Adrenal Text, LNMI
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Adrenal
Robert Udelsman
Anatomy
The adrenal glands are paired retroperitoneal organs located in close contact to the superior surface of either kidney. They are surrounded by a loose layer of areolar connective tissue and have multiple fibrous bands and vascular attachments through which they are associated with the superior poles of the kidneys. They are recognizable by their firm texture and chromate yellow color, which is distinctly darker than the pale retroperitoneal fat. The normal adrenal gland is slightly nodular and generally weighs between 4 and 5 g in the adult. The presence of adrenal nodules is not uncommon, and their frequency increases with age. The anatomical relationships of the adrenal glands are important and have significant surgical ramifications. The CT findings of the normal adrenal glands are easily visualized on most CT scans, and the width of each adrenal gland limb is similar to that of the nearby diaphragm. Their anatomical relationships have been summarized by Mihai and Farndon (Table 29.1). The location of the adrenal gland deep in the retroperitoneum has in the past made them relatively inaccessible. However, laparoscopic adrenalectomy has dramatically changed the surgical management of adrenal tumors. Each adrenal gland is supplied by small arterial branches that originate from three distinct sources. The major supplying vessels are the inferior phrenic artery, the aorta, and the ipsilateral renal artery. Occasional additional sources include the intercostal and ovarian vessels. The arterial branches ramify over the capsule of the gland and form a subcapsular plexus. The major source of adrenal medullary blood appears to be via the adrenal cortex from which blood rich in glucocorticoids flows from the cortical layers into the medulla. This intraadrenal “portal venous” circulation has significant physiological ramifications. The final pathway for the catacholamine epinephrine requires the enzyme phenylethanolamine N methyltransferase (PNMT), and glucocorticoids are required for this final step. Thus, there is significant functional interaction between the adrenal medulla and cortex. The venous drainage of the adrenal gland is more constant than the arterial supply. The right adrenal gland usually drains by one short vein, which empties directly into the vena cava. Accessory adrenal veins are not infrequently present. The left major adrenal vein is often joined by the inferior phrenic vein, which drains into the left renal vein. There may be associated small additional veins. Lymphatic drainage from the adrenal glands drains directly into adjacent, periaortic and paracaval nodes. These structures are important when operating for malignant adrenal lesions.3
Physiology
The adrenal gland is composed of two distinct organs, the adrenal cortex and the adrenal medulla. The cortex is divided into three functional zones: the outer glomerulosa, the intermediate fasciculata, and the inner reticularis. These three zones are associated with the production of mineralocorticoids, glucocorticoids, and sex steroids, respectively. Of these three hormone classes, the only one absolutely required for life is glucocorticoids. Glucocorticoids exert a myriad of effects on essentially every tissue in the body. A partial list of the effects of glucocorticoids is presented in Table 29.2. Cortisol is the major glucocorticoid in humans. The rate-limiting step in adrenal steroid synthesis, which is controlled by adrenocorticotropic hormone (ACTH), is the cleavage of the cholesterol side chain to yield p regnenolone. Glucocorticoids are secreted directly into the circulation immediately upon their synthesis. Cortisol circulates in both the bound form (95%) and in a free unbound state (5%). The free form passes into target cells by diffusion and binds to cytosolic receptors. All physiological actions of glucocorticoids are mediated through binding to steroid receptors, which are present in virtually every nucleated cell. The actions of glucocorticoids are both “permissive,” allowing other hormones to function in the basal state, as well as “regulatory,” which are observed under stress-induced conditions. The autonomic nervous system develops in parallel to the hypothalamic–pituitary–adrenal (HPA) axis. The adrenal medulla is embryologically analogous to a peripheral sympathetic ganglia. The medullary chromaffin cells have rudimentary nerve fibers and the ability to synthesize, store, and secrete catecholamines. The primary secretory product of the adrenal medulla is epinephrine. The proximity of the adrenal medulla and the adrenal cortex results in a unique site of catecholamine–glucocorticoid interactions.
The biosynthetic pathway for catecholamines is demonstrated in Figure 29.2. Epinephrine constitutes approximately 80% of adrenal medullary secretion.
Adrenal Imaging
The adrenal glands are relatively inaccessible retroperitoneal organs that are surrounded by perinephric fat. Plain abdominal films have a very limited role in adrenal imaging. However, they can detect calcifications, especially in children who have had neonatal hemorrhage or have neuroblastoma. In adults, calcifications of the adrenal glands are highly suggestive of granulomatous disease including tuberculosis, histoplasmosis, and sarcoidosis. Ultrasonography can detect adrenal lesions and is a relatively inexpensive method to serially follow small adrenal adenomas. Adrenal ultrasound has a limited role for diagnostic purposes and is largely supplemented by computed tomography (CT) or magnetic resonance imaging (MRI) scans. Intraoperative ultrasound performed during laparoscopic adrenalectomy has proven to be a useful modality. It can identify the location of small adrenal glands and delineate their vasculature.
CT Scans
Computed tomography scanning of the adrenal gland has proven to be the diagnostic procedure of choice for most patients. Simple cysts and myelolipomas can be diagnosed with virtual certainty based on their CT characteristics. Intravenous contrast is generally not required, and a low attenuation value on an unenhanced CT scan can help differentiate benign (low density) from malignant lesions as well as metastases, which generally have a higher density. Lesions with low Hounsfield units (HU) are most likely benign, whereas lesions that have a HU density greater than 20 are more likely to be malignant. Accordingly, it has been suggested that a cutoff point of 30 HU should be accepted for discriminating malignant and benign lesions.
