Anaesthesia and pituitary disease

Abstract

The pituitary gland occupies the sella turcica of the sphenoid bone at the base of the skull, the roof of which is created by an incomplete fold of dura, the diaphragma sella, through which passes the pituitary stalk. The fossa is limited posteriorly by the clivus of the sphenoid and anteriorly and inferiorly by the sphenoidal air sinuses. The pituitary is related to the third ventricle, hypothalamus, and visual pathways superiorly, and the cranial nerves III, IV, V, and VI, cavernous sinus and to the internal carotid arteries laterally. These structures can be damaged during pituitary surgery, leading to cranial nerve palsies, visual field defects, major haemorrhage, and cerebrospinal fluid (CSF) leaks. The gland weighs 500–900 mg and measures about 15 10 6 mm in an adult. Anatomically, the human pituitary has two lobes: anterior and posterior. The anterior lobe (adenohypophysis) constitutes two-thirds of the volume of the gland and the posterior lobe (neurohypophysis) constitutes the remainder. The adenohypophysis is further divided into pars distalis, pars tuberalis, and pars intermedia. The neurohypophysis is divided into pars nervosa and the infundibulum. Developmentally, the anterior lobe originates from Rathke’s pouch and the posterior lobe from neural crest cells. The two pituitary lobes function as separate endocrine organs, their cell populations classified by electron microscopic appearances, and functionality. The neurohypophysis is anatomically continuous with the hypothalamus via the hypothalamo-hypophyseal nerve tract. The pituitary gland lies outside the blood–brain barrier, but maintains anatomical and functional connections with the brain. The blood supply to the hypothalamopituitary axis is complex. The hypothalamus receives its blood supply from the circle of Willis, while the neurohypophysis and adenohypophysis receive blood from the inferior hypophyseal artery (IHA) and superior hypophyseal artery (SHA), respectively, which are branches of the internal carotid. The SHA and the IHA anastomose with each other forming a vascular plexus that encircles the gland. The capillary plexus of the IHA forms ‘short’ portal veins that drain into the anterior pituitary as well as the dural venous sinuses. The ‘long’ portal veins are formed from the capillary plexus of the SHA that supplies the nerve endings of the neurosecretory cells in the median eminence of the hypothalamus in addition to the adenohypophysis. The hypothalamic hormones are released into the long portal veins, through which they are transported to the anterior lobe. Here, the portal veins form a secondary capillary network into which the anterior pituitary hormones are secreted. Venous drainage from the gland is to the cavernous sinuses and into the internal jugular vein via the petrosal sinuses.