Retrospective Evaluation of Surgical Outcomes in Cats With Glaucoma Following Diode Endoscopic Cyclophotocoagulation

ABSTRACT

Objective

To describe the post‐operative IOP control and visual status up to 12 months following endocyclophotocoagulation (ECP), and to characterize post‐operative complications following the use of ECP for the treatment of glaucoma in cats.

Methods

A retrospective analysis of glaucomatous cats treated with ECP between 2020 and 2025 was performed. Signalment factors, individual case factors, and surgical factors were assessed and compared against surgical outcomes.

Results

Twenty‐two eyes from 15 cats met the inclusion criteria. Causes of glaucoma included primary open‐angle glaucoma, Feline aqueous humor misdirection syndrome, chronic uveitis, lens subluxation, anterior segment dysgenesis, and toxic anterior segment syndrome following phacoemulsification. All cats received a lensectomy via phacoemulsification or intracapsular lens extraction, followed by ECP. Successful IOP control (≤ 30 mmHg) was achieved in 86% and 94% of cases at three months and one year. Vision was present in 77% and 71% of cases at three months and one year. Of the eyes that were visual pre‐operatively, vision was retained in 93% and 100% of cases at three months and one year. There was no effect of signalment on IOP control or vision retention at any time point evaluated. The most common post‐operative complication was corneal ulceration, occurring in 36% of cases.

Conclusions

ECP combined with lensectomy appears to be a safe and successful procedure for short‐ to medium‐term vision preservation and long‐term control of IOP in cats with glaucoma.

1. Introduction

Glaucoma is a disease characterized by degeneration of retinal ganglion cells and their axons, accompanied by elevations in intraocular pressure (IOP) [1]. In non‐human animal species, elevations in IOP represent the principal and most consistent risk factor for glaucoma development [1]. Compared to dogs, glaucoma is less frequently diagnosed in cats, likely due to an insidious onset, subtle clinical signs, and gradual progression [2]. If left untreated, glaucoma leads to irreversible blindness and is considered a potential source of discomfort, although overt clinical signs of pain are rarely observed in the cat [2, 3]. Uncontrolled glaucoma accounts for approximately one‐third (29%) of feline globe submissions to the Comparative Ocular Pathology Lab of Wisconsin (COPLOW) [4], underscoring its role as a major cause of enucleation in this species.

As in the dog, feline glaucoma can be categorized as primary (congenital or acquired) or secondary. Primary glaucoma in the cat is uncommon, accounting for only 5% of feline glaucoma cases in one retrospective study [5]. It most frequently presents as open‐angle glaucoma, with narrow or closed‐angle glaucoma less commonly reported [5, 6, 7, 8]. Histopathologic studies of affected cats suggest post‐trabecular obstruction of aqueous outflow, associated with perivascular deposition of a myxomatous matrix around the intrascleral veins [7, 9]. While the Domestic Shorthair and Domestic Longhair are overrepresented, breed predispositions have been identified in Burmese, European Shorthair, and Persian cats, although a hereditary basis has not been identified [3, 6, 7, 10, 11, 12]. Secondary glaucoma, resulting from an underlying ocular disease, is the most common form of glaucoma in cats, and is most frequently caused by intraocular neoplasia and chronic lymphocytic‐plasmocytic uveitis [2, 3, 4, 5, 8, 13].

Treatment of glaucoma in the cat aims to reduce aqueous humor production or enhance aqueous humor outflow through various medical and/or surgical approaches. Surgical interventions for glaucoma in veterinary medicine are employed for cases refractory to medical management alone and include various cyclodestructive procedures and gonioimplantation [2, 14, 15, 16, 17, 18, 19, 20, 21, 22]. Diode endoscopic cyclophotocoagulation (ECP) is a surgical procedure that uses an endoscopically guided diode laser to selectively target and destroy the ciliary body epithelium to reduce aqueous humor production. The laser energy is absorbed by the pigmented ciliary body epithelium, causing coagulation necrosis, leading to secondary thermal injury to the aqueous‐producing non‐pigmented epithelium. Endoscopic cyclophotocoagulation has the advantage of direct visualization of the tissue of interest, leading to less collateral damage and lower cumulative energy levels compared to a transscleral approach [14, 23]. Endoscopic cyclophotocoagulation was first described in 1992 for the treatment of neovascular glaucoma in humans [24]. Since then, ECP has been widely used in human medicine and has more recently been introduced to veterinary patients. In dogs, ECP has demonstrated positive outcomes, with a recent study reporting 90% IOP control and 63% vision retention at one year [15].

