Week 2: Disorders of water and serum sodium concentration: polyuria and hypernatremia

Question 1

Distinguish between abnormal total body water balance and disorders of total body sodium balance.

Answer 1

• Abn TBW balance: causes hypo or hypernatremia. Regulated by ADH
• Abn total body sodium balance: causes volume contraction or expansion. Regulated by RAAS.

Question 2

What are causes of non-osmotic ADH release?

Answer 2

• Large decreases in intravascular volume
• emotion, CHF, liver disease, hypoxia, nicotine, morphine
• nausea/vomiting, pain, stress, AngII, hypoglycemia, hypotension, hypovolemia

Question 3

Define polyuria and polydipsia.

Answer 3

• polyuria=urine output>3 liters/day=loss o free water
• high frequency urination, not large volumes
• polydipsia is intake>3 liters/day

Question 4

What are causes of polyuria?

Answer 4

Classified into

• osmotic: uncontrolled DM, hyperosmolar ionic contrast, mannitol, electrolytes
• water diuresis: psychogenic polydipsia, central diabetes insipidus, nephroenic diabetes insipid is

Question 5

Define diabetes insipidus.

Answer 5

-passing of tasteless urine because of relatively low sodium content

Question 6

What are causes of central diabetes insipidus?

Answer 6

Decreased ADH production due to hypothalamic or high pituitary stock lesion

• surgery
• head trauma
• tumors
• CVA
• infections
• granulomatous disease (TB or sarcoidosis)
• autosomal dominant due to mutations in vasopressin gene-rare

Question 7

What are causes of nephrogenic diabetes insipidus?

Answer 7

Inability of kidney to respond to ADH
-Hypokalemia, Hypercalcemia
-Tubulointerstitial nephropathies:
Sickle cell disease
Myeloma
Obstructive uropathy
Recovery from ATN
Lithium 
-Familial

Question 8

How do we distinguish between causes of water diuresis?

Answer 8

1. Water deprivation test
   - measures changes in body weight, urine output, and urine composition when fluids are withheld
   - When 2 sequential urinary osmolalities vary by less than 30 mOsm/kg or when the weight decreases by more than 3%, or Posm reaches 300, give DDAVP
2. response to exogenous ADH-desmopressin (DDAVP)
   - primary polydipsia: Uosm>750
   - CDI: Uosm should increase >50% within 2 hours
   - complete NDI: no increase

Question 9

Describe principles of treatment of polyuria.

Answer 9

1. pyschogenic polydipsia: decrease water consumption
2. CDI: give ADH+adequate water intake
3. NDI: increase proximal water absorption by giving low sodium diet and a thiazide diuretic+adequate water intake
4. osmotic diuresis: remove osmolar load

Question 10

What are effects of thiazide diuretics in treatment of NDI?

Answer 10

Reduce free water excretion by

1) Mild volume contraction which increases water reabsorption in Proximal Tubules and decreases water excretion.
2) Impair urinary dilution by increasing expression of aquaporin2 in collecting duct.

Question 11

What is the pathogenesis of hypernatremia?

Answer 11

• decreased water intake +increased water loss
• water loss may be from renal, extrarenal, GI, sweat
• CDI or NDI
• no access to water
• hospitalized patients receiving IV hypertonic solutions
• When serum [Na]>145 mEq/L

Question 12

Clinical manifestations of hypernatremia?

Answer 12

when serum Na rises>158 mEq/L

• lethargy, weakness
• seizures
• coma
• death

Question 13

Describe compensatory mechanisms in hypernatremia.

Answer 13

• Increased serum osmolality–> shift of water from ICF to ECF
• brain shrinkage–> increase intracellular organic osmoles (1-3 days)
• leads to increased intracellular osmolality, leads to ECF–>ICF of water, and normalization of brain volume

Question 14

Describe diagnosis of hypernatremia.

Answer 14

Failure to concentrate the urine suggests that there is renal free water loss due to diabetes insipidus or osmotic diuresis.

1. If Usom800 mOsm/kg
   - decreased water intake +
   - insensible H20 loss or
   - GI water loss

Question 15

How do you calculate free water deficit?

Answer 15

1. Estimate total body water (TBW) = ~50% of lean body weight (Kg) in females and 60% in males.
2. Serum [Na+] x TBW
   = 140 mEq/L x TBW[Na+]140 mEq/L
3. TBWhypernatremia – TBW[Na+]140 mEq/L
   EXAMPLE
   Serum [Na+]=149. Weight 70kg
   TBWhyp=60%x70kg=42 L
   TBW[Na+]140=(149x42)/140=44.7
   Deficit=44.7-42=2.7

Question 16

Describe management of hypernatremia.

Answer 16

-Replace ½ of free water deficit every 24 hours
plus the ongoing losses,
Do not decrease serum [Na+] more than 0.5 mEq/L per hour or 8 to 10 mEq/L per day
-Ongoing WATER losses =
ALL insensible losses, ½ diarrhea, ~½ urine output + all of vomitus or NG suction
-When the brain has adapted to the hyperosmolality, overly rapid treatment of hypernatremia can lead to cerebral edema. Thus, the free water deficit should be calculated and replaced slowly.
-treat underlying cause