Ectopic Pituitary Adenoma

Abstract

Ectopic pituitary adenomas are defined by the presence of adenomatous pituitary tissue outside the sella and are distinctly separate from the pituitary gland. They are believed to originate from remaining pituitary cells resting along the path of the embryological formation of the pituitary gland. Ectopic pituitary adenomas can be nonfunctioning or secreting, particularly ACTH, GH, and prolactin secreting. The most common site is by far the sphenoid sinus, followed by the cavernous sinus, clivus, nasal cavity, nasopharynx, suprasellar region, and third ventricle. Sphenoid sinus ectopic pituitary adenomas can manifest themselves through a mass effect such as headache and nasal obstruction if large enough, or with symptoms of hormonal excess. Most case reports deal with Cushing disease, and it is accepted today that the sphenoid sinus must be carefully screened if pituitary MRI is negative in the setting of hypercortisolism (Fig. 18.1). Ectopic pituitary adenomas are generally heterogeneous with areas of T1 hypointensity and T2 hyperintensity, and moderate enhancement after contrast administration. Dynamic MRI can help to differentiate ectopic ACTH-secreting from nonfunctioning ectopic adenomas, the time–intensity curve featuring a rapidly enhancing and slow washout pattern in ACTH adenomas. Involvement of adjacent bone is frequent with large ectopic pituitary adenomas. Pituitary adenomas arising from the pituitary stalk are not stricto sensu ectopic adenomas, since the stalk is normally enfolded by an extension of normal anterior pituitary tissue (Fig. 18.2). An empty sella can coexist with an ectopic adenoma (Fig. 18.3). Ectopic pituitary adenomas must be distinguished from pituitary adenomas invading the sphenoid sinus, sometimes through a tiny hole pierced in the sellar floor best detected by CT (Fig. 18.4). Distinction sometimes can be almost impossible before surgery. Careful analysis of case reports of so-called ectopic pituitary adenomas have revealed that some are in fact intrasellar adenomas invading the sphenoid sinus.