Clinical features, anaesthetic management and perioperative complications seen in three horses with pheochromocytoma

Abstract

Three horses presenting with colic signs to the Equine Referral Hospital at The Royal Veterinary College underwent general anaesthesia between September 2013 and November 2017 for emergency exploratory laparotomy. No obvious cause for the colic signs was identified in two horses, while a haemoperitoneum was identified in the third. All horses were euthanased within 12 hours of surgery due to deteriorating haemodynamic instability and/or intractable pain. Postmortem examination revealed an adrenal mass in each case, confirmed to be a pheochromocytoma on histopathology. In retrospect, each horse had some hallmark characteristics consistent with a functional pheochromocytoma, including hyperglycaemia and hyperlactataemia. Extremely high packed cell volume (PCV) (>65 per cent) was also identified in two horses, with a high-normal PCV found in the haemoperitoneum case. Perioperative haemodynamic instability was predominantly characterised by episodes of intermittent hypertension and tachycardia.

Polymorphic tachycardia in an anaesthetised horse with an undiagnosed pheochromocytoma undergoing emergency coeliotomy

“…Ketamine was used as an induction agent despite its known effect to cause tachycardia and decrease the arrhythmogenic potential of adrenaline in dogs 32. If a pheochromocytoma is suspected, ketamine is still the safest choice, whereas thiopental should be avoided due to its potential to precipitate arrhythmias 1. Acepromazine, which in this case was not administered due to the risk of hypovolaemia and hypotension, could theoretically have been protective against catecholamine‐induced arrhythmias in this case 33.…”

“…Interestingly, if severe hyperlactataemia (>7 mmol/l) is persistent, the risk for postoperative death is 11.27 times higher 29. In horses with pheochromocytoma, hyperlactataemia is one of the most common signs1 resulting from catecholamine-mediated vasoconstriction, which can lead to impaired aerobic metabolism and hypoxia. In this case, normoxaemic hypoxia (normal pO 2 values) was probably present as a result of cardiovascular compromise and increased oxygen demand.…”

“…Pheochromocytomas are rare in horses undergoing emergency coeliotomy, with no short-term survival reported in the literature 1. Particularly, mortality in horses with ruptured pheochromocytoma is high due to associated haemorrhage and catecholamine release predisposing to arrhythmias 2–5.…”

“…In cases of acute haemorrhage, fast transfusions are essential for horses’ survival, especially when blood loss exceeds 30 per cent of blood volume, and tachycardia or hyperlactataemia is present 8. Also, the presence of excessive sweating, muscle fasciculations, hyperlactataemia (>4.9 mmol/l) and/or hyperglycaemia (>10.2 mmol/l) should raise suspicion for a pheochromocytoma 1. During anaesthesia, the most significant challenge that an anaesthetist faces in the presence of pheochromocytomas is the management of haemodynamic instability.…”

“…Pheochromocytomas are rare in horses undergoing emergency coeliotomy, with no short-term survival reported in the literature. 1 Particularly, mortality in horses with ruptured pheochromocytoma is high due to associated haemorrhage and catecholamine release predisposing to arrhythmias. [2][3][4][5] In this horse, several factors were associated with the increased risk of mortality, including advanced age, low haematocrit, persistent tachycardia, persistent hyperlactataemia, persistent hypotension as well as the extent of pain symptomatology.…”