Diagnostic Ophthalmology

History and clinical signs

A 10-month-old female golden retriever dog was referred to the ophthalmology service at the Pulse Veterinary Specialists and Emergency clinic with bilateral exophthalmos and mild exotropria (Figure 1). The referring veterinarian had tentatively diagnosed bilateral orbital disease and referred the dog for a complete diagnostic evaluation. The menace responses, direct and consensual pupillary light, palpebral and oculocephalic reflexes were present in both eyes. However, the dorsal, ventral, lateral, and medial ocular movements were all markedly reduced. Schirmer tear tests (Schirmer Tear Test Strips; Alcon Canada, Mississauga, Ontario) were 25 mm/min bilaterally. The intraocular pressures were estimated with a rebound tonometer (Tonovet; Tiolat Oy, Helsinki, Finland), and were 13 mmHg bilaterally. Topical ophthalmic tropicamide (Mydriacyl; Alcon Canada, Mississauga, Ontario) was applied to both corneas and both pupils dilated within 20 min. Biomicroscopic (Osram 64222; Carl Zeiss Canada, Don Mills, Ontario) and indirect ophthalmoscopic (Heine Omega 200; Heine Instruments Canada, Kitchener, Ontario) examinations were completed and no abnormalities were detected. Fluorescein stain (Fluorets; Bausch & Lomb Canada, Markham, Ontario) was applied and rinsed from the corneal surfaces and the eyes were examined under cobalt blue filtered light; staining was not evident.

Figure 1.

A 10-month-old golden retriever with bilateral exophthalmos and exotropia.

What are your tentative clinical diagnoses, diagnostic and therapeutic plans, and prognosis?

Our tentative clinical diagnosis was extraocular polymyositis. We advised and conducted a routine complete blood cell count, serum biochemical profile, a serum toxoplasmosis titer, and urinalysis. The database was within normal reference ranges and the serum toxoplasmosis titer was negative. We completed a routine sedation, general anesthetic induction, intubation and maintenance, were carried out while a computed tomography (CT) scan of the skull was completed. Bilateral enlargement and moderate contrast enhancement of all extraocular muscles were confirmed (Figure 2). The medial and ventral rectus muscles were most affected, followed by the ventral oblique, with the right ventral oblique slightly larger than the left. The remaining structures of the skull were normal. A routine bilateral presurgical ocular surgical preparation was completed with dilute betadine solution and bilateral superior rectus muscle biopsies were harvested and fixed in formalin and submitted for routine histologic examination.

Figure 2.

Post-contrast, transverse CT image in a soft tissue window (400W/50L) at the level of the orbit. The right side of the animal is on the left. The medial (arrowheads) and ventral (black arrows) rectus muscles of the globe are severely enlarged and contrast enhancing. There is slight enlargement of the right ventral oblique muscle (white arrow).

Discussion

The clinical signs and signalment are very typical of previous reports of an uncommon condition that predominates in young golden retrievers, extraocular polymyositis (1–5). The myositis is limited to the extraocular muscles and appears to be immunemediated, based on the T-lymphocyte driven reaction (3,5). The differential diagnostic considerations are limited and include Toxoplasma gondii orbital myositis/cellulitis and infiltrative orbital round cell neoplasia such as lymphosarcoma (5,6). Toxoplasma extraocular myositis and orbital cellulitis and neoplasia were ruled out with serologic testing and histologic examination of the biopsy. The CT scan confirmed marked enlargement of all the extraocular muscles and no other abnormalities. The diagnosis was confirmed histologically as a bilateral lymphocytic extraocular myositis (3).

This condition was first reported in 1989 by Carpenter et al (3). The bilateral exophthalmos without 3rd eyelid prolapse in the dog is consistent with the intraconal swelling (5). There are no reports of this condition with involvement of other muscles or organ systems. Some have reported that diagnostic testing beyond clinical examination is not required and even considered unethical (6). Although infiltrative neoplastic orbital disease or Toxoplasma gondii myositis are rare conditions, both have the potential to respond at least partially to many months of immune suppressive therapy. In our opinion, this warrants, at a minimum, extraocular muscle biopsies to confirm the diagnosis before initiation of immunosuppressive therapy. In the ideal situation, cross-sectional imaging (computed tomography or magnetic resonance imaging) should also be used to screen for other causes of retrobulbar intraconal disease.

Extraocular myositis in dogs is treated by systemic immune suppression for many months (5,6). Relapses and progressive extraocular muscle inflammation are to be avoided as enophthalmos and strabismus are known complications due to extraocular muscle fibrosis. These complications are difficult if not impossible to treat effectively. Systemic administration of corticosteroids, azothioprine, cyclosporine have been reported (5,6). After discussion with Dr. David Ramsey, oral cyclosporine was chosen in this case as it is an excellent T-lymphocyte suppressor with minimal systemic complications. A recheck at approximately 5 mo revealed marked improvement (Figure 3). Prolonged therapy is planned for 6 to 12 mo, with gradual reduction and careful follow-up evaluations. If recurrence of exophthalmos or enophthalmos and strabismus are noted, extension of the therapy is planned. At the time of publication, this dog remains on oral cyclosporine therapy, which is steadily being reduced; relapses have not been encountered.

Figure 3.

The same dog noted in Figure 1 after approximately 5 mo of oral cyclosporine therapy.