How to Use the Clinical Examination to Determine the Significance of Abnormalities of the Horse Cornea and Adnexa

1. Introduction

The normal tear film is continuous over the entire surface of the horse cornea and conjunctiva. The tear film “breaks up” from instability and discontinuity of the ocular surface if the cornea is scarred, infected or edematous, and if blinking does not occur often enough.^1,2

Precorneal Tear Film

The precorneal tear film is a trilayered mucin dominated gel produced by the meibomian glands (outer oily layer), the lacrimal and nictitans glands (aqueous layer), and the conjunctival goblet cells (inner mucin layer). The tear film serves as an extracellular matrix to the cornea, which provides an optically smooth surface and aids nutrition to the cornea. It contains vitamin A, proteinases, proteinase inhibitors, growth factors, and cytokines that affect corneal health. The tear film is replaced every 7 min in the horse!³

Third Eyelid

The nictitans or third eyelid (TE) of the horse is located at the medial canthus. It displays rapid, near horizontal movement as it protects the cornea and distributes the tear film. The TE conjunctiva may be completely pigmented or may contain no melanin. A large gland produces part of the tear film and is found at the base of a T-shaped piece of supportive TE cartilage. 

Conjunctiva

Conjunctiva is a mucous membrane that is pigmented near the limbus in some horses. The conjunctival epithelium covering the globe (bulbar conjunctiva) begins at the corneoscleral junction or limbus, lies superficial to the sclera, and reflects forward at the fornix where it becomes palpebral conjunctival epithelium. The palpebral conjunctival epithelium lines the inner eyelid surface and continues to the mucocutaneous junction at the lid margin. The ventromedial portion of the palpebral conjunctiva covers the nictitating membrane. Components of the ocular immune system and tear producing cells are present in the conjunctiva.

2. The Clinical Examination: Observations

The Cornea Looks "Dry"

Keratoconjunctivitis Sicca

“Dry eye” or keratoconjunctivitis sicca (KCS) is an abnormality of the tear film and may be commonly due to qualitative changes in the tear film or, rarely, due to reduced quantity of tears in the horse. Environmental, immunologic, traumatic, and infectious causes of KCS are reported. Mild to severe ulcerative keratitis and conjunctivitis can be present. The corneal surface is not shiny in KCS (Fig. 1). Treatment with a fly mask, topical serum, and cyclosporine A can be beneficial. This may not be a permanent condition.
 

The Conjunctiva is “Red”

Conjunctivitis

Because the eye has limited ways to react to injury, the conjunctiva gets red (Fig. 2) or becomes inflamed in nearly all types of eye disease. Conjunctivitis is a nonspecific finding that indicates ocular inflammation, but may also be seen in systemic diseases. Infectious and noninfectious diseases of the eyelids, cornea, sclera, anterior uvea, nasolacrimal system, and orbit can result in conjunctivitis in the horse.

Habronemiasis is a parasitic disease resulting in conjunctival and ocular granulomas. Thelazia lacrymalis is a commensal parasite of the conjunctival fornices and nasolacrimal ducts of horses, which can incite conjunctivitis, superficial keratitis, dacryocystitis, and mild eyelid swelling.
 

Foal conjunctivitis is associated with neonatal maladjustment syndrome, septicemia, immune-mediated hemolytic anemia, environmental allergens and irritants, dermoids, and subconjunctival or episcleral hemorrhages secondary to birth trauma.

Allergic conjunctivitis due to exposure to environmental antigens is treated with topical corticosteroids and is often difficult to eliminate completely due to the nature of the horse’s environment.

Conjunctival neoplasia may be focal or diffuse and includes squamous cell carcinoma, hemangiosarcoma, and lymphoma. Tumors of the conjunctiva may resemble conjunctivitis in the early stages. The eyelid, globe conjunctiva, and third eyelid conjunctiva are susceptible to neoplasia.

3. Cornea

The horse cornea should normally be clear, smooth and shiny.

The Cornea is “Red”

Vascularization of the cornea will result in the entire or focal regions of the cornea being red (Fig. 3). The position of the redness can indicate the location of inflammation. The higher the intensity of redness, the more the incitatory activity there is for the neovascularization at the corneal site.

The Cornea is Hazy or “Blue”

Edema causes a blue color of the cornea if severe and more subtle haziness if the edema is slight (Figs. 3 and 4). Severe edema can result in bullae or “bubbles” in the epithelium if very severe. Corneal edema may surround a focal ulcer, foreign body or abscess, or involve the entire cornea in uveitis and glaucoma.
 

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