Книги по МРТ КТ на английском языке / Advanced Imaging of the Abdomen - Jovitas Skucas

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acid lipase. It is characterized by abnormal storage of cholesteryl esters and triglycerides. Onset is after the first month of life and manifests as hepatosplenomegaly, abdominal distention, and failure to thrive. It is usually fatal in infancy.

Computed tomography in these infants shows hepatosplenomegaly, with the liver being hypodense, and bilateral adrenal calcifications. These imaging findings, in the appropriate clinical setting, should suggest the diagnosis.

Beckwith-Wiedemann Syndrome

Beckwith-Wiedemann syndrome is a congenital overgrowth syndrome having a sporadic occurrence and variable expressivity. These children develop cysts and various childhood solid tumors, including adrenocortical carcinomas. The kidneys are more often affected than the adrenals. Bilateral multilocular cystic adrenal tumors developed in a neonate with BeckwithWiedemann syndrome (4); resection revealed benign hemorrhagic macrocysts. Unilateral cysts are more common.

Trauma

Trauma to the adrenal and surrounding structures most often leads to hemorrhage and a hematoma. The right side is more often involved, and trauma to the overlying ribs and liver is common. Associated bleeding often is not confined to the adrenal gland but also involves adjacent extraperitoneal tissues. In some patients a hematoma is asymptomatic and is discovered only later, when trauma is already forgotten.

An adrenal injury classification scale, devised by the American Association for the Surgery of Trauma, is outlined in Table 16.1. Adrenal hemorrhage/hematoma is discussed later in this chapter.

Enlargement

Visualization of normal-appearing adrenal glands essentially excludes a neoplasm. An abnormal gland is identified as a focal bulge, diffuse gland enlargement, or both. A focal tumor can be either neoplastic or hyperplastic. In general, a functioning primary tumor is asso-

Table 16.1. Surgical adrenal injury scale

* Advanced one grade for multiple injuries, up to grade V. Modified from Moore et al (5).

ciated with either an atrophic or normal-size contralateral gland. Most small hyperplastic and neoplastic nodules are isodense on CT and isointense on MR to normal adrenal parenchyma on precontrast images and are identified by their contour abnormalities.

Determining whether a tumor is in the adrenal or is extraadrenal in location is usually straightforward with imaging. The presence of a normal adrenal gland adjacent to a tumor establishes their relationship. Especially when large, however, some extraadrenal tumors invade and obliterate the adrenal gland; the reverse is also true—an adrenal tumor can involve adjacent tissues. With both scenarios it may not be possible to establish the site of tumor origin.

Hyperplasia

A distinction between a normal-sized gland and an enlarged one is a borderline of gradation. With diffuse enlargement the usual adrenal shape is preserved, although on occasion some degree of nodularity to the gland outline is observed. Bilaterally enlargement signifies hyperplasia. It should be kept in mind, however, that some patients with clinical evidence of adrenal hyperfunction have normal-sized glands.

Hyperplasia of the medulla is less common; such hyperplasia appears to be a precursor to a pheochromocytoma.

Hyperplastic adrenal gland MR signal intensity is identical to a normal adrenal with all imaging sequences.

Infection

Adrenal abscesses are uncommon in adults. Some represent the sequelae of an infected

hematoma. These abscesses can be diagnosed by needle aspiration. At times an infected tumor is in the differential diagnosis.

Adrenal hydatidosis is rare (6). Imaging reveals a similar appearance to that seen in other organs.

Involvement by histoplasmosis or tuberculosis is usually bilateral but also asymmetrical. The imaging findings vary depending on the extent of involvement and the degree of necrosis. Initially, histoplasmosis results in bilateral adrenal enlargement, but eventually the glands become atrophic and develop punctate calcifications. Tuberculosis, a cause of adrenal insufficiency, has a similar adrenal enlargement followed by atrophy and calcification appearance.

