Biosynthesis

Catecholamine synthesis begins with the amino acid tyrosine, which comes from the diet or via hydroxylation of phenylalanine in the liver. l-Tyrosine is converted to dihydroxyphenylalanine (dopa) by tyrosine hydroxylase (TH) (Fig. 14-2). This reaction requires molecular oxygen and the cofactor tetrahydrobiopterin and represents the rate-limiting step in catecholamine biosynthesis (Fig. 14-3).³² Tissue sources of catecholamines therefore are principally dependent on the presence of this enzyme, which is largely confined to dopaminergic and noradrenergic neurons of the central nervous system, and to sympathetic nerves and chromaffin cells of the adrenal medulla and extramedullary paraganglia in the periphery.

Dopa is decarboxylated by aromatic l-amino acid decarboxylase to produce dopamine. The dopamine formed in neurons and chromaffin cells is translocated from the cytoplasm into vesicular storage granules. In dopaminergic neurons of the central nervous system, dopamine serves as the end product neurotransmitter. Large amounts of dopamine are produced as an end product of catecholamine synthesis in peripheral non-neuronal cells of the gastrointestinal tract and kidneys.³³ The dopamine present in urine is derived mainly from decarboxylation of dopa taken up from the circulation by the kidneys.

The dopamine formed in noradrenergic neurons and chromaffin cells is converted to norepinephrine by dopamine β-hydroxylase (DBH), an enzyme that is found only in the vesicles of cells that synthesize norepinephrine and epinephrine.

In adrenal medullary chromaffin cells, norepinephrine is metabolized by the cytosolic enzyme phenylethanolamine N-methyltransferase (PNMT) to form epinephrine.³⁴ Epinephrine then is translocated into chromaffin granules, where the amine is stored while awaiting release.³⁵ PNMT activity is induced by high levels of glucocorticoid production in the adrenal cortex.

Presumably because expression of PNMT is dependent on high local concentrations of glucocorticoids, pheochromocytomas that produce large amounts of epinephrine typically have an adrenal location, whereas those in extra-adrenal locations usually produce only norepinephrine.³⁶ Pheochromocytomas that lack both PNMT and DBH and that predominantly produce dopamine are extremely rare. Dopamine production is a more common finding in individuals with parasympathetic-associated paragangliomas.

Conversion of tyrosine to dopa by TH represents the pivotal step in regulating synthesis and in maintaining stores of catecholamines in response to changes in catecholamine turnover associated with variations in exocytotic release. Rapid activation of TH is achieved by phosphorylation of serine residues at the regulatory domain, under the control of multiple Ca²⁺ and cyclic adenosine monophosphate (cAMP)-dependent pathways influenced by changes in nerve activity and actions of peptides and other coactivators.³⁷ Long-term regulation involves induction of synthesis of the enzyme at the transcriptional level. Feedback inhibition by catecholamines provides a further mechanism for the short-term regulation of enzyme activity.

α-Methyl-l-tyrosine or metyrosine (Demser) is an analogue of tyrosine that inhibits TH, thereby decreasing catecholamine stores. The drug is used occasionally in patients with pheochromocytoma (particularly those with extensive metastatic disease) or preoperatively.

Storage and Release

Translocation of catecholamines into vesicular granules for storage is facilitated by two vesicular monoamine transporters.³⁸ Both transporters have wide specificity for different monoamine substrates. It is important to note that vesicular stores of catecholamines do not exist in a static state, simply waiting for a signal for exocytotic release. Rather, vesicular stores of catecholamines exist in a highly dynamic equilibrium with the surrounding cytoplasm, with passive outward leakage of catecholamines into the cytoplasm counterbalanced by inward active transport under control of the vesicular monoamine transporters.³¹

Catecholamines share the acidic environment of the storage granule matrix with adenosine triphosphate (ATP), peptides, and proteins, the most well known of which are the chromogranins.³⁹ The chromogranins are ubiquitous components of secretory vesicles, and their widespread presence among endocrine tissues has led to their measurement in plasma as useful, albeit relatively nonspecific, markers of neuroendocrine tumors, including pheochromocytomas.

Catecholamines are stored in several types of vesicular granules that differ in size and type of protein and peptide components, the specific functions of which are incompletely understood. Two populations of chromaffin cells have been described with morphologically distinct vesicles, which preferentially store norepinephrine or epinephrine, and which release the two catecholamines differentially in response to different stimuli. Most adrenal pheochromocytomas secrete both norepinephrine and epinephrine; about a third exclusively produce norepinephrine, and a much smaller proportion exclusively produce and secrete epinephrine.

