Cystic Thymoma-Associated Fulminant Myasthenia Gravis with Rapid Progression in a Cat: Diagnostic and Therapeutic Challenges.

Acquired myasthenia gravis (MG) is an autoimmune disorder of the neuromuscular junction caused by antibodies against the acetylcholine receptor, leading to generalized muscle weakness and, in severe cases, respiratory muscle paralysis. While MG is well described in dogs, it is rarely reported in cats, and fulminant cases with rapid progression have a poor prognosis. An 11-year-old neutered female Persian cat presented with acute hindlimb weakness, which progressed to tetraplegia within 4 days. The clinical course initially resembled acute polyradiculoneuritis, and electromyography revealed positive sharp waves and fibrillation potentials. Diagnostic imaging identified a cystic cranial mediastinal mass without megaesophagus. Worsening respiratory muscle paralysis prompted surgical excision of the mass via left intercostal thoracotomy. Postoperatively, the cat developed persistent respiratory failure requiring re-intubation, mechanical ventilation, and temporary tracheostomy. Histopathological and immunohistochemical examination confirmed a thymoma, and serum acetylcholine receptor antibody levels were markedly elevated, establishing the diagnosis of thymoma-associated fulminant MG. Despite tumor excision and medical therapy with pyridostigmine and prednisolone, the cat remained ventilator-dependent and was euthanized on postoperative day 3. This case emphasizes early recognition of respiratory compromise, prompt diagnostic confirmation, intensive ventilatory support, and consideration of adjunctive immunomodulatory therapies.