- Apr 1, 2002
This month we continue our in-depth look at the equine eye by focusing on eyelids. Without the eyelid to protect the sensitive structures of the eye, the horse would go blind very quickly. Therefore, even small injuries are important and require veterinary attention.
The functions of the eyelids are to protect the cornea from dust and injury, produce and distribute the tear film, pump tears into the tear drainage system, serve as a source of inflammatory cells in the event of corneal injury, and help control the amount of light entering the eye.
Anatomically, the eyelids are divided into the thin eyelid skin, the powerful orbicularis oculi muscle (that causes blinking), the tarsal plate, and the inner conjunctival layer. The upper eyelid joins the lower eyelid at the medial and lateral canthi (singular is canthus), which are the corners of the eye.
Horses have large eyelashes on their upper eyelid margin and none on the lower lid. Meibomian or tarsal glands, which secrete part of the tear film, are found at the eyelid margins. Long hairs or vibrissae provide sensation to the face near the eyes of horses. The eyelids have a very good blood supply.
Entropion is an inward rolling of the eyelid. It most commonly involves the lower lid. The entropion causes increased tearing, squinting, conjunctivitis or "pink eye," and corneal ulcers as hairs at the eyelid margin touch and irritate the cornea. Entropion can be a primary anatomical problem in foals due to the relatively small size of the foal eye compared to the lids, or can be secondary to dehydration and weight loss in sick foals. Eyelid scarring from trauma can result in entropion in foals and adult horses. The resulting eye pain might cause eyelid muscle spasm, worsening the entropion.
Permanent surgical correction of entropion in neonates is not recommended. The treatment in young foals is to temporarily evert the eyelid's margin with sutures for 14-21 days to prevent corneal damage as he grows. Concurrent eye problems such as blepharitis (eyelid inflammation, more on this later) and/or corneal disease should also be identified and treated. (For more on foal eye problems, see "Common Foal Eye Problems" in the March 2002 issue of The Horse, article Quick Find #3343 at www.TheHorse.com.)
Reconstructive entropion surgeries should be reserved for adult horses. The extent of required surgical correction must be determined with the animal awake. If the entropion has a large spastic component, it might be necessary to use topical corneal anesthetic to determine how much of the entropion is secondary to eye pain and how much is due to the anatomy of the eyelid.
Dermoids of the eyelids are benign tumors that contain hair, cartilage, and other tissues. These are usually reported in foals. Surgical removal of eyelid dermoids is indicated before the hairs grow enough to cause irritation of the eye.
Traumatic eyelid lacerations and forehead skin trauma can occur in horses. Lid lacerations might be accompanied by inflammation of the skin around the eye, skull fractures, and severe to minor corneal ulcers. Upper eyelid lesions are more serious than lower eyelid injuries, as upper eyelid movement distributes the tear film to prevent corneal ulcers. Surgical repair of the lacerated upper eyelid is imperative, as is preservation of the eyelid margins to prevent corneal ulcers.
Removal of lacerated pieces of eyelid marginal tissue should be avoided. The prominent blood supply to the eyelids generally allows for quick healing and acceptable, functional surgical repair. Wounds should be gently cleansed with disinfectant solution, systemic antibiotics administered, and warm compresses applied. Your veterinarian will use a two-layer closure of skin-orbicularis muscle and tarsal-conjunctival layers to minimize scarring, maximize healing, and return the horse to function quickly.
Large eyelid lacerations with loss of lid tissue require surgical repair using sophisticated skin grafting procedures and general anesthesia.
Squamous Cell Carcinoma
Squamous cell carcinoma (SCC) is the most common tumor of the eye in horses. The cause of SCC might be related to long-term exposure to the ultraviolet (UV) component of the sun's rays, lack of pigment around the eye, and/or a weakened immune system leading to increased susceptibility to cancer. The UV component is the most likely cancer-causing agent associated with SCC, as it targets the tumor suppressor gene p53 that is abnormal in equine SCC.
Horses older than 11 years of age are at risk. Belgians, Clydesdales, and other draft horses are most likely to develop SCC of the eye, followed by Appaloosas and paints, with the least chance of SCC occurring in Arabians due to their desert heritage. White, gray, and palomino hair colors predispose to SCC of the eye, with SCC less common in dark horses with bay, brown, and black hair coats.