To date, no larger studies have evaluated the success rate of diode endoscopic cyclophotocoagulation (ECP) as a primary surgical therapy for the treatment of glaucoma in cats. The objectives of this retrospective case series were therefore to (i) describe the post‐operative IOP control and visual status up to 12 months following ECP, and (ii) to characterize post‐operative complications following the use of ECP for the treatment of glaucoma in cats.

2. Materials and Methods

A digital retrospective review of medical records was performed for feline patients with primary or secondary glaucoma that received ECP across two Veterinary Ophthalmology facilities from 2020 to 2025. Cats were excluded if they did not have a minimum follow‐up period of three months. Patient data recorded included age, sex, breed, eye(s) affected, type and etiology of glaucoma if known, and concurrent ocular conditions. Surgical data collected included time from diagnosis to surgery, average ECP power, circumferential degrees of ciliary body treated, and concurrent surgical procedures including intracapsular lens extraction (ICLE) and phacoemulsification with or without intraocular lens placement. Intraocular pressure control and visual status were recorded at various time points during the follow‐up period. A diagnosis of primary glaucoma was made by assessment of signalment, clinical suspicion, and by ruling out causes of secondary glaucoma. While gonioscopy was not performed, the iridocorneal angle was examined using slit lamp biomicroscopy where possible.

In all cases, surgical treatment was pursued due to poor response to medical therapy or increasing IOP despite medical therapy. Pre‐operative topical medications were administered and included three rounds of tropicamide 1%, prednisolone acetate 1%, and either dorzolamide 2%/timolol maleate 0.5% or brinzolamide 1%/timolol maleate 0.5%. Pre‐medication was variable between patients and may have included methadone, medetomidine, acepromazine, ketamine, and/or midazolam. Propofol was used for induction in all cases, and inhalant anesthetic (isoflurane) was used for maintenance ofanesthesia. All cases received intravenous cefazolin shortly after induction of anesthesia. All cases received a non‐depolarizing neuromuscular blockade (atracurium or cis‐atracurium) and were mechanically ventilated.

All 22 eyes had surgical lens removal, either by phacoemulsification or intracapsular lens extraction. Nine eyes underwent phacoemulsification with intraocular lens (IOL) placement, nine eyes underwent phacoemulsification without IOL placement, and four eyes underwent intracapsular lens extraction (ICLE). One cat had a posterior continuous capsulorrhexis performed, and one cat had a complete capsulectomy performed following phacoemulsification. Three out of 22 eyes had a vitrectomy performed following lens extraction. Two eyes (one cat) had a core vitrectomy, and one eye had an anterior vitrectomy to retrieve posteriorly displaced lens material. If ICLE was performed, the corneal incision was partially closed to 2.75 mm to maintain the anterior chamber to facilitate ECP. Following lens removal, the ciliary sulcus was filled with 1.2%–1.6% visco‐cohesive gel (Anvision) to maximize visualization of the ciliary processes. Curved and/or straight endolaser probes were used in each case based on surgeon preference. Using the endolaser probe (Endo Optiks E4 + DioVet 810 nm Laser System/Endo Optiks E2 Endoscopy System), the entire ciliary process was treated in a brushstroke manner until blanching and shrinking of the tissue was observed. Each patient had between 160° and 270° treated, with power ranging from 200 to 500 mW, 9000 ms duration, with a repeat interval of 0, equivalent to continuous mode. If tissue pops were observed, power was decreased in 20–50 mW increments to prevent further pops. Visco material was removed from the eye using an irrigation/aspiration handpiece for a minimum of 60 s in the anterior chamber. The corneal incision was closed in a simple interrupted, cruciate, or a sawtooth pattern using 8/0–9/0 Vicryl (polyglactin) or 9/0 PGA (polyglycolic acid) depending on the case and surgeon preference. One eye received intracameral tissue plasminogen activator (TPA), and one eye received intracameral preservative‐free dexamethasone due to a reduced ability of the owners to medicate topically. Six eyes from three cats had an anterior chamber suture shunt placed using 5/0 Dermalon (Nylon) or 6/0 Prolene (polypropylene), two of which were placed at the end of surgery and four of which were placed 24–48 h post‐operatively due to an elevation in intraocular pressure.

