Chapter 140 Diseases of the Orbit
Disorders of the orbit are relatively uncommon in companion animals. Because the area cannot be directly visualized without sophisticated imaging devices or complicated surgical procedures, even the simplest of lesions can pose a diagnostic and therapeutic challenge.
ANATOMY
Orbital Cavity
• Only the medial wall, part of the orbital roof in the dog, and the entire orbital roof in the cat are bony. The medial wall is a thin septum that separates the orbit from the nasal cavity. The floor of the orbit is muscular, with the exception of a small shelf of maxillary bone in the cat. A ligament that bridges the space between the frontal and the zygomatic bones completes the lateral aspect of the canine and feline orbit.
• Three prominent openings at the apex of the orbit permit the passage of its major vessels and nerves.
Orbital Contents
• The contents of the orbit are enclosed by a fibrous membrane, the periorbita, that attaches to the orbital wall at the optic foramen and extends to the orbital rim. The periorbita contains sympathetically innervated smooth muscle fibers.
• The anterior limit of the orbit is formed by a sheet of connective tissue, the orbital septum, that extends from the periorbita at the orbital margin to blend with the tarsus of the lid.
• Of the seven extraocular muscles, six originate at the orbital apex. The inferior oblique arises from the anteromedial orbital wall.
• The orbital fat pad cushions the orbital contents and is found between the periorbita and the surrounding structures. Fat is also located within the periorbita between the extraocular muscles.
• In addition to numerous arteries, veins, and autonomic nerves, the second, third, fourth, and sixth cranial nerves and the ophthalmic branch of the trigeminal nerve traverse the orbit.
• The nictitans occupies the ventronasal portion of the orbit and moves passively in relation to globe movements in the dog. In addition, movement of the feline nictitans is sympathetically controlled.
• The lacrimal gland is contained within the periorbita near the superotemporal aspect of the globe.
EXOPHTHALMOS
Developmental Disorders
Congenital Cysts
• Orbital cysts develop in association with other ocular anomalies, appearing as appendages of microphthalmic eyes or entirely supplanting a recognizable globe.
Craniofacial Deformities
• Cyclopia is a rare lethal anomaly in which the orbits and their contents are fused into a single structure. A single midline orbit and a dysplastic eye appearing as either a single or a bilobed sphere characterize the anomaly. The nose and maxilla are rudimentary. The defect has been reported in offspring of cats administered griseofulvin during the first half of gestation. There is no treatment for this anomaly.
Arteriovenous Malformations
Clinical Signs
• Exophthalmos is associated with a pulsatile murmur auscultated over the orbit. The bruit diminishes when the carotid artery is compressed.
Diagnosis
• Confirm the malformation in the anesthetized patient using orbital ultrasonography, color Doppler imaging, or contrast venography.
Acquired Disorders
Inflammatory or Infectious Disorders
Orbital Cellulitis or Orbital Abscess
Orbital inflammatory disease is a common cause of acute, painful, unilateral exophthalmos.
Etiology
• Trauma: Trauma produces both septic and nonseptic inflammation. Infections follow breaching of the oral mucosa or eyelids by bite wounds or foreign objects, such as bones. Blunt trauma may damage the osseous and soft tissues of the orbit. Iatrogenic injury may occur as a consequence of aggressive or improper dental extraction techniques. Occasionally, a foreign body will be retained within the orbit, causing recurrent inflammation.
• Infection: Aerobic and anaerobic bacteria, fungi, and parasites infect the orbit primarily or extend from the paranasal sinuses and nasal cavity. Orbital fungal pyogranulomas can occur with blastomycosis, cryptococcosis, and coccidioidomycosis. In cats, Aspergillus and Penicillium have been implicated in orbital cellulitis secondary to sinusitis. Larvae of Dirofilaria immitis and Ancylostoma species, and Toxocara canis produce primary orbital cellulitis by invading canine orbital tissues. The nasal mite Pneumonyssus caninum is a documented cause of orbital cellulitis and sinusitis in a dog. Even Cuterebra larvae occasionally invade the orbit. Granulomatous nodules containing Onchocerca species Nematodes have been identified in the canine retrobulbar space.
