Defining the margins of orbital disease

Orbital diseases, particularly diffuse septic inflammation and expansive neoplasms, usually infiltrate the orbit within its fascial planes and confound determination of the disease margins, thereby limiting the success rate of surgery or radiotherapy. Before entering the orbit surgically, the disease process and its borders should be identified as best as possible, by the different imaging modalities, as thoroughly as possible. The orbit is highly vascular, and visualization of the different structures during surgery is often limited using only one technique. Vital nerves, blood vessels, and extraocular muscles span the orbital space between the different bony foramina and the globe, tear glands, nictitating membrane, conjunctiva, and eyelids. Although imaging procedures such as computed tomography (CT) and magnetic resonance imaging (MRI) are expensive, they often yield the best results. Fortunately, however, the orbit can be evaluated by a number of physical examination procedures, as follows.

When exophthalmia is the presenting primary clinical sign, a number of additional more subtle clinical signs may assist in determining whether the space-occupying disease is inflammatory, cystic, or neoplastic. Primary orbital diseases associated with exophthalmia are usually unilateral, and comparisons of the orbital position and size of both eyes can be informative. If the exophthalmia is bilateral, systemic disease should be suspected. The inability of the animal to retropulse the globe or retract the globe upon eyelid or corneal touch often signals a sizeable space-occupying disease. With orbital inflammatory diseases, retraction of the extraocular muscles and globe may also elicit pain. With orbital cysts or neoplasia, the retropulse reflex is impaired but usually elicits no pain.

Imaging techniques to evaluate the orbit initially include plain and special contrast radiologic procedures, and ultrasonography. More sophisticated diagnostic procedures such as CT and MRI offer the best imaging of orbital tissues, and fortunately their availability is increasing.

Strabismus and orbital diseases

Strabismus refers to deviation of the globe, and is traditionally divided into dorsal (hypertropia), ventral (hypotropia), lateral (exotropia), and medial (esotropia). Primary strabismus without ocular disease is rare in animals. It has been reported in horses and mules, cattle, and dogs, but appears most frequent in cats, especially the Siamese and Himalayan breeds. Uni- or even bilateral strabismus may occur in dogs, secondary to myositis of selected extraocular muscles, and may be complicated by fibrosis of these muscles. Secondary strabismus is frequently present with orbital diseases, and the direction of the deviation of the globe may suggest the location of the space-occupying lesion. When space-occupying orbital disease is directly behind the globe and confined to the retrobulbar muscle cone, the eye is usually displaced directly forward. The pressure on the globe may be sufficient to indent areas of the posterior segment and be detectable ophthalmoscopically.

If the space-occupying mass is located within the medial or ventromedial orbital or zygomatic salivary gland in animals, the resultant strabismus is usually lateral (exotropia) and/or dorsal (hypertropia). If the mass involves the rostral aspect of the medial orbit, the eye may be deviated upward, and the nictitating membrane is usually protracted. If the mass is located within the rostral aspect of the dorsal orbit, the eye is usually deviated downward (hypotropia). In spite of the strabismus, tonic eye reflexes and eye movements are usually normal.

Strabismus (rectus muscle) surgery

Strabismus surgery is infrequently performed because the ocular malalignment usually signals ophthalmic disease. Extraocular muscle surgery for strabismus in the Siamese cat with esotropia has not been successful because of the very small rectus muscles in the cat as well as the underlying neuro-ophthalmic tract malformation. The congenital exotropia occasionally seen in brachycephalic breeds of dogs has not been treated with rectus muscle surgery. Strabismus is infrequent in horses and mules, and some affected horses may also exhibit night blindness (e.g., Appaloosa breed). Strabismus in the horse has been successfully treated by either resection or recession of affected extraocular muscles. The surgery is carried out at their insertions to the anterior globe, and is relatively easy to perform.

Occasionally lateral strabismus develops after traumatic proptosis, and signals either nerve and/or muscle injury to the medial rectus muscle. If exotropia results from damage to the oculomotor nerve, spontaneous recovery often occurs. If the muscle or its insertion to the globe has been transected, recovery is unlikely. Attempts to reattach the severed ends of the medial rectus muscle are not usually successful as the deeper (proximal) aspects of the medial rectus cannot be located. Splitting the dorsal rectus muscle longitudinally and repositioning the medial portion to the medial rectus insertion provides satisfactory results.

A fairly recent disorder in dogs, extraocular muscle myositis (also called fibrosing strabismus), usually presents with uni- or bilateral restrictive ventromedial strabismus and enophthalmia. Several breeds of dogs appear affected, but the large breeds as well as the Chinese Shar Pei seem most often affected. Involved muscles include the ventral rectus, ventral oblique, and medial rectus. Medical therapy includes immunosuppressive systemic therapy (prednisone and/or azathioprine) and early cases are the most responsive. Surgical correction usually involves resection of the affected muscle(s) or their insertion(s) rather than recessing the opposite muscle to straighten the globe (because of the inflammation and subsequent fibrosis of the affected muscles).

Incomplete orbital walls in domestic animals

In contrast to humans, most small and large animal orbital walls are incomplete caudoventrally and laterally. In the primate the contents of the entire orbit are bounded by bone, thereby providing maximum protection for the globe. In many avian species orbits are unusually large and often limit access through only the frontal aspects (like human and non-human primates). The incomplete orbital walls in dogs and cats seem associated with the ability to open the jaw as widely as possible. As a result, the coronoid process of the mandible is medial of the zygomatic arch and in the caudolateral orbit. Hence, in orbital diseases, especially acute inflammations, manipulation of the mouth will usually elicit pain. Where the bony orbital walls are missing in carnivores, the wall is replaced with a variable thickness fascial layer, the endorbita (or periorbita).

