Successful long-term management of canine thymoma with myasthenia gravis by using pyridostigmine followed by single-dose radiotherapy

Abstract

A 12-year-old spayed golden retriever was brought to our institute because of ptyalism and regurgitation. Thymoma associated with myasthenia gravis (MG) and megaesophagus was tentatively diagnosed based on chest radiography of a 2.5-cm mediastinal mass and a dilated air-filled esophagus, and the level of the acetylcholine receptor (AChR) antibody titer was found to be elevated to 5.23 nmol/L. The dog was first treated with pyridostigmine for 4 weeks to alleviate the symptoms of MG, followed by a single dose of radiotherapy at 12 Gy for the thymoma. One month after radiotherapy, the symptoms of MG were alleviated, and megaesophagus was not detected on chest radiography. Two months after radiotherapy, the AChR antibody titer was reduced to 1.58 nmol/L and no tumor was detected on chest radiography. The pyridostigmine therapy was discontinued 5 months after radiotherapy. At the 1-year and a half follow-up, the dog remained free of tumor and MG symptoms, and the AChR antibody titer was reduced to 0.6 nmol/L. In addition, no side effects of radiation were detected. This case suggests that administration of the acetylcholinesterase inhibitors before single-dose radiotherapy may be efficient against canine thymoma with MG and megaesophagus.