Orbit

Chapter 17 Orbit





ANATOMY


The orbit is the cavity that encloses the eye. The two orbital patterns in domestic animals are as follows:









The orbit separates the eye from the cranial cavity, and the foramina and fissures in its walls determine the path of blood vessels and nerves from the brain to the eye. The walls of the equine orbit are formed by the frontal, lacrimal, zygomatic, temporal, presphenoid, palatine, and maxillary bones, which are similar in other species. In the dog and cat the dorsolateral portion of the orbit is spanned by the dense collagenous orbital ligament, which passes from the zygomatic process of the frontal bone to the frontal process of the zygomatic bone. The basic foramina and fissures of the orbit are the orbital, rostral and caudal alar, oval, supraorbital, ethmoidal, lacrimal, maxillary, sphenopalatine, round, and palatine. In cattle, the orbital foramen and the foramen rotundum fuse to form the foramen orbitorotundum. The vessels and nerves that pass through these foramina and fissures in the dog are shown in Figures 1-16, 1-17, and 1-19 to 1-22 in Chapter 1 and Figure 17-6.



The position of the orbit within the skull varies with species. In cattle, sheep, and horses the eyes are situated laterally, giving panoramic vision, whereas in dogs and cats the eyes are located more anteriorly, which emphasizes binocular overlap between the two eyes. The visual, orbital, and optic axes, defined as follows, do not coincide (Figure 17-7):






The angle formed by the optic axes, a measure of binoc-ular overlap, is shown in different species in Figures 17-8 to 17-10.





The relationships of the orbit to the paranasal sinuses, teeth, zygomatic gland, and ramus of the mandible are important, because they affect incidence, diagnosis, and pathogenesis of clinical diseases of the eye and orbit, as follows:


Infections of the sinuses or nasal cavity may enter the orbit in all domestic species (Figure 17-11). The junction of the frontal, lacrimal, and palatine bones in the medial wall of the canine orbit (see Figures 17-2 and 17-11) is often thin and may be eroded by disease processes in the nasal cavity, which then enter the orbit. The bone is thicker in horses (Figure 17-12).



Enlargement of the canine and feline zygomatic salivary gland may cause increased pressure within the orbit or protrusion of the gland into the ventral conjunctival fornix (Figure 17-13). When the mouth is opened, especially in dogs and cats with greater mobility of the mandible, the vertical ramus of the mandible moves forward, exerting pressure on the orbital contents. This is painful if orbital contents are inflamed.




The orbital contents are completely enclosed in a sheet of connective tissue—the periorbita—that lies next to the bone in the bony parts of the orbital wall and that is thicker laterally where the wall is incomplete (in carnivores). The periorbita is reflected over the extraocular muscles and forward over the globe to become Tenon’s capsule, lying beneath the conjunctiva (Figure 17-14). The periorbita is continuous with the periosteum of the facial bones at the orbital rim, with the orbital septum anteriorly, and with the dura mater of the optic nerve. The orbital fat pad lies between the periorbita and the extraocular muscles. Intraorbital fat lies between the muscles and fascial layers (Figure 17-15). In animals with an incomplete bony orbit, the masticatory muscles play a critical role in providing posterior support for the orbital contents. Orbital disease processes may thus be located in one of the following three planes:







The lacrimal gland lies beneath the orbital ligament on the dorsolateral surface of the globe (see Figure 17-14). The base of the third eyelid and gland is held down by the orbital retinaculum, which are poorly defined sheets of collagenous tissue continuous with the periorbita but that contain smooth muscle with sympathetic innervation.




PATHOLOGIC MECHANISMS


Because the orbit forms a semiclosed space, increases and decreases in the volume of its contents affect the position of the eye in relation to the orbital rim and to the other eye. Space-occupying lesions (Figure 17-17) push the eye forward, causing exophthalmos, and often the third eyelid also protrudes as it is passively forced out of the orbit. In dogs and cats orbital masses usually result in swelling of the tissues caudal to the last upper molar tooth, because the orbital floor is only soft tissue in this area. With decreased volume of the orbital contents (e.g., dehydration or atrophy of fat or muscle), the eye sinks further into the orbit—enophthalmos—and the third eyelid protrudes. Osteomyelitis of the bones forming the orbit due to organisms such as Cryptococcus and Actinomyces spp. may also cause exophthalmos.





The position of space-occupying lesions alters the direction of displacement of the globe and is used to determine the site of the offending mass (Figure 17-18) and the optimal route of surgical exploration.



Because the subconjunctival tissues and the orbit are connected, orbital diseases frequently cause chemosis. If the orbital lesion compresses the orbital veins, posterior venous drainage diminishes and chemosis is further increased. In horses, orbital swelling or inflammation commonly causes filling of the depression superior to the upper eyelid.




Because of the many tissue types present, numerous kinds of neoplasms may affect the orbit. The most common causes of exophthalmos in one case series of dogs and cats were neoplasia (52%), orbital abscesses/cellulitis (30%), hematoma (9%), zygomatic mucocele (5%), arteriovenous fistula (2%), and eosinophilic myositis (2%).



DIAGNOSTIC METHODS


The diagnosis of orbital disorders requires a complete ophthalmic examination and perhaps additional special diagnostic techniques, as follows (Figure 17-19).





B-scan ultrasonography (see Chapter 5) is very useful for evaluation of soft tissue masses within the orbit and may guide further diagnostic procedures, such as fine-needle aspiration.

Magnetic resonance imaging (MRI) and computed tomography (CT) (Figures 17-19 through 17-21) yield superior definition in localizing orbital lesions. They allow the extent of disease to be better estimated and enable more accurate surgical planning. CT may be used to guide fine-needle aspiration or biopsy, thus avoiding exploratory orbitotomy.














ORBITAL DISEASES


A summary of orbital diseases, classified by type, is given in Table 17-2.


Table 17-2 Summary of Orbital Diseases















TYPE OF DISORDER CONDITION CLINICAL SIGNS
Developmental abnormalities









Trauma





Aug 11, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Orbit
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