Other infections are rare. Anecdotal reports describe adrenal coccidioidomycosis or North American blastomycosis involving the adrenal glands even in an immunocompetent patient; the latter infection can also cause adrenal insufficiency. Computed tomography in a patient with adrenal paragonimiasis detected an enhancing right adrenal tumor, with US revealing a dumbbell-like hyperechoic tumor (7); a multicystic structure filled with creamy material was found at surgery.

Initially with gland enlargement a neoplasm is in the differential diagnosis.With an atrophic, calcified gland, however, a neoplasm is less likely.

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cyst without a cellular wall, but it has a fibrous capsule. Wall thickness tends to be several millimeters; a thicker or irregular wall should raise suspicion for a neoplasm. These cysts tend to be unilocular and vary in size considerably. Ultrasonography reveals a cyst with internal echoes. Magnetic resonance imaging shows a thickrimmed cyst. Similar to their US appearance, these cysts’ signal intensity varies depending on cyst content. Curvilinear wall or septal calcifications develop in some; an occasional one contains central calcifications.

The CT density of adrenal cysts varies from water to old blood. Some exhibit rim contrast enhancement, but the cystic component should not enhance. Fluid-fluid layering is occasionally found. The relationship of polycystic renal disease and adrenal cysts is not clear.

The differential diagnosis of a cystic adrenal tumor is between one of the above-listed cysts and a cystic or necrotic neoplasm. The presence of soft tissue density nodules suggests a neoplasm. Contrast enhancement of any solid component most often signifies a neoplasm (8). If needed, an aspiration biopsy is obtained, but keep in mind that cysts containing old blood are difficult to drain. If a lesion appears indeed to be a cyst, it can be observed.

One must ensure that a cystic structure is indeed of adrenal origin. For instance, CT or MRI of a gastric cardia diverticulum can minic a left adrenal cyst.

Cysts

Aside from cystic neoplasms, adrenal cysts are uncommon. Cystic neoplasms include a rare carcinoma, pheochromocytoma, or a postinjection therapy adenoma. Nonneoplastic cysts can be classified as parasitic, epithelial retention cysts, endothelial cysts, and pseudocysts due to prior hemorrhage, with a majority of cysts representing the latter two entities. Imaging cannot differentiate between these various nonneoplastic cysts, with a few exceptions. Endotheliumlined cysts consist of lymphatic and vascular degenerative cysts. They tend to be multilocular and are filled with clear or milky fluid.

Some adrenal hemorrhages evolve into a pseudocyst, a poor term because there is nothing “pseudo” about these cysts. They are not related to pancreatic pseudocysts, which have a different etiology. An adrenal pseudocyst is a cortical

Incidental Tumors

In some patients diffusely enlarged adrenal glands are discovered incidentally; most often this finding is of little significance. An incidentally detected focal adrenal tumor (incidentaloma), on the other hand, probably warrants further study. About 10% to 15% of patients with a focal tumor have autonomous cortisol hypersecretion without obvious stigmata of Cushing’s syndrome. Among newly detected single adrenal tumors about half are adenomas, yet even an incidental tumor has about a one-third chance of being a metastasis, even with no known primary.

What is the eventual outcome of incidentally discovered adrenal tumors (incidentalomas)? In patients with a unilateral incidental tumor, 17% had biochemical evidence of adrenal

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hyperfunction, consisting of hypercortisolism, hyperaldosteronism, and medullary hyperfunction (9); in patients with bilateral incidental tumors, abnormal adrenal function was detected in 29%, consisting of hypercortisolism, hyperaldosteronism, adrenal insufficiency, and congenital adrenal hyperplasia. A not uncommon incidental functioning adrenal tumor is a pheochromocytoma. Some of these tumors function subclinically and appropriate endocrine testing is useful. Most initially discovered metastases are unilateral and do not produce adrenal insufficiency; bilateral involvement suggests adrenal hemorrhage, especially in a setting of a coagulopathy, and possible lymphoma.

