A 42-year-old man presents with bitemporal hemianopia and hypopituitarism. MRI reveals a non-functioning pituitary macroadenoma compressing the pituitary stalk. In addition to anterior pituitary hormone deficiencies, his serum sodium is 148 mEq/L with urine osmolality of 200 mOsm/kg. Which neuroanatomical structure's dysfunction best explains the diabetes insipidus in this patient?

• A Destruction of the posterior pituitary gland
• B Infiltration of the supraoptic nucleus by tumor
• C Compression of the neurohypophyseal vessels in the infundibulum
• D Disruption of the hypothalamo-hypophyseal tract above the median eminence ✓

Explanation

Stalk compression disrupts the hypothalamo-hypophyseal tract (axons from magnocellular neurons in supraoptic and paraventricular nuclei) before they reach the posterior pituitary. If the lesion is above the median eminence and enough axon terminals are disrupted, ADH cannot be released. The posterior pituitary itself may remain intact initially (the 'first phase' of triple-phase response).

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.