A 9-year old Cockapoo with exercise-induced generalised weakness was diagnosed with AMG alongside with thymoma and cholangiocellular carcinoma. This dog was treated symptomatically with pyridostigmine bromide and complete surgical removal of thymoma and cholangiocellular carcinoma. This treatment resulted in resolution of the clinical signs and immune-remission as documented in this report by serial acetylcholine receptor antibody titers. Clinical remission was achieved after 8 months and immune remission was achieved after 18.5 months.

Acquired myasthenia gravis (AMG) is an immune-mediated disorder in which antibodies are targeted against the nicotinic acetylcholine receptor (AChR) of skeletal muscle. This results in muscular weakness and excessive fatiguability. Several conditions have been associated with canine AMG, including other autoimmune disorders and neoplastic disorders. For the latter, AMG has been linked to paraneoplastic syndromes secondary to thymoma, osteosarcoma, cholangiocellular carcinoma, anal sac adenocarcinoma and cutaneous lymphoma. Cranial mediastinal masses and thymomas have been associated with AMG in humans, dogs and cats. Thymoma cells may express antigenic epitopes similar to those of nicotinic AChRs. The immune response to those epitopes results in the lack of functional AChRs in skeletal muscles. Surgical removal of hyperplastic thymic tissue is associated with long-term clinical improvement in humans with generalised forms of AMG.

Case video courtesy of Thomas Mignan and Mark Lowrie at Dovecote Veterinary Hospital.

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