An incidental adrenal tumor discovered in a workup for another condition, without a known primary cancer, has only about a 1% rate of being malignant; most of these malignancies will be larger tumors. As a result, some investigators suggest that smaller tumors, for instance those <3cm or so in diameter, can be followed with imaging; an exception is with those exhibiting subtle hormone production or in a patient with hypertension or diabetes, and these should be resected. On the other hand, risk of malignancy is increased in a setting of a known primary cancer in another organ, but here also the prevalence of malignancy varies with tumor size.

Both CT and MRI are helpful in defining adrenal tumors (differentiation of benign and malignant tumors is discussed in more detail in the next section). A biopsy is helpful, especially for larger tumors. Similar to other sites, a positive biopsy is diagnostic but a negative one needs to be placed in the proper perspective. One biopsy caution: a pheochromocytoma should be excluded before a biopsy is performed.Also,biopsy rather than cytology is necessary; cytology often cannot differentiate a benign from a malignant tumor.

2-[18F]-fluoro-deoxy-D-glucose positron emission tomography (FDG-PET) appears to have a role in distinguishing benign from malignant adrenal tumors. Benign tumors show no FDG uptake; on the other hand, malignant adrenal tumors have increased FDG uptake, indicative of high glucose metabolism. Whole-body PET also identifies primary extraadrenal tumor sites, thus providing staging.

Occasionally prenatal US identifies a suprarenal tumor, at times even containing small cysts; eventually these tumors tend to disappear and thus a conservative approach with US followup appears reasonable in otherwise healthy neonates.Presumably some of these findings represent congenital adrenal hyperplasia.

Adenoma Versus Nonadenoma

A major problem in clinical practice is to differentiate an adrenal adenoma from other tumors, usually a metastasis. Although most adenomas are small, smooth in outline, and homogeneous, while metastases are larger, have an irregular outline, and are heterogeneous, sufficient overlap exists in these characteristics that they are of limited use in any one patient. With a nonfunctioning adenoma or carcinoma the contralateral gland is not atrophic.

Two specific imaging characteristics are useful in differentiating adrenal adenomas from nonadenomas. Initially, emphasis was placed on detecting an adenoma’s increased lipid content. While still a useful concept, some overlap exists, and this technique has been supplanted by another almost unique adenoma property—a more rapid contrast washout compared to nonadenomas. Computed tomography and MR differentiation of adenomas from nonadenomas rely primarily on these findings.

Using a variety of techniques, including chemical shift imaging and dynamic contrast enhancement, MRI could differentiate between benign and malignant adrenal tumors with a 89% sensitivity and 99% specificity (10). Sensitivity and specificity can be increased by combining results from several MR sequences, such as dynamic contrast enhanced sequences and chemical shift imaging.

Lipid Content

Most adrenocortical adenomas contain considerable lipid (lipid rich), but the amount of fat is still considerably less than in a myelolipoma.An inverse linear relationship exists between the percentage of lipid-rich cortical cells in adrenal adenomas and their unenhanced CT attenuation; a similar inverse linear relationship exists with MR chemical shift changes. It should be kept in mind, however, that about 20% of adenomas contain little or no lipid (lipid poor).

The reverse is also true—an occasional adrenal cortical carcinoma and metastasis contains sufficient fat to be detectable by CT and MRI, but these tend to be large and irregular in outline at initial presentation.

In general,unenhanced CT attenuation values of a nonfunctioning adrenal tumor are more useful in differentiating between an adenoma and a nonadenoma than size or CT homogeneity. Due to their fat content, most adenomas have lower attenuation values than malignant tumors, with the published sensitivities in making this differential varying depending on the assumed threshold HU; thus in 60 patients with single adrenal nodules, a CT threshold of 19HU on unenhanced and 41HU on late postcontrast scans achieved a sensitivity of 93% and specificity of 100% in differentiating benign from malignant tumors (11). As a rough guide, adrenal tumors having a CT density of 0 Houndsfield units (HU) or less can be assumed to be benign and those having a density >20HU can be assumed to be malignant. It is those with intermediate densities that present a diagnostic dilemma.

