Understanding catecholamine metabolism as a guide to the biochemical diagnosis of pheochromocytoma

Diagnostic value of various biochemical parameters for the diagnosis of pheochromocytoma in patients with adrenal mass

OBJECTIVE

Pheochromocytomas are neoplasms generally characterized by the autonomous production of catecholamines. This study compared various biochemical parameters for the diagnosis of adrenal pheochromocytoma in patients with adrenal mass.

DESIGN

One hundred and fifty subjects were studied, including 24 histologically proven pheochromocytomas, 17 aldosterone-secreting and 21 cortisol-secreting adrenal adenomas and 30 nonfunctioning adrenal masses, 16 patients with essential hypertension and 42 healthy normotensive volunteers. Spontaneous blood samples and 24-h urine samples were collected prospectively.

METHODS

Plasma and urinary epinephrine and norepinephrine levels were measured by high performance liquid chromatography, whereas plasma and urinary metanephrine and normetanephrine levels were determined by radioimmunoassay (RIA). Putative ratio thresholds were calculated by receiver operating characteristic (ROC) analysis to balance between sensitivity and specificity.

RESULTS

Plasma normetanephrine was found to be the best single parameter with the highest sensitivity (91.7%) and specificity (95.6%) using a threshold of 126 pg/ml. In combination, plasma normetanephrine and metanephrine had a higher sensitivity of 95.8% with lower specificity (79.4%). All other combinations of plasma and/or urinary parameters demonstrated a lower accuracy.

CONCLUSION

Plasma metanephrines measured by RIA are reliable screening parameters for the diagnosis of pheochromocytoma.

Catecholamine metabolism: a contemporary view with implications for physiology and medicine

This article provides an update about catecholamine metabolism, with emphasis on correcting common misconceptions relevant to catecholamine systems in health and disease. Importantly, most metabolism of catecholamines takes place within the same cells where the amines are synthesized. This mainly occurs secondary to leakage of catecholamines from vesicular stores into the cytoplasm. These stores exist in a highly dynamic equilibrium, with passive outward leakage counterbalanced by inward active transport controlled by vesicular monoamine transporters. In catecholaminergic neurons, the presence of monoamine oxidase leads to formation of reactive catecholaldehydes. Production of these toxic aldehydes depends on the dynamics of vesicular-axoplasmic monoamine exchange and enzyme-catalyzed conversion to nontoxic acids or alcohols. In sympathetic nerves, the aldehyde produced from norepinephrine is converted to 3,4-dihydroxyphenylglycol, not 3,4-dihydroxymandelic acid. Subsequent extraneuronal O-methylation consequently leads to production of 3-methoxy-4-hydroxyphenylglycol, not vanillylmandelic acid. Vanillylmandelic acid is instead formed in the liver by oxidation of 3-methoxy-4-hydroxyphenylglycol catalyzed by alcohol and aldehyde dehydrogenases. Compared to intraneuronal deamination, extraneuronal O-methylation of norepinephrine and epinephrine to metanephrines represent minor pathways of metabolism. The single largest source of metanephrines is the adrenal medulla. Similarly, pheochromocytoma tumor cells produce large amounts of metanephrines from catecholamines leaking from stores. Thus, these metabolites are particularly useful for detecting pheochromocytomas. The large contribution of intraneuronal deamination to catecholamine turnover, and dependence of this on the vesicular-axoplasmic monoamine exchange process, helps explain how synthesis, release, metabolism, turnover, and stores of catecholamines are regulated in a coordinated fashion during stress and in disease states.

Plasma metanephrines in renal failure

BACKGROUND

Diagnosis of pheochromocytoma in renal failure poses a diagnostic dilemma due to lack of reliability of conventional urinary measurements of catecholamine excess. Measurements of the plasma metanephrines, normetanephrine and metanephrine (the O-methylated metabolites of norepinephrine and epinephrine), provide an alternative diagnostic test. The metanephrines may be measured as free metabolites or after a deconjugation step where measurements reflect mainly sulfate-conjugated metabolites. The influence of renal insufficiency states on these various measurements is unclear.

METHODS

Plasma free and deconjugated metanephrines and catecholamines in 17 patients on dialysis with end-stage renal disease and 19 patients with renal insufficiency (creatinine clearance, 5-78 mL/min) were compared with levels in 89 hypertensives, 68 healthy normotensives, and 51 patients with von Hippel-Lindau syndrome.

RESULTS

Patients with renal failure had up to two-fold higher plasma concentrations of catecholamines and free metanephrines, and more than 12-fold higher plasma concentrations of deconjugated metanephrines than comparison groups. Plasma free metanephrines and catecholamines were, respectively, within the 95% confidence intervals of reference groups in 75% and 42% of the dialysis patients, and in 74% and 68% of patients with renal insufficiency. In contrast, no dialysis patient and only half the renal insufficiency patients had plasma levels of deconjugated metanephrines within the reference intervals. Plasma levels of deconjugated metanephrines, but not free metanephrines, showed strong inverse relationships with creatinine clearance.

CONCLUSION

Plasma concentrations of free metanephrines are relatively independent of renal function and are, therefore, more suitable for diagnosis of pheochromocytoma among patients with renal failure than measurements of deconjugated metanephrines.

Pheochromocytoma and myocardial infarction

Pheochromocytomas are rare chromaffin cell tumors, 90% of which arise from the adrenal glands. Pheochromocytomas presenting with true myocardial infarction are even more rare. We report a 76-year-old man who had a previously undiagnosed pheochromocytoma, and presented with the uncommon complication of myocardial infarction. Our high-risk patient was managed with the combination of simultaneous coronary artery bypass grafting and adrenalectomy.