MRI Scan
Magnetic resonance imaging (MRI) has a significant role in the evaluation of adrenal tumors. Nonfunctioning adenomas appear on T2-weighted images like normal adrenal tissue. Functional adenomas tend to demonstrate a slightly increased signal intensity, whereas adrenal metastases or primary adrenal cortical carcinomas tend to be relatively bright. Enhancement on T2-weighted images is particularly useful for pheochromocytomas and therefore MRI appears to be the imaging study of choice in patients with suspected pheochromocytomas.
Radioisotope Scan
Iodocholesterol-labeled agents including 131I-6-_-iodomethyl- 19-norcholesterol (NP59) are incorporated into steroidogenesis pathways in the form of intracellular cholesterol and therefore have the ability to visualize functional adrenal cortical lesions. However, NP59 is not readily available at most institutions and dexamethasone pretreatment is required. These two factors limit its clinical utility. Meta-iodobenzylguanidine (MIBG) is frequently used for the evaluation of pheochromocytoma as well as neuroblastoma. 131I-MIBG and 123I-MIBG are concentrated in catecholamine storage vesicles and therefore are useful in suspected cases of pheochromocytoma. In cases of extraadrenal disease, which has a lower propensity for MIBG uptake, positron emission tomography (PET) utilizing 2-fluorine-18-fluoro-2-deoxy-D-glucose (FDG) may be useful.
Angiography
Angiography and venography were at one time more commonly employed for the evaluation of adrenal tumors. These procedures have been largely replaced by noninvasive imaging.
Pertucaneous Biopsy
Percutaneous biopsy of the adrenal gland can be performed under either CT or ultrasound guidance. However, there are very few appropriate indications for this procedure. A percutaneous biopsy cannot reliably distinguish between an adrenal adenoma and an adrenal carcinoma. The most common indication is in the setting of suspected metastatic disease to the adrenal gland. In such a case, when a fine-needle aspiration demonstrates nonadrenal malignant tissue the diagnosis of metastasis is confirmed. This procedure should never be performed in a patient until a biochemical workup has been completed to rule out a pheochromocytoma because sudden death has been reported following biopsy of unsuspected pheochromocytoma.
Incidentaloma
Adrenal “incidentalomas” are adrenal tumors discovered on an imaging study that has been obtained for indications exclusive of adrenal-related conditions. The frequent use of CT scans, which can detect adrenal lesions greater than 1 cm, has resulted in their detection in 0.35% to 5% of studies. The evaluation and decision paradigm for an incidentaloma hinges on three issues: (1) Is it functional? (2) Is it likely to be a malignant adrenal tumor? (3) Is it metastatic? The evaluation is focused on answering each of the foregoing questions.
Hormone Evaluation
All evaluations begin with a detailed history and physical examination. If symptoms or signs suggesting a functional adrenal neoplasm are detected, then, in addition to a routine screening evaluation, specific hormone studies are indicated. However, most patients are asymptomatic. The CT findings of a specific subset of adrenal masses including simple adrenal cysts and myelolipomas can be pathognomonic. In these instances, hormonal screening studies are not required. Screening studies are directed at three specific syndromes: pheochromocytoma, aldosteronoma, and Cushing’s syndrome. Pheochromocytomas are rare. However, because the risk of complications associated with an occult pheochromocytoma is significant, virtually all investigators agree that all incidentaloma patients should be screened for catecholamine hypersecretion. Most commonly, urinary collections over 24 h are obtained in bottles containing acid. These collections are analyzed for metanephrines, vanillylmandelic acid (VMA), or fractionated catecholamines. The screen for aldosteronoma in the setting of an incidentaloma is often limited. If the patient is normotensive and not receiving hypertension or diuretic therapy and has a normal serum potassium (_3.5 mEq/l), then an aldosteronoma is very unlikely. If the patient does not satisfy these criteria, then an aldosteronoma evaluation is performed as delineated later in this chapter. Cushing’s syndrome is important to consider in all patients with adrenal tumors. Patients with advanced Cushing’s syndrome present with classic symptoms and signs of glucocorticoid excess and are therefore not difficult to diagnose. However, patients not uncommonly present with subtle stigmata of Cushing’s syndrome or with occult or “subclinical” disease. In this situation the patient has an adrenal adenoma that has attained functional autonomy in its ability to secrete glucocorticoids but has not yet manifest findings of Cushing’s syndrome. This silent but subtle hypercortisolism occurs in approximately 15% of patients with incidentalomas. It is important to rule out subclinical Cushing’s syndrome for two reasons: (1) if one elects not to perform an adrenalectomy, then the endocrinopathy will continue and deleterious effects will occur, and (2) if one does perform an adrenalectomy, the contralateral adrenal will be suppressed, and if perioperative glucocorticoids are not administered the patient will be at risk for Addisonian crisis. It is important to recognize that an incidentaloma may represent a metastatic lesion in the adrenal gland. The majority of patients with metastatic disease to one or both adrenal glands have both a history of malignant disease and metastases to multiple additional sites. In the setting of widespread metastatic disease, the adrenal disease is not treated directly as it represents only a small focus of total tumor burden. Patients with bilateral adrenal metastases are at some risk for adrenal insufficiency. An important and unresolved issue in the management of incidentalomas is the determination of what size of adrenal tumor is in itself an indication for extirpation. In the absence of scientific trials an empiric approach has been employed. Virtually all experts agree that any lesion greater than 5 cm ...
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