Three of 15 cats were hospitalized overnight following surgery to monitor IOP. The remaining patients were treated as outpatients and were rechecked the following morning. Based on clinician preference, some patients were admitted for IOP curves in the few days following surgery, for routine IOP monitoring, or due to an IOP elevation. Immediate post‐operative follow‐ups were scheduled for one week post‐operatively, two to three weeks post‐operatively, and one month post‐operatively unless more frequent rechecks were required (i.e., if IOP was significantly elevated or if glaucoma medications were adjusted). Post‐operative medications were variable between patients, but generally consisted of a topical antibiotic (chloramphenicol, neomycin/polymyxin/bacitracin, ofloxacin, or moxifloxacin), topical steroid (prednisolone acetate 1% or dexamethasone 0.1%) or non‐steroidal (ketorolac tromethamine 0.5%) and a carbonic anhydrase inhibitor (dorzolamide 2% or brinzolamide 1%) with or without a beta‐blocker (timolol maleate 0.5%). A small subset of patients also received a topical lubricant or prostaglandin analogue (latanoprost 0.005%). Oral medications were also variable between patients and included an oral steroid (prednisolone) or non‐steroidal (meloxicam or robenacoxib), antibiotics (amoxicillin‐clavulanic acid), and pain relief (gabapentin or buprenorphine). Topical anti‐inflammatory medications were tapered to effect over a three‐to‐six‐month period post‐operatively. Topical glaucoma medications were adjusted individually to each patient according to response to therapy (Table 1).

TABLE 1.

Average number of glaucoma drops administered per day, pre‐ and post‐operatively.

Time point	Pre‐op	1 month post‐op	3 months post‐op	6 months post‐op	12 months post‐op
Average number of glaucoma drops per day	4.8	2.7	3.0	2.6	3.0

Postoperative success of IOP control was defined primarily as achieving an IOP of 25 mmHg or less, with a secondary success rate calculated using a threshold of 30 mmHg or less [2]. Patients were considered visual if they had a positive menace response. If an eye was enucleated due to poor IOP control and/or other complications, this was considered an unsuccessful outcome at each subsequent time point for statistical purposes. Patients who were lost to follow‐up were excluded from further statistical analysis. For the purposes of this study, short‐term outcomes were defined as those assessed within three months of surgery, medium‐term outcomes as those between three and twelve months, and long‐term outcomes as those beyond one year.

All statistical analyses were conducted in Genstat (version 22, VSN International, Hemel Hempstead, UK) and a p‐value of < 0.05 was considered significant. Two‐sample binomial tests for equal proportions were used to assess the difference between treatment methods across the two outcomes (visual and IOP control) at multiple time points (at diagnosis, 24 h, one month, three months, six months, and one year post surgery). Logistic regression was performed to assess the effect of pre‐operative vision status on post‐operative visual outcomes at 3 and 6 months, and odds ratios were obtained. The effect of sex (M/F), breed (Burmese/domestic), and age (< 10 years/> 10 years) on vision retention and IOP control at six and 12 months post‐surgery was assessed using logistic regression with an underlying binomial distribution.

3. Results

A total of 22 eyes from 15 cats met the inclusion criteria for the study. Age at glaucoma diagnosis was variable, ranging from two to 13 years (mean = eight years) (Table 2). Sex included nine female spayed and six male neutered cats. Four breeds were recorded, including Domestic Shorthair (n = 8), Burmese (n = 5), Tonkinese (n = 1), and Domestic Medium Hair (n = 1). All Burmese cats were diagnosed with primary open‐angle glaucoma (POAG).

TABLE 2.

A summary of each case, including follow‐up time and outcomes.

