primary polydipsia

**Definition:** Excessive water intake (usually >3 L/day) that is not a response to physiologic stimuli (high plasma osmolality or hypovolemia). It is one of the causes of polyuria-polydipsia syndrome.

**Pathophysiology:** - Large water load suppresses ADH (appropriately) and causes a water diuresis with dilute urine (urine osm <100 mOsm/kg in a pure state). - Plasma osmolality and sodium are low-normal or low. If intake exceeds the kidney's maximal free-water excretion (~10-20 L/day), hyponatremia develops. - Chronic high-volume urine flow washes out the renal medullary concentration gradient. This blunts the response to ADH, so urine concentrating ability can be impaired initially.

**Etiologies:** - Psychogenic: schizophrenia (classic, and can also be linked to antipsychotic-related dry mouth), OCD, anxiety. - Dipsogenic: hypothalamic thirst-center dysfunction (sarcoidosis, TB, head trauma, tumors, multiple sclerosis). - Drugs causing dry mouth or thirst: anticholinergics, thioridazine, and others. - Iatrogenic: excessive IV or oral fluid instructions. - Other: MDMA (ecstasy) use, which also raises ADH and worsens hyponatremia.

**Clinical features:** - Polydipsia, polyuria, nocturia. - If severe: hyponatremia with headache, nausea, confusion, seizures, and cerebral edema. - Rapid correction of chronic hyponatremia risks osmotic demyelination syndrome (central pontine myelinolysis).

**Diagnosis (differentiating from diabetes insipidus):** - Labs: low or low-normal serum Na and serum osm, with urine osm low (dilute). - **Water deprivation test:** - Primary polydipsia: urine osm rises (>~500-600 mOsm/kg) with water restriction, and there is little further rise with exogenous desmopressin (<9% increase). - Central DI: urine stays dilute with deprivation, then concentrates after desmopressin (>50% rise). - Nephrogenic DI: urine stays dilute even after desmopressin. - Serum Na tends to be low-normal in primary polydipsia but high-normal or elevated in DI. - Copeptin (stimulated or baseline) is increasingly used and is a useful alternative to the water deprivation test.

**Management:** - Fluid restriction and treat the underlying cause (psychiatric or structural). - For symptomatic severe hyponatremia, use hypertonic saline (3% NaCl) with a cautious correction rate (generally ≤8 mEq/L per 24 h in high-risk patients). - Do NOT give desmopressin, which can cause severe hyponatremia. - Behavioral therapy and weight monitoring (daily weight changes can flag water intoxication in psychiatric patients).

**High-yield pearls:** - Classic vignette: a psychiatric patient (schizophrenia) with polyuria, low serum Na, and dilute urine that concentrates with water deprivation. - Primary polydipsia is a low-ADH state (ADH suppressed appropriately), unlike SIADH, in which ADH is inappropriately high and urine is concentrated. - Hyponatremia with low serum osm and low urine osm suggests primary polydipsia (or low solute intake, such as beer potomania or "tea and toast").