CHAPTER 143 Enucleation
Horses frequently develop serious ocular diseases that result in loss of function of the eye and chronic pain. Effects of injury or inflammation on the equine eye can be catastrophic. In addition to blindness, chronic equine ocular diseases can lead to permanent cosmetic defects. The cosmetic appearance of the eye, although not important to the horse, can be of serious concern to many owners. Enucleation has been the conventional treatment for these conditions and is indicated in horses with irreparable trauma, intraocular neoplasia, glaucoma causing blindness and pain, panophthalmitis, and intractable uveitis.
This chapter reviews the surgical techniques of enucleation in the horse, including performing the surgery with the horse standing. It also addresses the options available for improved cosmetic outcome in horses that require enucleation, including the use of orbital prosthesis and orbital prosthesis with corneoscleral shells.
ENUCLEATION AND ORBITAL PROSTHESIS
In general, enucleation, or surgical removal of the eye, is indicated when an eye is irrevocably blind and painful. Causes may include malignant intraocular neoplasm, panophthalmitis, chronic uveitis, glaucoma, and severe penetrating injuries. The enucleation can be performed via the transpalpebral or transconjunctival approach and with the horse under general anesthesia or standing after use of tranquilizers and local nerve blocks. However, removal of an equine globe is a major surgical procedure, and it should rarely be considered in a nonanesthetized horse and then only after careful surgical preparation, aggressive analgesia, adequate sedation, and careful postoperative care and monitoring.
The methods for surgical approach for enucleation are similar; however, the transpalpebral approach is indicated if neoplasia or ocular surface infection is involved, because the infected area is closed off to the surgical site by use of a temporary tarsorrhaphy. If the ocular disease is contained within the globe, then a transconjunctival approach may be used. Most surgeons prefer the subconjunctival approach because it is simpler, faster, and more easily performed than the transpalpebral approach.
Surgical Site Preparation
Careful and meticulous surgical site preparation is needed in all types of enucleation to minimize postoperative orbital infections. Routine preparation includes general cleansing of the face, surgical clipping of a 1- to 2-inch margin around the eyelids, removal of the eyelashes and possibly the vibrissae, and a surgical scrub of the skin with 1:10 diluted baby shampoo. The ocular surfaces should be cleansed with 1:10 dilute povidone iodine and flushed with sterile saline solution. Routine draping is performed to establish a sterile field. A retrobulbar block is recommended (see below) to improve anesthetic stability and reduce postoperative discomfort.
After placement of an eyelid speculum and creation of a lateral canthotomy, the transconjunctival enucleation is started by making an incision through the conjunctiva, 1 to 2 mm behind the limbus, 360 degrees around the eye (Figure 143-1, A). Using a combination of blunt and sharp dissection, the conjunctiva and overlying fibrous tissue are dissected off the globe. The opening is extended posteriorly and the extraocular muscles are identified and resected adjacent to the globe. The large retrobulbar muscles are then resected along with the optic nerve. A clamp or forceps should never be placed on the optic nerve; simple resection is all that is required. The globe is removed; if necessary, the orbit may be temporarily packed with sterile surgical gauze until hemorrhage ceases (usually within 1 to 2 minutes). The third eyelid and associated conjunctiva are removed, followed by the remaining tarsal conjunctiva. The eyelid margins are excised ensuring that the meibomian glands, which generally extend 5 mm into the eyelid margins, are removed. It is important also to remove all glistening conjunctiva to help prevent formation of mucoceles. The orbit is irrigated with sterile saline, but, to minimize irritation, other materials such as antimicrobials should not be infused into the orbit. Care must be exercised to ensure that the medial canthus is completely excised and that the angularis oculi vein, which lies near the medial canthus, is avoided.
Figure 143-1 A, Transconjunctival enucleation. An incision is made through the conjunctiva, 1 to 2 mm behind the limbus, 360 degrees around the eye (dotted line). The opening is extended posteriorly, and the extraocular muscles are identified and resected adjacent to the globe. The retrobulbar muscles are then resected along with the optic nerve. The globe, third eyelid, and conjunctiva are removed, followed by removal of the eyelid margin and remaining tarsal conjunctiva. Three-layer closure is performed. B, Transpalpebral enucleation. The eyelid margins are closed (tarsorrhaphy) with 3-0 nylon. This is generally indicated to seal off a contaminated ocular surface from the surgical field. An elliptical full-thickness skin incision is made 360 degrees around, and 5 mm posterior to, the eyelid margin (dotted line)