Diagnosis and localization of pheochromocytoma

This Hypertension Grand Rounds shows how applying new clinical laboratory techniques helped to diagnose pheochromocytoma in a difficult case. In the setting of long-standing, sustained hypertension, the patient had a hypertensive paroxysm during anesthesia induction for surgery, leading to suspicion of a pheochromocytoma. Conventional testing, including CT scanning and fractionated urinary metanephrine test, was not diagnostic. The patient had another hypertensive paroxysm during subsequent anesthesia induction, requiring intensive care. Consistently elevated plasma levels of free normetanephrine provided the first and only biochemical evidence for a pheochromocytoma in this case. 6-[18F]Fluorodopamine positron emission tomography and 123I-metaiodobenzylguanidine scintigraphy subsequently agreed on the existence of a small left adrenal mass, which when removed surgically proved to be a pheochromocytoma. Postoperatively, plasma levels of normetanephrine normalized, and there were no further hypertensive paroxysms, although the patient remained hypertensive. This case illustrates the superiority of plasma levels of free (unconjugated) metanephrines, compared with other biochemical tests, to detect pheochromocytoma. It also confirms that functional imaging by 6-[18F]fluorodopamine or 123I-metaiodobenzylguanidine scanning can localize pheochromocytoma in difficult cases in which other imaging tests are not diagnostic.

Advances in metanephrine testing for the diagnosis of pheochromocytoma

Pheochromocytoma is a lethal tumor of chromaffin cells of the adrenal medulla that produces episodes of hypertension with the symptoms of palpitations, severe headaches, and sweating. The diagnosis of pheochromocytoma is a challenging one; autopsy series suggest that many pheochromocytomas are not clinically suspected, and the undiagnosed tumor can be associated with morbid consequences. The testing of catecholamines, metanephrines, and Vanillylymandelic acid commonly is used for screening of pheochromocytoma. The diagnostic value of various biochemical tests,as reported in recent Mayo and National Institutes of Health studies, have been compared and discussed in detail. The recent developments in the methodologies of metanephrines testing also are presented.

Pheochromocytoma: rediscovery as a catecholamine-metabolizing tumor

Catecholamine-producing tumors are rare neoplasms derived mainly from chromaffin cells of the adrenal medulla (pheochromocytomas) or, in about 10% of cases, from paraganglia (paragangliomas). Diagnosis of these tumors relies heavily on measurements of urinary or plasma catecholamines or catecholamine metabolites. The metabolites are usually thought to be produced after release of catecholamines into the bloodstream. This, however, ignores observations of over 40 yr ago that catecholamines are metabolized within pheochromocytoma tumor cells. Development of improved methods for measurement of catecholamine metabolites, in particular, plasma concentrations of free normetanephrine and metanephrine, has reestablished the importance of intratumoral catecholamine metabolism. In patients with pheochromocytoma, over 90% of the elevations in plasma free normetanephrine and metanephrine result from metabolism of catecholamines within pheochromocytoma tumor cells. This process occurs continuously and independently of variations in catecholamine release. As a consequence, measurements of plasma concentrations and urinary outputs of normetanephrine and metanephrine provide more reliable methods for diagnosis of pheochromocytoma than measurements of the parent amines. Rediscovery of the importance of intratumoral catecholamine metabolism is leading to a reevaluation of the procedures used to diagnose pheochromocytoma. This review provides an update on the diagnosis of pheochromocytoma, with emphasis on identifying and correcting relevant misconceptions about catecholamine metabolism.

New advances in the biochemical diagnosis of pheochromocytoma: moving beyond catecholamines

Pheochromocytomas are dangerous tumors that, although a rare cause of hypertension, require consideration among large numbers of patients. The resulting low prevalence of the tumor among tested populations and the inadequacies of commonly used biochemical tests make excluding or confirming the tumor an often difficult and time-consuming task. Recognition that catecholamines are metabolized to free metanephrines within pheochromocytoma tumor cells, and that this process is independent of catecholamine release, provides a rationale for use of these metabolites in the biochemical diagnosis of pheochromocytoma. Here we briefly review the history of biochemical diagnosis of pheochromocytoma in relation to recent data about the diagnostic utility of plasma free metanephrines for detection of these tumors. Measurements of urinary or plasma catecholamines have reasonable sensitivity for detection of most pheochromocytomas, particularly those in patients with sustained hypertension. False-negative test results can, however, occur in asymptomatic patients tested because of an adrenal incidentaloma or a familial predisposition for pheochromocytoma, or when sampling is carried out between episodes of paroxysmal hypertension. Measurements of urinary total metanephrines or vanillylmandelic acid are less reliable and are of little value as initial screening tests. In contrast, measurements of plasma concentrations or free metanephrines or 24-hour urinary outputs of fractionated normetanephrine and metanephrine almost always reveal the tumor. Although, both tests have similarly high sensitivity, the relatively low specificity of urinary fractionated metanephrines means that pheochromocytomas can be more efficiently excluded or confirmed using measurements of plasma free metanephrines.

Choice of biochemical test for diagnosis of pheochromocytoma: validation of plasma metanephrines

Pheochromocytomas, although a rare cause of hypertension, are dangerous tumors that require consideration among large numbers of patients. The subsequent low prevalence of the tumor among those tested and inadequacies of commonly used biochemical tests make excluding or confirming the tumor an often difficult and time consuming task. Recognition that catecholamines are metabolized to free metanephrines within pheochromocytoma tumor cells, and that this process is independent of catecholamine release, provides a rationale for use of these metabolites in the biochemical diagnosis of pheochromocytoma. Measurements of plasma concentrations of free metanephrines thereby promise more reliable and efficient diagnosis of pheochromocytoma than offered by conventional biochemical tests.