A benign or malignant neoplasm arising from paraganglia located along the sympathetic or parasympathetic nerves. Infrequently, it may arise outside the usual distribution of the sympathetic and parasympathetic paraganglia. Tumors arising from the adrenal gland medulla are called pheochromocytomas. Morphologically, paragangliomas usually display a nesting (Zellballen) growth pattern. There are no reliable morphologic criteria to distinguish between benign and malignant paragangliomas. The only definitive indicator of malignancy is the presence of regional or distant metastases.