The process of exocytosis occurs at specialized locations on sympathetic varicosities dictated by the cell-surface expression of specialized docking proteins that interact with other proteins on the surfaces of secretory vesicles. The process is stimulated by an influx of Ca²⁺, which in neurons is controlled primarily by nerve impulse–mediated membrane depolarization, and in adrenal medullary cells by acetylcholine release from innervating splanchnic nerves. The wide range of voltage-, receptor-, G protein–, and second messenger–operated Ca²⁺ channels provide numerous points for regulation of Ca²⁺-triggered exocytosis. In this way, a variety of peptides, neurotransmitters, and humoral factors provide additional mechanisms for stimulation of exocytosis or may act to modulate nerve impulse–stimulated release of catecholamines. Norepinephrine also inhibits its own release through occupation of presynaptic α₂-adrenoceptors. The mechanisms regulating catecholamine release are closely coordinated so as to regulate the enzymes responsible for synthesis, thereby ensuring appropriate replenishment of the catecholamines lost through exocytosis.⁴⁰

Uptake and Metabolism

Because the enzymes responsible for the metabolism of catecholamines have intracellular locations, the primary mechanism limiting the life span of catecholamines in the extracellular space is uptake by active transport, not metabolism by enzymes.⁴¹ Uptake is facilitated by transporters that belong to two families with mainly neuronal and extraneuronal locations. The neuronal norepinephrine transporter provides the principal mechanism for rapid termination of the signal in sympathoneuronal transmission, whereas transporters at extraneuronal locations are more important for limiting the spread of the signal and for clearing catecholamines from the bloodstream. For norepinephrine released by sympathetic nerves, about 90% is removed back into nerves by neuronal uptake, 5% is removed by extraneuronal uptake, and 5% escapes these processes to enter the bloodstream. In contrast, for epinephrine released directly into the bloodstream from the adrenals, about 90% is removed by extraneuronal monoamine transport processes, the liver being particularly important. The presence of these highly active transport processes means that catecholamines are rapidly cleared from the bloodstream with a circulatory half-life of less than 2 minutes.

Irreversible inactivation of released catecholamines occurs in a series arrangement, with uptake followed by metabolism. Metabolism occurs through a multiplicity of pathways catalyzed by an array of enzymes and resulting in a variety of metabolites (see Figs. 14-3 and 14-4).⁴² Deamination of catecholamines by monoamine oxidase (MAO) yields reactive aldehyde intermediate metabolites that are metabolized further to deaminated acids (by aldehyde dehydrogenase) or to deaminated alcohols (by aldehyde or aldose reductase). The aldehyde intermediate formed from dopamine is a good substrate for aldehyde dehydrogenase but not for aldehyde or aldose reductase. In contrast, the aldehyde intermediates formed from the β-hydroxylated catecholamines (i.e., norepinephrine and epinephrine) are good substrates for aldehyde or aldose reductase but are poor substrates for aldehyde dehydrogenase. Therefore, norepinephrine and epinephrine are preferentially deaminated to 3,4-dihydroxyphenylglycol (DHPG), the alcohol metabolite. Deamination to the deaminated acid metabolite, 3,4-dihydroxymandelic acid (DHMA), is not a favored pathway.

Catechol-O-methyltransferase (COMT) is responsible for the second major pathway of catecholamine metabolism: catalyzing O-methylation of dopamine to methoxytyramine, norepinephrine to normetanephrine, and epinephrine to metanephrine. COMT is not present in catecholamine-producing neurons, which contain exclusively MAO, but it is present along with MAO in most extraneuronal tissues. The membrane-bound isoform of COMT, which has high affinity for catecholamines, is especially abundant in adrenal chromaffin and pheochromocytoma tumor cells.⁴³ As a result of differences in expression of metabolizing enzymes, catecholamines produced in neurons and adrenal medullary or pheochromocytoma tumor cells follow different neuronal and extraneuronal pathways of metabolism (see Figs. 14-3 and 14-4).

Neuronal pathways are quantitatively far more important than are extraneuronal pathways for the metabolism of norepinephrine produced in sympathetic nerves (see Fig. 14-4). The reasons for this are twofold. First, much more norepinephrine released by sympathetic nerves is removed by neuronal uptake than by extraneuronal uptake. Second, under resting conditions, much more of the norepinephrine metabolized intraneuronally is derived from transmitter leaking from storage vesicles than from transmitter recaptured after exocytotic release. Thus, most of the norepinephrine produced in the body is metabolized initially to DHPG, mainly from transmitter deaminated intraneuronally after leakage from storage vesicles or after release and reuptake.

