A thymoma is a neoplasm arising from thymic epithelium infiltrated by lymphocytes and reticular epithelial cells. It may occur alone or with multi-organ systemic lymphoma. It is the most common cause of cranial mediastinal masses in rabbits.
Clinical signs of thymoma
- exophthalmos, bilateral protrusion of the third eyelid from raised venous pressure due to mass effect (cranial vena cava syndrome);
- oedema of head, neck, or thoracic limbs;
- tachypnoea;
- dyspnoea;
- exfoliative dermatitis or sebaceous adenitis.
Differential diagnosis
- lymphoma;
- normal thymus (may not fully regress with age in rabbits);
- thyroid neoplasia;
- rarely other mediastinal masses;
- abscess;
- consolidated lung lobe;
- with bilateral exophthalmos — tooth root abscess, retrobulbar masses (usually painful and globe cannot be retropulsed).
Diagnosis
Thoracic radiographs show a cranial mediastinal mass with caudal displacement of the heart. Thoracic ultrasound helps differentiate residual normal thymus from neoplasia.
Fine-needle aspiration — cytology shows mature or pleomorphic lymphocytes with thymic epithelial cells.
Treatment
- surgical excision;
- radiation therapy;
- corticosteroids cause immunosuppression and may worsen subclinical Encephalitozoon cuniculi or bacterial infection, with fatal outcome.
Prognosis is guarded to poor.
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