Magnetic resonance signal intensities of both benign and malignant tumors vary, presumably due to their inhomogeneous histologic appearance. Nonfunctioning adenomas range from hypoto mostly isointense to liver parenchyma on both T1and T2-weighted sequences and show mild-to-moderate postcontrast enhancement. Most adenomas contain insufficient fat to produce a major change on fat-suppressed MR images, and a major decrease in intensity should suggest a myelolipoma. Nevertheless, chemical shift MRI is useful in differentiating lipid-rich adenomas from nonadenomas, achieving sensitivities and specificities similar to those of CT enhancement washout techniques (see next section). This technique is insensitive in characterizing lipid-poor adenomas.

An internal standard is needed to meaningfully evaluate adrenal MR signal intensity. Although liver signal intensity is readily measured, it varies considerably depending on liver fat content. The adrenal-to-spleen ratio appears more useful. Metastases have an adrenal-to- spleen ratio >0.8, while most adenomas have a lower ratio. An overlap exists, however, and the ratio is of limited value in establishing the benign or malignant nature of an adrenal

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lesion. Some investigators prefer muscle as their standard.

Most adenomas are isointense to muscle or spleen on in-phase spoiled gradient echo (SGE) images and appear hypointense on opposedphase images. With a chemical shift technique, most adenomas have a loss of signal intensity on opposed-phase images while most benign nonadenomas, pheochromocytomas, and metastases do not. Loss in signal intensity in bone marrow in an adjacent vertebral body is useful confirmation of this technique. With a tendency of adrenal adenomas to contain fat and the high sensitivity of opposed-phase imaging for detecting fat, in a setting of a known malignancy and an adrenal tumor, MRI can be sufficient to differentiate these entities and save these patients a biopsy.

Another option is to use a signal intensity (SI) index, defined as:

(SI in-phase - SI opposed-phase)/SI in-phase

This index differentiates adenomas, metastases and pheochromocytomas (12). One study achieved 100% accuracy in distinguishing adenomas from metastases if the SI index cutoff value was 11.2–16.5% (13); this index was more reliable in differentiating adenomas from metastases than an adrenal-to-liver, spleen, or muscle ratio for signal change on opposedphase MR imaging..

Which modality is more accurate in differentiating benign from malignant tumors—unen- hanced CT attenuation or chemical shift MRI? For most tumors both yield similar results; most adenomas measuring >10H on unenhanced CT can be characterized with chemical shift MRI (14). Nevertheless, diagnostic uncertainty still exists with a minority of these tumors.

Contrast Washout

Very useful in differentiating adrenal adenomas from nonadenomas is washout of contrast enhancement. Enhancement washout is equal to initial enhanced attenuation minus delayed enhanced attenuation. More rapid washout is a characteristic finding in a majority of adenomas (compared to nonadenomas). Mean CT attenuation measured at specific time delays postcontrast (10 to 30 minutes), is lower for adenomas than for nonadenomas. In one study, the per-

tumor of undetermined nature (19); 4 months later the patient presented with a metastatic cortisol and androgen-producing adrenocortical carcinoma.

Pain is more common with carcinomas. Precocious puberty develops in an occasional patient. About half of these tumors have some type of endocrine abnormality, but in general, endocrine testing per se does not differentiate between benign and malignant tumors.

The role of percutaneous ablation for adrenal tumors is mostly anecdotal.

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vena cava results in an acute Budd-Chiari syndrome. Imaging, including magnetic resonance angiography (MRA), is useful in evaluating malignant tumor extension into the adrenal veins and inferior vena cava.

A carcinoma tends to have patchy CT contrast enhancement, a somewhat nonspecific finding.

Nonfunctioning adenomas are hypointense to liver on T1and vary in intensity on T2weighted MR images (Fig. 16.1). Magnetic resonance is useful in distinguishing a cyst from an adenoma; on T2-weighted images fluid in a cyst is hyperintense.