DHPG is further O-methylated by COMT in non-neuronal tissues to 3-methoxy-4-hydroxyphenylglycol (MHPG), a metabolite that is also produced to a limited extent by deamination of normetanephrine and metanephrine. Compared with DHPG, normetanephrine and metanephrine are produced in small amounts and only at extraneuronal locations, with the single largest source representing adrenal chromaffin cells, which account for more than 90% of circulating metanephrine and 24% to 40% of circulating normetanephrine.⁴⁴ Within the adrenals, normetanephrine and metanephrine are produced similarly to DHPG in sympathetic nerves, from norepinephrine and epinephrine leaking from storage granules into the chromaffin cell cytoplasm.

The MHPG produced from DHPG and metanephrines is conjugated or metabolized to vanillylmandelic acid (VMA) by the sequential actions of alcohol dehydrogenase and aldehyde dehydrogenase. The former enzyme is localized largely to the liver. Thus, at least 90% of the VMA formed in the body is produced in the liver, mainly from hepatic uptake and metabolism of circulating DHPG and MHPG.⁴⁵

With the exception of VMA, all the catecholamines and their metabolites are metabolized to sulfate conjugates by a specific sulfotransferase isoenzyme type 1A3 (SULT1A3). In humans, a single amino acid substitution confers the enzyme with particularly high affinity for dopamine and the O-methylated metabolites of catecholamines, including normetanephrine, metanephrine, and methoxytyramine. The SULT1A3 isoenzyme is found in high concentrations in gastrointestinal tissues, which therefore represent a major source of sulfate conjugates.

In humans, VMA and the sulfate and glucuronide conjugates of MHPG represent the main end products of norepinephrine and epinephrine metabolism. HVA and the conjugates of HVA are the main metabolic end products of dopamine metabolism. These end products and the other conjugates are eliminated mainly by urinary excretion. As a result, their circulatory clearance is slow and their plasma concentrations are high relative to those of the precursor amines.

Signs and Symptoms

The presence of pheochromocytoma usually is characterized by clinical signs and symptoms (Table 14-1) that result from hemodynamic and metabolic actions of circulating catecholamines or, less commonly, of other amines and co-secreted neuropeptides.¹⁸ Clinical findings such as hypertension, headache, unusual sweating, frequent arrhythmias, and the presence of pallor during hypertensive episodes are highly suggestive of pheochromocytoma. Although headache, palpitations, and sweating are nonspecific symptoms, their presence in patients with hypertension should arouse immediate suspicion for a pheochromocytoma because this triad constitutes the most commonly encountered symptoms in patients with a pheochromocytoma.

Sustained or paroxysmal hypertension (equally present) is the most common clinical sign (85% to 90%). Up to 13% of patients typically present with persistently normal blood pressure,5,18 but this proportion can be much higher in patients with adrenal incidentalomas or in those who undergo periodic screening for familial pheochromocytoma.^12,46,47 In the latter group, tumors now are being found at an earlier stage, usually are quite small, and secrete low amounts of catecholamines. Other factors that may affect pheochromocytoma-associated elevations in blood pressure include the nature of catecholamine secretion, that is, receptor downregulation caused by consistently high levels of catecholamines, hypovolemia, and associated changes in sympathetic nerve function.⁴⁸ Pheochromocytoma also may present with hypotension, particularly with postural hypotension or with alternating episodes of high and low blood pressure.^5,49 This is commonly seen in patients who harbor tumors that are secreting epinephrine or compounds causing vasodilatation, or after higher doses of antihypertensive therapy. Hypotension also may occur secondary to hypovolemia, abnormal autonomic reflexes, or differential stimulation of α- and β-adrenergic receptors, or as a result of the type of neuropeptide that is co-secreted.

Headache, which occurs in up to 90% of patients with pheochromocytoma,1,18 may be mild or severe, short or long in duration, and may last for up to several days. In some patients, catecholamine-induced headache may be similar to tension headache.

Excessive generalized sweating occurs in approximately 60% to 70% of patients presenting with pheochromocytoma.1,18 Other complaints are palpitations and dyspnea, weight loss despite normal appetite (caused by catecholamine-induced glycogenolysis and lipolysis), and generalized weakness.^49,50

Some patients present with new and commonly more severe episodes of anxiety or panic attacks.⁴⁹ Palpitations, anxiety, and nervousness are more common in patients with pheochromocytomas that produce epinephrine.⁴⁹ Less common clinical manifestations include fever of unknown origin (hypermetabolic state) and constipation secondary to catecholamine-induced decrease in intestinal motility, or possibly enkephalins.⁵¹ Some patients say that they feel flushed^18,52; however, in our experience, the observation of an actual flush during a paroxysmal hypertensive episode in a patient with pheochromocytoma is an exceedingly rare event. Occasionally, Raynaud’s phenomenon is noted. Patients also may present with tremor, seizures, hyperglycemia, hypermetabolism, weight loss (usually noted only in patients with malignant pheochromocytoma), fever, and even mental changes.^18,52

