Update (July 5, 2026): Fellow-Eye Prophylaxis in Canine Primary Angle-Closure Glaucoma — 70.9% Convert Within 5 Years (Donohue 2025)

Bottom line

• In a 2025 multicenter retrospective cohort (117 dogs), 70.9% of dogs with unilateral primary angle-closure glaucoma (PACG) developed glaucoma in the fellow eye within 5 years, even with prophylaxis. [1]
• Median time to prophylactic-treatment failure was 2.15 years, and no drug class was superior: topical carbonic anhydrase inhibitors (CAIs) and beta-blockers performed equivalently, and adding a topical corticosteroid did not change time to failure. [1]
• This reframes fellow-eye therapy as delay, not prevention — set owner expectations for near-certain contralateral disease and schedule lifelong monitoring rather than promising protection.

Clinical facts

• PACG is a bilateral disease with asynchronous onset. Unilateral presentation is a snapshot in time; the predisposing goniodysgenesis and narrow drainage angle are present in both eyes. Prophylaxis buys vision-time in the second eye but has never been shown to prevent conversion. [1][2]
• Historic benchmark for benefit. The foundational prophylaxis trial (Miller et al., 2000) reported median time to fellow-eye glaucoma of ~8 months untreated versus ~31 months on demecarium bromide plus corticosteroid or on betaxolol — the origin of the "~2 extra years of vision" figure often quoted to owners. [2]
• Signalment. Predisposed breeds in the 2025 cohort included the Cocker Spaniel, Basset Hound, and Shih Tzu; mean age at diagnosis 8.3 years, 69.2% female. [1]
• A laterality signal, not a mechanism. Dogs first diagnosed in the left eye had a 1.8-fold higher risk of failure in the right eye — an association the authors flag as hypothesis-generating, not a treatment target. [1]
• What clinicians actually reach for. Aqueous-suppressant topicals (dorzolamide, timolol, or the fixed combination) dominate current practice; concurrent anti-inflammatory and neuroprotectant use is inconsistent. [3]

Have a fellow-eye case in front of you? Ask Voyage ForVets about this evidence.

What the evidence shows

Anchor study — Donohue, Bentley, Boush, et al., Veterinary Ophthalmology (2025; published 2026, 29(1):e70038). [1] This multicenter retrospective study followed 117 dogs presenting with unilateral PACG, recording patient factors, examination findings, prophylactic regimens, and the date of fellow-eye conversion (defined as IOP > 25 mmHg), with death handled as a competing risk. Two numbers anchor the takeaway: 70.9% of dogs converted in the contralateral eye within 5 years, and the median time to treatment failure was 2.15 years. Critically, time to failure did not differ between dogs on prophylactic topical CAIs versus beta-blockers, nor between dogs that did or did not receive a concurrent topical corticosteroid. The authors' conclusion is unambiguous: no prophylactic regimen was superior to another, and they call for prospective work on complementary strategies. The one patient-level association reaching significance was laterality — a left-eye index diagnosis carried a 1.8-fold higher risk of right-eye failure.

Supporting — Miller, Schmidt, Vainisi, et al., JAAHA (2000; 36(5):431–438). [2] The classic open-label multicenter trial established that prophylaxis delays conversion: untreated controls reached fellow-eye glaucoma at a median of ~8 months, versus ~31 months for the demecarium-bromide/corticosteroid arm and ~30.7 months for the 0.5% betaxolol arm, with untreated eyes failing significantly sooner (p < 0.001). This is the historical evidence base most practitioners were trained on — and it already showed comparable efficacy across drug classes, foreshadowing the 2025 finding that class choice does not drive outcome.

Context — Komáromy, Bras, Esson, et al., Veterinary Ophthalmology (2019; 22(5):726–740). [4] This state-of-the-field review situates prophylaxis within canine glaucoma's core problem: the disease is progressive and the durability of any medical benefit is limited. It underscores that poor adherence to topical dosing is a major contributor to progression — a modifiable variable that matters more when the drug itself offers only delay — and frames prophylaxis as one component of a management strategy that ultimately trends toward surgical or end-stage options in many patients. Current practice patterns (Plummer et al.) reinforce that aqueous suppressants are the mainstay while anti-inflammatory and neuroprotectant adjuncts remain inconsistently applied. [3]

How this fits clinical practice

The 2025 data do not change what you prescribe so much as how you frame it. Three practical shifts:

1. Recalibrate the owner conversation. With ~71% five-year conversion and a ~2-year median to failure, prophylaxis is realistically vision-time, not vision-insurance. Counsel owners that fellow-eye glaucoma is likely and that the goal is to buy comfortable, sighted months while preserving the option to intervene early.
2. Don't chase a "better" drug class. Since CAIs and beta-blockers were equivalent and corticosteroids added no measurable delay, drug selection can be driven by tolerability, comorbidity (e.g., cardiac disease with beta-blockers), dosing burden, and adherence rather than a presumed potency advantage. [1]
3. Make monitoring the intervention. Because conversion is expected and often the failure point, structured re-checks — tonometry on a defined cadence and clear owner instructions for acute pain/redness — are where outcomes are won. Adherence support is a first-class part of the plan, not an afterthought. [4]

This is a clinical evidence summary, not a treatment protocol; individual case management should follow a full ophthalmic examination and current specialist guidance.

Want to pressure-test a fellow-eye plan against this cohort? Bring the case to Voyage ForVets.

References

1. Donohue LK, Bentley E, Boush JJ, Lasarev MR, Pumphrey SA, Maggio F, Yang VY. Factors Influencing the Incidence and Onset of Primary Angle-Closure Glaucoma in the Unaffected Eye of Dogs. Veterinary Ophthalmology. 2025 (published 2026);29(1):e70038. https://pubmed.ncbi.nlm.nih.gov/40485308/
2. Miller PE, Schmidt GM, Vainisi SJ, Swanson JF, Herrmann MK. The efficacy of topical prophylactic antiglaucoma therapy in primary closed-angle glaucoma in dogs: a multicenter clinical trial. Journal of the American Animal Hospital Association. 2000;36(5):431–438. https://pubmed.ncbi.nlm.nih.gov/10997520/
3. Plummer CE, et al. Prophylactic anti-glaucoma therapy in dogs with primary glaucoma: A practitioner survey of current medical protocols. Veterinary Ophthalmology. 2021. https://onlinelibrary.wiley.com/doi/10.1111/vop.12820
4. Komáromy AM, Bras D, Esson DW, et al. The future of canine glaucoma therapy. Veterinary Ophthalmology. 2019;22(5):726–740. https://pmc.ncbi.nlm.nih.gov/articles/PMC6744300/

Changelog

• 2026-07-05: First published.