Discovering strange diseases: Cerebral–Renal Salt Wasting

Cerebral or the preferred term, renal salt wasting (RSW), remains an unresolved syndrome that has historically evolved from being considered nonexistent to acceptance as a distinct clinical syndrome. Yet, differences over its prevalence continue. Many with RSW are diagnosed and treated for SIADH, which has diametrically opposite therapeutic goals from RSW, that is, to water-restrict in SIADH and administer salt and water in RSW. The major obstacles to differentiating SIADH from RSW are the overlapping of significant findings and clinical associations of both syndromes and our inability to assess the volume status of these patients, being volume-depleted in RSW and expanded in SIADH. In this chapter, we (1) redefine RSW, (2) review the pathophysiology of RSW, (3) review relevant volume studies, which prove RSW to be much more common than SIADH in neurosurgical patients, (4) review the complexities of differentiating RSW from SIADH, focusing on how a previously increased FEurate normalizes in SIADH as compared to being persistently increased in RSW after correction of hyponatremia, (5) review the emerging importance of determining fractional excretion (FE) of urate, which surpasses serum urate in the evaluation of hyponatremic conditions, (6) increased FEurate in the presence of normonatremia is suggestive of RSW, (7, 8) A normal FEurate in nonedematous hyponatremia is highly suggestive of reset osmostat, (9) present an algorithm that uses FEurate as central to the evaluation of the hyponatremic patient, (10). demonstrate the presence of a natriuretic factor in RSW that has different characteristics from A/BNP (11) advocate changing cerebral salt wasting to RSW based on reports of RSW occurring in patients without clinical cerebral disease and eliminating reset osmostat as a subtype of SIADH based on a normal FEurate and the predictability of ADH response to changes in serum osmolality, and (12) awareness, that symptoms with potentially serious complications are associated with hyponatremia, creates a therapeutic urgency to improve methods of differentiating RSW from SIADH.

Hyponatremia 2013, pp 65-85