8-year-old male castrated Boston Terrier

• An 8 year-old, male, castrated Boston Terrier was referred to the Texas A&M Ophthalmology Service from his primary veterinarian for a two week history of exophthalmos of the oculus dexter (OD).
• Retrobulbar aspirates performed by the primary veterinarian two and a half months prior to referral were non-diagnostic.
• The patient responded minimally to antibiotics and anti-inflammatories. Upon presentation, ophthalmic examination revealed marked, fluctuant, non-painful, right-sided swelling of the temporalis and masseter muscles. Associated exophthalmos and lagophthalmos were also noted OD.
• The chemistry panel and complete blood count were within reference intervals except for mild hyperalbuminemia indicating mild dehydration. Thoracic radiographs were unremarkable. Due to the lack of response to conservative therapy, advanced imaging was pursued.

Imaging below includes (1) FLAIR and T2-Fat Saturation transverse; (2) T2 transverse; (3) RAGE Fat Saturation; (4) RAGE Fat Saturation, with/without contrast

• MRI and CT examinations reveal a large, irregularly shaped, well-demarcated mass in the right retrobulbar space and muscles of the head. On MRI, the mass is strongly T2W/FLAIR hyperintense, hypointense (to adjacent muscle) in T1W fat suppressed images, and hypointense on T2W fat saturated sequences.
• There is minimal contrast enhancement peripherally, and there are T2W hypointense and T1W hyperintense parallel linear strands through the dorsal and ventral aspects of the mass.
• On CT performed subsequently for radiation and/or surgery planning, the mass is predominantly fat attenuating with some linear soft tissue striations. The mass extends caudal, dorsal, and lateral to the right zygomatic arch and extends into the right temporalis and masseter muscles.
• Due to the mass, the right globe and zygomatic salivary gland are rostrally displaced and the right temporal and masseter regions are expanded.

Given the fatty characteristics of the mass, the primary differential was an infiltrative lipoma of the right side of the head with involvement of the right temporalis and masseter muscles and extension into the right retrobulbar space. A liposarcoma was deemed unlikely given the relative homogeneity of the mass and lack of a significant contrast-enhancing soft tissue component (1).

• Retrobulbar disease can be characterized as primary or secondary. Primary disease includes neoplasia, infection, and trauma of orbital structures, while secondary disease includes infection or neoplasia of adjacent structures that extends into the retrobulbar space. The limited capability of physical examination in assessing the retrobulbar space often necessitates cross sectional imaging to further evaluate the disease.
• Clinical features as well as CT, MRI, and ultrasound findings are used to determine the origin and characterize the disease. Regardless of disease type, retrobulbar masses present with a variety of clinical signs; however, the pathognomonic sign for a space occupying retrobulbar lesion is exophthalmos. Other findings associated with this lesion include decreased ocular retropulsion, strabismus, protrusion of the third eyelid, pain, and blindness. Tumors, in particular, cause slowly progressive and non-painful symptoms( 2). Of these symptoms, our patient displayed exophthalmos and decreased retropulsion of the right eye. Retrobulbar lipomas are not well documented in canines due to their low incidence rate.
• Only two case reports of canine retrobulbar lipomas have been published in the literature. In one case, a dog that presented with exophthalmos, protrusion of the nictitating membrane, and decreased retropulsion of the left eye was worked up as a retrobulbar mass. An ocular ultrasound revealed a hypoechoic mass that was aspirated. Like our patient’s fine needle aspirate, the results were inconclusive (3). A CT was then performed which revealed a space occupying hypoattenuating mass in the retrobulbar space that was not contrast enhancing. The mass had well defined margins and displaced the left globe.
• Thoracic radiographs and abdominal ultrasound were performed to stage the patient for spread of the tumor. Because of the lack of metastasis and CT findings, the primary differential was a primary lipid-based tumor. Excision was surgically performed with a ventral transpalpebral anterior orbitotomy (3). Another reported case of an orbital lipoma was diagnosed with ultrasound. This patient, unlike ours and other case reports, did not have exophthalmos. The patient presented with a subconjunctival swelling that was aspirated and diagnosed as an orbital corpus adiposum prolapse. Two years later, the mass swelled to the point of ocular irritation and the patient was reexamined. An ultrasound revealed a well demarcated hypoechoic mass that was presumed to be an orbital lipoma and was removed surgically (4).
• In human medicine, an orbital lipoma was diagnosed with MRI. The findings of the mass include high signal intensity mass on T1W and T2W/FLAIR sequences and complete suppression on STIR images. There was no contrast enhancement. The differentials in this case included an orbital lipoma and orbital fat prolapse (5). One of the more difficult aspects of identifying the lesion type of retrobulbar disease is that many of the clinical signs are similar among different disease processes.
• Advanced imaging such as CT is useful in determining the degree of infiltration of the lesion, but not always useful in determining the type of lesion. In a retrospective study of CT findings of retrobulbar disease, orbital osteolysis, periosteal reaction, and the presence of retrobulbar mass were significantly associated with neoplasia, with osteolysis and periosteal reactions being more frequently associated with malignant neoplasia. These bone changes were not seen in our patient’s CT or MRI scans, supporting the suspicion that this is a benign but locally invasive tumor (2).
• Though retrobulbar neoplasms are more likely to be malignant, benign tumors occur 10% to 25% of the time (3). This case illustrates the value of MRI and CT in characterizing retrobulbar space disease and highlights an unusual cause of exophthalmos. Case Outcome Referral to an oncologist was recommended to discuss treatment options, including surgery with radiation therapy. At the time of this writing, the referral has not been pursued by the client and the patient has been lost to follow up.

1. Spoldi E, Schwarz T, Sabattini S, Vignoli M, Cancedda S, Rossi F. Comparisons Among Computed Tomographic Features Of Adipose Masses In Dogs And Cats. Vet Radiol Ultrasound. 2017 Jan;58(1):29-37.

2. Winer JN, Verstraete FJM, Cissell DD, Le C, Vapniarsky N, Good KL, Gutierrez CJ, Arzi B. Clinical Features and Computed Tomography Findings Are Utilized to Characterize Retrobulbar Disease in Dogs. Front Vet Sci. 2018 Aug 21;5:186.

3. Charnock L, Doran B, Milley E, Preston T. Canine retrobulbar lipoma excision through a ventral transpalpebral anterior orbitotomy. Can Vet J. 2020 Mar;61(3):257-262.

4. Williams DL, Haggett E. Surgical removal of a canine orbital lipoma. J Small Anim Pract. 2006 Jan;47(1):35-7.

5. Shrestha P, Shrestha GB. Orbital Lipoma as an Uncommon Cause of Unilateral Proptosis: A Case Report. Int Med Case Rep J. 2020 Sep 11;13:415-418.