Physical Examination
General physical exam was unremarkable with the exception of the ocular exam. Exophthalmos without strabismus was apparent in both eyes (Fig. 1). Menace response and palpebral and corneal reflexes were normal OU. There was a slight increase in resistance to retropulsion OU. No pain was elicited on retropulsion, palpation of the masseter or temporal muscle regions, or on manipulation of the mandible. No restrictions to ocular movement were noted in either eye. The nictitating membranes were in normal position and mobile. Anterior segment examination revealed bulbar conjuctival injection and chemosis, with multifocal 2-4 mm subconjunctival hemorrhages adjacent to the dorsal limbus OU as the only abnormalities. Pupillary light reflexes were brisk OU. Funduscopic examination was normal OU.

Figure 1.

Diagnostic Testing

Schirmer tear tests (STT): 17 mm/minute right eye (OD), 20 mm/min left eye (OS) (reference range 15-20 mm/min)

Intraocular pressure (IOP): 19 mmHg OD, 15 mmHg OS (reference range 15-25 mmHg)

Ocular Ultrasound: Diffuse increase in echogenicity in the region of the extraocular cone consistent with inflammation of retrobulbar structures

CBC: Unremarkable

Serum Chemistry: ALP 129 IU/L (reference range 20-115 IU/L)

Urinalysis: Unremarkable

Toxoplasma gondii IgM and IgG titers: Negative

Neospora caninum: Negative

Leptospira titers: Negative

Electromyography: Right lateral rectus showed increased spontaneous activity including fibrillation potentials and positive sharp waves.

2M antibody: Negative

Muscle Biopsies: Biopsies were taken from the right lateral rectus (Fig. 2A,B) and temporalis (Fig. 2C) muscles. The lateral rectus muscle showed areas of myonecrosis, fibrosis and mononuclear cell infiltrates composed predominantly of lymphocytes with occasional macrophages. No abnormalities were noted in the biopsy from the temporalis muscle.

Diagnosis
Extraocular muscle myositis with moderate fibrosis.

Treatment
Prednisone was initiated at an immunosuppressive dosage of 2.7 mg/kg (80 mg BID, weight 30 kg). Famotidine 0.67 mg/kg (20 mg SID) was prescribed for gastric protection during glucocorticoid therapy. Ampicillin and clavulonic acid 14.6 mg/kg (437 mg BID) and neomycin-polymyxin-dexamethasone ophthalmic solution (one drop OD TID) were prescribed to prevent post-operative infection and the latter for anti-inflammatory effect.

Outcome
The owners reported that there was rapid resolution of the clinical signs with the prescribed therapy (Fig. 3, 1 week post treatment). The first recheck was two weeks after initiation of glucocorticoid therapy and there was complete resolution of the conjunctival hyperemia, swelling, and exophthalmos. There was no strabismus, which may result from fibrosis post-inflammation and ocular motility was normal in both eyes. Prednisone was continued at decreasing doses for a total of 11 weeks. Follow up calls at 1,2,4, and 6 months revealed that the dog was free of clinical signs. At 10 months post diagnosis, mild, bilateral exophthalmos was again noted. Routine vaccinations had been administered 6 weeks prior to this episode and the significance of this was undetermined. Systemic glucocorticoid therapy (prednisone 2 mg/kg) was reinstituted by the RDVM with rapid resolution of the exophthalmos.

Comments
Bilateral exophthalmos is uncommon and is suggestive of myositis or infiltrative disease such as lymphoma. The signalment in this case of a young, female, Golden retriever with initially transient clinical signs of bilateral, acute onset of ocular swelling was consistent with the limited reports of EOM in the veterinary literature. EOM is distinct in that it involves only the extraocular muscles. The diagnosis of EOM is based on clinical signs, orbital imaging to localize affected tissues and response to treatment. Ocular ultrasound in this case was consistent with inflammation in the extraocular muscles though this imaging modality cannot precisely identify which tissues in the retrobulbar space are involved. MRI or CT are the diagnostics of choice for identifying affected structures, but these options were declined by the owner. Biopsies may be useful for differentiating EOM from other myositides and for prognostic purposes. Prognosis for complete recovery in cases of EOM is guarded to good depending on the degree of fibrosis. Recurrences are reported in up to 81% of the cases; however, these episodes are rapidly responsive to glucocorticoid therapy as well. Long term follow up has not been available in most cases.

Figure 3.