Pheochromocytoma-induced metabolic or hemodynamic attacks may last from a few seconds to several hours, with intervals between attacks varying widely; some occur only once every few months. A typical paroxysm is characterized by a sudden major increase in blood pressure; a severe, often pounding headache; profuse sweating over most of the body, especially the trunk; palpitations with or without tachycardia; prominent anxiety or a sense of doom; skin pallor; nausea, with or without emesis; and pain in the abdomen, the chest, or both.18,52,53 After an episode, patients usually feel drained and exhausted, and some may urinate more frequently. Initially, the episodes may be mild, of short duration, and infrequent.

Unusual symptoms related to paroxysmal blood pressure elevations during diagnostic procedures such as endoscopy, administration of anesthesia (caused by a sudden fall in blood pressure or any activation of the sympathetic nervous system, such as occurs during the induction phase of general anesthesia), or ingestion of food or beverages containing tyramine (e.g., certain cheeses, beers, wines, bananas, and chocolate) should arouse immediate suspicion of pheochromocytoma. The use of certain drugs such as histamine, metoclopramide, adrenocorticotropic hormone (ACTH), phenothiazine, methyldopa, monoamine oxidase inhibitors, tricyclic antidepressants, opiates (e.g., morphine, fentanyl), naloxone, droperidol, metoclopramide, glucagon, and chemotherapy may precipitate a hypertensive episode.18,52,54–60 Moreover, micturition or bladder distention in the case of a pheochromocytoma of the urinary bladder (more than half of these individuals have painless hematuria) should promptly arouse suspicion of the presence of this tumor. Causes that account for episodic catecholamine secretion often remain unestablished, but in some situations, it may be caused by intentional or accidental tumor manipulation coupled with an increase in intra-abdominal pressure from palpation, defecation, a fall, an automobile accident, or pregnancy.^18,52,61 Psychological stress does not usually precipitate a hypertensive crisis.^18,52,61 Timing of attacks may be unpredictable, and attacks may also occur at rest. However, about 8% to 10% of patients may be completely asymptomatic, usually because of a very small (less than 1 cm) tumor associated with nonsignificant catecholamine secretion or tumor dedifferentiation characterized by the absence of catecholamine-synthesizing enzymes and resulting in no production of catecholamines.⁵ Normal plasma and urine catecholamines and metanephrines due to absence of catecholamine production have been observed in patients with metastatic pheochromocytoma, often due to an underlying mutation of the succinate dehydrogenase subunit B gene.⁶² The defect was identified as an absence of tyrosine hydroxylase, the enzyme that catalyzes the initial and rate-limiting step in catecholamine biosynthesis (personal communication). These patients with so-called biochemically silent pheochromocytoma usually present at an advanced stage of the disease with symptoms and signs related to tumor mass effects rather than with symptoms and signs of catecholamine excess.

The hyperglycemia is usually mild, occurs with the hypertensive episodes, is accompanied by a subnormal level of plasma insulin (because of α-adrenergic inhibition of insulin release),⁶³ and usually does not require treatment; however, it can be sustained and severe enough to require insulin and even to present as diabetic ketoacidosis.⁶⁴ Hypoglycemia has also been reported.

Hypercalcemia has been reported in some patients with pheochromocytoma, sometimes as part of multiple endocrine neoplasia type 2 (MEN2)⁵⁴ but at other times without evidence of parathyroid disease⁶⁵ because the hypercalcemia disappears after the tumor is resected. Likewise, high levels of serum calcitonin, suggestive of medullary thyroid carcinoma, have been reported; these return to normal after the tumor is removed.⁶⁶ In addition, pheochromocytoma has presented as Cushing’s syndrome with the tumor as the ectopic source of ACTH.⁶⁷ Because catecholamines suppress intestinal motility, constipation is common, and occasionally even adynamic ileus occurs. Rarely, pheochromocytoma has produced vasoactive intestinal peptide with resultant watery diarrhea, hypokalemia, and achlorhydria (Verner-Morrison syndrome).⁶⁸ Lactic acidosis without shock or sepsis has been reported.⁶⁹ Interleukin-6 (IL-6) may have caused fever and multiorgan failure in one patient with pheochromocytoma with increased serum IL-6 and in another with pheochromocytoma and Castleman’s disease (IL-6–mediated B cell proliferation) because these conditions were reversed in each patient when the tumor was removed.^70,71 Many patients with pheochromocytoma are asymptomatic or have only minor signs and symptoms; therefore, the diagnosis is easily missed, often with tragic consequences. Several studies of routine autopsies have indicated that most pheochromocytomas are first discovered after death.⁴ This pattern has continued into more recent studies, which show that the diagnosis is missed much more often in elderly patients.⁷² This may result from the fact that elderly patients often have other, more common diseases (e.g., coronary or cerebral atherosclerosis) that could easily explain the minor symptoms that patients report.⁷² A list of emergency situations characteristic for the presence of pheochromocytoma are described in Table 14-2.

