Thymic Haemorrhage in a 6 Month Old Dog
Written by: Dr Wey Yen Loh BVSc BVMS
Viv is a six month old female entire Italian Greyhound. She presented to the Animal Emergency Centre (AEC) with a non-weight bearing lameness in her right forelimb after jumping off a couch. Radiographs revealed a fracture of the distal radius and ulna. Analgesia was provided overnight and fracture repair performed the following day at the Veterinary Referral Hospital utilising external fixation.
One week after discharge, Viv represented to the AEC in a collapsed state. Her owners reported lethargy and hyporexia since discharge. She had been confined at home and did not have any access to poisons. On physical examination, she had marked tachycardia, pale mucous membranes, absent capillary refill time, hypodynamic pulses and was hypothermic. Mild dyspnea was present and thoracic auscultation was normal.
An intravenous catheter was placed and blood collected for a packed cell volume (PCV), total solids (TS), serum electrolytes, acid-base, venous blood gases, prothrombin time (PT) and activated partial thromboplastin time (APTT). Packed cell volume was 22% and total solids measured by refractometry were 34 g/L. Venous blood gases revealed a respiratory acidosis and a severe hyperlactataemic (lactate 11.7 mmol/L) metabolic acidosis. Electrolytes, PT and APTT were normal.
Acute haemorrhage was considered the most likely cause of the moderate anaemia and hypoproteinaemia, with subsequent hypovolaemic shock. Treatment was commenced with lactated Ringer’s solution and hydroxyethyl starch (Voluven) boluses, to correct the hypovolaemia, followed by a whole blood transfusion.
Further diagnostics at the AEC included a complete blood count (CBC), serum biochemistry, thoracic radiographs and limited abdominal and thoracic ultrasound using a focused assessment with sonography for trauma (FAST) technique. A mild regenerative anaemia and mild thrombocytopenia were present on the CBC, and a mild increase in alkaline phosphatase and glucose, and a marked hypoproteinaemia were present on serum biochemistry. Moderate free fluid was identified in the pleural space and an increased mediastinal soft tissue density was seen on thoracic radiographs. A thoracocentesis was not performed.
Following stabilization, Viv was transferred to the Veterinary Referral Hospital for further investigation. A full thoracic ultrasound revealed pleural fluid and mass lesions within the cranial mediastinum and right caudal, ventral thorax, with no evidence of vascular flow. The rest of the thorax was unremarkable. Thoracocentesis was performed and a serosanguinous fluid was recovered with a PCV and TS concentration consistent with blood. Thoracic computed tomography with contrast study demonstrated non-contrast enhancing soft tissue masses in the right caudal hemithorax and in the cranial mediastinum adjacent to the thymus. This was thought to be consistent with haematoma formation originating from the thymus. Based on the above findings, Viv was diagnosed with thymic haemorrhage.
Viv improved with supportive care and was discharged home after two days. She represented the following day for haematuria and a delayed blood transfusion reaction was diagnosed. She was diuresed for two days with lactated Ringer’s solution and she recovered uneventfully.
The thymus is an organ of major immunological importance, being the site of maturation of T lymphocytes exported as precursors from the bone marrow. It is an organ that is present at birth and starts involuting after the onset of sexual maturity; usually between 6 – 12 months in dogs. Thymic disease is an uncommon disease of dogs and cats. It occurs mostly in older animals and the most frequently seen conditions are thymic lymphoma and thymoma. Other conditions reported include thymic branchial cyst, thymic hyperplasia, thymic haemorrhage and thymic amyloidosis.
Thymic haemorrhage is an uncommon condition that occurs primarily in dogs and rarely in cats. It has been described in dogs younger than two years of age, most commonly between 3 and 9 months. Presenting signs include a peracute to acute onset of lethargy, tachypnoea or dyspnoea, pallor, muffled heart sounds or decreased lung sounds and sudden death. Anticoagulant rodenticide toxicosis is the most commonly reported cause of thymic haemorrhage, however other reported aetiologies include trauma, dissecting aneurysm of the aorta, neoplasia, and idiopathic. The veterinary literature reporting thymic haemorrhage is limited to case reports where the prognosis for survival appears to be low; however this may reflect a reporting bias.
It has been hypothesised that the involuting thymus in younger dogs is prone to haemorrhage. Two mechanisms have been proposed: 1) haemorrhage from acute hypertension since the thin walled veins and arteries in an involuting thymus do not receive enough lateral supportive pressure from adjacent adipose and loose connective tissue; 2) mild trauma could cause overstretching of the neck leading to rupture of the vessels.
In the case reported here, disorders of secondary haemostasis (including anticoagulant rodenticide toxicosis) can be excluded as the PT and APTT were normal. The platelet count was normal eliminating thrombocytopenia as a cause of the haemorrhage; however a thrombocytopathia affecting primary haemostasis cannot be excluded. It is less likely that the traumatic episode a week prior would have directly caused the thymic haemorrhage since published case reports suggest that presentation tends to be peracute to acute and death is common within 24 hours. It is more plausible that a thymic cyst formed from the trauma and this cyst subsequently ruptured. Since, neither thoracic surgery nor histopathology were required as part of the case management an exact aetiology was not determined.
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