Pituitary apoplexy is attached to the hypothalamus, a part of the brain, by a stalk that also contains the blood vessels that supply the gland. It is unclear why pituitary tumors are five times more likely to bleed than other tumors in the brain. There are various proposed mechanisms by which a tumor can increase the risk of either infarction (insufficient blood supply leading to tissue dysfunction) or hemorrhage. The pituitary gland normally derives its blood supply from vessels that pass through the hypothalamus, but tumors develop a blood supply from the nearby inferior hypophyseal artery that generates a higher blood pressure,
Pituitary adenoma shown there are to date scant studies – of poor quality – to either support or refute this assumption. Adenomas which exceed in size are defined as "macroadenomas", with those smaller than 10 mm referred to as "microadenomas". Most pituitary adenomas are microadenomas, and have an estimated prevalence of 16.7% (14.4% in autopsy studies and 22.5% in radiologic studies). A majority of pituitary microadenomas often remain undiagnosed and those that are diagnosed are often found as an "incidental finding", and are referred to as "incidentalomas". Pituitary macroadenomas are the most common cause of hypopituitarism, and in the majority of cases