DIABETES INSIPIDUS - Diagnosis



In both central and nephrogenic diabetes insi­pidus the principal findings are polyuria and po­lydypsia with urinary volumes generally in excess of 3 L per day, and occasionally, depending upon the concomitant water intake, exceeding 5 to 10 L per day. The diagnosis is generally suspected in patients who excrete large quantities of a dilute urine in which the specific gravity is less than 1.010 or osmolality is less than 300 mOsm/kg. The major diagnostic challenge is to distinguish dia­betes insipidus of either type from compulsive water drinking (psychogenic polydipsia), in which maximal urinary concentrating ability may be impaired as a consequence of “washout” of the normally hypertonic medulla by continuous ex­cretion of a dilute urine.’Demonstration of frank hypertonicity (serum osmolality >295 mOsm/kg) will exclude primary polydypsia, but usually a protocol employing water deprivation (overnight dehydration) followed by administration of aqueous pitressin is required’to distinguish be­tween these three polyuric states. In this proce­dure, water is withheld until the osmolality of hourly voided urines reaches a plateau. In pa­tients with primary polydypsia, urine osmolality is generally much greater than plasma osmolality and increases minimally in response to the sub­sequent administration of aqueous vasopressin, 5 units subcutanepusly. In patients with severe cen­tral diabetes insipidus, urine osmolality is usually much less than plasma osmolality and increases by at least 50 per cent in response to vasopressin; those with nephrogenic diabetes are distin­guished by the failure of a low urine osmolality to respond normally to vasopressin. Occasionally, patients with partial defects in ADH secretion will require further investigation using hypertonic sa­line infusion or more elaborate water deprivation tests.