Third degree atrioventricular block and acquired myasthenia gravis in four dogs.

Third degree atrioventricular block was diagnosed in 4 dogs with acquired myasthenia gravis (serum acetylcholine receptor antibody titer > 0.6 nmol/L). All 4 dogs had megaesophagus. Two dogs also had mediastinal thymomas, which were resected. One dog with thymoma received a permanent pacemaker at the same time that it underwent thymectomy; the other did not develop third degree atrioventricular block until 3 months after thymectomy. Both dogs with thymoma died of severe aspiration pneumonitis within 3 months after surgery. The third dog received a permanent pacemaker and was treated with pyridostigmine bromide, but also died of aspiration pneumonitis 1 month after the pacemaker was implanted. The fourth dog was treated with prednisolone and pyridostigmine bromide and improved, but did not become clinically normal. Because third degree atrioventricular block as well as myasthenia gravis can cause signs of weakness, acquired myasthenia gravis should be considered in dogs with idiopathic cardiac conduction disturbances. Likewise, an ECG should be evaluated in dogs with acquired myasthenia gravis.