Two patients presented to us with polyuria and polydipsia.
Case 1 A 34 year old gentleman was referred with polyuria, polydipsia, diffuse headaches and poor sleep. Initial plasma and urine osmolality were 306 and 272 milliosmol/kg respectively. Water deprivation test demonstrated normal concentrating ability. Starting urinary osmolality was 788 milliosmol/kg, rising to 877 milliosmol/kg. MRI pituitary was normal. Hypertonic saline infusion test produced an increase of the sodium from 149 to 155 mmol/L, plasma osmolality from 306 to 321 milliosmol/kg, thirst score from 5/10 to 10/10 and AVP from 3.5 to 7.8 pg/ml.
Case 2 A 53 year old lady was referred after a CT brain and sinuses for frontal headaches and tenderness over sinuses. The CT findings prompted a MRI showing mild chiari malformation and a partial empty sella. She gave a history polyuria, polydipsia and headaches across her eyes and forehead. Anterior pituitary hormone profile was normal. Initial plasma and urine osmolality were 297 and 306 milliosmol/kg respectively. Water deprivation test showed concentration to 710 milliosmol/kg pre DDAVP and only 728 milliosmol/kg 2 hours post DDAVP. Hypertonic saline infusion produced an increase of sodium from 150 to 159 mmol/L, plasma osmolality from 311 to 328 milliosmol/kg, thirst score from 4/10 to 10/10 and AVP from 1.2 to 21.5 pg/ml.
Discussion: The adequate rise of AVP to a higher level of sodium and osmolality reflect the osmostat to be reset at a higher level for both the cases. Literature search reveals that upward resetting of osmostat is quite rare. Elevated osmotic threshold for AVP can present with normal or reduced thirst sensation. The former group presents with polyuria and polydipsia. In view of AVP release at a higher osmolality we might wrongly label these patients with diabetes insipidus unless we proceed to hypertonic saline infusion test.