Polyuria

Today we had a discussion about polyuria, and how to differentiate its different causes.

Polyuria is defined as greater than 3L/d. Always get a urine specific gravity. 1.010 is equivalent to plasma osmolality (~300)


General approach:

1. Solute- mediated: glucose, mannitol, NaCl (saline infusion, diuretics, post-obstructive), urea from high protein feeds.
Labs for solute show urine osm over 300, 24h urine osmoles over 900.
2. Psychogenic- urine osmol less than 300, esp probable if serum Na less than 137
3. Central DI- hypothalamic or pituitary stalk disease- urine osm less than 300. Serum Na usu over 142. May see urine osm over 300 if it is partial central DI and pt is water deprived.
4. Nephrogenic DI- X-linked, Li-induced, hyperCa2+, HypoK+, amyloid, sjogren's. Urine osm less than 300, Serum Na usu over 142. May see urine osm over 300 if it is partial central DI and pt is water deprived.


A word about psychogenic polydipsia and beer potomania:

Minimum achievable urine osmolality is ~50mEq/L (i.e. no ADH). Metabolism of a normal diet yields ~600mOsm/d. This means maximum urine output is ~12L/d.

In primary polydipsia, maximum output is exceeded by free water intake, leading to dilutional hyponatremia despite minimally concentrated, high volume urine output. Result is low (below 100) urine osm, low serum osm, hyponatremia, high urine output.

In beer potomania, protein intake is low, and ~250mOsm/d are produced, leading to a theoretical maximum urine output of 5L/d. If free water intake exceeds this (i.e. in beer), the same situation develops, with low urine osmolality, low serum osmolality, and relatively lower urine output.


Water deprivation test: Used for diagnosis of diabetes insipidus.

Patient is admitted to hospital first thing in AM. Do hourly weights, urine vol, urine lytes, osmol, serum Na, serum osm, ADH. Stop the test if urine osm is over 600. This means psychogenic polydipsia (i.e. kidneys can concentrate urine)
Give DDAVP 2ug sc when 1 of:
over 3% of body wt lost
serum osm over 300 or serum Na over 145
serum osm increasing, but urine osm unchanged x 2-3h

After DDAVP give, collect urine and plasma for 2 more hours.


Results

Complete nephrogenic DI: urine osm lower than 300, no resp to DDAVP
Partial nephrogenic DI: urine osm over 300 pre DDAVP, then no response to DDAVP
Complete central: urine osm never over 300, then over 50% increase after DDAVP
Partial central: urine osm over 300, rises 10-50% post DDAVP


Nephrogenic- fix underlying, salt restrict, thiazides
Central- give ddAVP


Bonus:

Secondary causes of central DI

1) Infection - TB, Whipple's
2) Malignancy- Germ cell tumor, lymphoma
3) Infiltration- Hemochromatosis, amyloidosis
4) Autoimmune- Wegner's, sarcoidosis, lymphocytic infundibulitis
5) Histiocytoses- Langerhans' cell or non-langerhans' cell