Imaging

Regions of hemorrhage and necrosis are common in carcinomas and result in a heterogeneous appearance. Necrosis is not specific for a carcinoma and is also seen in adrenal hemangiomas and other larger tumors. Some adenomas and carcinomas develop calcification. Calcifications are more common in larger, heterogeneous-appearing tumors containing central necrosis or hemorrhage; the imaging appearance of these larger tumors does not permit ready differentiation between adenomas and carcinomas.

Especially with the larger adrenal carcinomas, the vena cava should be studied for possible invasion. A right adrenal gland adrenocortical carcinoma can grow as an intracaval tumor thrombus, at times even extending into the atrium. Or, a neoplasm invading the inferior

Metastases

An adrenal metastasis without a known primary tumor is sufficiently rare that it is generally not considered in the differential diagnosis of an incidental adrenal tumor. On the other hand, adrenal gland metastases are common in a setting of a known cancer in another organ. Given the known prevalence of adrenal adenomas, in a setting of a known primary malignancy the chance that an enlarged adrenal gland represents an incidental adenoma is roughly 50%.

Lung and breast carcinoma and melanoma are common metastases. Less common metastases to the adrenal glands include thyroid carcinoma and hepatocellular carcinoma.

Patients with a renal cell carcinoma are at approximately a 2% to 3% risk of ipsilateral

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adrenal metastasis or direct invasion, although these risks increase with larger tumors. Occasionally renal cell carcinoma metastases develop to the contralateral or even both adrenals. After a right radical nephrectomy in a 74-year-old patient CT identified a hypervascular tumor on the left, initially believed to be a splenic artery aneurysm, but arteriography suggested a hypervascular adrenal tumor and adrenalectomy confirmed a metastasis (20).

Small metastases blend into a normalappearing adrenal gland; with growth, a slight bulge in gland outline may be the only finding. The larger metastases are readily visualized by CT and MRI. Some eventually invade surrounding structures. Small metastases tend to be homogeneous; central necrosis develops with growth, and the imaging findings vary accordingly. Necrosis, however, is seen in both metastases and some primary adrenal neoplasms. Calcifications in a metastasis are rare.

Magnetic resonance signal intensity of metastases varies considerably and is,per se,not a useful guide. Most metastases exhibit heterogeneous contrast enhancement, and the homogeneous capillary blush seen with adenomas is uncommon. Their lack of lipid results in no appreciable signal loss on opposed phase MR images, thus distinguishing most metastases from adenomas. A possible exception is with some renal clear cell carcinomas, which contain intracytoplasmic lipid, and such an adrenal metastasis can potentially be confused with an adenoma.

Iodine-131–NP-59 scintigraphy reveals lack of tracer accumulation in most metastases. A word of caution is also necessary here: some adrenal metastases from renal cell carcinomas accumulate NP-59 (21). Some thyroid carcinoma metastases to the adrenal gland show I 131 uptake.

If needed, a percutaneous biopsy provides confirmation.As in other sites, a negative biopsy is less significant than a positive biopsy.

Whether an adrenal-sparing nephrectomy is a viable option in a setting of a renal cell carcinoma and a normal-appearing adrenal gland by imaging is controversial; preoperative CT does not detect adrenal invasion in some of these patients, because of metastases below the resolution of CT or other reasons.

Percutaneous ethanol injection has been performed on adrenal metastases from hepatocel-

lular carcinoma (22); preliminary results are encouraging. CT-guided radiofrequency ablation appears to be effective local control of adrenal metastases (23).

Cortical Hormone-Producing Tumors

Hyperfunctional cortical adrenal conditions include Cushing’s syndrome and aldosteronism. A single predominant hormone activity is evident in the majority of patients with hormone-producing gland hyperplasia or neoplasm. Occasionally encountered are unusual combinations, such as coexisting pheochromocytoma and primary adrenal Cushing’s syndrome.

