Polyuria & Diabetes Insipidus

Is it polyuria, and what kind?

Definition. Polyuria is urine output above about 3 L/day in adults. It differs from urinary frequency or nocturia with small voids, which point to the bladder or prostate (BPH, cystitis, overactive bladder). Confirm the volume with a 24-hour collection, then measure urine osmolality.

Solute diuresis. A high urine volume with urine osmolality above 300 mOsm/kg means water is being dragged out by an osmotically active solute:

• Glucose: uncontrolled diabetes mellitus.
• Urea: relief of urinary obstruction, high-protein tube feeds, recovery from AKI, catabolic states.
• Mannitol or large volumes of saline.

Water diuresis. Dilute urine (osmolality below 300 mOsm/kg) with high volume has three causes: primary polydipsia, central diabetes insipidus (now called AVP deficiency), and nephrogenic diabetes insipidus (AVP resistance). Serum sodium offers the first hint. In primary polydipsia, drinking comes first, so sodium runs low-normal; in DI, water is lost first, so sodium is high-normal and thirst follows. Long-standing polydipsia also washes out the medullary concentration gradient, so even normal kidneys concentrate imperfectly at first.

Causes of AVP deficiency and resistance

Central DI. Results from pituitary or hypothalamic surgery, head trauma, tumors (craniopharyngioma, germinoma, metastases), infiltrative disease (sarcoidosis, Langerhans cell histiocytosis), hypoxic brain injury, or idiopathic autoimmune destruction; rare familial forms exist. Polyuria appears once roughly 80–90% of AVP-producing neurons are lost. Pituitary adenomas rarely cause DI before surgery, so DI with a sellar mass suggests craniopharyngioma, metastasis, or infiltrative disease.

Nephrogenic DI. Lithium is the most common acquired cause. Others are hypercalcemia, hypokalemia, relief of urinary obstruction, sickle cell disease, and demeclocycline. Congenital forms are X-linked AVPR2 (V2-receptor) mutations in about 90%, affecting boys, and autosomal aquaporin-2 mutations in the rest.

Gestational DI. Placental vasopressinase degrades AVP late in pregnancy. Desmopressin resists the enzyme and is the treatment; the disorder resolves after delivery.

Diagnostic testing

Copeptin. Copeptin is cleaved from the same precursor as AVP and is stable and easy to measure. A baseline level above 21 pmol/L without prior fluid restriction indicates nephrogenic DI. After hypertonic saline infusion raises serum sodium to at least 150 mEq/L, a copeptin of 4.9 pmol/L or less indicates central DI rather than primary polydipsia. This saline-stimulated test is more accurate than classic water deprivation, particularly for partial forms, which give intermediate desmopressin responses.

Imaging. Every new central DI needs pituitary MRI. Loss of the posterior pituitary T1 bright spot is typical; a thickened stalk suggests Langerhans cell histiocytosis, germinoma, sarcoidosis, or lymphocytic hypophysitis. Anterior pituitary function should be tested as well.

Treatment

Central DI. Desmopressin, a selective V2 agonist given orally, intranasally, or by injection, controls polyuria and nocturia. Its main danger is hyponatremia, so patients should drink only to thirst and have serum sodium checked periodically.

Nephrogenic DI. Correct calcium or potassium and stop the offending drug when possible. Then reduce urine volume by lowering the solute load and blunting the free-water loss:

• Low-salt, low-protein diet: less solute to excrete means less obligatory water.
• Thiazide diuretic: mild volume contraction increases proximal sodium and water reabsorption, so less fluid reaches the collecting duct.
• NSAID such as indomethacin: removes prostaglandin opposition to AVP.
• Amiloride for lithium: lithium enters principal cells through ENaC and downregulates aquaporin-2; amiloride blocks that entry.

Thiazides reduce lithium clearance and raise lithium levels, so lithium must be monitored closely if the two are combined. After years of lithium, the concentrating defect may be irreversible.

Hypernatremia. With intact thirst and free access to water, DI patients keep serum sodium near normal. Severe hypernatremia develops when thirst is impaired (adipsic DI) or water is unavailable (infants, frail elderly, postoperative or unconscious patients). Replace free water orally or as D5W, add desmopressin for central DI, and lower chronic hypernatremia by no more than about 10–12 mEq/L per day.

After pituitary surgery

Triphasic response. Manipulation of the stalk or posterior pituitary can produce three sequential phases:

Practical points. Many patients show only one phase, and isolated hyponatremia around the end of the first week is common, so sodium is rechecked about a week after surgery even after discharge. Untreated adrenal insufficiency can mask DI, because cortisol is needed to excrete free water; polyuria may appear only after glucocorticoid replacement begins.