Patients who have macroadenomas usually have expanded pituitary fossae. One caveat, however, is necessary: the patient may have cisternal herniation (a partially empty fossa). The aetiology of this condition is unknown but it may be due to a defect in the diaphragma sellae, allowing the transmitted pressure changes of the cerebrospinal fluid to extend into the fossa, thereby expanding it. Another explanation may be that the fossa once contained a pituitary tumour that subsequently infarcted and therefore become smaller. The presence of cisternal herniation may be diagnosed either by CT scanning or MRI.
