Which of the following is incorrect?
A. In both SIADH and CSW, laboratory tests alone cannot easily differentiate because in both conditions, serum Na is low, plasma osmolality low, urine osmolality high, and urine Na high.
B. Serum uric acid is low in CSW, and high in SIADH.
C. CSW is associated with negative fluid balance and clinical evidence of volume depletion, whereas in SIADH no negative fluid balance or volume depletion is seen.
D. Urea/Creatinine ratio may increase in CSW and is normal in SIADH.
Answer: B
Serum uric acid is normal in CSW and low in SIADH.
The cerebral salt wasting syndrome (CSW) was first defined in 1950. After recognition of the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) in 1957, the concept of CSW was abandoned for a long time until later when people began to find cases who met the criteria for SIADH but showed reduced plasma volume. The condition was found to be due to inappropriate natriuresis rather than inappropriate renal free water retention.
SIADH is caused by inappropriate secretion of ADH/vasopressin, resulting in renal water retention and hence dilutional hyponatraemia. It is defined as plasma sodium <130 mmol/l, plasma osmolality <280 mosmol/l, urine sodium >25 mmol/l, and urine osmolality higher than plasma osmolality in the absence of thyroid, adrenal, renal, liver or cardiac dysfunction.
CSW causes similar blood test profile, but is caused by the secretion of natriuretic peptides, chiefly ANP and BNP, by the heart, resulting in loss of Na in urine together with water.
Clinical differentiation may be difficult based on laboratory parameters alone, and must be diagnosed with clinical assessment of fluid status. On the background of low serum Na, low plasma osmolality, high urine osmolality, and high urine Na, a practical approach for differentiation is as follows:
1. greater than 20% negative fluid balance and/or a rise of haematocrit without transfusion at the beginning of hyponatraemia = CSW
2. greater than 20% positive fluid balance and/or a decline in haematocrit in the absence of haemorrhage at the beginning of hyponatraemia = SIADH
Table. Comparison between CSW and SIADH (reference 1)
Reference
1. Bussmann C, Bast T, Rating D.Hyponatraemia in children with acute CNS disease: SIADH or cerebral salt wasting?Childs Nerv Syst. 2001 Jan;17(1-2):58-62; discussion 63. Erratum in: Childs Nerv Syst 2001 Sep;17(9):575.