When one encounters really high Na levels- 170-200meq/L range, adipsic hypernatremia should be in the differential; especially if they are asymptomatic. The key question to ask is “ are you thirsty?” Once called essential hypernatremia, this disorder is now called adipsic diabetes insipidus or central diabetes insipidus with deficient thirst or easier to call them adipsic hypernatremia. The most common reasons for these are lesions that affect the thirst center in the brain- craniopharyngiomas, CNS sarcoidosis, germinomas and clipping or rupture aneurysms of the anterior communicating artery of the circle of Willis.
Tight regulation of water balance is accomplished via the thirst mechanism and ADH. Both are crucial to maintaining a remarkably narrow range of plasma osmolarity of 282–298 mOsm/kg. Osmoregulation of ADH is mediated by osmoreceptors located in the anteromedial hypothalamus near the neurohypophyseal cell bodies in the supraoptic nucleus. These osmoreceptors are extremely sensitive to changes in osmotic pressure. For example, an increase in osmolarity of 1 to 2 percent increases ADH secretion. However, ADH secretion alone is not adequate to prevent dehydration, and an intact thirst mechanism is vital for water homeostasis. Thirst is regulated by hypothalamic osmoreceptors that are sensitive to changes in effective osmotic pressure of body fluids. The osmotic threshold at which the thirst mechanism is activated begins approximately 5–10 mOsm higher than the threshold for ADH release. These two systems work together to maintain plasma osmolality. With both systems intact, hypernatremia is a rare development, but can occur in patients who have lost their ability to maintain or increase free water intake, for example hospitalized patients and particularly the geriatric population.
There are four variants of adipsic hypernatremia. Type A adipsia is characterized by an upward setting of the osmotic threshold for both thirst and vasopressin release, sometimes called essential hypernatremia. Type B adipsia is characterized by subnormal thirst and vasopressin responses to osmotic stimuli. This is due to partial destruction of the osmoreceptors. Complete destruction of these receptors is classified as type C adipsia, and these patients have complete absence of ADH release and a lack of thirst mechanism. Type D is an extremely rare form that manifests as only a thirst mechanism failure with an intact ADH production.
In all patients with adipsic hypernatremia, a careful neurologic and radiologic evaluation should be performed, looking for a possible treatable disease (such as a benign tumor) that might restore osmoreceptor function.
Forced drinking to make patients eunatremic is the treatment—that is, scheduled water drinking because there is no thirst mechanism, with some desmopressin if need be—is usually what helps. Surgical correction of the cause will be helpful in cases where it is possible.
An old case of detective nephron that discusses this in a fun way at ASN Kidney News.