Estrogen, growth hormone, vitamin D, and retinol A (Accutane) have been shown to induce pheochromocytoma in experimental animals.⁷³ Whether these hormones contribute to the higher incidence of clinical pheochromocytoma or to an increase in malignant potential is unknown; however, recently at least two patients chronically receiving Accutane treatment were found to have pheochromocytoma (personal communications).

In summary, the following patients should be evaluated for a pheochromocytoma: (1) anyone with the triad of headaches, sweating, and tachycardia, whether or not the subject has hypertension; (2) anyone with a known mutation of one of the susceptibility genes and/or a family history of pheochromocytoma; (3) anyone with an incidental adrenal mass; (4) anyone whose hypertension is associated with borderline increases in catecholamine production reflected by elevated plasma catecholamine or metanephrine levels; (5) anyone whose blood pressure is poorly responsive to standard therapy; and (6) anyone who has had hypertension, tachycardia, or an arrhythmia in response to anesthesia, surgery, or medications known to precipitate symptoms in patients with pheochromocytoma.⁴⁸

Differential Diagnosis

The differential diagnosis of pheochromocytoma includes a long list of conditions that may suggest the presence of the tumor (Table 14-3).^18,52,61 Many of these conditions can be excluded readily on the basis of a good history and physical examination. The most common mimic is hyperadrenergic hypertension, which is characterized by tachycardia, sweating, anxiety, and increased cardiac output.^74,75 These patients often have increased levels of catecholamines in blood and urine and may be excluded by use of the clonidine suppression test, which shows that the excess catecholamines are caused by excess central sympathetic nervous activity and are not caused by a tumor (see the following).⁴⁸ Another common problem involves differentiating patients with pheochromocytoma from those with anxiety or panic attacks. This generally requires close observation of the patient during an episode.

Differentiation from an acute myocardial infarction may be difficult because angina pectoris and myocardial damage may occur in the absence of coronary artery disease in a patient with a pheochromocytoma. Many nonspecific electrocardiographic (ECG) changes have been reported, as well as various supraventricular and ventricular tachycardias, in patients with pheochromocytoma.

In the very rare situation in which a strong discordance is noted between symptoms and laboratory findings, one must consider the possibility of factitious administration of catecholamines. In this rare situation, as in other cases of factitious illness, an association of the individual with someone in the medical profession appears to be one of the common elements and should raise appropriate suspicion.

Patterns of biochemical test results that are more suggestive of sympathoadrenomedullary activation than of a tumor (e.g., as occurs in hypernoradrenergic hypertension, renovascular hypertension, congestive heart failure, panic disorder, and dumping syndrome) include proportionally larger elevations above the normal limits of plasma norepinephrine or epinephrine than of plasma normetanephrine or metanephrine.

Although severe paroxysmal hypertension should always raise suspicion of pheochromocytoma, it can also reflect a clinical entity called pseudopheochromocytoma. Pseudopheochromocytoma refers to the large majority of individuals (often women) with severe paroxysmal hypertension, whether normotensive or hypertensive between episodes, in whom pheochromocytoma has been ruled out.76,77 Recent evidence indicates that pseudopheochromocytoma is a heterogeneous clinical condition subdivided into a primary and a secondary form. In contrast to a primary form, a secondary form is associated with various pathologies (e.g., hypoglycemia, autonomic epilepsy, baroreceptor failure) and medications, or with drug abuse.

The most common clinical characteristics of this syndrome might be attributable in many cases to short-term activation of the sympathetic nervous system. Paroxysmal hypertension usually is associated with tachycardia, palpitations, nervousness, tremor, weakness, excessive sweating, pounding headache, feeling hot, and facial paleness or (rarely) redness. In contrast to pheochromocytoma, patients with pseudopheochromocytoma more often present with panic attacks or anxiety, flushing, nausea, and polyuria.76–78 All these symptoms well resemble a syndrome described by Page.⁷⁹ Equally interesting were his observations that “an attack is brought on by excitement” and that the syndrome has a clear predominance in women. Among the important features distinctive from pheochromocytoma are the circumstances under which episodes occur. In pseudopheochromocytoma, symptoms may be provoked rarely by some identifiable event. It is important, therefore, in questioning these patients, to search for specific provocative factors that may have precipitated these episodes. Similar to pheochromocytoma, episodes may last from a few minutes to several hours and may occur daily or once every few months. Between episodes, blood pressure is normal or may be mildly elevated. Pseudopheochromocytoma is sometimes treatable by antihypertensive drugs or psychotherapy.