In the caudal, dorsal, and lateral orbital walls, the temporalis, masseter, and pterygoid muscles are adjacent to the endorbita. The endorbita constitutes a reasonable barrier to inflammation, trauma, and neoplasia, but still does not provide the protection provided by bony walls. As a result, orbital diseases in small animals may extend from the orbit into the mouth via the caudal floor, perforate anteriorly to the conjunctival surfaces, or extend laterally into the subcutaneous tissues of the lateral orbit and face. While these regional soft-tissue orbital walls represent incomplete and weaker barriers, these same areas provide potential entry routes for surgical invasion of the orbital space.

Enophthalmia and surgery

Enophthalmia (recession of the globe within the orbit) is a common clinical sign when the orbital contents are less than normal. The palpebral fissure may be reduced in size. The most frequent condition with enophthalmia is microphthalmia, a congenital and sometimes inherited condition, and phthisis bulbi or atrophy of the eye, an acquired disorder. Enophthalmia can also accompany Horner’s syndrome, the loss of fat associated with debility, dehydration, and pain. An emerging clinical problem in large and giant breeds of dogs is the ‘medial canthal pocket syndrome’, which consists of bilateral enophthalmia, persistent and chronic conjunctivitis, and entropion and/or ectropion secondary to the lack of support and contact between the globe and eyelids. This condition, occurring more frequently in male dogs, is associated with very large heads and deeply recessed eyes. It is difficult to treat medically and/or surgically and resolve. Augmentation of the volume of the orbit with a visual eye has been attempted by adding autogenous fat (usually about 70% is lost before its blood supply is restored), sterile glass beads (about 4–5 mm diameter), medical grade silicone, and newer microporous implants that permit vascular ingrowth into the device.

Canine orbit

The bones that envelop and confine the orbital tissues in the dog consist of the zygomatic process of the frontal bone dorsally; the frontal bone and palatine bones medially; and the zygomatic arch and vertical ramus of the mandible laterally (Fig. 4.1a). The bony orbital floor is comprised of the sphenoid bone. The orbital rim consists of the zygomatic process of the frontal bone dorsally, the lateral orbital ligament, and parts of the zygomatic, maxillary, and lacrimal bones ventrally. Soft tissues that support the orbital walls include the temporalis muscles posteromedially, the temporalis and pterygoid muscles medially, the masseter muscle laterally, and the medial pterygoid muscle ventrally. The zygomatic or orbital salivary gland is located in the dog in the rostrolateral orbit. In brachycephalic breeds of dogs the orbit is very shallow, while in dolichocephalic breeds the orbit is considerably deeper and difficult to access surgically.

Fig. 4.1 The bones of the canine orbit. The dorsolateral wall and caudal floor of the canine orbit consist of soft tissues. Entry into the canine orbit is restricted to these soft tissues avenues, as well as through the palpebral fissure and orbital opening. (a) The canine orbital anatomy varies considerably based on breed and skull type. The bony orbital rim is incomplete, and laterally is formed by the lateral orbital ligament. The bones that comprise the canine orbit include the frontal bone and its zygomatic process (A), palatine bone (B), zygomatic arch (C), and vertical ramus of the mandible (D). The bony orbital floor consists of the sphenoid bone (not visible). The orbital rim consists of the zygomatic process of the frontal bone (see A), and parts of the zygomatic (see C), maxillary (E), and lacrimal (F) bones. The lateral orbital ligament (missing in this specimen) extends from the zygomatic process of the frontal bone to the zygomatic arch (arrows). Surgical access to the canine orbit is generally through the orbital rim or through the lateral orbital wall, with or without the central zygomatic arch removed. (G) indicates the lacrimal fossa which contains the lacrimal sac. (b) The three important foramina in the apex of the canine orbit are: (A) optic foramen, through which the optic nerve and internal ophthalmic artery pass; (B) orbital fissure, through which the third, fourth and sixth cranial nerves, the ophthalmic division of the trigeminal (fifth) nerve, and the orbital vein pass; (C) rotundum foramen, through which the internal maxillary nerve and artery pass.

The major arteries in the dog orbit are located along the ventral and ventromedial floor, and consist of the pterygopalatine portion of the maxillary artery with its various branches, including the external ophthalmic (orbital) artery, the infraorbital artery, the minor palatine artery, and a trunk that gives rise to the major palatine and sphenopalatine arteries. From the external ophthalmic artery are branches that include the external ethmoidal artery and the anastomotic ramus to the internal carotid artery. Branches from the external ethmoidal artery include, in part, the ventral and dorsal muscular branches to the extraocular muscles, the lacrimal and zygomatic branches. With the union of the external and internal ophthalmic arteries emerge two to four long posterior ciliary arteries to supply the globe. The orbital veins tend to follow the respective arteries, and, in addition, form an extensive and highly variable orbital venous plexus.

With the origin of the majority of the extraocular rectus muscles in the apex of the orbit, these muscles span the orbit to insert anterior to the globe’s equator. The four bellies of the retractor oculi muscle envelop the optic nerve, and attach to the globe posterior to the rectus muscle insertions. The ventral oblique muscle has its unique origin in the medial orbital wall. Immediately above the dorsal rectus muscle is the levator palpebrae superioris muscle, with its common origin with the other rectus muscles but its insertion in the tarsal layer of the upper eyelid. In addition to an extensive arterial and venous supply, and space-filling adipose tissues, all the orbital tissues are covered with endorbita or the periorbital fascia that eventually merges with the fascia bulbi (or Tenon’s capsule) that surrounds the globe and attaches at the limbus, the periosteum or endorbita lining the orbital walls, and the tarsal layer within the eyelids as the septum orbitale anteriorly. The sub-Tenon’s space between the fascia bulbi and the sclera is an important surgical plane that permits dissection around the globe with minimal hemorrhage.

