Proximal tubules reabsorbs 67% of water and Na. Loop (25%) and DCT (3-5%) absorbs the remaining Na but can’t absorb any water. So the fluid reaching the collecting tubule has 30% water and 2-3% Na. If water is not absorbed at this point, all this will go in urine causing polyuria and dilute urine.
Absorption of water at collecting tubules needs following;
1: Collecting tubules with water channels
2: ADH to enhance production and opening of the water channels.
3: The 25% Na absorbed by the loop stays in the interstitium to provide osmotic force to pull the water out of lumen of collecting tubules when ADH opens water channels.
Damage to any of the above will cause excessive water loss in urine & this disease is known as Diabetes Inspidus. So the causes of DI include:
1: Lack of ADH (Hypothalamic damage) such as stroke, SOL, granulomatous diseases and many more). MRI and CSF analysis will be needed as clinically indicated.
2: Damage to collecting tubules (Pyelonephritis, Hydronephrosis, Tubulointerstitial Nephritis, ATN, long standing hypokalemia, hypercalcemia).
3: Congenital absence of water channels or response to ADH.
1: Blood is concentrated with hypernatremia and high osmolarity.
2: Dehydration & consequence of hypovolemia.
3: Polyuria, dilute urine with low osmolarity.
1: Psychogenic polydipsia & Beer potomania also cause polyuria but there is dilute blood and dilute urine because of excessive drinking of water or alcohol, respectively. Patient is not hypovolemic. Plus these two cause low serum Na (unlike DI which cause high serum Na).
2: Tubular disease, diuretic abuse etc also cause polyuria but low serum Na and other clues for tubular diseases such as RTA etc.
3: Cerebral salt wasting also causes polyuria and dehydration but serum Na is low and urine Na is high. Underlying cause may also be evident such as head injury or neurosurgery etc.
4: Cranial cause vs renal cause: high ADH and lack of response to ADH during water deprivation test mean nephrogenic cause. Water deprivation test mainly differentiated b/w psychogenic polydipsia from DI by improving urine osmolarity (DI will not improve). Then lack of response to ADH injection will mean renal cause.
Hypertonic saline stimulation test helps to diagnose partial cranial DI, where clinical picture isn’t that classic. Normally hypertonic saline increases ADH production from Hypothalamus but in partial cranial DI this resolves is blunted / impaired.
Free water access. IV hypotonic fluid replacement when unable to swallow.
ADH replacement for cranial causes
Thiazide diuretics for renal causes as they make urine concentrated by losing Na.
Treat the cause when possible.