Acquired myasthenia gravis associated with a non-invasive thymic carcinoma in a dog

Case report: Immune remission from generalized myasthenia gravis in a dog with a thymoma and cholangiocellular carcinoma

A 9-year-old male neutered Cockapoo was presented with an acute and progressive history of exercise induced weakness involving all limbs, and bilateral decreased ability to blink. Investigations revealed generalized myasthenia gravis alongside the presence of a thymoma and a cholangiocellular carcinoma. Symptomatic treatment through pyridostigmine bromide was used to control clinical signs, and complete surgical removal of the thymoma and cholangiocellular carcinoma was performed. Serum acetylcholine receptor antibody concentration was measured serially. Clinical remission defined as resolution of clinical signs alongside discontinuation of treatment was achieved by day 251 (8.2 months). Immune remission defined as normalization of serum acetylcholine receptor antibody concentration alongside resolution of clinical signs and discontinuation of treatment was achieved by day 566 (18.5 months). Neurological examination was normal, and the owners did not report any clinical deterioration during the final follow-up appointment on day 752 (24 months), hence outcome was considered excellent. This is the first report describing the temporal evolution of serum acetylcholine receptor antibody concentration in a dog with thymoma-associated myasthenia gravis which achieved immune remission following thymectomy. Treatment was successfully discontinued without any evidence of clinical deterioration thereafter despite serum acetylcholine receptor antibody concentration not normalizing for another 315 days (10 months).

Thymoma related myasthenia gravis in humans and potential animal models

Thymoma-associated Myasthenia gravis (TAMG) is one of the anti-acetylcholine receptor MG (AChR-MG) subtypes. The clinico-pathological features of TAMG and its pathogenesis are described here in comparison with pathogenetic models suggested for the more common non-thymoma AChR-MG subtypes, early onset MG and late onset MG. Emphasis is put on the role of abnormal intratumorous T cell selection and activation, lack of intratumorous myoid cells and regulatory T cells as well as deficient expression of the autoimmune regulator (AIRE) by neoplastic thymic epithelial cells. We review spontaneous and genetically engineered thymoma models in a spectrum of animals and the extensive clinical and immunological overlap between canine, feline and human TAMG. Finally, limitations and perspectives of the transplantation of human and murine thymoma tissue into nude mice, as potential models for TAMG, are addressed.

Perioperative Mortality and Long-Term Survival in 80 Dogs and 32 Cats Undergoing Excision of Thymic Epithelial Tumors

OBJECTIVE

To examine perioperative mortality, long-term survival, causes of death, and prognostic factors for dogs and cats undergoing surgical excision of thymic epithelial tumors (TETs).

STUDY DESIGN

Multi-institutional case series.

ANIMALS

Eighty dogs and 32 cats.

METHODS

Follow-up information was obtained for dogs and cats that underwent surgical excision of a TET between 2001 and 2012.

RESULTS

Perioperative mortality was 20% in dogs and 22% in cats. No independent risk factors for perioperative mortality were identified. The estimated median survival time for all dogs was 1.69 years (95% CI 0.56-4.32) and the 1- and 4-year survival rates were 55% (95% CI 44-67) and 44% (95% CI 32-56). The estimated median survival time for all cats was 3.71 years (95% CI 0.56-unestimatable) and the 1- and 4-year survival rates were 70% (95% CI 53-87) and 47% (95% CI 0-100). Of animals that survived to discharge, 42% of dogs and 20% of cats eventually died of TET-related causes. The presence of paraneoplastic syndromes (hazard ratio [HR] 5.78, 95% CI 1.64-20.45, P = .007) or incomplete histologic margins (HR 6.09, 95% CI 1.50-24.72, P = .01) were independently associated with decreased survival in dogs. No significant predictors of survival were identified in cats. Conclusions regarding the effect of chemotherapy or radiation therapy could not be made.

CONCLUSIONS

While there is substantial risk of perioperative death in dogs and cats undergoing surgery for TETs, many animals that survive to discharge have prolonged survival. Survival is significantly decreased in dogs with paraneoplastic syndromes or incomplete histologic margins.

Histologic and immunohistochemical characterization of thymic epithelial tumours in the dog

Thymic epithelial tumour (TET) histologic subclassification has not been well described in the veterinary literature as it has in humans. The objective of this study was to identify and describe TET subtypes in dogs and to determine the utility of immunohistochemistry (IHC) in differentiating these subtypes. Samples were reviewed and classified according to a modified World Health Organization (WHO) criteria for human tumours of thymic origin. Signallment, presenting signs, treatment and survival data was collected from medical records. Histologic review confirmed the same subtypes as described in humans. Presence of high stage disease, pleomorphism, mitotic figures and capsular invasion was more common in atypical thymomas and thymic carcinomas than in thymomas. IHC was performed for GLUT-1, CD5, CD117 and CK8/18; however, this was not useful in classifying the tumours.