The third (oculomotor), fourth (trochlear), and sixth (abducens) cranial nerves innervate the retrobulbar muscles, and the second (optic), fifth (trigeminal), and branches of the seventh (facial) cranial nerves permit vision, sensation, and lacrimation (Fig. 4.1b). The orbit also contains autonomic fibers, with the sympathetic fibers extending from the superior cervical ganglion, and parasympathetic fibers entering the orbit to synapse in the ciliary ganglion. Parasympathetic fibers from the ciliary ganglion then continue to innervate the iris sphincter and ciliary body muscles.

Canine orbit size varies by breed and skull type. In general, skull length, width, and height range from about 156 × 27 × 29 mm in mesaticephalics, 79 × 28 × 30 mm in brachycephalics and 214 × 33 × 29 mm in dolichocephalics. There is an increase in orbital size from the toy to giant breeds, but the increase is not directly proportional. The canine globe size ranges from 19.7 to 25 mm transverse, 18.7 to 25 mm vertical, and 20.0 to 25 mm anteroposterior.

Feline orbit

The orbit of the cat is similar but not identical to the dog. In contrast to the dog, the feline orbit is only slightly larger than the globe, which greatly restricts orbital exploration unless the globe is first removed. The bones that compose the orbital walls include the sphenoid, maxillary, lacrimal, zygomatic, and frontal (Fig. 4.2). The lateral orbital ligament joins the frontal and zygomatic processes. The bony floor of the feline orbit consists of only a small shelf of maxillary bone that holds the last molar teeth. The extraocular muscles are small and ocular mobility is limited. The zygomatic or intraorbital salivary gland is small in the cat and lies close to the maxillary nerve. The cat’s orbital measurements are about 87 mm long, 26 mm wide, and 23 mm high. The feline globe ranges in size from 20 to 22 mm anteroposteriorly, 19 to 20.7 mm vertically and 18 to 21 mm transversely. The larger Siamese breed globes measure 22.5 mm anteroposteriorly and 22.5 mm transversely.

Fig. 4.2 The bones of the feline orbit. Entry into the feline orbit is generally limited to the palpebral fissure and orbital opening. View of the feline skull from the side (a) and from the front (b). The bony orbit provides little more than the essential space to accommodate the cat globe. Like the dog, the bony orbital rim is incomplete laterally and this area is formed by the short lateral orbital ligament. The bones that comprise the cat orbit are, from the side (a): frontal (A), lacrimal (B), maxillary (C), and zygomatic (D). The medial orbital wall (E) consists of the frontal bone dorsally and the sphenoid and palatine bones ventrally. The cat orbital floor is incomplete and very thin. As viewed in the apex of the orbit (b), the dorsal optic foremen and ventrolateral orbital fissure (E) permit passage of the essential ophthalmic nerves and vessels. The globe and orbit accommodate fairly short optic nerves, and very small and limited mobility extraocular muscles.

Horse orbit

Both the horse and cow orbits are among the largest that clinically confront the veterinarian. The orbital bones in the horse include the frontal, lacrimal, zygomatic, temporal, sphenoid, palatine and maxillary (Fig. 4.3a). The bones contributing to the equine orbital rim include the lacrimal (ventromedial orbital rim), the frontal and its zygomatic process (dorsal orbital rim), and the zygomatic processes of the temporal and zygomatic bones (incomplete lateral wall and lateral canthus). The zygomatic process of the frontal bone contains the supraorbital foramen, an important landmark for supraorbital nerve blocks to produce upper eyelid regional anesthesia and paralysis. Hence, the entire orbital rim in horses consists of bones and no fascial tissues or ligaments. The lacrimal bone contains both a shallow fossa for the poorly developed lacrimal sac as well as the entry of the nasolacrimal system into the nasal turbinates.

Fig. 4.3 The bones of the equine orbit. Entry into the equine orbit is generally limited to the palpebral fissure and orbital opening. Although the dorsolateral orbital wall and caudal floor of the orbit consist of soft tissues, these avenues provide very limited access to the caudal orbit as the extraocular muscle cone, vital blood vessels, and cranial nerves are so deep! (a) The bones that comprise the equine orbit, as viewed laterally, consist of the frontal bone with large supraorbital process (A), zygomatic arch (B), the coronoid process of the mandible (C, not part of the orbit but within the orbit), lacrimal bone (D), and zygomatic bone (E). The medial bony orbital wall is formed by the frontal, lacrimal, and wing of the presphenoid bones. The dorsal orbital wall is formed by the frontal and small part of the lacrimal bones. The ventral floor is formed by the zygomatic bone, zygomatic process of the temporal bone and small part of the maxillary bone. The incomplete lateral wall is formed by the zygomatic bones. In contrast to the dog and cat orbital rims, the horse orbital rim is all bone with limited access to the dorsal orbit structures. (b) At the apex of the equine orbit are four important foramina (arrow): most dorsal and medial is the ethmoidal foramen (passage of the ethmoidal artery, vein, and nerve); more ventrad and further caudad is the optic foramen (passage for the optic nerve and internal ophthalmic artery); just ventral is the orbital fissure (carrying the third, sixth and often the fourth or trochlear and ophthalmic division of the trigeminal nerves); and lastly is the furthest ventral foramen, the round foramen (passage of the maxillary nerve). The apex of the orbit is a considerable distance from the globe (longer than the cow), and generally approached immediately caudal of the supraorbital arch.