Multiple Endocrine Neoplasia Syndromes

MEN2 is an autosomal dominantly inherited syndrome (Sipple’s syndrome) that consists of pheochromocytoma, medullary carcinoma of the thyroid, and hyperparathyroidism.94–96 It affects about 1 in 40,000 individuals and is characterized by medullary thyroid carcinoma, pheochromocytoma, and parathyroid hyperplasia/adenoma.⁹⁷

The gene responsible for MEN2 is a proto-oncogene called RET.⁹⁸ In contrast to MEN1, RET is specifically expressed in neural crest–derived cells such as calcitonin-producing C cells in the thyroid gland and catecholamine-producing chromaffin cells in the adrenal gland. Whether it is also expressed in the parathyroid glands remains to be ascertained, especially when the low rate of hyperparathyroidism in patients with MEN2A and the lack of hyperparathyroidism in those with MEN2B are considered, although both conditions are caused by mutations in the RET gene. RET plays a role in normal gastrointestinal neuronal and kidney development, as exemplified by the RET knockout mouse, which has a Hirschsprung-like phenotype and renal cyst or agenesis.⁹⁹ RET is located on chromosome 10q11.2 and encodes a receptor tyrosine kinase, RET protein. As an oncogene, activation of RET leads to hyperplasia of target cells in vivo. Subsequent secondary events then lead to tumor formation.¹⁰⁰ RET consists of 21 exons with 6 so-called “hot spot exons” (exons 10, 11, 13, 14, 15, and 16) in which mutations are identified in 97% of patients with MEN2. Recently, it has been proposed that in addition to other events, RET protein accumulation, secondary to absent or reduced VHL protein, may then cause transformation of selected chromaffin cells to pheochromocytoma.¹⁰⁰ Additional studies are needed to clarify whether such somatic VHL gene alterations in MEN2-associated tumors play a role in early or late tumorigenesis or rather in tumor progression. RET germline mutation screening is commercially available (in the US, http://endocrine.mdacc.tmc.edu; Mayo Clinic, Rochester, MN) and has widely replaced the cumbersome provocative testing of calcitonin stimulation (with calcium and/or pentagastrin), which has been unreliable.

Pheochromocytomas (at least 70% of which are bilateral) develop on a background of adrenomedullary hyperplasia and become manifest (e.g., biochemically or on imaging) in about 50% of patients. MEN2-associated pheochromocytomas are almost exclusively benign (with <5% reported to be malignant) and localized to the adrenals. The peak age is around 40 years, but children as young as 10 years can be affected.101,102

Patients with MEN2-related pheochromocytoma often lack sustained hypertension or other symptoms (they occur only in about 50%). Because MEN-related pheochromocytomas secrete epinephrine, stimulation of β-adrenergic receptors causes palpitations and tachycardia. Therefore, their detection is based mainly on elevated plasma metanephrine and epinephrine levels. In patients with pheochromocytomas that exclusively produce normetanephrine, MEN2 can be excluded. MEN2-related pheochromocytomas are almost always intra-adrenal, are often bilateral (≈30% at diagnosis), and are rarely malignant (<5%).46,103 In addition, as with most epinephrine-secreting pheochromocytomas, the hypertension is more likely to be paroxysmal than sustained. For these reasons, the diagnosis is easy to miss.

Medullary thyroid carcinoma is found in most patients with MEN2A, pheochromocytoma in a somewhat lesser percentage, and hyperparathyroidism in only about 25% of affected patients. Pheochromocytomas in MEN 2A most often are diagnosed at between 30 and 40 years of age. The syndrome results from germline mutations in the RET proto-oncogene on chromosome 10 (10q11.2).⁹⁸

Patients with MEN2B have pheochromocytoma, medullary carcinoma of the thyroid, ganglioneuromatosis, multiple mucosal neuromas of eyelids, lips, and tongue, and some connective tissue disorders that include marfanoid habitus, scoliosis, kyphosis, pectus excavatum, slipped femoral epiphysis, and pes cavus.¹⁰⁴ This syndrome also appears to be caused by germline mutations in the RET proto-oncogene on chromosome 10, but these mutations affect the tyrosine kinase catalytic site of the protein.¹⁰⁵ In children with MEN2B-associated pheochromocytomas, a higher risk of malignancy compared with MEN2A or sporadic disease is found.¹⁰⁶

