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Keywords:

  • ciliary body;
  • cyclophotocoagulation;
  • equine;
  • glaucoma;
  • intraocular pressure;
  • semiconductor diode laser

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Funding
  9. References

Objective  To describe the long-term histologic and intraocular pressure (IOP) lowering effects of diode laser transscleral cyclophotocoagulation (TSCP) on the normal equine eye.

Animals  Eight normal adult horses.

Procedures  TSCP was performed in one randomly assigned eye. Sixty spots were treated at settings of 1500 ms and 1500 mW. Two horses were randomly selected for euthanasia at 2, 4, 12, or 24 weeks post-TSCP. Both eyes were enucleated and histologically evaluated. Intraocular pressure was measured by applanation tonometry prior to TSCP, immediately post-TSCP, twice daily for 7 days post-TSCP and then monthly until study conclusion. A longitudinal model estimated the average IOP values for the treated and untreated eyes at 1 week, 1, 3, and 6 months post-TSCP.

Results  All treated eyes at all time periods exhibited four characteristic histologic lesions: scleral collagen hyalinization, ciliary body pigment dispersion and clumping, focal disruption of the ciliary body epithelium, and focal ciliary process atrophy. After TSCP, there were no significant changes in IOP from baseline for the control eyes, while the IOP in treated eyes was significantly decreased from baseline (P < 0.05) at all time periods. The estimated decrease in IOP in the treated eyes compared to baseline IOP at 6 months was -3.76 mmHg for an average decrease in IOP of 20% from baseline.

Conclusion  Diode laser TSCP produces histologic lesions in the equine ciliary body that result in a significant and sustained decrease in IOP. TSCP may be an effective management for equine glaucoma.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Funding
  9. References

Glaucoma is a recognized clinical problem in horses resulting in retinal and optic nerve degeneration, blindness, and variable degrees of pain. The prevalence of equine glaucoma was 0.07% in a Veterinary Medical Data Base search from 1981 to 1991.1 The prevalence is likely higher now because of the availability of portable tonometers and an increased awareness of glaucoma by veterinarians. Medical treatment of glaucoma with daily topical therapy is often ineffective long term as well as being costly and labor intensive. Performed correctly, semiconductor diode TSCP is a reliable method for intraocular pressure (IOP) control that can augment or replace medical therapy, thereby prolonging vision and improving comfort.2 Intraocular pressure is determined by aqueous humor production and outflow. Glaucoma results from an alteration in aqueous humor dynamics causing an IOP that is not compatible with ocular function leading to optic nerve atrophy, visual field loss, and blindness.3 Increased IOP results in ocular pain and can produce behavioral changes that impair performance. Equine eyes blinded by glaucoma are often enucleated. Loss of an eye reduces esthetic appearance, limits showability in most breeds, and may be emotionally difficult for the horse owner.

Glaucoma is diagnosed based on IOP and clinical signs. The normal equine IOP ranges from 17 to 28 mmHg with a mean of 23 mmHg.4–7 The goals of glaucoma therapy are to maintain IOP within a range that is compatible with intraocular health and vision and to provide patient comfort. Therapy is aimed at either improving aqueous humor outflow from the eye or reducing aqueous humor production. Medical management is the first line of glaucoma therapy in humans and animals. Several glaucoma medications have been evaluated in the equine eye, but to date, no studies demonstrate adequate control of IOP by medical therapy in the equine eye.7–10

A common surgical therapy for glaucoma in animals is TSCP.11 Cyclophotocoagulation refers to coagulation necrosis of the ciliary body processes with light energy.12 The goal of TSCP is to destroy enough ciliary body epithelium to lower aqueous humor production and subsequently IOP. The diode laser emits light energy with a wavelength of 810 nm that has a high degree of absorption by melanin pigment in the ciliary process epithelium.12 The success of TSCP depends on the appropriate probe location and energy settings.13 A previous equine TSCP study determined that probe application should be 4 mm posterior to the limbus in the dorsotemporal and ventrotemporal quadrants with an energy of 2.25 J/site.14 At this energy level, the diode laser caused coagulation of the ciliary process epithelium and stroma. Complications of diode laser TSCP are generally mild but can include conjunctival hyperemia, corneal ulceration, intraocular hemorrhage and/or inflammation, retinal detachment, cataract, and failure to control IOP.1,11,15

The acute histologic effects of TSCP on normal equine intraocular tissue include ciliary body epithelial coagulation necrosis, epithelial separation, pigment dispersion, vascular congestion, and scleral collagen coagulation.14 No long-term histologic or IOP studies have been performed on the equine eye post-diode TSCP. This study was designed to describe the long-term histologic lesions and IOP lowering effects of diode laser TSCP on the normal equine eye.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Funding
  9. References