The complete medial orbital wall consists of contributions from the frontal and lacrimal bones and the wing of the presphenoid bone. The dorsal wall is formed by the frontal and, to a smaller extent, the lacrimal bones. The incomplete ventral wall is formed by the zygomatic bone and, to a limited extent, the maxillary and palatine bones. The incomplete lateral wall is formed by the zygomatic processes from both the temporal and zygomatic bones, and the periorbita. A significant lateral barrier to the deep orbital structures is the large coronoid process of the mandible.

Four important foramina are sited in the apex of the orbit (Fig. 4.3b). They include:

In the placement of successful retrobulbar nerve blocks in the horse, at least 8–10 cm of distance must be traversed by the hypodermic needle to inject local regional anesthetic in the vicinity of these four foramina.

The orbital dimensions of the adult horse are estimated to be 62 mm wide, 59 mm high, 98 mm deep, and 173 mm between the eyes. The adult horse globe measures 43.7 mm on the meridional anterior–posterior axis, 47.6 mm on the equatorial axis vertical, and 48.5 mm on the horizontal axis.

Cow orbit

The cow orbit has many similarities to the horse, but also significant differences. For instance, the frontal bone is very large and well developed to accommodate the cow’s horns. The bones which contribute to the bovine orbit include the frontal, lacrimal, zygomatic, palatine, maxillary, and sphenoid. The bovine orbital rim, composed of three bony structures around 360°, consists of: 1) the frontal bone (dorsal rim); 2) the lacrimal bone (medial canthus); and 3) the zygomatic bone and frontal process of the zygomatic bone (entire ventral rim and lateral canthus). The coronoid process of the mandible is well developed and positioned just caudal to the lateral rim to permit hypodermic needle insertion into the deep orbital tissues from behind the lateral orbital rim (Fig. 4.4a). The bony orbital walls consist of: 1) the lacrimal and sphenoid bones medially; 2) the palatine and sphenoid bones ventrally; 3) the frontal bone dorsally, and 4) the temporal and zygomatic bones laterally.

Fig. 4.4 The bones of the bovine orbit. Entry into the bovine orbit is generally through the palpebral fissure and orbital opening. (a) The bones that comprise the cow orbit, as viewed laterally, are the frontal bone with zygomatic process (A), lacrimal bone (B); zygomatic bone with frontal process (C); zygomatic process of the temporal bone (D); and coronoid process of the mandible (E). (b) Viewed through the orbital rim and into the orbital base or apex, are three foramina: the ethmoidal foramen (A); optic foramen (B), lateral to which is the pterygoid crest; and lastly the foramen orbitorotundum (C). Like the horse, the cow orbital rim consists totally of bony structures, but the orbit is shallower than the horse. The pterygoid crest presents a sizeable barrier to orbital nerve blocks (shielding all of the important foramina), and generally local anesthetic is injected deep to it (the pterygoid fossa) or just anterior to successfully block all of the nerves supplying the orbit and globe.

Important orbital foramina include: 1) the ethmoidal foramen (ethmoidal blood vessels and nerves); 2) the optic foramen (passage of the optic nerve); and 3) the orbitorotundum (a combination of the orbital fissure and the foramen rotundum) for passage of the oculomotor (third), the trochlear (fourth), the trigeminal (fifth) and the abducens (sixth) nerves, and retinal and maxillary blood vessels (Fig. 4.4b).

The orbital dimensions of the adult cow are estimated to be 65 mm wide, 64 mm high, 120 mm deep, and 151 mm between the eyes. The adult cow globe measures 35.3 mm on the meridional anterior–posterior axis, 40.8 mm on the equatorial axis vertical, and 41.9 mm on the horizontal axis. Hence, for a successful retrobulbar injection in cattle, the hypodermic needle, if positioned near the optic and orbitorotundum foramina, must traverse distance of about 12 cm.

Rabbit orbit

The rabbit has become an increasing popular household pet, as it can be house broken and trained to use a litter box. The rabbit has a large orbit as well as a large globe which occupies most of the orbital space, and a large venous sinus within the orbit. Hence, orbital surgery in this species must be limited to the space between the sclera and Tenon’s capsule, and avoid entering the retrobulbar tissues directly. The bones of the rabbit orbit include the maxillary, orbitosphenoid, alisphenoid, lacrimal, palatine, frontal, pterygoid, and zygomatic. The orbital rim consists of the following bones: frontal bone dorsally, lacrimal bone anteriorly, zygomatic processes of the maxilla, and zygomatic bones (ventral orbital rim). The globe’s lateral displacement allows the temporal bone to also contribute to the lateral orbital rim.

The orbit is roughly circular, about 25 mm diameter, with the skull being about 108 mm long and 50 mm wide. The rabbit’s globe measures 16–19 mm anteroposteriorly, 17 mm vertically, and 18–20 mm horizontally. A large Harderian gland (19 mm long, 12–15 mm wide, and 4–6 mm thick at its largest point) occupies the lower anterior part of the orbit. It is medial to the lacrimal gland and almost completely surrounded by a large venous sinus. A very small intraorbital gland is beneath the zygomatic arch.

Avian orbit

Avian species that are presented to veterinarians for eye disease are generally in the raptor group (owls, falcons, and hawks) and the psittacines (parrots, cockatiels, and parakeets). Trauma in the raptor group is probably the most frequent single cause of eye disease in this group, and the most treatable. Orbital anatomy varies markedly in the avian species, based on the shape of the skull and beak (Fig. 4.5).

