Application of established pathophysiologic processes brings greater clarity to diagnosis and treatment of hyponatremia

Table 1 Listing the clinical features that are found in syndrome of inappropriate anti-diuretic hormone and cerebral salt wasting/renal salt wasting

Findings common to both SIADH and RSW
Association with intracranial disease
Hyponatremia
Concentrated urine
Urine sodium [Na] usually > 20 mEq/L
Non-edematous
Hypouricemia, with increased fractional excretion urate [FEurate]
Only difference between SIADH and RSW
Volume state: Normal/high in SIADH, low in RSW

Note: How the overlapping features are the most commonly encountered clinically in both syndromes and how the presence of edema can be found in CSW/RSW. SIADH: Syndrome of inappropriate anti-diuretic hormone; RSW: Renal salt wasting; FE: Fractional excretion; CSW: Cerebral salt wasting.

Table 2 Summary of extracellular volume expansion with isotonic, hypotonic and hypertonic saline on fractional excretion of sodium [FEsodium] and urate [FEurate] at control and experimental periods after saline administration. Note the meager changes in FEurate despite very high FENa

	FENa (%)		FEurate (%)		Ref.
	Control	Exp	Control	Exp
Isotonic	1.04	4.43	7.98	9.76	[36]
	1.6	8.2	5.0	5.8	[35]
Hypertonic	2.9	18.6	5.4	12.1	[35]
	1.4	14.5	12.5	18.7	[34]
Hypotonic	1.1	6.1	4.0	7.3	[35]

Printed with permission Ref. [26]. Exp: Experimental.

Table 3 Summary of volume studies by gold standard radio-isotope dilution methods in hyponatremic and normonatremic neurosurgical patients[45,46,50]

Ref.	n of patients	Low blood volume RSW	Increased blood volume SIADH	Urine Na mEq/L
Nelson et al[45] HN	12	10 (83%)	2	41-203
Wijdicks et al[46] HN	9	8 (89%)	1	--
NN	12	8 (67%)	4
Sivakumar et al[50] HN	18	17 (94%)		43-210

RSW much more common than SIAD. Printed with permission, Ref. [26]. SIADH: Syndrome of inappropriate anti-diuretic hormone; RSW: Renal salt wasting; HN: Hyponatremic; NN: Normonatremic.