Number (Case)	Age (years)	Breed	Sex	Eye affected	Diagnosis	Surgery performed	Visual pre‐operatively?	Time of last follow‐up	IOP controlled at last follow‐up? (≤ 30 mmHg)	Visual at last follow‐up?	Pertinent information
1	3	DSH	FS	OD	Secondary glaucoma, Feline aqueous humor misdirection syndrome (FAHMS)	Phaco IOL + ECP	Yes	3 years	Yes	No	Lost vision 2 years post‐operatively, presumed glaucoma‐related retinal degeneration
2	12	DSH	FS	OS	Secondary glaucoma, anterior uveitis, immature cataract	ICLE + ECP	Yes	1.5 years	Yes	Yes	Repeat ECP performed at 3 months post‐operatively
3	10	Burmese	FS	OD	POAG	Phaco IOL + ECP (suture shunt placed 2 days post‐operatively)	Yes	3 years	Yes	Yes
3	10	Burmese	FS	OS	POAG	Phaco IOL + ECP (suture shunt placed 2 days post‐operatively)	Yes	3 years	Yes	Yes
4	7	DMH	MN	OS	Secondary glaucoma, FAHMS	Phaco + ECP + vitrectomy + suture shunt	No	3 years	Yes	No	Glaucoma‐ related vision loss prior to surgery; never regained vision
4	7	DMH	MN	OD	Secondary glaucoma, FAHMS	Phaco + ECP + suture shunt*	No	3 years	Yes	No	Glaucoma‐ related vision loss prior to ECP surgery; never regained vision Pars plana vitrectomy performed 1 month before ECP.
5	10	Burmese	FS	OD	POAG	Phaco IOL + ECP	Yes	1 year	Yes	Yes
5	10	Burmese	FS	OS	POAG	Phaco IOL + ECP	Yes	1 year	Yes	Yes
6	13	DSH	FS	OD	Secondary glaucoma, FAHMS	Phaco + ECP	No	6 months	Yes	Yes
6	13	DSH	FS	OS	Secondary glaucoma, FAHMS	Phaco + ECP	Yes	6 months	Yes	Yes
7	7	DSH	MN	OD	Secondary glaucoma, lens subluxation	ICLE + ECP	Yes	3 years	Yes	Yes
7	7	DSH	MN	OS	Secondary glaucoma, lens subluxation, chorioretinitis, anterior uveitis	ICLE + ECP	No	3 years	Yes	No	Regained vision, then lost vision 1 year post‐operatively due to retinal degeneration related to previous chorioretinitis and glaucoma‐related injury.
8	5	DSH	FS	OD	Primary glaucoma	Phaco + ECP	Yes	9 months	Yes	Yes
9	13	DSH	FS	OS	Secondary glaucoma, FAHMS	Phaco/capsulectomy + vitrectomy + ECP	Yes	3 months	Yes	No	Blindness due to suspected hypertensive retinopathy
10	10	Burmese	FS	OD	POAG	Phaco IOL + ECP	Yes	3 years	No	No	Lost vision between 1 and 2 years post‐operatively, presumed glaucoma‐related retinal degeneration
10	10	Burmese	FS	OS	POAG	Phaco + ECP	No	3 years	No	No	Glaucoma‐ related vision loss prior to surgery; never regained vision
11	3	Burmese	MN	OD	POAG	Phaco IOL + ECP (suture shunt placed 1 day post‐operatively)	Yes	3 years	Yes	Yes
11	3	Burmese	MN	OS	POAG	Phaco IOL + ECP (suture shunt placed 1 day post‐operatively)	Yes	3 years	Yes	Yes
12	12	Burmese	FS	OD	POAG (confirmed on histopathology)	Phaco IOL + ECP	No	3 months	No	No	Enucleated at 3 months due to infected corneal ulcer. Histopathology did not indicate chronic uveitis or uveitis‐related changes for surgical failure
13	8	Tonkineese	MN	OD	Secondary glaucoma, anterior uveitis, mature cataract	ICLE + ECP	No (cataract)	6 months	Yes	Yes
14	6	DSH	MN	OD	Secondary glaucoma following phacoemulsification (suspected Toxic Anterior Segment Syndrome)	IOL removal + posterior CCC + ECP (phaco 48 h prior)	Yes	1 year	Yes	Yes
15	2	DSH	MN	OD	Primary glaucoma, anterior segment dysgenesis	Phaco + ECP	Yes	6 months	Yes	Yes

Note: * identifies information related to the “surgery performed” section.