Fig. 4.5 The bones of the bird orbit. The avian skull varies markedly in size and shape, and is directly influenced by the bird’s beak. Also, the orbits in birds are generally quite large as compared to the associated skull. (a) In general, the avian bony orbit consists of the following bones: (A) frontal bone; (B) lacrimal bone; (C) the interorbital septum (separating both globes), at the caudal border of which is the optic foramen; and (D) nasal bone (forms the basis for the beak). In this Rhea the orbital rim is much smaller than the bony sclerotic ring (E, representing the globe). In general, the avian orbital rim consists of the following bones: sphenoid, lacrimal (or prefrontals) which form the dorsal orbital rim, nasal and frontal. There may also be an ectethmoid bone. (b) In this Macaw parrot skull, the orbit is very large compared to the bird’s skull, and its shape is influenced by its massive beak. The globes are larger than the orbital rim, requiring special surgical procedures for surgical removal (enucleation) of the globe.

The avian orbit and globe are unusually large relative to the bird’s head and body. The large globes result in restricted access to the extraocular muscles as well as limited orbital space during surgery. In fact, sometimes it is best to rupture the globe at the beginning of the enucleation procedure to facilitate surgery and globe removal. Since there is considerable variation in skull and orbital osteology, only some generalizations are possible. In most birds the orbit is almost completely enclosed by bones, the floor being the main exception (contains muscles related to jaw movements).

Bones contributing, in part, to the avian orbit include: 1) prefrontal or lacrimal bone; 2) frontal bone; 3) ethmoid or ectethmoidale bone (part of the rostral wall of the orbit, separating it from the nasal cavity); 4) laterosphenoidale bone (ventral caudal wall of orbit); and 5) zygomatic bone. Often pneumatization of the skull bones is present; the reduction in weight of these bones is probably an adaptation for flight. Both globes are separated by a thin bony partition, the interorbital septum (ethmoid bone), which can be fractured easily during enucleation if one is not careful. Another interesting adaptation in birds are two muscles, the M. quadratus membranae nictitantis (originates from beneath the origin of the dorsal oblique muscle) and M. pyramidalis membranae nictitantis (originates near the ventral rectus muscle), which combine and rotate round the optic nerve en route to provide motion to the highly mobile nictitating membrane.

Effect of globe development on orbit growth

Development of puppy, kitten and foal orbits is partially determined by concurrent growth and expansion in the size of the eye. In animals that lose an eye to trauma and/or inflammation in early life and while still growing, orbital development will markedly slow and result in noticeable orbital asymmetry at adulthood. The earlier in life that the globe is destroyed, the more pronounced the orbital defect. Orbital asymmetry also occurs in puppies, kittens and foals with unilateral microphthalmia: the more severe the microphthalmia, the more extensive the orbital maldevelopment. Hence in young animals, enucleation of the globe and use of an intraorbital implant after surgery usually help to reduce the orbital deformity to a minimum.

Traumatic proptosis/avulsion/luxation of the globe

In proptosis or luxation of the globe, the entire globe is displaced forward. In mild cases, secondary pressure from retrobulbar hemorrhage and edema will force the globe forward sufficiently to induce exophthalmia, exposure keratitis, and an impaired blink reflex. Proptosis occurs most frequently in dogs and certain breeds, especially the brachycephalic breeds, and in cats is usually catastrophic. In the horse, traumatic proptosis is usually incomplete and is typically exhibited by intraorbital hemorrhage, exposure keratitis, exophthalmia, and impaired blink reflex. In other species, proptosis appears rare.

When trauma is extensive, the globe may be thrust forward with such force and speed that the equator of the globe extends beyond the palpebral fissure. The compensatory eyelid contractions that should retain the globe within the orbit are delayed, and with the globe already forward of the eyelid margins, the orbicularis oculi muscle spasms prevent retraction of the globe into the orbit. At the same time, forward stretching of the orbital tissues results in intraorbital hemorrhage and edema which can displace the globe even further forward. The stretching, direct pressure, and perhaps thrombosis and ischemia can result in optic nerve inflammation and subsequent atrophy. Elevated intraorbital pressure, and nerve and vascular damage to the lacrimal gland, may cause sufficient destruction to result in keratoconjunctivitis malacia. The extraocular muscles are stretched considerably in traumatic proptosis, and the shortest medial rectus muscle may be transected near its insertion. The impaired blink reflex results in acute corneal exposure and rapidly progressing malacia. Unchecked, the corneal integrity can be compromised within hours. Medical and surgical treatment strategies that directly address the primary and secondary events that can occur in traumatic proptosis are the most successful.

Orbital fractures

As the orbital shell is composed primarily of bony tissues, the globe is fairly well protected against trauma. However, considerable trauma can cause orbital fractures of the temporal, zygomatic and frontal bones in most domestic species. With the concurrent hemorrhage and swelling, globe displacement, strabismus, impaired mobility, hemorrhage, pain, and orbital asymmetry result. If the adjacent sinuses are involved, orbital and/or subcutaneous emphysema with crepitus occur.

In general, orbital fractures with minimal displacement of the fractured bone heal without surgery; however, if displacement is considerable and unstable, reapposition and internal fixation of the fractured fragments is recommended. A vigorous blink reflex must be maintained in spite of the orbital swelling, and the cornea protected by topical tear substitutes. Temporary complete tarsorrhaphy may be indicted to protect the outer eye and prevent secondary corneal ulceration.