Imaging is not a substitute for an appropriate clinical endocrinological investigation in a patient suspected of having a hyperfunctional adrenal condition.

Cushing’s Syndrome

Cushing’s syndrome, or hypercortisolism, results from excessive cortisol secretion by the adrenal cortex. Affected individuals have increased urinary free cortisol excretion and an elevated serum cortisol level, with loss of a circadian rhythm. Screening includes an overnight dexamethasone suppression test. It should be emphasized that not all patients appear clinically cushingoid.

Secondary Hyperplasia

Most often Cushing’s syndrome is due to adrenal hyperplasia secondary to excessive adrenocorticotropic hormone (ACTH) secretion. The most common cause of excessive ACTH secretion is a pituitary tumor (Cushing’s disease), although an occasional pancreatic neuroendocrine tumor, pheochromocytoma, bronchial carcinoid, gastrinoma, functioning ovarian tumor, or a tumor at some other site is responsible for ectopic ACTH production. A pituitary adenoma and an adrenocortical adenoma can coexist, resulting in a complex clinical presentation.

Computed tomography shows that only about 70% of patients with ACTH-dependent Cushing’s syndrome have enlarged adrenal glands (24); the adrenals are larger in patients with ectopic ACTH syndrome than in those with

Cushing’s disease. These glands are most often smooth in outline, with only a minority containing nodules. The less common nodular appearance mimics an adrenal adenoma, especially if hyperplasia is unilateral.

Primary Adrenal Causes

Primary adrenal causes of Cushing’s syndrome, or ACTH-independent causes, are less common and include adenoma, carcinoma, and primary adrenal hyperplasia. A rare variant encountered in pediatrics and young adults is primary pigmented nodular adrenocortical disease,which is due to hypersecretion of cortisol by multiple intraadrenal pigmented or “black” cortical adenomas.

About two-thirds of patients with primary Cushing’s syndrome have adenomas and onethird carcinomas. Rarely, adenomas develop bilaterally. Even an adrenocortical carcinoma in one gland and a contralateral adenoma have been described (25); CT simply showed bilateral adrenal tumors.

Primary adrenal hyperplasia is uncommon. It tends to be bilateral, with the adrenal glands having a somewhat nodular appearance. One patient developed unilateral nodular hyperplasia and an atrophic contralateral gland (26).

The 5-year survival rate for those with these carcinomas is about 30%.

Imaging

In a patient with hypercortisolism, a CT or MR finding of an enlarged unilateral adrenal gland is suggestive of an adenoma. A symptomatic adenoma generally is round or oval and several centimeters in diameter. Calcifications are uncommon. An adenoma is homogeneous and of soft-tissue–to-water density. Some of the less dense adenomas mimic a cyst, but their functioning status cannot be predicted from their attenuation values. The contralateral gland tends to be atrophic secondary to ACTH suppression by the hyperfunctioning adenoma. They exhibit little contrast enhancement.

Functioning adenomas have a similar MR appearance to that of nonfunctioning adenomas (discussed previously), although some functioning adenomas contain little lipid and thus

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do not lose signal intensity on opposed-phase chemical shift images (an exception is with aldosterone-producing tissue, discussed below; see Aldosteronism).

Computed tomography in 11 patients with primary adrenal Cushing’s syndrome and later proven primary adrenal hyperplasia revealed massively enlarged, multinodular adrenal glands (27); these glands were hypointense to liver on T1and hyperintense on T2-weighted MR images.

Magnetic resonance imaging is at least equal to CT and superior to US in detecting hyperfunctioning adrenocortical tumors in patients with Cushing’s syndrome; these tumors are hyperintense relative to liver on T2-weighted sequences, thus differing from most nonfunctioning adrenal adenomas, which tend to be hypoto isointense.