Eleven eyes were diagnosed with primary glaucoma, and 11 eyes were diagnosed with secondary glaucoma. Causes of secondary glaucoma included aqueous humor misdirection syndrome (n = 6), chronic lens‐induced uveitis (n = 2), lens subluxation and chronic anterior uveitis (n = 2), and toxic anterior segment syndrome (TASS) following cataract surgery (n = 1). One case had a repeat ECP procedure performed three months following the first ECP procedure.

Success rate for IOP control at 25 mmHg or below was 50% at 24 h (n = 11/22), 86% at one month (n = 19/22), 82% at three months (n = 18/22), 80% at six months (n = 16/20), and 71% at one year (n = 12/17) (Table 3). Success rate for IOP control at 30 mmHg or below was 82% at 24 h (n = 18/22), 91% at one month (n = 20/22), 86% at three months (n = 19/22), 90% at six months (n = 18/20), and 94% at one year (n = 16/17). When comparing cases that received phacoemulsification with IOL placement (n = 9) vs. without IOL placement (n = 8), there was no significant difference in IOP control (≤ 30 mmHg) at three months (8/9 vs. 8/8; p = 0.33), six months (7/9 vs. 6/6; p = 0.22), or one year (8/9 vs. 3/3; p = 0.55). When comparing cases that received phacoemulsification (n = 17) vs. intracapsular lens extraction (ICLE) (n = 4), cases that received ICLE were significantly less likely to have IOP control at three months (16/17 vs. 2/4; p = 0.02), but there was no difference in IOP control at six months (13/15 vs. 4/4; p = 0.38) or one year (11/12 vs. 3/3; p = 0.1). There was no significant difference in IOP control between cases that received a vitrectomy during surgery (n = 3) vs. those that did not (n = 19) at any time point (three months, 15/19 vs. 3/3; p = 0.4; six months, 16/18 vs. 2/2; p = 0.2; one year, 12/13 vs. 3/3; p = 0.33). There was no effect of glaucoma diagnosis (primary glaucoma, n = 11; secondary glaucoma, n = 11) on IOP control at three months (p = 0.65), six months (p = 0.08), or one year (p = 0.3). There was no effect of pre‐operative IOP control (IOP ≤ 25 mmHg, n = 3; IOP > 25 mmHg, n = 19) on IOP control at three months (p = 0.33), six months (p = 0.76), or one year (p = 0.13). There was no significant effect of Sex (M/F), Breed (Burmese/Domestic), and Age (< 10 years/> 10 years) on IOP control at six months or one year (p ≥ 0.06).

TABLE 3.

Success rates for IOP control and vision across various time points.

Success rates	24 h	1 month	3 months	6 months	1 year	2 years	3 years
IOP Control (≤ 25 mmHg)	50% (11/22)	86% (19/22)	82% (18/22)	80% (16/20)	71% (12/17)	75% (9/12)	75% (9/12)
IOP Control (≤ 30 mmHg)	82% (18/22)	91% (20/22)	86% (19/22)	90% (18/20)	94% (16/17)	92% (11/12)	75% (9/12)
Vision	55% (12/22)	77% (17/22)	77% (17/22)	80% (16/20)	71% (12/17)	42% (5/12)	42% (5/12)

Success rate for vision was 55% at 24 h (n = 12/22), 77% at one month (n = 17/22), 77% at three months (n = 17/22), 80% at six months (n = 16/20), and 71% at one year (n = 12/17). Cases that received phacoemulsification with IOL implantation (n = 9) were significantly more likely to retain vision at one year, compared to cases that received phacoemulsification without IOL implantation (n = 3) (8/9 vs. 0/3; p = 0.005), however there was no significant difference at three months (8/9 vs. 3/8; p = 0.08) or six months (8/9 vs. 3/6; p = 0.1). When comparing cases that received phacoemulsification (n = 17) vs. ICLE (n = 4), there was no significant difference in visual status at any time point (three months, 12/17 vs. 4/4, p = 0.21; six months, 11/15 vs. 4/4, p = 0.24; or one year, 8/12 vs. 2/3, p = 1). All cases that received an anterior vitrectomy were diagnosed with feline aqueous humor misdirection syndrome (FAHMS). Cases that received an anterior vitrectomy (n = 3) were significantly more likely to be non‐visual than cases that did not (n = 19) at three months (0/3 vs. 17/19; p < 0.001), six months (0/2 vs. 16/18; p = 0.003) and one year (0/2 vs. 10/13; p = 0.03). There was no effect of glaucoma diagnosis (primary glaucoma, n = 11; secondary glaucoma, n = 11) on post‐operative visual status at three months (p = 0.78), six months (p = 1), or one year (p = 0.27). There was no effect of pre‐operative IOP control (IOP ≤ 25 mmHg, n = 3; IOP > 25 mmHg, n = 19) on post‐operative visual status at three months (p = 0.33), six months (p = 0.11), or one year (p = 0.13). There was no significant effect of Sex (M/F), Breed (Burmese/Domestic), and Age (< 10 years/> 10 years) on vision retention at six months or one year (p ≥ 0.2).