Orbital inflammation: acute and chronic

The animal orbit is susceptible to bacterial infections (Fig. 4.6). Orbital cellulitis may present as acute or chronic, and is usually associated with bacterial or fungal infections (often entry cannot be ascertained), as well as foreign bodies. Orbital cellulitis occurs most frequently in dogs (especially the hunting breeds), and is rare in cats. In horses, cattle, and certain species of birds orbital cellulitis may be secondary to adjacent sinus infections or as a sequel of de-horning in cattle. Fungal infections are infrequent in the dog, and are usually associated with foreign bodies. Infection may enter the orbit through several routes. Infectious agents can enter from the mouth, conjunctivae, the adjacent sinuses and nasal cavity, the subcutaneous and skin surfaces of the incomplete lateral and dorsolateral orbital walls, and hematogenously. In a recent report on orbital abscesses in dogs and cats, the most common bacterial genera isolated from dogs were Staphylococcus, Escherichia, Bacteroides, Clostridium, and Pasteurella. The most frequent bacteria isolated from orbital abscesses in cats were Pasteurella and Bacteroides. The highly vascular orbit and the endorbita that covers the orbital tissues usually respond quickly to antibiotic therapy. This orbital compartmentalization can also impede the spread of the infectious nidus, but also foster the development of focal septic areas that impede antibiotic penetration. As a result, surgical excision of chronic orbital abscesses and focal granulomas may be necessary for complete resolution of the condition.

Fig. 4.6 English Springer Spaniel with acute orbital cellulitis. Note the swelling of the eyelids and dorsal orbital subcutaneous tissues.

For orbital abscesses in dogs and cats, based on in-vitro susceptibility testing of aerobic bacterial isolates, cephalosporins, extended-spectrum penicillins, potentiated penicillins, and carbapenems are recommended for the initial antimicrobial therapy of orbital abscesses in dogs and cats. Antimicrobial culture is recommended for any severe orbital abscess and in-vitro antimicrobial susceptibility determined to assist in antibiotic selection.

Orbital neoplasms

Orbital neoplasms are not infrequent in dogs, but are less common in cats. In both horses and cattle, intraorbital lymphomas, lymphosarcomas, and squamous cell carcinomas are the most frequent types. In dogs, orbital neoplasms consist of a large number of different tumor types, while in cats the most frequent orbital neoplasm is squamous cell carcinoma. Primary orbital neoplasms can arise from any tissue (epithelial, vascular, neural, and connective tissues) within the orbit. Secondary orbital neoplasms also occur and invade locally from the nasal, sinus, and cranial cavities, as well as metastasize from distant sites. The clinical signs of orbital neoplasia are usually associated with a slowly enlarging and painless mass within the orbit (Fig. 4.7). Depending on its position, a neoplasm within the orbit can produce strabismus; the direction of the ocular deviation may assist to localize the mass.

Fig. 4.7 Left orbital neoplasm (meningioma) affecting the medial orbital floor in a 15-year-old Beagle. Note the exophthalmos and dorsolateral deviation of the globe.

The majority of information on orbital neoplasia is on dogs. The mean age of affected dogs with orbital neoplasms is 8–9 years old. Females may be at higher risk. There is no breed predisposition. Younger dogs may demonstrate more rapidly growing orbital masses. Most neoplasms external to the extraocular cone affect the medial orbital space and wall. This area has the most difficult and limited surgical exposure. In dogs, about 60% of orbital neoplasms are primary. As a result, when orbital neoplasia is suspected, a complete and comprehensive general physical examination is required. The remaining 40% of orbital neoplasms usually invade the orbit from the adjacent nasal and oral cavities, and the sinuses. Unfortunately, 90% of canine orbital neoplasms are malignant.

The prognosis for orbital neoplasms is poor, because conservative surgery in an attempt to maintain the globe and vision results in unacceptably high rates of tumor recurrence. Patients with orbital osteolysis usually have a poor prognosis. Most clients do not accept the aggressive attempts of orbitectomy with the resultant loss of the globe and vision, and postoperative facial deformities. A recent study reported that surgical intervention and chemotherapy can prolong life; about 40% of the dogs were alive 6 months after diagnosis, and about 19% were still alive 1 year later. Unfortunately, the other 60% of patients, with advanced orbital neoplasia and most with no therapy, were euthanized within 6 months of diagnosis.

The treatment of choice is usually exenteration, which involves excision of the entire orbital contents including the globe. Orbital neoplasms affecting the rostral and lateral orbit may be successfully excised while preserving the eye. Unfortunately, masses involving the ventromedial and posterior orbit, which are the most frequent, generally require removal of the eye during attempts at excising the neoplasm. A major difficulty during surgery is the differentiation of normal and cancerous tissues, often resulting in an incomplete excision of the neoplasm. When considering extensive therapy for advanced orbital neoplasia in small animals and horses, careful education of the client is very important as the postoperative results can markedly affect the facial appearance.

Orbital neoplasms in cats are usually malignant. The orbital neoplasms reported most frequently include squamous cell carcinomas, followed by lymphosarcoma–leukemia complex, undifferentiated sarcomas, osteogenic sarcomas, and rhabdomyosarcoma. Orbital neoplasia in cats necessitates a guarded to very poor prognosis.

Orbital neoplasia is infrequent in horses and cattle, and careful systemic patient evaluation is essential. Conjunctival squamous cell carcinomas may invade the orbit, especially in the medial canthus. Retrobulbar lymphosarcoma occurs in both species, and not infrequently affects both orbits. The appearance after enucleation or exenteration in horses is a greatly shrunken orbit, and an intraorbital prosthesis may prevent most of the shrinkage.

Enucleation procedures in small animals

In enucleation, the globe and its contents are excised. In animals, the indications for enucleation include: 1) ocular congenital defects, such as microphthalmia, that result in chronic problems such as conjunctivitis and keratitis; 2) intraocular infections that have destroyed the globe, and are potential sources of systemic infection; 3) intraocular tumors not amenable to local excision or laser therapy and still confined to the globe (Fig. 4.8); 4) proptosis of the globe with several of the extraocular muscles and/or the optic nerve severed; 5) intraocular inflammation that has destroyed the intraocular tissues and resulted in blindness; and 6) extensive trauma to the globe with the loss of intraocular tissues and without the possibility of successful repair.

