Editor's Note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 American Association of Equine Practitioners' Focus on Ophthalmology conference, held Sept. 6-8 in Raleigh, N.C.
Equine eye problems--ranging from corneal ulcers to uveitis--aren't uncommon, and many horse owners are familiar with the trouble they can cause. Less common, however, are disorders involving the horse's orbit and periorbita, the bone around the eye and the area around the orbit, respectively.
At the 2012 American Association of Equine Practitioners' Focus on Ophthalmology conference, held Sept. 6-8 in Raleigh, N.C., Brian C. Gilger, DVM, MS, Dipl. ACVO, professor of ophthalmology at the North Carolina State University College of Veterinary Medicine, reviewed some of the equine orbit and periorbita disorders practitioners might encounter in the field.
"Orbital disease may not be as common (as other eye problems), but it's a devastating problem," he began. "Not only is it vision threatening, it can leave long-term cosmetic damage as well."
Anatomy
"We don't necessarily need to remember the names of these bones, but it's important to understand the basic anatomy of the horse's skull," Gilger said.
The eye is surrounded by bone, and there is little protection for the delicate structure behind it, he explained. Additionally, the upper rim of the orbit protrudes further to the side than any other bone on the horse's skull, he said, which likely explains the fact that the horse sustains comparatively more orbital fractures than any other domestic animal. The periorbita consists of several sinuses and drainage ducts.
This combination of anatomical factors makes the horse's eye and surrounding structures vulnerable to blunt trauma injuries. Gilger explained: "If there's blunt trauma to the eye it's pushed back into the orbit, which is mostly hard bone. The eye bounces off the orbit, which causes problems. (Further,) the upper orbital rim often fractures off."
Identifying Orbital Disorders
Unless the horse has been injured in an accident or falling over backwards, Gilger said it can be difficult to identify exactly when and how an injury occurred. Therefore, it's important to be familiar with the clinical signs that can point to orbital disorders, including:
- Acute periorbital swelling;
- Chronic exophthalmos (the eye bulging out of the orbit);
- A prominent third eyelid;
- Emphysema;
- Bone deformation; and
- Swelling of the suborbital fossa (the temple-like structure located just above the eye).
Gilger said that when a horse presents with clinical signs consistent with an orbital problem, the veterinarian should perform a complete diagnostic examination to determine the exact problem. He recommends first ultrasounding the eye to determine if the globe appears normal.
"This is one of the most satisfying, easy diagnostic procedures you can do," he relayed.
In some cases, the veterinarian will ultrasound the other eye to compare the two structures, he said. Common problems identified in this stage of the exam include vitreal hemorrhage and retinal detachment, he said, adding that both are poor indicators for return to vision.
If the globe appears normal, Gilger recommended more advanced imaging--skull radiographs or the gold-standard, computed tomography (CT)--to try to identify the problem.
Following imaging, Gilger recommended veterinarians palpate the eye; he stressed that the horse needs to be sedated for this procedure, and a topical anesthetic should be used as well. Palpation, he said, is useful for locating fractures and replacing fragments to their appropriate positions ("Because these bones are non-weight bearing, they will often heal once replaced to proper position," he added). Palpation is not as useful for assessing sinus problems, he cautioned.
Once a diagnosis has been made, treatment can begin.
Specific Problems
Gilger touched on the treatment of several specific orbital and periorbital problems:
Orbit Fractures--Fracture treatment will depend on whether the surrounding skin is open or closed, he said. As mentioned, closed skin fractures typically heal well if the fragment is replaced to the proper position. In his experience, Gilger said the horses' eyes returned to normal function post-treatment.
When dealing with an open fracture, Gilger said that, just like with open wounds, the veterinarian should clean the defect well and consider minimal debridement before wiring the bone back into place.
"Depending on the extent of the contamination, some or all the wound is left open for adequate drainage, or drains are place to facilitate healing," he added.
Orbital Neoplasia--Fortunately, tumors on the orbit is a rare occurrence in horses, Gilger said. When they do develop, the most common types are squamous cell carcinoma, lymphosarcoma, and anaplastic sarcoma, he noted.
Orbital neoplasia is treated surgically, with or without adjunct therapy, he added. "Although surgical removal and salvage of the eye is possible early in the disease process, many cases require exenteration of the orbit," he said.
Nasolacrimal System Disorders
Finally, Gilger switched gears to "talk a little plumbing" and discussed some disorders of the nasolacromal duct (NLD, the duct that connects the eye to the nasal passage; essentially the tear duct).
He explained that signs of NLD problems are evidenced by several clinical signs, including:
- Ocular discharge;
- Nasal discharge;
- Canthus (where the upper and lower eyelids meet) swelling and discharge; and
- A lack of ocular pain.
NLD problem diagnoses typically begin with flourescein staining and watching for fluorescent nasal discharge, he said; no drainage means there is a blockage of some sort in the NDL.
In some mild cases, simply irrigating the NLD via a catheter can correct the problem, he said. In these cases, he recommended topical broad spectrum antibiotics and low-dose systemic non-steroidal anti-inflammatory drugs as adjunct treatments.
In the event irrigation isn't successful, he recommended locating the obstruction via a procedure called a dacryocystorhinography . Endoscopic exams or CT scans are useful if needed, he added.
If the NLD can't be opened with irrigation, it could mean that the entire duct is damaged, Gilger said. In these cases, he said a dacrystorhinotomy--a procedure that recreates the NLD--should be used.
NLD atresia (no opening in the lacrimal duct in the nose) is also a possibility. In these cases, Gilger said using electrocautery to make an opening in the nostril is the most effective option; however this condition is difficult to manage, he cautioned.
Take-Home Message
"Diseases of the equine orbit may not be as common as diseases in other parts of the equine eye, but the impact of orbital disease is very high," Gilger concluded. Staying vigilant and watching horses' eyes for signs of trouble can allow treatment to begin earlier, increasing the chances for a successful recovery.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.