History 1: Soft Tissue Surgery: Thymoma in a DSH Cat

Lilly was referred for investigation of a short history of dyspnoea.The owner reported the cat had been retching and coughing for 2 months prior to referral. More recently she had developed laboured breathing. Examination by the referring veterinary surgeon identified an increased respiratory rate.No heart sounds could be auscultated on either side of the thorax. Thoracic radiography revealed a large soft tissue opacity within the mediastinum displacing the heart.

CLINICAL EXAMINATION

Examination revealed the cat was bright,alert and responsive, but in poor bodily condition. Respiratory rate was increased and respirations were shallow. Auscultation revealed muffling of heart sounds bilaterally and also caudal displacement of heart sounds. Decreased lung sounds were evident cranially and the chest was dull to percussion,except in the caudodorsal lung fields.

INVESTIGATION

Routine haematology and biochemistry were unremarkable except for a mild eosinophilia (1.1x109 /l) and hyperglycaemia (10.6mmol/l) FeLV and FIV status was negative. The cat was sedated with a combination of midazolam(0.25mg/kg) and ketamine(5mg/kg) to perform ultrasonography. Ultrasound revealed a cavitated lesion with fluid filled areas. Fluid was aspirated from the anechoic areas of the mass and fine needle aspirate biopsies were taken from the solid part of the mass. Cytology of the mass was suggestive of thymoma.

TREATMENT:ANAESTHESIA & SURGERY

Exploratory thoracotomy via a median sternotomy approach was carried out. Premedication was undertaken using a combination of acepromazine(0.03mg/kg) and morphine(0.4mg/kg) Oxygen was administered via face mask prior to induction.Anaesthesia was induced with propofol(10mg/kg) and maintained by endotracheal intubation and a combination isoflurane. Nitrous oxide was avoided due to respiratory compromise. Neuromuscular blocking agents (atracurium 0.5mg/kg) were used with positive pressure ventilation for the duration of the surgery whilst the thoracic cavity was open.

Prophylactic intravenous antibiotics were given perioperatively, as the cat was debilitated & at increased risk of developing pulmonary infection. The entire ventral thorax was clipped & prepared aseptically for surgery.A median approach was made through the sternum using an oscillating bone saw, leaving the most caudal sternebrae intact to reduce movement,post-operative pain and delayed healing. Finochietto chest retractors and moistened swabs were used to expose the thoracic cavity.

A 9cm long cone-shaped mass was visible filling the entire cranial thorax.The heart was displaced dorsally and caudally by the mass.A small amount of inflated lung was present on the right, but no inflated lung on the left. The rest of the thorax was examined for other abnormalities including metastases.

The cranial vena cava and phrenic nerves were identified so they could be avoided.The mass was carefully resected using a combination of meticulous ligation of the small blood vessels and gentle blunt dissection. A chest drain was placed at the end of surgery.An intercostals nerve block was carried out with bupivicaine (2mg/kg) All the resected tissue was submitted for histopathology.

POST-OPERATIVE CARE

Post-operative thoracic radiography was carried out. Intravenous fluid therapy was maintained.The cat was carefully monitored for any evidence of pneumothorax or haemorrhage. Analgesia was provided.The chest drain was removed 24hours post-operatively following further thoracic radiographs

OUTCOME

Lilly made an excellent recovery from surgery. She was discharged from the hospital five days after surgery. Histopathology confirmed a thymoma. Follow-up at three months revealed she was bright and alert with improved bodily condition and no evidence of respiratory difficulties.

DISCUSSION

The main differentials for mediastinal masses in cats are thymoma and lymphoma.Anterior mediastinal cysts are very rare.Anterior mediastinal lymphomas are more common in young cats, whereas thymomas are more common in older cats.

Fine needle aspiration can usually be performed without great risk with post-biopsy haemorrhage being a rare complication. Cytologically it can be difficult to differentiate between thymomas and lymphomas. Lymphomas are usually composed of a monomorphic population of lymphocytes and the immature lymphocytes are often heavily vacuolated. Thymomas are uncommon tumours in both the cat and dog. Tumour development is most common in older dogs and cats over nine years.There is no reported sex or breed predisposition. All thymomas are epithelial in origin, but are infiltrated by mature lymphocytes to a greater or lesser degree. Eosinophils and mast cells can be also be seen in low numbers. Interestingly this cat had a mild circulating eosinophilia and eosinophils were evident in the mediastinal mass and the cystic fluid. Although different histological cell types are identified, histological type cannot be used to predict prognosis. Distant metastasis is rare.It is reported that pleural effusions are not common in association with thymoma but are often reported with mediastinal lymphoma, although this cannot be used reliably to differentiate between the two tumour types.

The prognosis for thymoma depends on the possibility of surgical resection and the presence of associated paraneoplastic syndromes which include hypercalcaemia and myasthenia gravis. Neither were evident in this case. Myasthenia gravis is seen in up to 40% of dogs with thymomas, but is much rarer in the cat. The prognosis is good if there is no evidence of megaoesophagus/myasthenia gravis and the tumour is surgically resectable. The prognosis is poor if there is myasthenia gravis or the tumour is non-resectable.Although myasthenia gravis can resolve following successful surgery this may take several months.Therefore if myasthenia gravis is complicated by megaoesophagus the dog may not survive long enough after surgery for the improvement to be seen. The surgical resection of thymoma in cats appears to have a favourable prognosis. Gores et al reported a median survival rate after surgery approaching 2 years (3).Dogs have reported to have an 83% survival rate at one year following surgery.

REFERENCES

1. Day M. (1997) Thymic pathology in 30 cats and 36 dogs. Journal of Small Animal Practice 38 393-403
2. Davies C,Forrester S.D. (1996) Pleural effusion in cats 82 cases. Journal of Small Animal Practice 37 217-224
3. Gores BR, Berg J, Carpenter JL & Aronsohn MG(1994) Surgical treatment of thymoma in cats:12 cases(1987-1992) Journal of American Veterinary Medical Association 204 1782-1785