Seven out of 22 eyes were non‐visual prior to surgery. Three out of seven of these eyes (43%) regained vision within one month post‐operatively, two of which were visual at the time of their last follow‐up at six and 12 months post‐operatively, and one of which lost vision between six months and one year post‐operatively. Of the 15 eyes that were visual pre‐operatively, vision retention was achieved in 93% at one month (n = 14/15), 93% at three months (n = 14/15), 100% at six months (n = 13/13), and 100% at one year (n = 11/11). Eyes that were visual pre‐operatively were significantly more likely to be visual post‐operatively at three (p = 0.01; 93% vs. 43%) and six months (p < 0.001; 100% vs. 57%).

Table one outlines the average number of glaucoma drops that were administered daily pre‐ and post‐operatively. Between pre‐operatively and six months post‐operatively, the number of glaucoma drop administrations per day reduced by an average of 2.2 drops. Prostaglandin analogues (PA) (latanoprost 0.005% or travoprost 0.004%) were used variably in the pre‐ and post‐operative periods. Six eyes never required a PA; five eyes were taken off a PA in the post‐operative period; four eyes that were on a PA pre‐operatively were tapered to a lower frequency post‐operatively, while four eyes stayed at the same frequency. Only two eyes were started on a PA in the post‐operative period that were not previously on one.

Four eyes experienced intraoperative complications, which included an iris tear (n = 1/22), focal iris hemorrhage (n = 1/22), posterior lens capsule tear during phacoemulsification with lens material lost in the anterior vitreous (n = 1/22), and iris prolapse through the corneal incision with subsequent focal hemorrhage (n = 1/22). Twelve eyes experienced post‐operative complications, including corneal ulceration (n = 8/22), post‐operative hypertension (n = 4/22), focal bullous retinal detachment which spontaneously resolved (n = 2/22), uveitis development two years post‐operatively (n = 3/22), and fibrinous uveitis necessitating TPA injection (n = 2/22). Three of the eight eyes that developed a corneal ulcer did so within two weeks after surgery, and the remaining five eyes did so between two weeks and eight months after surgery. One case that developed a corneal ulcer did so at three months post‐operatively, which subsequently became infected and resulted in enucleation. Focal bullous retinal detachment was observed in two eyes from the same cat and was presumed to be related to the surgery, as no other ophthalmic or systemic disease was identified. One case lost vision two weeks post‐operatively and was diagnosed with systemic hypertension and suspected hypertensive retinopathy, which was poorly responsive to medical therapy. Three cats (five eyes) died within the longer follow‐up period, of which all eyes had controlled pressures (≤ 30 mmHg) and four eyes remained visual at the last recorded follow‐up. Three cats (four eyes) were lost to follow‐up in the longer follow‐up period. Three of four eyes were visual with controlled IOPs, and the fourth eye was non‐visual with controlled IOP at the last recheck appointment.

4. Discussion

Endoscopic cyclophotocoagulation was found to be effective for short‐ to medium‐term vision preservation and long‐term control of IOP in cats with glaucoma, with success rates similar to those reported in dogs [15]. Our study reported IOP control (≤ 30 mmHg) in 94% of cats and vision in 6% of cats at one year, compared to 90% IOP control (≤ 25 mmHg) and 63% vision retention in dogs at one year post ECP [25]. Furthermore, signalment factors, type of glaucoma, and pre‐operative pressure control did not influence the likelihood of pressure control or vision retention at any time point during the follow‐up period.