Fig. 4.8 Primary mast cell sarcoma involving multiple areas of the limbus with secondary glaucoma in an American Cocker Spaniel. The recommended treatment is enucleation.

Enlarged and blind glaucomatous globes may also be treated by enucleation; however, the evisceration procedure followed by insertion of an intraocular prosthesis has largely replaced the enucleation procedure because of superior cosmetic results. Advanced glaucoma secondary to intraocular neoplasms and non-specific panophthalmitis is best managed by enucleation.

In the enucleation procedure in small animals, the eye, eyelid margins, nictitating membrane, and lacrimal gland are excised. Surgical approaches for enucleation include the subconjunctival (through the bulbar conjunctiva), transpalpebral (through the eyelids), and lateral (a modified palpebral procedure starting at the lateral canthus and removing the inner (deeper) one-half of the upper and lower eyelids). During enucleation of the eye in cats, minimal traction on the globe during the procedure is recommended. Excessive traction on the feline globe undergoing enucleation may damage the optic chiasm and the opposite optic nerve.

All orbital tissues (including the globes) that are excised should be examined histologically. Microscopic examination of these tissues can confirm the clinical diagnosis, as well as provide additional information that could affect the postoperative clinical management and long-term prognosis for the animal.

Subconjunctival enucleation

The subconjunctival enucleation technique is the simplest and most rapid of these procedures, and the most frequently performed in small animals. Using this method, the globe is excised from its surrounding Tenon’s capsule with the majority of the surgical dissection limited to the sub-Tenon’s space. As a result, this method usually has less hemorrhage intraoperatively and less serum accumulation postoperatively. This technique does not usually remove the conjunctivae and lacrimal gland; however, the entire nictitating membrane is excised. Exposure of the deeper orbital tissues may be limited with this procedure because of the edematous bulbar conjunctiva, but can be enhanced by a lateral canthotomy.

In the subconjunctival procedure for enucleation, entry into the orbit is through the bulbar conjunctiva. After completion of draping around the palpebral fissure, a 5–10 mm lateral canthotomy may be performed to increase exposure (Fig. 4.9a). With blunt-tipped tenotomy, strabismus, or Metzenbaum scissors, the full-thickness lateral canthus is cut. Hemostasis is usually achieved by direct pressure with a surgical sponge, if necessary supplemented by point electrocautery. The bulbar conjunctiva and Tenon’s capsule are incised at the 12 o’clock position by curved Steven’s tenotomy, strabismus, or Metzenbaum scissors with blunt tips for about 3–5 mm posterior to the limbus, and the incision extended for 360° (Fig. 4.9b). Using the scissors’ blunt tips, the dissection plane between the sclera and Tenon’s capsule is extended deeper into the orbit until each extraocular muscle insertion is identified (Fig. 4.9c). After isolation with a muscle hook, the tendinous insertions of all of the extraocular muscles are incised. Transection of the extraocular muscle insertions, rather than through the muscle per se, minimizes hemorrhage. As each of the four major rectus muscle insertions is incised, the globe becomes more mobile. After incision of the retractor muscle and oblique muscle insertions, the globe will displace slightly forward.

Fig. 4.9 Enucleation – subconjunctival approach: In this procedure the globe is removed from Tenon’s capsule through a bulbar conjunctival incision at the limbus. After removal of the nictitating membrane, the eyelid margins are removed and permanently apposed. (a) The palpebral fissure is temporarily enlarged by a lateral canthotomy. The lateral canthus is incised by small tenotomy scissors for 5–10 mm. (b) The bulbar conjunctiva and Tenon’s capsule are incised 360° by curved Steven’s tenotomy or strabismus scissors a few millimeters behind the limbus. About 2–4 mm of bulbar conjunctiva are left attached at the limbus, to permit manipulation of the globe with forceps during the enucleation procedure. (c) By blunt–sharp dissection with curved tenotomy scissors, the extraocular muscle insertions to the globe are excised. The globe is rotated in different directions to provide the optimal exposure during the dissection process. (d) The optic nerve is clamped by curved hemostat and transected by curved enucleation or Metzenbaum scissors. Special care is required to prevent touching the posterior globe with the tips of the scissors during optic nerve incision. Once the optic nerve has been cut, the globe can be rotated forward for incision of any remaining fascial attachments. (e) The nictitating membrane is protracted by thumb forceps and its base clamped with two curved hemostats. The structure is excised by Mayo scissors. The lacrimal gland may be removed at this time from beneath the lateral orbital ligament. (f) The two layers of closure include apposition of the rostral portion of Tenon’s capsule by simple interrupted absorbable sutures. The skin–orbicularis muscle layer is apposed with simple interrupted non-absorbable sutures. (g) Two weeks following enucleation in a young Labrador Retriever and suture removal.

To sever the optic nerve and the adjacent posterior ciliary arteries, a small curved hemostat or enucleation forceps are carefully positioned posterior to the globe (Fig. 4.9d). With curved Metzenbaum scissors or the specially curved enucleation scissors, the optic nerve and surrounding blood vessels are transected just anterior to the hemostat. Placement of the scissors is critical to avoid any contact with the posterior sclera and to prevent inadvertent incision of the posterior segment of the eye. The globe is carefully removed from the orbit to permit placement of a ligature deep to the hemostat still clamped to the optic nerve and accompanying blood vessels. The orbit is now carefully examined for any bleeders, and ligatures or point electrocautery applied if needed.