MEN1 (Wermer’s syndrome) consists of hyperparathyroidism, pituitary adenomas, and pancreatic islet cell tumors. Pheochromocytoma is not usually part of this complex; however, the occurrence of pheochromocytoma and pancreatic islet cell tumors has been reported in some families.¹⁰⁷ Often, the islet cell tumors are nonfunctional. Various “crossover” syndromes have been reported in which pheochromocytoma has been associated with characteristics of MEN1, MEN2A, MEN2B, von Recklinghausen’s neurofibromatosis (NF), VHL, and the Zollinger-Ellison syndrome.¹⁰⁸ It has been proposed that the combination of neurofibromatosis, duodenal carcinoid, and pheochromocytoma constitutes a neuroendocrine syndrome that is separate from the combination of VHL, islet cell tumor, and pheochromocytoma.¹⁰⁹

Von Hippel-Lindau Syndrome

Another neuroectodermal syndrome commonly associated with pheochromocytoma is VHL syndrome, which is caused by mutations in chromosome 3 (3p25-26), which encodes the VHL tumor suppressor gene.¹¹⁰ Pheochromocytomas in VHL disease typically develop according to Knudson’s two-hit model, including an inherited germline mutation of VHL and loss of function of the wild-type allele of the VHL gene. The disease has been divided into two types based on the significant genotype-phenotype correlations observed. Type 1 includes mainly large deletions or mutations and expresses the full phenotype of vascular lesions of the retina, cysts or solid tumors in the brain or spinal cord, pancreatic cysts, renal cell carcinoma, epididymal cystadenoma, and endolymphatic sac tumors, but no pheochromocytoma. Type 2 has missense mutations, pheochromocytoma, and the full phenotype.¹¹¹ Patients often are asymptomatic when they present with other aspects of this disease. This syndrome is variable in terms of the different organ systems involved and the extent of involvement from patient to patient and from family to family. Overall, less than 30% of patients with a VHL germline mutation develop a pheochromocytoma. Pheochromocytomas as part of the VHL syndrome have an exclusively noradrenergic phenotype, reflecting the production of norepinephrine only.¹¹² These tumors are located mainly intra-adrenally and are bilateral in about 50% of patients, with a less than 7% incidence of metastases. Because they do not express glucagon receptors, the glucagon test should not be used for detection of these tumors. These tumors are commonly found on the basis of periodic annual screening, or during searches for other tumors that are part of this syndrome. Therefore, when detected, these tumors are commonly small; they often fail to be detected by nuclear imaging methods. Furthermore, about 80% of pheochromocytomas found in patients with VHL during screening are asymptomatic and are not associated with hypertension.

Neurofibromatosis Type 1

Von Recklinghausen’s NF is now divided into two types: NF-1 has neurofibromas of peripheral nerves, whereas NF-2 has central neurofibromas. NF-1 is inherited in an autosomal dominant pattern. The association with pheochromocytoma is one between a relatively common disease and a rare disease. Thus, although less than 1% to 2% of patients with NF have pheochromocytoma, about 5% of patients with pheochromocytoma have NF.¹¹³ Pheochromocytoma associated with NF-1 is caused by germline mutations in chromosome 17 (17q11), where the NF-1 gene encodes for the protein neurofibromin. These mutations lead to inactivation of this tumor suppressor gene and its protein.¹¹⁴ Similar mutations introduced into the NF-1 gene in mice lead to pheochromocytoma, which is otherwise rare in these animals.¹¹⁵ Pheochromocytoma in patients with NF-1 is rarely seen in children, because it usually occurs at a later age (around 50 years). Only 12% of patients with NF-1 are diagnosed with bilateral and multifocal pheochromocytomas, and less than 6% of patients have metastatic pheochromocytoma.¹¹⁶

The incidence of pheochromocytoma in NF-1 is relatively low (about 1%) compared with other hereditary syndromes, and routine screening of such patients generally is not recommended. However, if a patient with NF-1 has hypertension, then a pheochromocytoma should be considered and excluded.¹¹³