While glaucoma is conventionally defined as an intraocular pressure exceeding 25mmHg [1, 2, 26], 15 out of 22 eyes in the present study were found to present with pressures slightly above this threshold (26 to 33 mmHg) at one or more postoperative recheck examinations. Despite these elevations, all affected cats consistently showed no signs of ocular discomfort and were reported to be comfortable and behaving normally at home. Vision was also maintained in the majority of these patients (12/15 eyes at one month; 12/15 eyes at three months; 11/14 eyes at six months; 9/12 eyes at one year), even when IOP measurements exceeded 25 mmHg. A notable increase in success rate was noted when the definition of pressure control was defined more leniently as an IOP ≤ 30 mmHg. Given that many patients remained comfortable and retained vision despite a pressure slightly over 25 mmHg, the authors considered this adjusted criterion to more accurately reflect the clinical success rate of the procedure. In a comprehensive review of feline glaucoma, the authors state that the feline eye is remarkably resilient in terms of susceptibility to glaucomatous damage, less commonly displaying observable clinical signs (corneal edema, Haab's stria, pan‐retinal degeneration) than their canine counterparts, and maintaining functional vision despite appreciable buphthalmos in some cases [2]. An early case series describing a form of primary glaucoma seen in Burmese cats reported findings consistent with these statements, including a lack of corneal edema, delayed or absent retinal degeneration, and maintenance of vision in affected eyes for two to four years when appropriately managed [6]. The authors reported that surgical glaucoma therapy was frequently required to regain normotension in eyes in which IOP was greater than 30 mmHg, demonstrating this value as an appropriate threshold.

Success rate for vision was achieved in 71%–80% of cases overall between three months and one year. Retention of vision in eyes that were visual prior to surgery was achieved in 93%–100% of eyes between three months and one year, and return of vision was achieved in 43% of non‐visual eyes within the follow‐up period. These findings suggest that early surgical intervention before vision compromise increases the likelihood of vision retention post‐operatively.

Individuals who received a vitrectomy were significantly less likely to be visual at any time point. This finding should be interpreted with caution due to the low number of eyes that received a vitrectomy during surgery (n = 3). One eye was visual prior to surgery, and all three eyes were non‐visual from one month post‐operatively. While none of these eyes had a documented retinal detachment, all eyes had focal areas of tapetal hyperreflectivity noted on fundic exam. The definitive cause of vision loss was determined to be related to glaucoma in two eyes, and was not identified in the third eye but was presumed to be due to retinal degeneration. While it is known that the vitreous plays an important role in the maintenance of retinal health [27], the role of vitreous degeneration (age‐related or surgical via vitrectomy) on retinal pathology is still an emerging topic in both human and veterinary literature. The link between vitrectomy and retinal degeneration in these cases is unknown, and a larger cohort study is required to further elucidate these findings.

Lensectomy was performed in all cases in the present study. Adjunctive lensectomy is commonly performed in humans with glaucoma to improve patient outcomes, although the exact mechanism of reducing IOP is not fully known [28]. There is evidence to support that lens removal alone has some IOP‐lowering effects in human patients with POAG and primary angle‐closure glaucoma [28, 29, 30, 31, 32], which has also been suggested to be true in dogs [15]. It is the authors' opinion that lensectomy is necessary in the cat to achieve adequate access to the ciliary processes for treatment, due to the relatively large size of the feline lens. The possibility of post‐operative cataract formation secondary to physical or thermal injury to the lens is also present, and thus adjunctive lensectomy at the time of ECP is considered to reduce the need for surgery in the future.

Eyes that received an IOL implant were more likely to be visual at 1 year post‐operatively, compared to those that did not. This may reflect cases receiving IOL implants being better surgical candidates to start with. Placement of an IOL likely results in better visual acuity compared to an eye that remains aphakic. However, aphakic patients should still maintain a menace response despite ametropia, thus making this unlikely as a sole explanation for this finding. Investigating the benefits of lensectomy and IOL implantation in cats receiving ECP should be an area of future investigation.

