Severe hyperosmolarity and hypernatremia in an adipsic young woman.

BACKGROUND: Combined deficits in arginine vasopressin secretion (AVP) and thirst sensation can result in life threatening hyperosmolality and hypernatremia. Complications include seizures, profound volume contraction and renal failure. Fortunately, this is an uncommon clinical condition, with approximately 70 cases reported in the literature over the past 47 years [1]. Defects in AVP secretion and/or synthesis produce central diabetes insipidus (DI), polyuria with polydipsia, hypernatremia and hyperosmolality. Most awake and alert patients with an intact thirst stimulus will "drink" themselves back to a normal serum sodium and osmolality. However, if there is concomitant destruction of the osmoreceptors that regulate thirst, osmolal and volume homeostasis cannot be maintained. The relationships between urine osmolarity and serum osmolarity and plasma vasopressin levels are vital for distinguishing a reset osmostat from central DI.

METHODS: After obtaining approval from our institutional review board, we retrospectively reviewed the medical record of a 37-year-old patient who presented to our institution with a serum sodium of 176 mEq/l.

RESULTS: Admission laboratory examination revealed: hemoglobin 12.8 g/dl; white blood cell count 4.7 × 103/µl, with a normal differential; random serum glucose 91 mg/dl ; sodium 176 mEq/l; plasma osmolality 366 mOsm/kg; BUN 33 mg/dl; serum creatinine 1 mg/dl; calcium 9.5 mg/dl; urine specific gravity 1.032; and urine osmolality 1,172 mOsm/kg. An MRI with contrast of the sella/ pituitary revealed an enhancing mass centered within the suprasellar cistern and anterior third ventricle, measuring 3.0 × 3.9 × 3.4 cm. The lesion appeared to involve the hypothalamus and displaced the optic chiasm inferiorly. Evaluation of pituitary function revealed normal serum levels of thyroid stimulating hormone, AM cortisol, luteinizing hormone, follicle stimulating hormone and prolactin. Figure 1 illustrates the relationship between measured serum AVP levels and serum osmolality. Figure 2 shows the relationship between measured urine and serum osmolality. If the serum AVP levels were not available, it would appear as though the patient had a reset osmostat. The kidneys appear to appropriately generate maximally concentrated urine at a serum osmolality above 348 but are unable to below this value.

CONCLUSIONS: When compared with the normal curve, our patient's AVP levels were lower than expected for the corresponding osmolality. This pattern is consistent with a partial central DI. She does not have a reset osmostat. In the presence of significant volume contraction and a reduced GFR, her kidneys produced more concentrated urine despite markedly decreased central vasopressin production. As the volume contraction abated and the GFR improved, polyuria recurred, despite persistent hyperosmolarity and hypernatremia.