Exophthalmia Versus Enophthalmia

Changes in orbital volume manifest as exophthalmos or enophthalmos, depending on whether the orbital volume has increased or decreased respectively, and are influenced by the specific location of tissue changes within the orbit. Diffuse orbital volume increases, or mass lesions located inside the muscle cone behind the globe, typically displace the globe directly forward. Focal lesions outside the muscle cone in a more nasal, temporal, inferior, or superior position in relation to the globe will displace or rotate the globe off‐axis into a direction opposite to the mass lesion. These often subtle changes can help localize the lesion within the orbit and assist planning an approach for biopsy acquisition and/or surgical exploration.

The degree of exophthalmos or enophthalmos is usually estimated by determining the position of the axial cornea relative to the orbital ligament and other eye, both from a distance and from above. Exophthalmos can be measured directly with a Luedde or Hertel exophthalmometer, but these instruments have not been utilized in veterinary ophthalmology. Nictitating membrane protraction in the dog results from contraction of the retractor oculi muscles with the resultant forward displacement of orbital fat to push its base outward. A most passive nictitating membrane protrusion can accompany increased orbital volume. Conversely, with any decrease in volume or/and orbital fibrosis, passive nictitans protrusion occurs due to enophthalmos.

Palpation of Orbital Structures

Some of the orbital contents can be palpated both caudal to the orbital ligament and from the oral cavity through the pterygoid muscle. The immediate retrobulbar tissues can be palpated by retropulsion of the globe. In normal mesaticephalic and dolichocephalic dogs but not in brachycephalic breeds, the globe can be displaced caudally for some distance. In the presence of space‐occupying orbital lesions, retropulsion of the globe will be restricted or impossible, and may be painful.

The bony rim of the orbit and the walls of the nasal cavity and paranasal sinuses should be carefully palpated and evaluated for any asymmetry.

Percussion of the paranasal sinuses can be helpful to determine the presence of sinus occupying inflammatory or neoplastic material. The presence of a symmetric flow of air through both nostrils should be evaluated, as sinonasal tumors can cause tumor invasion of the orbit.

The oral cavity is routinely inspected, because disease processes from the oral cavity or teeth may adversely affect the orbit and its contents. Conversely, orbital diseases may affect, or be visualized through observation of, adjacent structures, such as the oral cavity, e.g., swelling behind the last upper molar tooth. Restrictive myopathies, like masticatory muscle myositis (MMM), can cause restrictions or even an inability to open the mouth.

Clinical Signs

Inflammatory changes within the orbit are usually accompanied by pain, especially upon globe retropulsion and when opening the mouth. Movement of the vertical ramus of the mandible into the orbital space upon opening the mouth increases pressure on the orbital contents. Most dogs with orbital pain will therefore display difficulties with, or will vocalize during, some activities requiring jaw movement: chewing and eating (especially dry food, rawhides, and bones), playing with balls and sticks, barking, or yawning. Inflammatory changes in the orbit (cellulitis, abscess formation), which cause tissue edema, can be accompanied by conjunctival hyperemia and chemosis, and periocular swelling.

Other Clinical Signs

Impaired function of orbital structures can also manifest as reduced ocular motility, strabismus, abnormal globe position, anisocoria, blindness, and increased or reduced tear production. These clinical signs can be caused by mechanical entrapment, pressure atrophy and loss of nerve function, and neural stimulation of affected tissues. Congestion of episcleral vessels is caused by a decreased venous return, which is due to an increased orbital tissue and intravenous pressure.

In more extreme cases of exophthalmos, associated with orbital cellulitis, trauma, and advanced masses, lagophthalmos can ensue due to a decreased ability to close the eyelids. This quickly results in exposure keratitis, ulceration, and potential loss of the eye.

Ophthalmoscopy

Posterior indentation of the globe can be observed by ultrasound or ophthalmoscopically as an area of fundus elevation and altered reflectivity due to the altered angles of incident and reflected light (Figure 5.2). This indented fundus area should be observed during spontaneous eye movements. The indentation will not change position if it is caused by a mass attached to the globe, whereas unattached masses will cause an indentation that shifts position during globe movement. Even with marked deformation of the globe, intraocular pressure usually remains within normal range. However, in dogs with a relatively short palpebral fissure, the globe can be pressed against the lids by a space‐occupying lesion, slightly elevating the intraocular pressure.

Auscultation

In rare cases of pulsating or intermittent exophthalmos associated with arteriovenous fistula or varix, auscultation of the orbit may reveal systolic murmur (“bruit”).

Anophthalmos

Complete absence of the eye, or anophthalmos, is very rare. In most cases, some remnants of ocular tissue can be identified histologically.

Cystic Eye, Microphthalmia, and Nanophthalmia

If remnants of ocular tissue are present histologically, a diagnosis of either a cystic eye or microphthalmia can be made. In true microphthalmia, the globe contains evidence of the presence of surface ectoderm (lens), neural crest migration, and neuroectodermal differentiation. Microphthalmia therefore presents as an abnormally small globe, with various other ocular anomalies involving the cornea, lens, uvea, vitreous, and retina. Vision may be normal, reduced, or absent. In contrast, a small but otherwise normal globe is called nanophthalmia. In the Doberman Pinscher, microphthalmia is associated with anterior segment and retinal dysplasia. In the homologous merle Australian Shepherd and other breeds, the microphthalmia is associated with equatorial staphylomas, persistent pupillary membranes, and retinal dysplasia, and an excessively white hair coat. Poorly pigmented animals (>90% white) are most affected. Collies and Shetland Sheepdogs with Collie eye anomaly often exhibit variable degrees of microphthalmia; this is especially true for merles (see Appendix A or access http://Optigen.com for the most recent information on DNA tests for canine eye diseases).