Animals studied

Eight adult geldings ranging in age from 3 to 25 years from The Ohio State University Alice Lloyd Finley Memorial Veterinary Research Farm were utilized. All horses were donated to the OSU research herd for use in terminal research studies because of chronic problems, primarily lameness. A complete ophthalmic examination including biomicroscopy, direct and indirect ophthalmoscopy, and applanation tonometry was normal in all horses. Two horses were randomly assigned to each of four time groups for euthanasia and enucleation: 2, 4, 12, and 24 weeks post-TSCP. The treated eye was randomly assigned. The Institutional Animal Care and Use Committee of The Ohio State University approved all procedures.

TSCP Procedure

To reduce the inflammatory effects of TSCP, each horse received topical prednisolone acetate 1% in the treated eye every 30 min starting 90 min prior to TSCP and one dose of flunixin meglumine (1.1 mg/kg, IV) at anesthetic induction. Intravenous general anesthesia was maintained during TSCP. The probe was applied 4 mm posterior to the limbus. Thirty sites were treated in both the dorsotemporal and ventrotemporal quadrants for a total of 60 sites per eye (Fig. 1). The laser energy was 2.25 J per site with settings of 1500 ms and 1500 mW. The IOP was measured in both eyes immediately post-TSCP. If the IOP was >30 mmHg at the conclusion of the laser procedure, a passive aqueocentesis was performed using a 30-gauge needle. The needle was introduced into the anterior chamber at the limbus, the needle hub was allowed to fill with aqueous humor, and then the needle was removed. Intraocular pressure was measured immediately post-aqueocentesis. The target IOP post-aqueocentesis was < 20 mmHg. All horses recovered uneventfully from general anesthesia.

image

Figure 1.  A schematic of the equine globe. The dots depict the locations of the laser probe application. The solid lines depict the two histologic sections examined. D = dorsal, T = temporal, V = ventral, N = nasal.

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Post-operative management

Post-operative management consisted of oral flunixin meglumine (1.1 mg/kg daily) tapered over 7 days and topical neomycin/polymyxin B/dexamethasone 0.1% ointment in the treated eye every 8 h for 7 days. Immediately after TSCP, a single dose of topical atropine sulfate 1% was administered to both eyes to induce cycloplegia. Intraocular pressure was monitored every 12 h during the first 7 days post-TSCP at approximately 7 am and 7 pm. A complete ocular examination was performed daily during the first 7 days post-TSCP, during which time the horses were housed at The OSU Veterinary Teaching Hospital. At 1-week post-TSCP, the horses were returned to Finley Farm until study conclusion. Ocular examination and IOP evaluation were performed at study conclusion and monthly for all surviving animals throughout the study. The horses were euthanized at their assigned time of 2, 4, 12, or 24 weeks post-TSCP. A complete ophthalmic examination was performed prior to euthanasia. Euthanasia was performed with pentobarbital sodium (85 mg/kg, IV)/phenytoin sodium (11 mg/kg, IV) after which both eyes were enucleated immediately via a transconjunctival approach and placed in 10% neutral buffered formalin for 48 h.

Histologic evaluation

After formalin fixation, the temporal globe was sectioned sagitally into two blocks to evaluate the dorsotemporal and ventrotemporal quadrants of the ciliary body (Fig. 1). Histologic sections were taken every 1 mm to obtain five slides per block yielding 10 slides per eye. The slides were stained with hematoxylin and eosin and examined by light microscopy. All slides of both eyes were examined for histologic lesions. Histologic lesions assessed included the following: corneal edema, corneal fibrosis, cataract, retinal detachment, retinal hemorrhage, chorioretinal scar, ciliary body vascular congestion, ciliary body pigment dispersion, ciliary body epithelial coagulation necrosis, ciliary body stromal necrosis, ciliary body epithelial separation, scleral collagen hyalinization, scleral fibrosis, destruction of the bilayered ciliary body epithelium, ciliary process atrophy, ciliary process fibrosis, loss of ciliary processes, and evidence of ciliary process regeneration. Lesions were assessed as present or absent for each eye.