Succinate Dehydrogenase Gene Family

Recently, pheochromocytoma susceptibility has been associated with germline mutations of the succinate dehydrogenase (SDH) gene family.117,118 The SDH genes (SDHA, SDHB, SDHC, and SDHD) encode the four subunits of complex II of the mitochondrial electron transport chain,¹¹⁹ which is essential for the generation of ATP (oxidative phosphorylation). SDHB and SDHD mutations can lead to complete loss of SDH enzymatic activity, a phenomenon that has been linked to tumorigenesis through upregulation of hypoxic-angiogenetic responsive genes.¹²⁰ Unlike the SDHA gene, mutations of SDHB, SDHC, and SDHD genes are associated with the presence of familial and nonfamilial pheochromocytoma and parasympathetic paraganglioma. Frameshift, missense, and nonsense mutations were identified for the SDH gene family. In recent studies, it has been found that about 4% to 12% of sporadic pheochromocytomas^12,121 and up to 50% of familial pheochromocytomas¹²¹ have SDHD or SDHB mutations. The SDHB, SDHC, and SDHD traits are inherited in an autosomal dominant fashion and give rise to familial paraganglioma (PGL) syndromes 4, 3, and 1, respectively (see Table 14-4). The penetrance of these traits is incomplete, however. Furthermore, SDHD-related disease is characterized by maternal genomic imprinting. Because of silencing of the maternal allele by methylation, individuals who inherit a mutation from their mother remain free of paraganglioma but still may pass on the mutation to their offspring. SDHB mutations predispose to mainly extra-adrenal pheochromocytomas with a high malignant potential, and less frequently to benign parasympathetic head and neck paragangliomas.^62,122–124 SDHD mutations typically are associated with multifocal parasympathetic head and neck paragangliomas and usually with benign extra-adrenal and adrenal pheochromocytomas.^122,124 Metastastic pheochromocytoma is rare in SDHD mutation carriers, but it can occur.^123,125 SDHC mutations are rare and are almost exclusively associated with parasympathetic head and neck paraganglioma,¹²⁶ although rare cases of SDHC-associated extra-adrenal pheochromocytoma have been reported.^127,128 Most SDHB-related paragangliomas secrete norepinephrine or both norepinephrine and dopamine,⁶² a profile that is consistent with mainly extra-adrenal tumors. Some SDHB-related PGLs exclusively overproduce dopamine, but not other catecholamines ^62,129–131 Therefore, measurement of plasma levels of dopamine or its O-methylated metabolite methoxytyramine should be considered in SDHB-related pheochromocytoma. As mentioned, ≈10% of SDHB-related sympathetic paragangliomas are “biochemically silent.” The biochemical phenotype of SDHD-associated pheochromocytoma has not been studied consistently.

We recommend to offer genetic testing, preceded by careful genetic counseling, to all first-degree family members of patients with SDH-related pheochromocytoma and paraganglioma,¹³² and to subsequently carry out a periodic clinical, biochemical, and radiologic tumor screening among mutation carriers, especially those with an SDHB mutation. No evidence-based protocol has been used for tumor screening among children with a known SDHB, or any other mutation. We recommend periodic biochemical and total body magnetic resonance imaging (MRI) screening from as early as age 7.

Sporadic and Other Pheochromocytomas

Genetic analyses of DNA from sporadic pheochromocytoma have yielded variable results, with up to 20% RET mutations (half MEN2A and half MEN2B) in some series, and up to 20% VHL mutations in others.46,133,134 In another study, 45% of sporadic adrenal pheochromocytomas had loss of heterozygosity at the VHL gene locus, but no genetic abnormalities were found in sporadic extra-adrenal pheochromocytomas.¹³⁵ Genetic analysis provides positive identification of carriers in familial syndromes and thus identifies who should be screened and who need not be screened for the various traits of the phenotype.

Pheochromocytoma also may occur as part of Carney’s triad (i.e., gastric leiomyosarcoma, pulmonary chondroma, and extra-adrenal pheochromocytoma).¹³⁶ The syndrome is very rare; fewer than 30 cases have been reported, and only 25% of patients manifest all three parts of the triad. It occurs sporadically and is clearly nonfamilial.¹³⁷ Recently, a new syndrome called the Stratakis-Carney syndrome, or the “dyad of gastrointestinal stromal tumor and paraganglioma” associated with SDHB, SDHC, and SDHD mutations, was identified.¹³⁸

For the genetic diagnosis of pheochromocytoma, a panel of experts at the International Symposium on Pheochromocytoma 2005 held in Bethesda, Maryland, agreed that, although there is now a reasonable argument for more widespread genetic testing, it is neither appropriate nor currently cost-effective to test for every disease-causing gene in every patient with a pheochromocytoma or paraganglioma. Rather, it was stressed that the decision to test, and which genes to test, requires judicious consideration of numerous factors, several of which are noted in Fig. 14-6.

Initial Biochemical Testing

Missing a pheochromocytoma can have deadly consequences. Therefore, one of the most important considerations in the choice of an initial biochemical test is a high level of reliability that the test will provide a positive result in that rare patient with the tumor. Such a test also provides confidence that a negative result reliably excludes the tumor, thereby avoiding the need for multiple or repeat biochemical tests or even costly and unnecessary imaging studies to rule out the tumor. Therefore, the initial workup of a patient with suspected pheochromocytoma should include a suitably sensitive biochemical test.