Hyponatremia & Hypernatremia

Question 1

Hypertonic Hyponatremia

Answer 1

sOsm > 300mOsm / kg

Hyponatremia is caused by a shift of water from cells to the interstitium in response to elevated serum osmolarity, usually hyperglycemia

Question 2

sOsm adjustment for hyperglycemia

Answer 2

For every 100 mg/dL excess (above normal = 100) in serum glucose, serum Na+ falls by 1.6mEq/L

Question 3

Isotonic Hyponatremia

Answer 3

sOsm = 280-300mOsm/kg

Due to a lab artifact caused by any condition that reduces plasma water (hyperlipidemia, hyperprotenemia); the machine “sees” less Na+ because more of the blood volume is occupied by other solutes than by plasma water

Question 4

Hypotonic Hyponatremia

Answer 4

sOsm < 280 mOsm/kg

True alteration of the ratio between ECF Na+ and ECF water; may be hypervolemic, euvolemic, or hypovolemic

Question 5

Hypervolemic hyponatremia

Answer 5

Total body water is elevated more than total body Na+

Associated with heart failure, cirrhosis, and nephrotic syndrome which decrease EABV, causing ADH-mediated retention of water and salt; UNa is usually low

Question 6

Hypovolemic Hyponatremia - 2 categories

Answer 6

Non-osmotic release of ADH in response to decreased volume dilutes total body Na+

Associated with salt and water loss, either renal (UNa > 20) or extra-renal (UNa < 20)

Question 7

Euvolemic Hyponatremia

Answer 7

Inappropriate release of ADH; caused by:

SIADH
Hypothyroidism
Cortisol deficiency
Potomania

Question 8

SIADH - Diagnostic criteria and etiologies

Answer 8

Hypoosmotic, euvolemic hyponatremia with less-than maximally dilute urine (Uosm > 100)

Caused by carcinomas, pulmonary diseases, CNS disorders, and medications

Question 9

Clinical presentation of hyponatremia

Answer 9

Anorexia
Nausea / vomiting
Seizures
Cerebral edema

Question 10

Treatment of hyponatremia

Answer 10

Water restriction 
Furosemide - increases free water excretion 
Hypertonic NaCl (3%) 

Avoid raising serum Na too quickly to avoid central pontine myelinolysis

Question 11

Central pontine myelinolysis

Answer 11

In order to prevent cerebral edema in the setting of hyponatremia, the brain loses some of its own osmoles; if you then overload the patient with hypertonic fluid, water will leave the brain to enter the ECF and cause brain shrinkage

Question 12

3 categorical causes of hypernatremia

Answer 12

1. Decreased total body Na+ due to loss of hypotonic fluid (i.e. free water loss > Na+ loss); loss may be renal (UNa > 20) or extrarenal (UNa < 20)
2. Increased total body Na+ due to administration of hypertonic fluids
3. Normal total body Na+ due to ADH deficiency (Centrial Diabetes Insipidus) or ADH resistance (Nephrogenic Diabetes Insipidus)

Question 13

Central Diabetes Insipidus

Answer 13

i.e. ADH deficiency; caused by head trauma, surgery, neoplasm

Characterized by increased urine volume (2-15L / day) and dilute urine osmolality (<200mOsm/kg)

Kidneys respond to exogenous ADH (DDAVP)

Question 14

Nephrogenic Diabetes Insipidus

Answer 14

Renal collecting duct does not respond appropriately to ADH; may be congenital (rare, X-linked) or acquired (chronic renal failure, hypercalcemia, hypokalemia, ethanol)

Administration of DDAVP does not increase urine osmolality

Question 15

Treatment of hypernatremia & Risk

Answer 15

Fluids +/- sodium

Risk of cerebral edema with fluid push; the brain protects itself from dehydration by increasing its own osmolality with production of free AAs (“idiogenic osmoles”); water repletion must occur slowly in order to allow inactivation of these solutes and avoid cerebral edema

Question 16

Clinical presentation of hypernatremia

Answer 16

Cellular dehydration occurs as water shifts out of cells, causing:

Neuromuscular irritability - hyperreflexia, twitches
Seizures
Coma
Death

Mortality rate 60-75% and 2/3 of survivors will have peermanent neurological injury

Question 17

Water replacement formula for treatment of hypernatremia

Answer 17

Water needed (L) = 0.6 x body weight (kg) x [(actual Na/desired Na) - 1]