Neurocritical Care

injury (12%–43%). The most common form of hyponatremia in the neurocritically ill patient is hypotonic

hyponatremia (Syndrome of Inappropriate Antidiuretic Hormone [SIADH] and Cerebral Salt-Wasting

Syndrome [CSWS]), which are the only types of hyponatremia addressed in this chapter.

Laboratory tests (serum sodium) are needed to diagnose hyponatremia.

help determine the specific pathogenesis for hyponatremia.

Consideration of iatrogenic hyponatremia

Typically made by assessing intravascular volume. Patients with SIADH tend to be euvolemic or

hypervolemic with hyponatremia because of excessive antidiuretic hormone (ADH) release, whereas

patients with CSWS tend to be hypovolemic with hyponatremia because of inappropriate urinary

excretion of sodium and extracellular fluid.

Noninvasive hemodynamic monitoring devices (e.g., blood pressure and heart rate)

Monitor fluid balance, weights, skin turgor

Echocardiogram to estimate ventricular filling pressures

Hyponatremia may result in increased brain edema and elevated intracranial pressure (ICP).

Serum Sodium

(mEq/L)

Serum

Osmolality

(mOsm/L)

Urine Sodium

(mEq/L)

Urine Osmolality

(mOsm/L)

Intravascular

Volume Status

SIADH

< 135

< 285

> 25

> 200

Euvolemia

CSWS

< 135

< 285

> 25

> 200

Hypovolemia

aNote: Medications, particularly diuretics, may alter serum or urine measurements of osmolality or sodium concentration.

CSWS = cerebral salt-wasting syndrome; SIADH = syndrome of inappropriate antidiuretic hormone.