30 - Pheochromocytoma

Summary

Although pheochromocytomas are not a common medical/surgical problem (they are estimated to cause only 0.1% to 0.5% of all cases of hypertension, and are operated on only once or twice per year in most centers), medical consultants are likely to be asked to evaluate and prepare for surgery patients with suspected pheochromocytomas at some time during their careers. Because catecholamines have major regulatory effects on many different body systems, it is vital that these be anticipated and properly managed in the perioperative period. Pheochromocytomas are associated with an increased risk of adverse reactions to many commonly prescribed drugs and clinicians must also be aware of this potential hazard. The surgical removal of a pheochromocytoma has great potential for intra- and postoperative complications because of the release of catecholamines during manipulation or stimulation of the tumor.

Pathophysiology

Pheochromocytomas arise from chromaffin cells of the neural crest that migrate to form the adult adrenal medulla and sympathetic ganglia. These cells synthesize catecholamines through a series of enzymatically controlled steps, starting with the conversion of tyrosine to dihydroxyphenylalanine (dopa) by tyrosine hydroxylase. This is the rate-limiting step in catecholamine synthesis. Dopa is then converted to dopamine, which is subsequently decarboxylated to norepinephrine. The methylation of norepinephrine to epinephrine is accomplished through the action of phenylethanilamine-N-methyl transferase, an enzyme that is induced by glucocorticoids that reach the adrenal medulla in high concentrations through the corticomedullary venous sinuses from the adrenal cortex. Norepinephrine and epinephrine are the major products of most pheochromocytomas.