Pathology in Practice. Metastatic lymphocyte-rich thymoma in a dog

Classification of myasthenia gravis and congenital myasthenic syndromes in dogs and cats

Myasthenia, a syndrome of impaired neuromuscular transmission, occurs as either an acquired or congenital condition. Myasthenia gravis (MG) is an acquired autoimmune disorder with autoantibodies against the neuromuscular junction (NMJ) of skeletal muscle whereas congenital myasthenic syndromes (CMSs) are a clinically heterogeneous group of genetic disorders affecting the NMJ with a young age of onset. Both conditions are diseases for which recognition is important with regard to treatment and outcome. We review the published literature on MG and CMSs in dogs and cats, and by comparison with published classification used in humans, propose a classification system for MG and CMSs in dogs and cats. Myasthenia gravis is first classified based on focal, generalized, or acute fulminating presentation. It then is subclassified according to the autoimmune disease mechanism or seronegativity. Autoimmune disease mechanism relates to the presence or absence of a thymoma, or administration of thiourylene medication in cats. Congenital myasthenic syndromes are classified according to the affected NMJ component, the mechanism of the defect of neuromuscular transmission, the affected protein, and ultimately the mutated gene responsible. In proposing this categorization of MG and CMSs, we hope to aid recognition of the disease groups for both conditions, as well as guide treatment, refine prognosis, and provide a framework for additional studies of these conditions.

CT imaging features of canine thymomas

Canine thymomas have been evaluated based on clinical features, treatment options, surgical excision, and outcomes with limited information on specific CT features. The objective of this retrospective, descriptive, cross-sectional study was to describe the CT characteristics of confirmed thymomas and to compare these imaging features to outcome. A total of 22 dogs met the inclusion criteria of histologically confirmed thymomas with concurrent CT imaging. Tumor size varied widely ranging from small and well-circumscribed to large and invasive. Delayed-phase, contrast-enhanced CT studies were best for determining the degree of contrast enhancement in tumors. Of these, 19 of 22 masses had heterogeneous enhancement and three of 22 masses had homogeneous enhancement. Vascular invasion was present in seven of 22 cases. Larger tumors were associated with vascular invasion (height: P = .04; width and volume: P = .02). On precontrast CT, larger tumors (16/21) were heterogeneous and cystic, with smaller tumors (5/21) being more homogeneous (all values P < .05). A larger size was associated with recurrence in fully resected masses (height: P = .03), but not a shorter outcome (P > .3 for all size dimensions). Postoperative complications and incomplete tumor resection were associated with shorter outcome (both values P < .01). Metastasis was confirmed in four cases. There were six cases with lymphadenopathy noted on CT; five of the six cases did not have evidence of metastasis. Larger tumors were more likely to be cystic and associated with vascular invasion.

Metastatic thymoma in the liver of a dog

A 12-y-old neutered male Portuguese Water dog was presented because of a 1-y history of persistent hyporexia, diarrhea, and recurrent pyelonephritis. Abdominal ultrasound revealed hepatic nodules and diffuse splenomegaly, and radiographs revealed a mediastinal mass. Fine-needle aspirates of the liver, spleen, and mediastinal mass were suspicious for lymphoma. Flow cytometry identified small T cells that co-expressed CD4 and CD8 at all sites, most suspicious for thymoma, but lymphoma could not be ruled out. PCR for antigen receptor rearrangements analysis identified polyclonal amplification of the T-cell receptor genes, more consistent with thymoma than lymphoma. Histopathology of the liver and thymic mass confirmed thymoma with hepatic metastasis.