The adrenal glands are paired retroperitoneal organs superomedial to the kidneys at the level of the 12th rib. They are surrounded by loosely attached fat posteriorly to diaphragmatic muscle. This fat can obscure the visualization and identification of adrenal tumors. Left-sided adrenal tumors lie adjacent to and can invade the spleen, pancreas tail, liver, kidney, or renal hilum. If not careful, it is possible to mistake the tail of the pancreas for the left adrenal gland given the similar texture and size. Right-sided adrenal tumors lie adjacent to and can invade the liver or inferior vena cava (IVC).
The arterial supply to the adrenals originates from the inferior phrenic arteries, aorta, and renal arteries. Despite being quite variable, the majority of the arterial supply approaches from the medial and inferior borders of the adrenals with few substantial arteries from the superior, posterior, or lateral sides. The adrenal arteries are generally small and amenable to electrocautery or vessel sealing devices.
Generally, the venous drainage from the right adrenal was thought to consist of a single, large, short vein draining into the IVC. On the left, drainage was thought to proceed to the left renal vein or inferior phrenic vein via a longer, single vein. These anatomic descriptions were based largely on cadaveric studies on non-diseased adrenal glands. In the 1940s, Anson and Caudwell identified only a single venous variant in nearly 900 adrenals examined.1 However, others have found significant heterogeneity in venous anatomy during operative intervention for adrenal pathology. Scholten et al. found 13% variance in venous anatomy in 546 consecutive adrenalectomies—no main adrenal vein, a single main vein with multiple small veins, double adrenal veins, and drainage sites including the IVC, hepatic vein, or inferior phrenic vein. The incidence of variant anatomy was more likely on the right side, with larger tumors, and with pheochromocytomas. Further, variant anatomy is associated with higher rates of transfusions due to operative complications.2
Each gland is divided into an outer cortex and an inner medulla, which are histologically and functionally distinct layers derived from separate embryologic origin. The cortex originates from mesodermal cells that form into cords of endocrine cells. In the adult, the cortex is composed of three zones. From outermost to innermost, they are: (1) zona glomerulosa, which regulates electrolyte homeostasis via production of aldosterone in response to the renin-angiotensin system, potassium concentration, and atrial natriuretic peptide, (2) zona fasciculata, which produces cortisol to promote gluconeogenesis and delivery of glucose to tissues, and (3) zona reticularis, which develops after roughly age 5 and produces the androgens dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulfate (DHEAS) in response to adrenocorticotrophic hormone (ACTH) stimulation.3
The medulla is derived from neural crest cells, called chromaffin cells, which migrate and become imbedded into the inner portion of the gland. They develop into modified postganglionic sympathetic neurons ...