Introduction

Enucleation refers to the removal of the globe and is a commonly performed surgery in our animal companions for a variety of reasons. There are two main approaches to enucleation: the transconjunctival approach (also called the subconjunctival approach) and the transpalpebral approach. The author prefers the transconjunctival approach, however, that is a purely personal preference, and there are occasions when the transpalpebral approach is optimal. Both techniques will be discussed in this chapter.

Anatomy

As with any surgery, it is important to consider globe and orbital anatomy before performing a procedure. The globe refers to the spherical portion of the eye and is enclosed within the orbit and covered externally by the eyelids. The orbit surrounds and protects the globe, and in dogs and cats is made up of portions of the frontal, lacrimal, zygomatic, maxillary, sphenoid, and palatine bones. The orbit in these species is incomplete, meaning it is not fully surrounded by bone.¹ Instead, the temporal orbit and part of the ventral aspect of the orbit are covered by dense periorbital tissue that encloses the globe along with the orbital ligament that spans the caudolateral opening from the frontal process of the zygomatic bone to the zygomatic process of the frontal bone.² The surrounding muscles also provide additional support and structure for the orbit and include the temporalis, masseter, and pterygoid muscles. The incomplete orbit in carnivores allows them to open their jaws wider than herbivores, making consuming prey easier.¹ Also within the orbit is the zygomatic salivary gland, which lies dorsal and lateral to the pterygoid muscle and ventral to the globe.³

The bony orbit is lined by orbital facia, which is a dense connective tissue that is made up of the periorbita, Tenon’s capsule, and the facial sheaths that surround the extraocular muscles. The periorbita is a cone‐shaped fibrous membrane that lines the orbit and encloses the globe, extraocular muscles, nerves, and blood vessels.¹ Tenon’s capsule is a thin, fibrous capsule that envelops the globe from the level of the limbus to the optic nerve. The limbus is the point where the cornea meets the sclera. Tenon’s capsule is separated from the sclera by a thin space known as the episcleral space.³ There are six extraocular muscles enclosed in the facial sheath, including the dorsal and ventral oblique muscles and the dorsal, ventral, medial, and lateral recti muscles. These muscles are attached directly to the globe and are responsible for the suspension and centralization of the globe within the orbit and the movement of the globe.¹ These muscles are severed during enucleation.

The main arterial supply to the eye is primarily from the maxillary and the external ophthalmic arteries and their branches that arise from the external carotid artery.³ The main arteries to avoid course along the ventral and ventromedial aspect of the orbit, and these can be avoided by remaining close to the ventral aspect of the globe during dissection. Also, the angularis oculi vein should be avoided when removing the eyelids. It lies at the dorsomedial aspect of the orbital rim (Figure 5.1) and can be avoided by not removing more than ~5 mm of the superior eyelid in this area.

There is inherent bleeding that occurs during enucleation due to the many arterial and venous branches that supply blood to and drain blood from the many intraocular and extraocular structures within the orbit, and as long as the main vessels are not punctured or transected, the smaller branches should vasoconstrict without ligation. The venous supply to the globe largely parallels the arterial supply.³

The optic nerves from the right and left eyes join at the optic chiasm, which is the site where there is a significant cross‐over of the nerve fibers to the contralateral optic tract. There is a 75% crossover in the dog and 65% in the cat. This is significant because too much tension on the globe during removal prior to severing the optic nerve could result in blindness in the contralateral eye. This is of greatest concern in the feline species due to anatomical differences between dogs and cats. Mammalian optic nerves have a sigmoid flexure within their optic nerves, presumably allowing for ocular motility and some globe protrusion without traction on the optic nerve. However, cats lack the degree of sigmoid flexure present in dogs, thus, they are more prone to tractional injuries of the optic nerve due to the shortened space between the posterior aspect of the globe and the optic chiasm. Also, the feline globe is only slightly smaller than the orbit, making exposure of the optic nerve more difficult during enucleation and increasing the risk of excessive traction on the optic nerve during globe removal.⁵

Something else to be aware of is the risk of the oculocardiac reflex (OCR) when manipulating the globe or periocular tissues. The OCR occurs due to stimulation of the ophthalmic branch of the trigeminal nerve, which in turn results in stimulation of the vagus nerve to then cause inhibitory effects on the cardiac myocardium. This can result in a significant decrease in heart rate and blood pressure, which could compromise organ perfusion and lead to death if not corrected immediately. Typically, temporary cessation of surgical manipulation along with administration of an anticholinergic allows the heart rate and blood pressure to return to normal. The incidence of the OCR in a multicenter retrospective study showed an incidence of 4.8% during enucleation, which is overall low, but a retrobulbar block can prevent the OCR from being an issue and can improve intraoperative comfort, thus, allowing a lighter plane of anesthesia.⁶ How to perform this block is beyond the scope of this chapter; however, it is a useful skill to reduce the risk of the OCR and to improve patient comfort.

Indications for Enucleation

Enucleation is performed for many different reasons including trauma, ocular congenital defects, intraocular infection, blindness, pain, neoplasia, and the inability to treat a chronic condition that would cause discomfort to the animal if left untreated.⁷ See Figure 5.2 for a picture of a globe proptosis, which is one of the most common globe‐threatening ocular emergencies seen in practice.

Techniques

The transconjunctival and transpalpebral approaches are the two techniques primarily used for enucleation. The periocular hair and eyelids should be shaved, and the skin, eyelids, and conjunctival sac of the globe should be antiseptically prepped. Use dilute povidone‐iodine (Betadine^®) solution (1:50) around the eyes and be sure to use the povidone‐iodine solution and not the scrub

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