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

Abstract

Canine thymomas are rare tumors of the mediastinum that originate in the epithelium of the thymus ［14］. Canine thy-moma is known to be occasionally associated with acquired myasthenia gravis （MG） ［1-4］. This neuromuscular disease is caused by an autoimmune response to AChR ［11］. The most common symptom of MG is regurgitation because muscle weakness prevents peristalsis in the enlarged esophagus （megaesophagus）. A poor prognosis has been reported for thymo-mas complicated with megaesophagus because the animals are at a risk of developing aspiration pneumonia ［18］. The average survival time of these animals are 4 days; none of these dogs survived more than 71 days ［2］. The preferred treatment option for thymoma without MG remains radical excision of the thymus. However, dogs with thymoma and MG often die following thymectomy due to worsened megaesophagus or aspiration pneumonia ［2］. Administration of acetylcholinesterase inhibitors with or without surgery increased survival time; however most cases died within 4 months ［9, 10, 15, 16］. Therefore, new treatment options for dogs with concurrent thymomas associated with MG and megaesophagus are the need of the hour. Here, we report a rare case of canine thymoma accompanied with MG and megaesophagus that was treated using the acetylcholinesterase inhibitor pyridostigmine and a single-dose of radiotherapy. The animal was found to remain free of tumor and MG symptoms at the 1-year and a half follow-up. Abstract A 12-year-old spayed golden retriever was brought to our institute because of ptyalism and regurgita-tion. 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 thy-moma with MG and megaesophagus.