Desmopressin treatment in dipsogenic form of primary polydipsia: keep serum osmolality marginally below thirst threshold for symptomatic relief.

Dipsogenic polydipsia (DP), a distinct variety of primary polydipsia, is characterised by selective diminution of osmotic threshold for thirst leading to polydipsia and subsequent hypotonic polyuria. Seen in patients without underlying psychiatric illness, DP closely mimics central diabetes insipidus (CDI), making it difficult for clinicians to discriminate these two conditions from each other. Carefully performed osmotic stimulation study, incorporating objective assessment of threshold for thirst and arginine vasopressin (AVP) release is the key to differentiate DP from CDI or psychogenic polydipsia, also termed compulsive water drinking (CWD). Low thirst threshold and high AVP release threshold separate DP from CDI and CWD, respectively. Unlike CWD, desmopressin may be successfully used in DP without concomitant risk of hyponatremia. We describe a child, in whom an initial diagnosis of partial CDI was subsequently revised to DP based on osmotic stimulation test. The child was treated successfully with desmopressin therapy with a target to keep serum osmolality close to thirst threshold.