Vascular Anomalies

Orbital varices and arteriovenous fistulas are very rare orbital anomalies with few reported cases. Varices can be congenital as well as acquired. Dogs usually present with a nonpainful, pulsating or intermittent exophthalmos. A decrease in venous return from the head can influence the extent of exophthalmos if an orbital varix is present, with the exophthalmos worsening when the head is kept low or pressure applied to the jugular vein region. In arteriovenous fistula cases, not varices, a systolic murmur (“bruit”) can be auscultated in the orbital region. Treatment is difficult in all cases, and the prognosis is grave. In one case, attempted ligation of orbital vessels caused massive hemorrhage as well as subsequent enucleation of the globe and ligation of the common carotid artery. Orbital exenteration and careful hemostasis should be curative.

Orbital Cysts

Orbital cysts are rare in the dog. A retrobulbar dermoid cyst in a Dachshund, containing a yellowish, viscous fluid and long black hair, has been described. On histopathological examination, the cyst wall consisted of a keratinized squamous epithelium. Surgical excision is recommended.

Inflammatory Lesions: Orbital Cellulitis/Abscess

In contrast to large animals, orbital inflammatory diseases are rather common in the dog. According to most studies, dogs presenting with non‐neoplastic orbital diseases are significantly younger than dogs presenting with neoplastic diseases (orbital abscesses at 4 years of age on average and mean age of tumor patients at 9.5 years).

Typically, dogs present with acute, unilateral exophthalmos, protrusion of the nictitating membrane, conjunctival hyperemia and chemosis, episcleral venous congestion, periocular swelling, serous to mucopurulent ocular discharge, and pain. The globe itself is usually normal and normotensive. Affected dogs are usually febrile and inappetent. The white blood cell count is often elevated with a neutrophilia.

The cause often remains unidentified. Foreign material is sometimes encountered with porcupine quills not infrequent. The point of entry for both foreign bodies and gunshot pellets is not always easily identified. Foreign bodies may enter the orbit from the oral cavity, percutaneously or through the conjunctival sac (Figure 5.4a–c). Hematogenous infection of orbital tissues is possible with the microorganisms invading from the oral cavity, sinuses, or tooth roots. Infections of the zygomatic gland may present as orbital cellulitis or abscess, and in these cases a swollen excretory duct may be seen lateral to the second molar tooth. Significant inflammatory diseases of the globe itself (panophthalmitis, scleritis) can also cause orbital cellulitis.

Extension from adjacent structures, exogenous trauma, and foreign bodies were the most common causes of infectious orbital disease. Staphylococcus spp. were isolated in 25%, Escherichia coli in 16.7%, Pasteurella multocida in 8.3%, and anaerobic bacteria (mostly Bacteroides and Clostridium spp.) in 30.5% of the canine patients in this study. In another study, Pasteurella spp. were the most common isolates from orbital abscesses. Fungal organisms are an uncommon cause of orbital disease. Onchocerciasis occurs in the southwestern United States and south‐central Europe, and in chronic cases, the live and dead worms are incorporated in subconjunctival, episcleral, and orbital granulomas causing corresponding clinical signs. Treatment consists of surgical excision and the postoperative use of microfilaricidal drugs.

Ultrasonography is a cost‐effective method to determine the presence of a drainable retrobulbar abscess and to screen for the presence of a foreign body. The type of foreign body material dictates which imaging modality is most useful for identification. Most foreign bodies are not recognized on plain radiographs, but metallic foreign bodies are easily demonstrated. MRI scans are contraindicated when a suspicion for a metallic foreign body exists, due to the risk of foreign body displacement under the influence of the strong magnetic field. A CT scan can directly pick up foreign bodies that are sufficiently dense (metal, glass, bone). Foreign bodies of lower density (plant material, wood, plastic, porcupine quills) may be identified in postcontrast studies as a filling defect with a contrast‐enhancing rim of reactive tissue. Obtaining samples for cytology and culture and sensitivity assays is an important step in the diagnostic process. FNAs and tissue biopsies can be used for these purposes.

The first and most important step in treating an orbital abscess is drainage (Box 5.1 and Figure 5.5). The oral mucosa behind the last upper molar tooth, which may show a fluctuating swelling, is incised. A closed hemostat is advanced through the pterygoid muscle and then opened and withdrawn without closing. Irrigation of the retrobulbar area is a controversial issue; however, irrigation with crystalline penicillin has been recommended. Systemic antibiotic therapy using broad‐spectrum antibiotics (cephalosporins, extended‐spectrum penicillins [e.g., ticarcillin], potentiated penicillins [e.g., amoxycillin–clavulanic acid], and carbapenems) is indicated pending the results of bacterial culture and sensitivity testing. Additionally, if no contraindications exist, these patients are usually treated with nonsteroidal anti‐inflammatory drugs. Hot packs also are beneficial and well tolerated by most dogs.

The globe itself must be treated symptomatically. In cases of lagophthalmos, lubrication of the ocular surface is important. In most cases, application of an antibiotic ointment three to four times daily is sufficient. The degree of exophthalmos often increases temporarily after drainage, and a temporary tarsorrhaphy may be necessary. Soft food should be offered until the globe is back in its normal position. The prognosis is usually good. Once diagnosed and treated with the appropriate systemic antibiotics, clinical signs often begin to regress within 48 h. In a series of 17 cases, 15 healed within a week, 1 healed after one recurrence, and 1 was lost to follow‐up. In most cases, the exophthalmos regresses within 36–48 h, and the general condition of the animal markedly improves.