Trends in Endocrinology & Metabolism
Hyponatremia in patients with central nervous system disease: SIADH versus CSW
Section snippets
SIADH is a volume-expanded state
The primary pathogenic mechanism underlying SIADH is excessive antidiuretic hormone (ADH) release causing renal water reabsorption and resulting in expansion of the ECF volume. Evidence for a volume-expanded state in SIADH initially came from studies of normal individuals given exogenous pitressin [7]. In these experiments, administration of pitressin resulted in an abrupt decrease in urine volume and increase in urine osmolality. The water retention produced by this antidiuretic effect
CSW is a volume-depleted state
The newly found appreciation for the diagnosis of CSW can be traced to reports in which blood and plasma volume were found to be decreased in patients who met the traditional laboratory criteria for SIADH. Nelson et al. [9] studied 12 unselected hyponatremic neurosurgical patients with subarachnoid hemorrhage, intracranial aneurysm and head injury. On an average, hyponatremia developed on the tenth day of illness and was associated with increased urine Na+ concentrations (>25 mEq l−1) and an
Pathophysiology of CSW
The mechanism by which cerebral disease leads to renal salt wasting is poorly understood. The most probable process involves disruption of neural input into the kidney and/or central elaboration of a circulating natriuretic factor (Fig. 2). By either or both mechanisms, increased urinary Na+ excretion would lead to a decrease in effective arterial blood volume (EABV), and thus provide a baroreceptor stimulus for the release of arginine vasopressin (AVP). In turn, increased AVP levels would
Differentiation of SIADH and CSW
Distinguishing between CSW and SIADH in clinical practice can be difficult, given the similarity in laboratory values and the overlap in associated intracranial diseases. Determination of ECF volume remains the primary means of distinguishing these disorders (Table 1). ECF volume is increased in SIADH, whereas it is low in CSW. Physical findings that support a diagnosis of CSW include orthostatic changes in blood pressure and pulse, dry mucous membranes and flat neck veins. Weight loss or
Treatment of CSW and SIADH
Making the distinction between CSW and SIADH is of particular importance with regard to treatment [31]. Fluid restriction is employed in SIADH because the primary abnormality is expansion of the ECF volume with water. Administration of NaCl is indicated in CSW because ECF volume is decreased as a result of renal salt wasting. Failure to distinguish properly between these disorders so that treatment indicated for one disorder is inappropriately used for the other can potentially result in an
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