• Extension of disease: Tooth root abscesses (particularly maxillary PM3, PM4, M1, and M2) are the most common cause of orbital cellulitis in dogs and cats. Disorders of the frontal or paranasal sinuses, nasolacrimal system, temporal and pterygoid muscles, or zygomatic salivary and lacrimal glands may also cause secondary orbital inflammation or infection. Inflammation of the posterior sclera may affect adjacent orbital tissues.
• Rare proliferative inflammatory disorders: Referred to as pseudotumors, these may mimic orbital neoplasms. Examples include fibrous histiocytoma, undifferentiated inflammatory pseudotumor, and eosinophilic granuloma. In cats, proliferation of spindle cells and fibrovascular tissue within and adjacent to the sclera decreases ocular motility and dictates enucleation for relief of progressive lagophthalmia and keratitis.
Clinical Signs
• Orbital inflammation is characterized by a relatively acute unilateral exophthalmos, with conjunctival hyperemia, chemosis, third eyelid protrusion, and pain on opening the mouth.
Diagnosis
• Diagnosis is based upon history and physical examination. Consider sedation in painful patients objecting to examination.
• Close the eyelids and retropulse (i.e., push) the globe into the orbit to rule out space-occupying masses.
• Thoroughly evaluate the oral cavity. If dental disease is not readily apparent, examine the caudal maxillary teeth with a dental explorer and periodontal probe.
• Radiographically localize orbital foreign bodies with four views: lateral, ventrodorsal, oblique, and frontal. A wire ring positioned over the limbus may serve as a helpful point of reference. Dental radiography should also be performed.
• Orbital ultrasonography of diffuse cellulitis is characterized by a generalized loss of orbital tissue definition. The optic nerve and extraocular muscles are less easily visualized when compared with the normal eye. An abscess may appear as a hypoechoic area within a well-defined hyperechoic wall.
Treatment
• Administer a broad-spectrum systemic antibiotic such as amoxicillin clavulanate for at least 10 days. Response to antibiotic therapy is generally rapid, with resolution within 1 to 2 weeks.
• Protect the corneal surface with an antibiotic ophthalmic ointment in cases of extreme exophthalmos.
• Alleviate the acute swelling and discomfort with warm periocular compresses and a systemic nonsteroidal anti-inflammatory agent (aspirin at a dosage of 10–25 mg/kg q12h PO in dogs). Corticosteroid therapy is controversial because of the potential for sepsis in some patients.
• Establish ventral drainage through the mouth if no response is seen within 48 to 72 hours. Incise the mucous membrane behind the last upper molar with a #15 Bard-Parker scalpel blade. Carefully insert a hemostat through the pterygoid muscle and open gently in the retrobulbar space. Insert a sterile, open-ended tomcat catheter through the mucosal opening to obtain aspirates for cytology and culture.
Masticatory Muscle Myositis (Eosinophilic Myositis)
Clinical Signs
• Temporalis, masseter, and pterygoid muscle swelling is accompanied by fever, pain on opening the mouth, difficulty eating, and trismus (spasm of masticatory muscles).
• Ocular signs include exophthalmos, conjunctival hyperemia, eyelid edema, and third eyelid protrusion.
• The German shepherd appears most susceptible. Shetland sheepdogs, golden retrievers, and Weimaraners are also predisposed, but any canine breed may be affected. The mean age of onset is 3 years.
• Acute episodes may last 10 to 20 days. Recurrences lead to muscle atrophy and decreased prominence of the globe.
Diagnosis
• Confirm the clinical diagnosis with temporalis muscle biopsy samples to demonstrate mononuclear and eosinophilic infiltrates and type 2M myofiber antigen-antibody complexes (see Chapter 130).
Bilateral Extraocular Polymyositis
Clinical Signs
• Affected dogs range in age from 5 to 72 months; 80% are between 7 and 10 months of age. Females are affected twice as often as males. Golden retrievers, Doberman pinschers, and springer spaniels are the most commonly affected breeds.
Diagnosis
Document the extraocular muscle swelling with orbital echography, computed tomography, or magnetic resonance imaging. Confirm the clinical diagnosis with extraocular muscle biopsy to demonstrate the mononuclear cell infiltrate of CD3+ T lymphocytes and macrophages directed against fine extraocular myofibers (see Chapter 130). Complete blood counts and serum chemistry values are normal.
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