SCC is diagnosed by biopsy. Ocular SCC must be differentiated from abscesses, scar tissue, other tumors, and parasites such as Habronema, Onchocerca, and Thelazia.
Treatment selection depends on tumor location, size, extent of invasion into surrounding tissues, vision status, the animal's use, therapy equipment available, and the owner's budget. Untreated ocular SCC can invade local soft tissues, the bony orbit, sinuses, and the brain. It can metastasize to regional lymph nodes, salivary glands, and the lungs. Tumor recurrence is highest in the eyelid or nictitans (third eyelid). Treatment results are best for smaller SCC.
Radiation, cryotherapy with liquid nitrogen or nitrous oxide, radiofrequency hyperthermia, carbon dioxide (CO2) laser ablation, or intralesional chemotherapy should follow surgical removal of equine ocular SCC. Treatment can include surgical resection alone if adequate surgical margins can be attained, although combinations of surgical debulking and adjunct therapy are best for a large SCC.
Immunotherapy with Bacillus Calmette-Guerin (BCG) cell wall extract has been used successfully for large SCC in horses. I have used chemotherapy with intralesional cisplatin for large eyelid and orbital SCC with effective results, with and without surgical debulking. Topical 5-fluorouracil or mitomycin C might be effective for superficial SCC surrounding the horse eye.
Sarcoids can be either solitary tumors or found in groups in the eyelids and surrounding regions of the same horse. They are generally found in horses less than seven years of age, and foals can be affected. Horses living in some parts of the world, such as in Europe, suffer from sarcoid types that are horribly aggressive and can be fatal. Viruses might be the cause of some sarcoids, with fly transmission involved in some cases.
Treatment includes surgical resection of the sarcoid tissue and immunotherapy with the immunostimulant BCG. Surgical debulking alone results in a high recurrence rate for sarcoids, but success rates can be improved if surgery is combined with BCG treatment.
Cryotherapy, hyperthermia (exceptionally high fever induced artificially for therapeutic purposes), CO2 laser ablation, radiation, and homeopathic therapy can also be effective for sarcoids.
Eyelid melanomas are found in gray horses (Arabians and Percherons are at increased risk). Melanomas can be single or multiple; treatment includes surgical excision and cryotherapy.
Blepharitis is inflammation of one or both eyelids. Trauma, bacteria, fungi, parasites, and insects can all cause inflammation of the eyelids of foals and adult horses. Hair loss, skin abrasions, lid swelling, and loss of skin pigmentation are associated with blepharitis in both foals and adult horses. Medications given orally or by injection might be necessary to treat blepharitis, and fly masks and fly repellants are among the defenses that can be employed to reduce eyelid inflammation from biting flies.
Habronemiasis is a common cause of ulcerated abscesses of the medial canthus of the eyelids, conjunctiva, and nictitans of horses. Non-healing, elevated, ulcerated abscesses with a thick, yellow, gritty exudate are found on these horses in the warm weather seasons, when large populations of house and stable flies serve as vectors. Topical therapy for solitary, focal lesions consists of intralesional corticosteroids. Large multifocal lesions should be treated with ivermectin.
The nematode parasite Thelazia lacrymalis can inhabit the conjunctiva and nasolacrimal drainage system of some horses. Most horses are asymptomatic, but signs of attack from this parasite can include mild lid swelling. There is no effective treatment.
If your horse has an eyelid problem, get a veterinary ophthalmalogist to evaluate him as soon as possible to preserve his valuable eyesight.
Editor's note: This is the fourth article in a series of equine eye articles by Dr. Brooks. See the first article, "Eye Anatomy and Physiology," article Quick Find #2797 at www.TheHorse.com for more information on eye anatomy.
About the Author
Dennis E. Brooks, DVM, PhD, Dipl. ACVO, is a professor of ophthalmology at the University of Florida. He has lectured extensively, nationally and internationally, in comparative ophthalmology and glaucoma, and has more than 140 refereed publications. He is a recognized authority on canine glaucoma, and infectious keratitis, corneal transplantation, and glaucoma of horses.