Diagnostic Considerations
Syndrome of inappropriate antidiuretic hormone (SIADH)
Postneurosurgery
Tumor
SIADH can be induced by medications (eg, carbamazepine, major tranquilizers, antidepressants).
Phenytoin and chlorpromazine inhibit the release of ADH.
Lithium may block the action of ADH peripherally at the kidney.
Diabetes insipidus (DI)
Hypothalamic (post-TBI) versus peripheral (nephrogenic) DI
Familial - X-linked recessive or autosomal dominant DI
Acquired DI - TBI, postneurosurgery, tumors, granulomatous, infections, vascular disorders, circulating antibodies to vasopressin, autoimmunity, and idiopathic
Cerebral salt wasting
Hypothalamic/nephrogenic DI
SIADH
Anterior hypopituitarism
Postneurosurgery
Tumors
Vascular (postpartum)
Infections
Granulomatous disease
Idiopathic
Primary adrenal insufficiency
Autoimmune (idiopathic adrenalitis)
Malignancy
Acute sepsis (including systemic fungal infections)
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Effects of hyponatremia on the brain and adaptive responses. Within minutes after the development of hypotonicity, water gain causes swelling of the brain and a decrease in osmolality of the brain. Partial restoration of brain volume occurs within a few hours as a result of cellular loss of electrolytes (rapid adaptation). The normalization of brain volume is completed within several days through loss of organic osmolytes from brain cells (slow adaptation). Low osmolality in the brain persists despite the normalization of brain volume. Proper correction of hypotonicity reestablishes normal osmolality without risking damage to the brain. Overly aggressive correction of hyponatremia can lead to irreversible brain damage.
Tables
| Formula* | Clinical Use | |
| Change in serum Na+ = |
Infusate Na+ — serum Na+
—————————— total body water +1 |
Estimate the effect of 1 L on any infusate serum Na+ |
| Change in serum Na+ = |
(Infusate Na+ + infusate K+)—serum Na+
———————————————— total body water +1 |
Estimate the effect of 1 L of any infusate containing Na+ and K+ on serum Na+ |
| Infusate | Infusate Na+ |
Extracellular—Fluid
Distribution |
| mmol/L | % | |
| 5% sodium chloride in water | 855 | 100† |
| 3% sodium chloride in water | 515 | 100† |
| 0.9% sodium chloride in water | 154 | 100 |
| Ringer lactate solution | 130 | 97 |
| 0.45% sodium chloride in water | 77 | 73 |
| 0.2% sodium chloride in 5% dextrose in water | 34 | 55 |
| 5% dextrose in water | 0 | 40 |
| *The number in formula 1 is a simplification of the expression (infusate Na+ —serum Na+) X 1 liter, with the value yielded by the equation in mmol/L. The estimated total body water (in liters) is calculated as a fraction of body weight. The fraction is 0.6 in children; 0.6 and 0.5 in nonelderly men and women, respectively; and 0.5 and 0.45 in elderly men and women, respectively.†In addition to its complete distribution in the extracellular compartment, this infusate induces osmotic removal of water from the intracellular compartment. |
