The Dreaded Corneal Stromal Abscess

The corneal stromal abscess is a very serious and potentially vision-threatening condition that can follow apparently minor corneal ulceration in the horse (see Figures 1 and 4 on page 56). They can be infected and cause severe eye pain. Aggressive medical and surgical therapies have been developed for this frightening and dreaded disease in the horse.

First, a bit of background on eyes and diseases. The corneal stroma is sandwiched between the superficial corneal epithelium and the deep endothelium and comprises the majority of the corneal tissue thickness (see diagram below). An abscess is a localized collection of dead tissue and white blood cells.

If an eye becomes punctured (such as a small corneal epithelial puncture from a branch or other foreign object), an ulcer will develop. The epithelial cells around the ulcer will try to cover the wound, but in doing so trap bacteria, fungi, and/or foreign bodies in the stroma under the epithelium. The corneal epithelium then forms a barrier to drug penetration, which protects the bacteria or fungi from topically administered anti-bacterial and anti-fungal medications.

Growth factors and proteins released by microorganisms and the dead tissue in the abscess then cause the migration of white blood cells into the area, and inhibit blood vessels in the corneal lesion. These white blood cells (neutrophils) release enzymes into the stroma to cause further collagen degeneration and sustain the stimulus for more neutrophil invasion.

Diagnosis

It might be difficult to differentiate a chronic stromal abscess with secondary uveitis from other painful horse eye diseases. A history of previous trauma, evidence of corneal ulceration, a yellow/white stromal infiltrate, and the varying corneal position of stromal abscesses help make the distinction from a corneal ulcer, equine recurrent uveitis, or a corneal tumor.

Although melting and infected ulcers are also associated with severe anterior uveitis, in the horse, each is quite distinct in general appearance from a stromal abscess--ulcers invariably retain fluorescein stain over the majority of the lesion, while stromal abscesses only retain stain over a small area of the lesion in the acute stages, if at all.

The diagnosis of a stromal abscess is based on the presence of a focal, yellow-white, stromal opacity with various levels of corneal edema (swelling). Abscesses can be in the superficial or deep corneal stroma. Single or multiple abscesses can be present. A mild to severe anterior uveitis that causes eye pain and possibly blindness occurs secondary to what first appears to be a relatively benign corneal disease.

The majority of horses have initial clinical signs suggestive of minor corneal trauma. Fluorescein dye retention is either negative, or only positive over an area much smaller than the diameter of the corneal opacity.

Corneal blood vessel formation is rarely present in the early stages of corneal abscessation (see Figure 1 above), although stromal abscesses do not heal until blood vessels grow into them. In later stages, corneal blood vessels might stop at the edge of the abscess, suggesting that chemicals released from the abscess prevent blood vessel growth into the lesion.

Therefore, vascularization (increasing of the supply of blood vessels required for delivery of fluids and factors needed for healing, and removal of infection and waste products) of deep stromal abscesses can only occur near the abscess or to the abscess margin, but not in the deeper lesion itself. This is why surgical removal of deep abscesses might be necessary to eliminate pain, promote vascularization, and save the horse's vision.

Medical Therapy

Medical and surgical therapies for stromal abscesses in horses have been developed. Superficial stromal abscesses tend to be associated with bacterial infection and will respond to medical therapy with a lessening of the signs of painful anterior uveitis. Topical atropine, topical and systemic antibiotics (antibacterial and antifungal drugs), and non-steroidal anti-inflammatory drugs (NSAIDs) are administered initially. If significant improvement in the signs associated with a stromal abscess does not occur in the first 48-72 hours of aggressive medical therapy, surgery is recommended to reduce the pain and speed healing.

The use of systemic NSAIDs such as phenylbutazone (Bute) or flunixin meglumine (Banamine) should be carefully monitored to allow the control of anterior uveitis without significantly inhibiting the corneal vascularization necessary to heal corneal stromal abscesses.

Surgical removal of the abscess might be the only way to obtain a definitive etiologic diagnosis in order to institute proper antimicrobial therapy. Placing a conjunctival graft over a corneal transplant site rapidly restores the physical integrity of the cornea by supplying tissue to fill in the stromal defect, and the graft delivers a focal blood supply so that the need for abscess vascularization is met. Laboratory findings from corneal specimens taken at surgery can give the practitioner significant information as to any cause of the abscess that is not seen on preoperative specimens.

Corneal Transplantation

Medical treatment is generally most effective for superficial abscesses. If the horse does not quickly show a positive response to medical therapy, especially when the stromal abscesses are deep or severe uveitis is present, surgery should be considered.

Deep corneal abscesses are often caused by fungi or foreign bodies and do not generally respond well to medical therapy alone. Corneal transplantation is utilized in eyes with abscesses near Descemet's membrane and in eyes with rupture of the abscess into the anterior chamber (see Figures 2 and 3 on page 56). This aggressive surgical therapy can be very successful, and it is done to eliminate inflammation from the trapped bacteria and fungi and to remove the necrotic (dead) debris, metabolites, and toxins from the degenerating white blood cells and microbes in the abscess. Partial-thickness and full-thickness corneal transplantations in horses are used for deep corneal stromal abscesses. The decision to perform corneal transplant surgery is based on continued progression of anterior uveitis despite intense medical therapy, imminent or pre-existing rupture of the abscess into the eye, severe and unrelenting intraocular (within the eye) inflammation, or anticipated poor visual outcome due to lack of vascularization of the stromal abscess.

The stromal abscess is carefully removed (keratectomy) using microsurgical technique (an operating microscope). Donor cornea is harvested from fresh or frozen equine cadaver eyes within 24 hours of death. Horse corneas have been donated to us from owners following the death of their horses from old age or serious systemic diseases. These horse owners wish us to use the donated corneas in order to save sight in other horses. Small sutures are placed to secure the donor cornea in place. A conjunctival graft then can be sutured over the keratectomy/graft site (see Figures 2 and 3 on page 56).

Successful results from corneal transplantation can be achieved in over 90% of deep stromal abscesses in horses. Horses which undergo early surgery in the course of this disease tend to have a more rapid recovery than those in which surgery is delayed. Although donor corneal grafts in most equine cases superbly perform their role, I have seen no horse have a completely clear graft site postoperatively (see Figure 3 on page 56). More scientific work is needed to understand this disease.


Editor's note: This is the eighth article in a series of eye articles by Brooks. See the first article, "Eye Anatomy and Physiology," article Quick Find #2797 at www.TheHorse.com, for more information on normal eye anatomy.

About the Author

Dennis E. Brooks, DVM, PhD, Dipl. ACVO

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.

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