Expert Q & A

Managing equine eye conditions

By Adam Marcus

Eye conditions are among the most common and challenging conditions for equine veterinarians to manage. From complex abscesses to chronic uveitis, choosing the right intervention can mean the difference between success and permanent vision loss. For insights into a few of these challenges, Modern Equine Vet spoke with Amanda Davis, DVM, DACVO, an expert in equine eye care and an assistant clinical professor in the Department of Veterinary Clinical Sciences at The Ohio State University College of Veterinary Medicine, in Columbus. This interview has been lightly edited for length and clarity.

Complicated corneal ulcers
MEV: What’s the surgical threshold when managing a deep stromal ulcer?

Dr. Davis: If the ulcer is extremely deep and at risk for perforating and/or not responding appropriately to aggressive medical management, surgery would be recommended. However, a lot of factors may play into this decision, including the size of the ulcer, how difficult the horse is to treat, and whether the animal is a good surgical candidate from an anesthetic standpoint.

MEV: What specific clinical markers tell you that medical management has failed and a conjunctival graft or corneal transplant is no longer an option?

Dr. Davis: If the ulcer continues to deepen and/or appears infected despite our medical management, and the horse remains in pain with significant signs of inflammation, this may indicate that our treatment is not working or that the infection is more severe and difficult to control. If the eye were to perforate despite our best efforts, it may still be a candidate for surgery. However, the decision depends on many factors, but mainly whether trauma has affected the remaining structures of the eye. For example, are the retina and optic nerve affected? We can tell this from the response to certain light reflexes, such as the dazzle reflex and the consensual pupillary light reflex. If the eye is damaged to the point at which it will not regain vision despite surgical repair, we likely would not move forward with graft placement.

MEV: Do you have any “red alert” signs in the first 48 hours following a surgical repair of a perforated ulcer?

Dr. Davis: For any graft procedure—either for a deep ulcer or perforated ulcer—we look for a few things in the first 48 hours. The graft should still be in place, and if there is an overlying conjunctival graft, it should be pink, indicating an active blood supply. Additionally, the eye should not be actively leaking, the anterior chamber should be well-formed, and the inflammation within the eye should be improving. If there is leakage, severe inflammation, or the graft has pulled away or appears pale, these may be signs that the graft will not do well in the long term.

Photodynamic therapy
MEV: Where do you see photodynamic therapy (PDT) in the hierarchy of equine eye care?

Dr. Davis: PDT is a good option for horses with squamous cell carcinoma of the eyelid. This condition is difficult to manage because you need to remove a significant amount of eyelid tissue to eliminate the microscopic disease. However, by doing this, we would be left with a nonfunctional eyelid, which is damaging to the globe itself. With PDT, we can remove part of the tumor while salvaging the eyelid and then try to kill the remaining cancer cells by injecting a photoactive agent into the affected area and locally irradiating the area. Horses tend to develop significant swelling after the procedure, but otherwise can do quite well. This procedure can be performed standing, thus avoiding potential risks with general anesthesia. PDT has also reportedly been used to treat immune-mediated keratitis in horses, although I am not very familiar with it personally.

MEV: Is PDT a primary replacement for cryotherapy or strictly an adjunctive treatment for margins? Do you anticipate using it primarily for bulbar conjunctival squamous cell carcinoma (SCC) or for eyelid lesions as well?

Dr. Davis: Both cryotherapy and PDT are adjunctive treatment modalities for eyelid SCC and are usually utilized after debulking or surgical removal of as much of the tumor as is feasible without causing major distortion of the eyelid margin. We use PDT for eyelid SCC, not for bulbar conjunctival SCC. Usually, bulbar corneoconjunctival SCC is treated via keratectomy followed by local beta irradiation or a topical chemotherapeutic such as mitomycin-C.

Uveitis
MEV: With the shift toward more surgical interventions, how do you decide between a suprachoroidal cyclosporine implant and low-dose intravitreal gentamicin injections for an animal with chronic uveitis?

Dr. Davis: Both treatment modalities are designed to reduce the number and severity of flare-ups. Low-dose intravitreal gentamicin injections are a newer option. These injections are easier to perform compared with cyclosporine implants and quicker compared with surgery.

Whether this approach works as well as cyclosporine implants long-term is a bit of an unknown, however, and there are reported adverse effects to consider, such as cataract formation or retinal degeneration that ultimately can be blinding.
Cyclosporine implants are a well-established, effective, and safe treatment option. However, the treatment can be more technically challenging because the implant must be inserted between the sclera and the choroid. The intervention is often performed under general anesthesia. Ultimately, we offer both options and review the pros and cons with owners to help them make an informed decision on what might be the best option for their horse, knowing all this information.

Standing sedation versus general anesthesia
MEV: Standing procedures may be better for systemic safety, but at what point does the risk of a sudden head movement outweigh the anesthetic risk during delicate intraocular maneuvers?

Dr. Davis: We try to ensure they are in a good plane of sedation and utilize a variety of local blocks around the eye, and a retrobulbar block if needed, based on the procedure being performed, to limit movement and sensation during the procedure. We do expect some subtle movement. Some procedures are more amenable to standing surgeries, such as enucleations, eyelid laceration repairs, third eyelid excision, episcleral cyclosporine implants for immune-mediated keratitis, and keratectomies for more superficial, well-localized corneoconjunctival cancers such as SCC. Some graft surgeries can be performed standing, but depending on the size and severity of the lesion, we may often opt to perform this under general anesthesia, as these require more delicate movements and precision to maximize success.

MEV: What is your favorite local anesthetic cocktail for a standing enucleation to ensure the horse remains completely desensitized throughout the procedure?

Dr. Davis: I rely on the help of our amazing anesthesia service to decide on the best cocktail, as this is very dependent on the horse’s temperament. I would say there is not a single cocktail used for every patient.

MEV: Are there specific ocular pathologies—perhaps deep stromal abscesses—where you feel the general anesthesia is non-negotiable compared with a standing approach?

Dr. Davis: Yes, an abscess that would likely require a more complicated corneal graft would be performed under general anesthesia to maximize visualization, reduce movement, and optimize chances of success. Most deep, complicated ulcers and corneal perforations also would likely require general anesthesia for graft placement. MeV