Selenium-75-methylnorcholesterol and I- 131–NP-59 scintigraphy detect endocrine functioning adrenal tumors. A rough relationship exists between radiotracer uptake and the degree of functional autonomy and I-131– NP-59 scintigraphy is both a functional and a localization test in Cushing’s syndrome. It is an adjunct to CT and MRI. This test has almost 100% specificity and sensitivity in detecting these tumors. Cortical adenomas have increased-to-normal uptake and malignant tumors a decreased-to-absent radiotracer uptake. Symmetric visualization or mild asymmetry of the adrenals in a setting of hypercortisolism almost always represents adrenal hyperplasia. More marked asymmetry suggests an adenoma. Unilateral adrenal gland visualization is typical for an adenoma; the adenomaproduced cortisol decreases pituitary ACTH production, which in turn shuts off function in a normally functioning contralateral adrenal gland. Cysts likewise have no uptake. Prior surgery also results in asymmetric uptake. Scintigraphy allows the localization of any residual postoperative adrenal tissue.

Aldosteronism

Clinical

Primary aldosteronism (Conn’s syndrome) is secondary to either an aldosterone-secreting neoplasm or adrenal hyperplasia. Over half of

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primary aldosteronism is due to a functioning adrenocorticoid adenoma (aldosteronoma). Hyperplasia, usually bilateral, accounts for most of the rest, with a carcinoma being rare. Bilateral adenomas have been reported. Some patients with Conn’s syndrome have grossly normal-appearing glands. A pheochromocytoma and primary hyperaldosteronism have occurred simultaneously; whether this is a coincidence or an unknown interreaction is conjecture.

The mineralocorticoid aldosterone is involved in blood volume and serum potassium homeostasis, which in turn regulate aldosterone secretion by the zona glomerulosa in the adrenal cortex. Excessive secretion leads to hypertension, hypokalemia, and suppression of plasma renin activity, a condition also known as mineralocorticoid hypertension (28). This is not a simple condition; although in most patients the two stimuli for aldosterone production (potassium and angiotensin II) tend to be low, some patients have normal serum potassium levels. Familial hyperaldosteronism is described. A curious sideshow is pseudohypermineralocorticism, caused by an excess of mineralotropics other than aldosterone.

Although mineralocorticoid hypertension is not common, its significance lies in its being a potentially correctable cause of high blood pressure. One should keep in mind that aldosteronism also develops in primary renovascular hypertension but the latter entity is associated with high serum renin levels while in primary aldosteronism the renin levels are low.

Hypoaldosteronism is rare. It appears to be due to inadequate stimulation of aldosterone secretion or a defect in the adrenal synthesis of aldosterone. An unusual cause of secondary hypoaldosteronism (called pseudohypoaldosteronism by some) is seen in some infants with urinary tract infection, with or without urinary tract obstruction. Clinically, hypoaldosteronism results in hypotension and hyperkalemia. Imaging has no role in its diagnosis.

Imaging

importance. The adrenal glands are significantly larger in patients with bilateral adrenal hyperplasia than in those with an aldosteronoma. One study achieved 100% sensitivity when a CT mean limb width of >3mm was used to diagnose bilateral adrenal hyperplasia, and 100% specificity when limb width was 5mm or greater (29). Unless imaging identifies a tumor, such differentiation is generally sought by bilateral adrenal venous sampling.

Aldosteronomas tend to be small; discrete nodules are difficult to visualize. Thus among 18 aldosterone-producing adrenal adenomas, 89% were detected with CT but only 28% with US (30). Adrenal hyperplasia in Conn’s syndrome ranges from diffuse and bilateral to nodular (Fig. 16.2). Thus one or more nodule may represent either an adenoma or nodular hyperplasia. Complicating this picture is the presence of the occasional unrelated incidental adrenal tumor.

Similar to Cushing adenomas, aldosteronomas contain varying amounts of lipid. As a result, some have CT attenuation values close to that of water and their CT appearance can mimic a cyst.

Calcifications develop only in an occasional benign aldosteronoma.

A distinction between unilateral aldosteronomas, which are treated surgically, and bilateral hyperplasia, treated medically, is of obvious

Figure 16.2. Conn’s syndrome. Computed tomography (CT) reveals bilateral adrenal hyperplasia. (Courtesy of Algidas Basevicius, M.D., Kaunas Medical University, Kaunas, Lithuania.)

