Pheochromocytomas are chromaffin cell tumors that produce, store, metabolize, and secrete catecholamines. The metabolism of catecholamines is a more consistent process than that of catecholamine secretion . The 2004 World Health Organization classification of endocrine tumors defines pheochromocytoma as a tumor arising from catecholamine-producing chromaffin cells in the adrenal medulla — an intra-adrenal paraganglioma. Closely related tumors of extra-adrenal paraganglia are classified as extra-adrenal paragangliomas. While these definitions serve to distinguish the two types of tumor based on location, this does not take into account differences in functional characteristics related to other differences in cellular origin. More specifically, while extra-adrenal paragangliomas derived from sympathetic nervous system-associated chromaffin tissue almost always produce catecholamines and often lead to hypertension, those derived from parasympathetic tissue (mainly head and neck paragangliomas) produce significant amounts of catecholamines in only less than 5 % and usually do not cause hypertension. It therefore seems likely that pheochromocytomas will be continued to be defined as catecholamine-producing tumors of intra- and extra-adrenal chromaffin cells, with those derived from from the latter types of chromaffin cells classified as extra-adrenal pheochromocytomas.

Pheochromocytomas typically occur in about 85% of cases from adrenal medullary chromaffin tissue and in about 15% of cases from extra-adrenal chromaffin tissues . Extra-adrenal pheochromocytomas in the abdomen most commonly arise from a collection of chromaffin tissue around the origin of the inferior mesenteric artery (the organ of Zuckerkandl) or aortic bifurcation . Most pheochromocytomas represent sporadic tumors and about 20-30% of pheochromocytomas are familial. Sporadic pheochromocytomas are usually unicentric and unilateral while familial pheochromocytomas are often multicentric and bilateral. Both adrenal and extra-adrenal paragangliomas display similar histopathological characteristics. Unusual sites in the abdomen and pelvis include kidney, urethra, prostate, spermatic cord, genital tract, and liver. About 4-10% of patients with pheochromocytoma present with adrenal incidentaloma, whereas approximately 5% are diagnosed at surgery . Although metastases may be rare for adrenal (about 10%) and familial (less than 5%) pheochromocytomas , the prevalence is up to 36-50% for extra-adrenal abdominal pheochromocytomas . Finally, up to 10% of intra-adrenal pheochromocytomas show local recurrence .

Pheochromocytomas occur in about 0.05% to 0.1% of patients with sustained hypertension. However, this probably accounts for only 50% of persons harboring pheochromocytoma, when it is considered that about half the patients with pheochromocytoma have only paroxysmal hypertension or are normotensive. Also, it must also be considered that the prevalence of sustained hypertension in the adult population of Western countries is between 15 to 20% . Thus, in Western countries the prevalence of pheochromocytoma can be estimated to lie between 1:6,500 to 1:2,500 with an annual incidence in the United States of 500 to 1,100 cases per year. Despite this low incidence, pheochromocytoma must always be considered since if it is identified the condition can be cured in about 90% cases, whereas left untreated the tumor is likely to be fatal due catecholamine-induced malignant hypertension, heart failure, myocardial infarction, stroke, ventricular arrhythmias, or metastatic disease.