Intraocular pressure measurements

The IOP was recorded by applanation tonometry (Tonopen-XL, Mentor Ophthalmic, Norwell, MA, USA) during the initial ophthalmic examination, prior to anesthetic induction, immediately before TSCP, immediately following TSCP, twice daily for the first seven post-operative days, monthly until study conclusion, and immediately prior to euthanasia. One investigator (VKC) measured IOP throughout the study. The horses were sedated with xylazine (0.2–0.3 mg/kg, IV), and an auriculopalpebral nerve block with carbocaine (2 mL, SQ) was performed on all horses for all IOP readings. The head was maintained in a normal upright position during all measurements. The eyelids were minimally manipulated to avoid excess pressure on the globe, and the cornea was topically anesthetized with 0.5% proparacaine solution. The tonopen was calibrated daily according to the manufacturer’s instructions, and three measurements with <5% variance were obtained and averaged for each reading. Because reports indicate that xylazine can decrease IOP, the dose of xylazine was standardized in each horse and used for all IOP measurements.16–18

Statistical analysis

A repeated measures model was applied to the data to estimate the average IOP values for both the treated and untreated eyes at 1, 4, 12, and 24 weeks post-TSCP. Age, weight, and the eye treated (OS or OD) were considered as potential covariates in the model; only age was significant. Based on the model, IOP estimates for both untreated and treated eyes at specific time points were calculated, with 95% confidence intervals (CI). For these estimates, an age of 17 years, the average age of the horses in our study, was used. Comparisons in IOP between the untreated and treated eyes over time were also made using an 0.05 level of significance. All analyses were performed using sas v9.1. (SAS Institute, Inc., Cary, NC, USA)

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Funding
  9. References

None of the horses developed corneal ulceration, cataract, or retinal detachment. All of the horses had mild conjunctival hyperemia for 4 days following TSCP. Four of the eight treated eyes (50%) had mild aqueous flare and/or fibrin in the anterior chamber one day post-TSCP. All four eyes that developed aqueous flare and fibrin had undergone a passive aqueocentesis because of increased IOP post-TSCP (see IOP section). All intraocular inflammation resolved by 5 days post-TSCP. Ocular discomfort was not observed in any of the horses.

Histologic lesions

None of the control eyes had histologic lesions in the cornea, iris, lens, ciliary body, or retina. In the laser treated eyes, no histologic lesions were noted in the cornea, iris, lens, or retina. All treated eyes at all time periods exhibited histologic lesions in the ciliary body. The frequency and severity of the histologic lesions was similar among the 8 treated eyes. The four characteristic histologic lesions noted across all time groups were scleral collagen hyalinization with loss of differential staining, pigment dispersion and clumping in the ciliary body, focal disruption of the bilayered ciliary body epithelium, and focal ciliary process atrophy (Figs 2 and 3). Two of the eight eyes exhibited vascular congestion adjacent to the laser sites. Both of these eyes were in the 2-week post-TSCP group. All treated eyes (n = 8) exhibited pigment dispersion, ciliary body epithelial coagulation necrosis, ciliary body stromal necrosis, scleral collagen hyalinization, scleral fibrosis, focal destruction of the bilayered ciliary body epithelium, ciliary process atrophy, and focal loss of ciliary processes. Four of the eight treated eyes exhibited separation of the ciliary body epithelium from the ciliary body stroma. None of the treated eyes showed evidence of ciliary body regeneration.

image

Figure 2.  Photomicrograph illustrating scleral collagen hyalinization and focal loss of ciliary body processes at 2 weeks post-TSCP. Note the normal ciliary processes on either side of the TSCP laser site. Hematoxylin and eosin, 4×.

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image

Figure 3.  Photomicrograph illustrating scleral hyalinization, ciliary body pigment dispersion, focal loss and disorganization of the bilayered ciliary epithelium, and ciliary processes at 24 weeks post-transscleral cyclophotocoagulation. Hematoxylin and eosin, 10×.

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Intraocular pressure

At baseline, prior to TSCP, there was no significant difference in the average IOP values between the control (18.82 mmHg) and treated (18.69 mmHg) eyes (P value = 0.83; Table 1). Table 2 provides IOP data for all treated and control eyes for the first 12 h post-TSCP. Five of the eight treated eyes (62.5%) had an increase in IOP ≥30 mmHg immediately post-TSCP. The mean IOP of these eyes was 34.8 mmHg with a range of 30–42 mmHg. All 5 of these eyes had a single passive aqueocentesis performed using a 30-gauge needle. Four of these five eyes developed mild aqueous flare and/or fibrin immediately post-TSCP. The mean IOP post-aqueocentesis was 14.8 mmHg with a range of 10–20 mmHg. Twelve hours after TSCP, the mean IOP of the five eyes in which aqueocentesis was performed was 20 mmHg with a range of 14–28 mmHg. The mean IOP of all treated eyes at twelve hours post-aqueocentesis was 20.4 mmHg and the range was 15–28 mmHg. The mean IOP of the control eyes was 18 mmHg with a range of 11–24 mmHg. From day two to day seven after TSCP, the IOP of all treated eyes ranged from 10 to 23 mmHg and the IOP of all control eyes ranged from 10 to 25 mmHg.