In 20 patients with primary hyperaldosteronism, 50% had aldosterone-producing adenomas and 50% bilateral adrenal hyperplasia (31); MRI detected adenomas with a sensitivity of 70% and specificity of 100%, with adenomas being isoto hypointense relative to liver on T1and slightly hyperintense on T2-weighted images. Of interest is that signal intensity decreased on out-of-phase chemical shift images in 86% of adenomas and 89% of adrenal hyperplasia, indicating the presence of lipid.

Iodine-131–NP-59 scintigraphy appears to be complementary to CT and MR in differentiating between adenomas and adrenal hyperplasia, being especially useful with a unilateral hyperplastic nodule. Scintigraphy visualizes these tumors as hot nodules, with an occasional warm nodule.

Bilateral adrenal venous sampling distinguishes most but not all adenomas from hyperplasia. Blood samples are obtained after stimulation with ACTH. With bilateral hyperplasia, after stimulation aldosterone levels increase in blood samples from both adrenals; on the other hand, a more marked unilateral increase is detected with an aldosteronoma.

Therapy

Patients with bilateral glomerulosa hyperplasia and those amenable to glucocorticoid therapy are treated medically. Adenomas are resected, but keep in mind that hypertension persists in 30% to 50% of patients after resection even if they are biochemically cured. Such persistent postoperative hypertension suggests coexisting essential hypertension.

Several patients with Conn syndrome and Cushing’s syndrome have been treated by CTguided acetic acid injected into their adrenal nodules (32); follow-up revealed cystic degeneration. A few aldosteronomas have also been treated by transcatheter arterial embolization with absolute ethanol.

Medullary Tumors

With some adrenal medullary tumors even a combination of histology, immunochemistry, and cytophotometric techniques achieves only a differentiation between benign and malignant states, and even then at times with difficulty.

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Pheochromocytoma (Paraganglioma)

Clinical

A pheochromocytoma is a paraganglioma located in the adrenal medulla. An inconsistent terminology is in use when describing corresponding extraadrenal neoplasms; some authors refer to them as extraadrenal pheochromocytomas if they are functioning and paragangliomas if nonfunctioning, while others call all extraadrenal tumors paragangliomas and simply specify the site and functioning status.

A paraganglioma originates from chromaffin neural crest tissue that has migrated to form the paraganglionic system. Most are located between the diaphragm and the inferior renal pole, with the most common extraadrenal site being in the organ of Zuckerkandl near the inferior mesenteric artery origin. An occasional one involves the inferior vena cava, urinary bladder, or even the broad ligament. About 10% occur in children, where a familial prevalence is evident and is more likely to be extraadrenal and multicentric. A number of pheochromocytomas have been detected during pregnancy and postpartum.

A pheochromocytoma produces an excess of catecholamines, and most of these patients have elevated catecholamine levels. An occasional one is part of a complex tumor; thus it can contain mesenchymal elements. Or, a cortical carcinoma or adenoma exhibits neuroendocrine differentiation. About 10% of pheochromocytomas are malignant. In general, extraadrenal paragangliomas are more malignant and metastasize more readily than their adrenal counterparts. The malignant potential of some is difficult to establish even by histology, the one definite finding of malignancy being the presence of metastases at sites normally devoid of chromaffin cells.

Although many patients with a pheochromocytoma are hypertensive, overall this condition is a rare cause of hypertension. Pheochromocy- toma-induced hypertension tends to be paroxysmal, but differentiation from other causes of hypertension is difficult. A rare paraganglioma (pheochromocytoma) undergoes spontaneous rupture and extraperitoneal hemorrhage, at times resulting in an acute abdomen (Fig. 16.3).

The prevalence of pheochromocytomas is increased in several disorders—neurofibro- matosis, von Hippel-Lindau disease, Sturge-