Orbital cysts are relatively uncommon in dogs. Congenital epithelial cysts have been most frequently reported and are divided
into categories based on the type of epithelium lining them: epidermoid, dermoid, conjunctival-dermoid, and neuroepithelial.10 Neuroepithelial cysts arise from developmental abnormalities in early ocular embryogenesis.10
Acquired cysts that have been reported are mucoceles and implantation cysts.10,13 These types of cysts often result from trauma to the orbital area that results in leakage of saliva from the zygomatic salivary
gland. The saliva causes inflammation and, eventually, necrosis of the surrounding tissue. The cysts present as fluctuant,
often nonpainful swellings in the conjunctiva, either dorsally or ventrally to the globe, and as exophthalmos.
Conjunctival dacryops were recently reported in two unrelated golden retrievers.14 This type of cyst develops from pockets of the lacrimal glandular tissue and can form wherever lacrimal gland tissue is
located. If a cyst is in the main lacrimal gland ductules, it can cause exophthalmos and ocular irritation. These cysts are
thought to develop with trauma, which leads to excessive secretion of tears and secondary ductal inflammation.14
Neoplasms should be suspected in all cases of exophthalmos in dogs but are more commonly seen in older animals. They can be
either primary or secondary and can be benign, locally invasive, or metastatic.15,16
Any tissue in the orbit may give rise to a neoplasm, but the most common primary orbital tumor in dogs is a meningioma.10,15,16 This tumor generally surrounds the optic nerve and causes exophthalmos and vision loss due to pressure on the optic nerve.
Although any primary malignant tumor has the capability to secondarily metastasize to the orbit, the most common is lymphoma.
When this type of tumor is diagnosed, it should always be considered a manifestation of a systemic process.
Orbital neoplasms, in contrast to abscesses, are generally minimally painful and slowly progressive. They often cause exophthalmos,
but tumors arising from the rostral portion of the orbit may produce enophthalmos and protrusion of the third eyelid. Retropulsion
of the globe is restricted.
Parasites and systemic mycotic infections can enter the orbit hematogenously or by local extension from the sinuses, nasal
cavity, brain, or globe. Cryptococcus neoformans, Blastomyces dermatitidis, Coccidioides immitis, Candida species, and Aspergillus species are mycoses that have been reported to invade the orbits of dogs and cats.10 Generally, animals with orbital mycoses have concurrent or preceding systemic clinical signs. On occasion, orbital biopsy
specimens are needed to make a diagnosis.10
Ocular onchocerciasis cases are more commonly seen in Europe but are becoming more frequent in the western United States,
particularly in California.10 Clinical signs consist of single or multiple red, raised lesions on the bulbar conjunctiva. Exophthalmos, protrusion of
the third eyelid, corneal edema, and anterior uveitis may also be present.
Hematomas of the orbit are generally associated with a history of trauma. They are characterized by exophthalmos, pain, and
other head and ocular injuries.
Swelling of the muscles of mastication adjacent to the orbit will often lead to exophthalmos because of the absence of a lateral
orbital wall causing pressure on the globe.17,18 Cases of eosinophilic myositis of the muscles of mastication present acutely and may be painful, making it difficult to
differentiate these cases from retrobulbar abscesses.17,18 In addition, both of these conditions often occur in young, large-breed dogs. One distinguishing feature is that abscesses
tend to be unilateral, while myositis tends to be bilateral.
Extraocular myositis may also cause exophthalmos.17,18 This disease is most common in young golden retrievers, and females are overrepresented. Characteristic clinical signs include
bilateral exophthalmos, chemosis, and retraction of the eyelids without protrusion of the third eyelid.
Other less common inflammatory diseases of the canine orbit include immune-mediated orbital disease, involving tissue such
as the periosteum, and zygomatic sialadenitis.10-14
Both orbital varices and arteriovenous fistulas have been reported in dogs, but they are rare. They can be congenital or associated
with trauma.19,20 Interestingly, cases of arteriovenous fistulas usually occur in young dogs and present with a nonpainful exophthalmos that
may be intermittent. The eye may also pulsate along with the systemic pulse.19,20
In one case of a puppy with a swelling in the orbit below the globe, the swelling pulsated and changed size with the changing
of the head position (Gionfriddo J, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort
Collins, CO: Unpublished data, 2012; Figures 3 & 4).
3 & 4. A 5-month-old puppy with its head held downward. Notice the large swelling, an orbital varix, ventral to the globe.
When the puppy’s head is held up, the varix shrinks in size.