Case Report: Overlap syndrome with concurrent polymyositis, masticatory myositis, and lymphocytic thyroiditis in a dog.

CASE PRESENTATION: A 4-year-old spayed female mixed breed dog presented for chronic upper airway noises and dysphagia. The dog presented for inspiratory stridor and expiratory stertor. Serum creatine kinase activity was 3,296 IU/L, and cholesterol and triglyceride levels were elevated at 353 mg/dL and 201 mg/dL, respectively. In-house thyroxine levels were below the reference range, prompting the submission of a comprehensive thyroid profile to an outside reference laboratory. Head and cervical CT with contrast showed perivertebral/perilaryngeal muscle thickening and contrast enhancement. Electrodiagnostics of the pelvic limbs and head showed diffuse spontaneous activity with normal motor nerve conduction velocity. Muscle biopsies revealed an inflammatory myopathy in both masticatory muscles and pelvic limb muscles. The serum 2M antibody titer was positive. Comprehensive infectious PCR and serology testing were negative. The constellation of findings is compatible with an overlap syndrome of masticatory myositis and immune-mediated polymyositis. A comprehensive thyroid panel consisting of total thyroxine, total triiodothyronine, free thyroxine by dialysis, T4 and T3 autoantibodies, thyroid-stimulating hormone, and thyroglobulin autoantibodies later returned supported results for lymphocytic thyroiditis.

INTERVENTION AND OUTCOME: Treatment with immunosuppressive doses of prednisone was effective in reducing, but not eliminating, the respiratory noises and dysphagia, with improved quality of life 763 days after initial presentation.

CONCLUSION: This is a clinical description of an overlap syndrome of masticatory myositis and polymyositis, further complicated by lymphocytic thyroiditis. This case report highlights the importance of differentiating euthyroid sick syndrome and lymphocytic thyroiditis in dogs with chronic immune-mediated illness.