| Discussion: |
The pituitary gland, located at the base of the brain, is composed of the adenohypophysis (anterior, derived from Rathke's pouch) and the neurohypophysis (posterior, derived from neural tissue). The adenohypophysis secretes GH, ACTH, TSH, LH, FSH, and prolactin. The release of these hormones is controlled by factors released from the hypothalamus. The neurohypophysis releases ADH and oxytocin. Abnormalities of the pituitary, infundibulum, or hypothalamus can result in a wide range of hormonal deficiencies and clinical presentations. Hypopituitarism can be congenital (perinatal insults, genetic, developmental CNS defects, and pituitary aplasia/hypoplasia) or acquired (radiation, inflitrative disorders like sarcoid or TB, hemochromatosis, or tumors).
Ectopic location of the pituitary gland is frequently accompanied by multiple endocrine deficiencies, and children may present because of growth delay. The MRI appearance is characteristic. The "bright spot" or neurohypophysis is found in the floor of the third ventricle or along the infundibulum. The sella turcica and anterior pituitray may be small and the infundibular stalk is absent or hypoplastic. Midline anormailities including agenesis of the corpus callosum, tonsillar ectopia, or optic pathway abnormalities may also be present.
The differential diagnosis of a suprasellar bright spot includes dermoid tumor, thrombosed aneurysm or lipoma. Distinction from dermoid tumor or lipoma can be confirmed by images obtained using fat saturation.
T1 hyperintensity in the posterior pituitary gland is thought to be related to neurosecretory vesicles. The descent of the posterior pituitary may become arrested at any point from the hypothalamus through the pituiatry stalk. Defective induction of the mediobasal structures of the brain in early embryogenesis may account for both the morphological abnormalities and the clinico-endocrinological features encountered in patients with posterior pituitary ectopia.
The pathogenesis of these findings was explained originally by a traumatic transection of the pituitary stalk during delivery. A high incidence of breech delivery has been reported in this group (about 32%) but the traumatic hypothesis cannot explain the findings in the patients with normal delivery. Nor does this theory adequately explain the midline anomalies that are sometimes seen with this condition. Breech delivery should be thought of not as cause of ectopic neurohypophysis, but as a result of embryonic pituitary hypothalamic abnormalities.
Ectopic neurohypophysis has been demonstrated to be present in 87% of patients with multiple hormone deficiencies and 10% of patients with isolated growth hormone deficiency. The anterior pituitary dysfunction in those with an ectopic neurohypophysis was thought to be related to the absent infundibulum, which normally houses the portal system.