Presumptive spontaneous hemothorax associated to thymic involution in a dog with steroid responsive meningitis-arteritis (SRMA)

To describe an unusual case of spontaneous hemothorax resulting from thymic involution in a dog with suspected acquired bleeding dyscrasia associated with steroid-responsive meningitis-arteritis (SRMA). A 6-month-old spayed female Golden Retriever was referred due to the sudden onset of lethargy, fever (pyrexia), loss of appetite (anorexia), and moderate neck pain. These symptoms emerged six days after an ovariohysterectomy performed by the primary veterinarian. Upon admission, the patient exhibited pale mucous membranes, tachycardia (180 bpm), bilateral muffled heart sounds and tachypnea. Abdominal and thoracic point-of-care ultrasound (POCUS) were performed and revealed bilateral pleural effusion. Due to the patient's unstable condition, emergent thoracocentesis and transfusion of packed red blood cells was required. The initial work-up performed included a complete blood cell count (CBC), biochemistry profile, venous blood gas and coagulation panel (PT, APTT, fibrinogen). Pleural effusion analysis was compatible with hemothorax. Bloodwork was unremarkable including the initial coagulation panel. Further coagulation test was performed including buccal mucosal bleeding time, viscoelastic-based clot detection tests (TEG) and Von Willebrand factor antigen measurement. TEG revealed marked hyperfibrinolysis. Angiostrongylus vasorum and 4DX snap test were performed and yielded a negative result. Thoracic CT scan revealed the presence of a soft tissue-attenuating mass in the ventral mediastinum, thymic involution, and enlargement of the sternal and mediastinal lymph nodes. Therapy with tranexamic acid and corticosteroids at anti-inflammatory doses was initiated. Marked clinical improvement was observed within 24 hours, and after three days of hospitalization the patient was discharged. One month later, the dog was referred again for acute pyrexia, hyporexia, and neck pain which progressed to non-ambulatory tetraparesis. Neurological examination was compatible with C6-T2 lesion. MRI and cerebrospinal fluid analysis were performed and revealed a final diagnosis of steroid-responsive meningitis-arteritis (SRMA) with associated intramedullary hemorrhage. Corticosteroids were started again, and the patient showed a dramatic improvement over the next 24 hours. Three weeks after the diagnosis, the dog returned to a clinically normal state. The treatment was gradually tapered over the following months, guided by regular neurological and clinical examinations and CRP me