Diagnosis

Biochemical testing for pheochromocytoma, in addition to patients with obvious characteristic paroxysmal episodes, should include patients with unusually labile or intermittent hypertension or pregnant patients with new hypertension (in the absence of preeclampsia), those with a family history of pheochromocytoma or one of the associated conditions (see later), and children with hypertension. Also, when an adrenal tumor is discovered by computed tomography (CT) or magnetic resonance imaging (MRI) scans in the evaluation of other symptoms, screening tests for pheochromocytoma should be performed.

Even though some controversy may exist regarding the optimal single test to establish the diagnosis, we have long relied on 24-hour urine collection for metanephrines and fractionated catecholamines, determined by high-pressure liquid chromatography. Increases in "mets and cats" have been diagnostic in 99% of our pheochromocytoma patients in the last 20 years.14 This is dependent on eliminating interfering substances or recognizing other situations that might render the tests inaccurate. For example, methylglucamine, a component of many iodine-containing contrast media, may falsely lower metanephrine levels for up to 72 hours after their use.7 In addition, labetalol, a combination of a and (3 blockers, must be avoided prior to testing. Tricyclic antidepressants are the agents that interfere most frequently with the interpretation of 24-hour urine metanephrines and catecholamines. They should be tapered and discontinued prior to hormone determinations.15 Also, measurements of urinary catecholamines and metabolites may be invalid in patients with advanced renal insufficiency, patients taking levodopa, or patients under major physical stress (e.g., stroke, surgery, or obstructive sleep apnea).

Provocative pharmacologic testing using glucagon, histamine, metoclopramide, or naloxone, coupled with timed samples for plasma catecholamines, was useful previously. However, because in 542 patients with normal urinary metanephrines and catecholamines, not a single patient tested positively with provocative tests, they are no longer utilized.16

Plasma free metanephrine has an extremely high sensitivity rate of 99% for the presence of pheochromocytoma17 and is valuable in testing for the presence of a pheochromocytoma in genetically predisposed patients. However, used as a general screening test, it has a troubling 10% false-positive rate. We rely on urinary testing as the primary method of diagnosis and screening.

Tumor size has been related to the ratio of unmetabolized catecholamines versus their metabolic products. Tumors weighing less than 50 g have more rapid turnover rates, releasing norepinephrine and epinephrine directly into the circulation. In contrast, a larger fraction of the catecholamines in larger tumors are metabolized into metanephrines and vanillylmandelic acid prior to release into the circulation.18 None of the biochemical tests is specific for the presence of malignancy.

FIGURE 71-2. Patient with multiple endocrine neoplasia type 2A with gross bilateral pheochromocytomas imaged well by CT scan (A) and showing the tumors (B).

FIGURE 71-2. Patient with multiple endocrine neoplasia type 2A with gross bilateral pheochromocytomas imaged well by CT scan (A) and showing the tumors (B).

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