Table 1.   Estimate of average intraocular pressure (IOP) values of untreated and treated eyes over time
TreatmentTimeEstimated IOP (mmHg)95% CI
  1. IOP = intraocular pressure; CI = confidence interval.

Untreated eye0 (baseline)18.82(16.86, 20.78)
1 week17.39(15.71, 19.08)
1 month17.65(15.38, 19.91)
3 months17.52(15.01, 20.03)
6 months20.25(16.81, 23.69)
Treated Eye0 (baseline)18.69(16.73, 20.65)
1 week14.76(13.08, 16.44)
1 month14.19(11.92, 16.46)
3 months14.48(11.97, 16.99)
6 months14.93(11.49, 18.37)
Table 2.   Intraocular pressure (IOP) in treated and control eyes immediately post-transscleral cyclophotocoagulation (TSCP), immediately post-aqueocentesis (performed for post-TSCP IOP≥30 mmHg), and 12 h post-TSCP
 IOP post-TSCPIOP post-tapIOP 12 h post-TSCP
  1. IOP = intraocular pressure; TSCP = transscleral cyclophotocoagulation; tap = aqueocentesis; unbracketed numbers = treated eye; () indicates control eye; IOP is reported in mmHg.

Horse 110 (12) 24 (21)
Horse 232 (21)1028 (17)
Horse 335 (18)1414 (18)
Horse 415 (15) 15 (11)
Horse 517 (15) 24 (16)
Horse 635 (22)1818 (19)
Horse 730 (17)1225 (24)
Horse 842 (24)2015 (18)

The average IOP values of the treated eyes were lower and remained lower than the control eyes at both baseline and throughout follow-up (Tables 1 and 3, Fig. 4). The lower IOP observed in both treated and untreated eyes was attributed to acclimatization of the horses during the course of the study. After TSCP, the slightly lower IOP compared to baseline in the control eyes was not significant and was also attributed to acclimatization of the horses during the study. However, after TSCP, the treated eyes showed an average decrease of 3.76 mmHg (20%) from baseline over time that was significant (−95% CI: −7.51 to −0.01, P = 0.0493; Table 3). At 6 months, the average IOP for the control eyes was 5.19 mmHg higher than the IOP for treated eyes (95% CI: 0.91–9.47, P = 0.0178).

Table 3.   Change in intraocular pressure (IOP) values from baseline in the untreated and treated eyes over time
TreatmentComparisonEstimated difference in IOP (mmHg)95% CI P value
  1. IOP = intraocular pressure; CI = confidence interval.

Untreated eyeIOP at 1 week vs. baseline−1.43(−3.68, 0.82)0.2115
IOP at 6 months vs. baseline1.43(−2.32, 5.18)0.4518
Treated EyeIOP at 1 week vs. baseline−3.93(−6.18, −1.68)0.0007
IOP at 6 months vs. baseline−3.76(−7.51, −0.01)0.0493
image

Figure 4.  Estimated average intraocular pressure over time for untreated and treated eyes.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Funding
  9. References

In this study of the normal equine eye, semiconductor diode laser TSCP produced histologically significant lesions in the ciliary body along with a decrease in IOP that was maintained over 6 months. The histologic findings of scleral collagen hyalinization with loss of differential staining, pigment dispersion and clumping, focal disruption of the bilayered ciliary body epithelium, and focal ciliary process atrophy are similar findings to other diode laser TSCP studies in humans and animals.12,14,19 Along with the histologic lesions, a significant reduction in IOP was maintained throughout the study. The IOP in the treated eyes remained significantly lower than the baseline IOP throughout the entire study. The 3.8–4.5 mmHg (20–24%) reduction in IOP in the treated eyes from baseline is similar to other laser TSCP studies in normal dogs and cats.19,20 It was not within the scope of this study to evaluate the reduction in IOP in glaucomatous equine eyes; however, in the retrospective study by Annear et al.,2 the mean IOP 3–5 weeks post-TSCP was 17.81 mmHg (48%) lower than the mean IOP of horses prior to surgery. Because the surgical technique used in the retrospective and our study was similar, we propose that a greater IOP lowering effect of TSCP on glaucomatous equine eyes can be expected.

