Extraocular myositis in a young pit bull

Patient presentation

History

A 32-kg spayed pit bull age 14 months was presented April 21, 2020, to the Mount Laurel Animal Hospital primary care service in New Jersey for evaluation of intermittently bulging eyes, yawning, and refusing hard treats. On presentation, the patient was friendly but nervous. A comprehensive physical examination was attempted, but the dog was resistant to both a rectal and an oral examination. Abnormalities identified on physical examination included a class III malocclusion and ocular changes.

An in-house ophthalmic consultation and examination revealed exophthalmos affecting both eyes, with no protrusion of the third eyelid and moderate conjunctival hyperemia Figure 1). The dog was exhibiting no visual deficits. The left eye had a focal, triangular, temporal, anterior cortical cataract. The patient exhibited no obvious discomfort on palpation of the masticatory muscles bilaterally and no restriction of jaw mobility.

Diagnostics

• Serologic testing for masticatory muscle myositis, neosporosis, toxoplasmosis, and Lyme borreliosis was recommended but declined by the owners.
• Ophthalmic diagnostics were normal, including absent fluorescein stain uptake and a normal intraocular pressure bilaterally.

Summary

In summary, this patient was a young, large-breed dog with nonpainful exophthalmos. There was no change in positioning of the nictitating membrane bilaterally, no visual deficits, no systemic signs, and no comorbidities on presentation.

Treatment

After discussion with the owners emphasizing presumptive extraocular myositis based on presenting signalment, clinical signs, and examination findings, they elected to pursue empiric treatment.

• A slowly tapering course of oral immunosuppressive steroids was started at 2 mg/kg divided into twice-daily administration.
• A 3-week course of twice-daily oral doxycycline at 5 mg/kg was also prescribed to cover for potential infections.
• A recheck examination with the ophthalmology service 2 weeks after treatment started was recommended.

Ten days after initial presentation, the client called with an update, reporting that the dog’s eyes were back to normal but the dog was experiencing significant adverse effects (AEs), including a ravenous appetite and increased water consumption.

• Oral prednisone was tapered to 1 mg/kg divided into twice-daily administration.
• A second update 8 days later (ie, 18 days after initial presentation) revealed worsening of the steroidal AEs, including persistent ravenous appetite, significant loss of muscle mass, urinary accidents in the house, lethargy, and panting most notable at night.
• The dose of oral prednisone was tapered to ≈0.6 mg/kg, still divided into twice-daily administration.
• A recheck examination was recommended for the following day.

Outcome

• Three weeks after initial presentation (ie, a few days after the client’s second report), the patient was returned for a recheck evaluation (Figure 2).
• The dog had lost 2.2 kg over the course of treatment and had moderate muscle wasting.
• Her eyes had returned to a normal position, with no apparent permanent changes affecting either eye.
• Because of the significant steroidal AEs, prednisone was tapered and ended over the following 2 weeks.
• A 1-year follow-up was recommended to monitor progression of the cataract.

Discussion

Extraocular myositis is a focal inflammatory myopathy observed most in young, predominantly large- or giant-breed dogs.^1-3 The typical presenting signs are exophthalmos without protrusion of the nictitating membrane and congestion of the episcleral vessels.^1-3

• If advanced imaging is pursued— including ultrasonography, CT, or MRI—it would reveal swollen extraocular muscles.¹
• Histopathology of the inflamed muscles would reveal CD3+ lymphocytes and macrophages as the predominant cell types.^1-3
• Most patients respond well to a course of immunosuppressive steroids administered over 3 to 4 weeks.¹
• In patients with chronic inflammation, detrimental AEs can develop, and in patients with uncontrolled or chronic inflammation, fibrosis of the extraocular muscles can result in enophthalmos with strabismus.^2,3
• If the changes are severe, a corrective procedure may be required to reposition the eye.^2,3
• While the appearance is similar to Graves syndrome in humans, extraocular myositis has no known association with hypothyroidism or hyperthyroidism.¹

Diagnostic testing was recommended because of the varied differential diagnoses, but the owners elected to treat with the presumptive diagnosis of extraocular myositis. Although considering differential diagnoses, masticatory muscle myositis is also a focal inflammatory myopathy, but there are typically a few differences when these patients present clinically for ophthalmic and physical examinations.^2,3 Masticatory muscle myositis also results in exophthalmos, but the third eyelid is typically elevated from swelling of the pterygoid muscle.^1-3

• On physical examination, these dogs are predominantly young to middle-age large-breed dogs that present with fever, lethargy, weight loss, potential blindness, and restricted jaw movement from swelling of the masseter, temporal, and pterygoid muscles.^1-3
• The masticatory muscles affected all contain type 2M myofibers, which can be identified with serology, allowing for diagnosis.¹
• If the dogs present blind with this disease, it is from compression or tension on the optic nerve.^1-3
• Other infectious causes that should be ruled out based on location are neosporosis, toxoplasmosis, leishmaniasis, and Lyme borreliosis.¹

References

1. Spiess BM, Pot SA. Diseases and surgery of the canine orbit. In: Gellatt KN, Gilgerl BC, Kern TK, eds. Veterinary Ophthalmology. 5th ed. WileyBlackwell; 2013:808-810.
2. Webb AA, Cullen CL. Neuroophthalmology. In: Gellatt KN, Gilgerl BC, Kern TK, eds. Veterinary Ophthalmology. 5th ed. WileyBlackwell; 2013:1862-1863.
3. Cullen CL, Webb AA. Ocular manifestations of systemic disease part 1: the dog. In: Gellatt KN, Gilgerl BC, Kern TK, eds. Veterinary Ophthalmology. 5th ed. Wiley-Blackwell; 2013:1916-1917.