Topical medications were variable in the pre‐ and post‐operative periods. While the overall number of topical medications (number of drops per day) in the immediate post‐operative period was greater with the addition of antibiotics and anti‐inflammatories, the average number of glaucoma drops per day was reduced at every time point post‐operatively. Topical antibiotics were discontinued two to three weeks post‐operatively once the corneal incision was considered healed, and topical anti‐inflammatories were weaned and discontinued in every case over a three‐to‐six‐month period. Glaucoma medications were overall reduced but were continued indefinitely at some frequency post‐operatively in all but one case (due to an inability to medicate). Prior to surgery, owners should be counseled that ECP will most likely not eliminate the need for glaucoma medications in the future.

Intra‐operative complication rate was low (n = 4). While the lens capsule tear required an anterior vitrectomy to retrieve lost lens material, other complications were considered minor and unlikely to affect the outcome of the surgery. Corneal ulceration was the most common post‐operative complication, occurring in eight out of 22 eyes (36%). Six of these cases were on topical steroids post‐operatively; however, ten of 14 patients who did not develop a corneal ulcer in the post‐operative period were also on topical steroids. Based on this, there does not appear to be an increased risk of corneal ulcer development with topical steroid use in this case series. Proposed mechanisms of corneal ulcer development include stress‐induced flare‐up of Feline Herpesvirus (FHV‐1), excessive corneal exposure during surgery, secondary to anesthesia‐induced decrease in tear production, or due to corneal hypoesthesia and neurotrophic keratopathy secondary to damage to the long ciliary nerves that innervate the cornea, as described in dogs following transscleral cyclophotocoagulation [33, 34]. The authors speculate that stress‐induced flare‐up of FHV‐1 is the most likely cause of the majority of corneal ulcers in this case series, likely related to frequent medication events and regular follow‐up visits in a clinic setting. Sosnowik et al. also reported corneal ulceration as the most common observed complication in 28% of dogs following ECP. They speculated that corneal hypoesthesia, either secondary to brachycephaly or laser‐induced damage to the long ciliary nerves, and their post‐operative tonometry protocol using 0.5% topical proparacaine hydrochloride and applanation tonometry for hourly IOP readings were the most likely contributors. While applanation tonometry was not used in this case series, corneal hypoesthesia cannot be ruled out as a contributor to corneal ulceration. Seven out of eight eyes with corneal ulcers were successfully managed with medical therapy, while one became infected and was eventually enucleated. Only one eye in this case series was enucleated during the follow‐up period, primarily due to a complicated corneal ulcer. This eye also had an uncontrolled IOP at one and three months (30–35 mmHg), was not visual prior to surgery, and did not regain vision at any time point; thus, enucleation was elected to achieve patient comfort.

Limitations of this study include those associated with its retrospective nature and small sample size. Surgical factors and post‐op care were not standardized, which may have contributed to different outcomes. As with any retrospective case series, data and case numbers were reliant on detailed clinical records and client compliance, which inevitably led to lower case numbers. Four cases were lost to follow‐up after three months, and while these all had controlled IOPs and three out of four eyes were visual at their last follow‐up appointment, the long‐term outcome of these cases is unknown.

In conclusion, endoscopic cyclophotocoagulation combined with lensectomy appears to be a safe and successful procedure for short‐ to medium‐term vision preservation and long‐term control of IOP in cats with glaucoma. Importantly, signalment factors, type of glaucoma, and pre‐operative pressure control do not seem to impact success rates, suggesting this procedure may have widespread application in cats with glaucoma. Further prospective or large cohort retrospective studies would likely provide more insight into this treatment in cats.

Author Contributions

Emily N. Perry: conceptualization, investigation, writing – original draft, methodology, writing – review and editing, data curation. Mayara Galetti: data curation. Kelly Caruso: writing – review and editing, resources. Evelyn Hall: formal analysis, investigation. Cameron Whittaker: writing – review and editing, resources. Paul M. G. McCarthy: writing – review and editing, methodology. Cassandra Bliss: conceptualization, writing – review and editing, supervision, resources, investigation, methodology. William M. Irving: writing – review and editing.

Disclosure

The authors have not used AI to generate any part of the manuscript.

Ethics Statement

This study complies with the Guidelines for Ethical Research in Veterinary Ophthalmology (GERVO) and is exempt from approval by an ethics committee. Animal owners or owners' representatives provided written consent for the treatment provided.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.