If an intraorbital implant is not used, parts of the remaining extraocular muscles and periorbital fascia are apposed with 2-0 to 4-0 simple interrupted absorbable sutures to reduce the dead space within the orbit. The remaining bulbar conjunctiva and anterior Tenon’s capsule are apposed with 2-0 to 4-0 simple interrupted absorbable sutures. With closure of the bulbar conjunctiva, 4–6 mm of the eyelid margins (including the medial and lateral canthi, and nictitating membrane) are excised circumferentially with tenotomy or strabismus scissors. The nictitating membrane is protracted, and two hemostats are overlapped and clamped at its base (Fig. 4.9e). The remaining nictitating membrane, complete with gland, is excised by tenotomy or strabismus scissors. The remaining eyelids (including the septum orbitale) are closed and apposed with 3-0 to 5-0 simple interrupted non-absorbable sutures (Fig. 4.9f,g).

If an orbital prosthesis is planned, an 18–22 mm sterile silicone sphere (Jardon Eye Prosthetics Inc., Southfield, MI) or methyl methacrylate sphere (Storz Instrument Company, St Louis, MO) is usually selected (Fig. 4.10). The surface of the silicone sphere is scarified or roughened with several incisions via scalpel blade to roughen its smooth surface and facilitate orbital retention. The sphere is inserted, and the extraocular muscles and endorbita are apposed about the sphere. An alternative method is the placement of mesh implants on the anterior surface of the orbital rim to prevent postoperative eyelid and orbital shrinkage.

Fig. 4.10 Placement of a 14–22 mm silicone or methacrylate sphere after enucleation in the dog and cat. Two-layer closure is recommended with apposition of the dorsal and ventral periorbita and Tenon’s capsule with simple interrupted absorbable sutures, and the eyelid skin and orbicularis oculi muscle layer with simple interrupted non-absorbable sutures. In both cats and horses these orbital implants are more apt to extrude than in dogs.

(Modified with permission from Nasisse MP, van Ee RT, Munger RJ, Davidson MG 1988 Use of methyl methacrylate orbital prostheses in dogs and cats: 78 cases (1980–1986). Journal of the American Veterinary Medical Association 192:539–542.)

Transpalpebral (‘en bloc’) enucleation

The transpalpebral enucleation technique differs from the subconjunctival procedure in that the surgical entry starts at the level of the eyelids, and the deeper aspects of the eyelids and the entire palpebral, fornix, and bulbar conjunctivae, and nictitating membrane are excised (‘en bloc’ method). This technique is performed more frequently in the large animal species. Although more tissues are excised in this procedure, the conjunctival and corneal surfaces are avoided, thereby reducing the chance of orbital contamination and postoperative infection. This method is preferred when infections of the globe and conjunctival surfaces are present. Because the entire conjunctiva is excised, exposure and visualization of the deeper orbital tissues are facilitated.

After draping, the eyelids are apposed with simple continuous 3-0 to 4-0 sutures, thereby closing the palpebral fissure (Fig. 4.11a). The eyelid skin is incised circumferentially by scalpel blade about 6–8 mm from the eyelid margins to avoid the bases of the meibomian or tarsal glands (Fig. 4.11b). The skin incision is carefully deepened until the submucosa of the palpebral conjunctiva is reached. Then, with blunt dissection with Steven’s tenotomy, strabismus or Metzenbaum scissors, the incision is continued under the conjunctival fornices, and onto the globe and under the bulbar conjunctiva (Fig. 4.11c). The procedure continues using the same steps as the subconjunctival method. Dissection within the sub-Tenon’s space between the sclera and Tenon’s capsule will usually minimize hemorrhage. All of the extraocular muscles are severed at their insertions (Fig. 4.11d). Isolation, clamping by curved hemostat, incision of the optic nerve, and removal of the globe follow (Fig. 4.11e). The Vicryl® ligature is carefully positioned deep to the hemostat on the optic nerve stump.

Fig. 4.11 Enucleation – transpalpebral approach: In this technique the globe is removed with the eyelids sutured or clamped together. (a) The palpebral fissure is closed by suturing the eyelids together with a continuous non-absorbable suture. Alternatively, the eyelids can be clamped together by Allis or towel forceps. (b) The eyelid skin and orbicularis oculi muscle layers are incised for 360° to the level of the tarsoconjunctiva. The incision is usually about 6–8 mm from the eyelid margins to avoid the bases of the meibomian glands. (c) With the sutured eyelids clamped by Allis forceps, the dissection is continued by small curved Metzenbaum scissors around the conjunctival fornices and onto the globe. (d) Once the sub-Tenon’s space is entered about the globe, the different extraocular muscle insertions are isolated and transected. Hemorrhage is usually minimal as long as the surgical plane remains in the sub-Tenon’s space. (e) Once the posterior orbit is entered, the optic nerve is carefully isolated, clamped by a curved hemostat, and incised by curved scissors posterior to the clamp. (f) The first of two layers of closure consists of apposition of the orbital septum with simple interrupted or simple mattress absorbable sutures. (g) The second and last closure is apposition of the eyelid and orbicularis oculi muscle layer with simple interrupted non-absorbable sutures.

Closure of the anterior periorbital fascial tissues with simple interrupted 3-0 to 5-0 absorbable sutures, with or without an orbital prosthesis, helps to reduce the dead space within the orbit. The orbital septum within the eyelids is apposed with 3-0 to 4-0 simple interrupted or horizontal mattress absorbable sutures (Fig. 4.11f). The eyelid–subcutaneous layer is apposed using the same type of suture and suture pattern. The eyelid skin is apposed with several 3-0 to 4-0 simple interrupted non-absorbable sutures (Fig. 4.11g).

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