocular pressure in horses with glaucoma. Overall, the most accurate anatomical position on the sclera for cyclophotocoagulation of the equine eye is 4-6 mm posterior to the limbus, avoiding the nasal quadrants. Accurate transscleral cyclophotocoagulation should optimize the therapeutic outcome and minimize potential side-effects such as retinal detachment and cataract formation.