The clinical findings in this study of mild conjunctival hyperemia (100%) and mild intraocular inflammation (50%) were also common side effects in other transscleral laser studies.11,21 Diode laser studies in dogs with and without glaucoma found mild aqueous flare in 80–100%.11,15,19 The mild aqueous flare and fibrin noted in this study occurred less often than in previous studies and only occurred in horses that had an aqueocentesis post-TSCP. This clinical finding was not surprising because aqueocentesis has been used as a method for inducing breakdown of the blood-aqueous barrier resulting in transient increases in protein and cells in aqueous humor.22 The flare and fibrin resolved within 5 days and the horses’ IOP values and histologic findings did not differ from the horses not requiring an aqueocentesis.

Previous diode laser studies in dogs with and without glaucoma reported corneal ulceration in 5–12.5% and cataracts in 3–37% post-TSCP.11,15,19 In one study of glaucomatous horses treated with a Nd:YAG laser, 13% developed corneal ulceration.21 Neither corneal ulcers nor cataracts were reported in a more recent retrospective study of the efficacy of semiconductor diode laser TSCP for horses with glaucoma; however, mild transient hyphema occurred in 5 of 42 (12%) eyes.2 Unlike other animal studies, none of the horses in this study developed corneal ulceration, cataract, or hyphema. Three possible reasons for the lower incidence of complications in this study are the following: (i) a lower amount of energy is required by the diode laser to induce ciliary body damage compared to other lasers such as the Nd:YAG, (ii) treatment was limited to the temporal quadrants of the globe instead of treating all quadrants, and (iii) the laser procedure was performed on normal healthy equine eyes. Because the diode laser targets pigmented tissue in the ciliary body, less energy is required to damage the ciliary body, thus decreasing damage to the surrounding ocular tissue and lowering the complication rate. There is less distance between the ciliary body and the retina in the nasal quadrants of the equine eye compared to the temporal quadrants.13 Therefore, avoiding the nasal quadrants and treating only the temporal quadrants decreases the chance of retinal detachment. Lasering only 180 degrees of the globe also limits damage to the nasal corneal nerve bundles decreasing the risk of decreased corneal sensation, neurotrophic keratitis, and corneal ulceration.23 Directing the angle of the probe slightly posteriorly decreases the risk of cataract formation.

Endoscopic ciliary body photocoagulation (ECP) is a newer modality utilizing a fiber-optic probe to directly visualize and treat the ciliary processes. Use in small animal patients has been increasing over recent years and clinical results are promising, although published reports on the efficacy are limited.24,25 The advantages of ECP include ability to precisely and completely ablate ciliary processes with direct visualization of the effect. However, in comparison with non-invasive TSCP, the procedure requires a limbal or pars plana incision and has a risk of inducing cataract formation. Species-specific protocols and specialized longer laser probes need to be developed before this modality can be clinically evaluated in horses.26

Ciliary body epithelial regeneration has been hypothesized to be a reason for long-term failure of laser TSCP.19 Our study found no histologic evidence of ciliary body epithelial regeneration, and clinically, the IOP remained significantly lower than baseline IOP throughout the 24-week study. Our study supports the theory of focal ciliary process atrophy with a decrease in production and secretion of aqueous humor as the major cause of the sustained decrease in IOP after diode laser TSCP. This study documents defined and sustained histologic lesions in the equine ciliary body along with a significant long-term reduction in IOP in the equine eye post-semiconductor diode laser transscleral cyclophotocoagulation. These histologic and clinical results are compatible with the clinical results of retrospective studies of the effects of TSCP in horses where Whigham et al. found IOP control in 70% of horses at ≥20 weeks post-operatively and Annear et al. found that 100% of eyes were sighted and IOP was ≤25 mmHg in 68% of eyes at 20–68 weeks post-operatively. However, the inherent limitations of data acquisition in retrospective studies as well as the potential differences in laser effects on normal globes versus those with uveitis and glaucoma temper the validity of comparison. However, in light of the information available, diode laser TSCP appears to offer an efficacious treatment option for equine glaucoma with the capability of providing long-term IOP reduction.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Funding
  9. References

Our thanks go out to Dr Tina Fife, Ms. Chris Basham, and Ms. Kelley Norris for their assistance in performing the laser procedures and data collection.

Funding

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Funding
  9. References

The Ohio State University College of Veterinary Medicine Equine Research Funds and ACVO Vision for Animals Foundation.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Funding
  9. References
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