Plasma and Urine Osmolality in the Differential Diagnosis of Syndromes of Inappropriate ADH Secretion and Cerebral Salt Wasting

Abstract

To evaluate the usefulness of the measurements of plasma and urine osmolality to make a differential diagnosis in the early period of hyponatremia occurring children hospitalized due to cerebral damage. This study was conducted between March 2008 and December 2010 in patients admitted to the Children's Hospital of Ondokuz Mayis University because of acute CNS disease, and developed hyponatremia that persisted for more than 24 hours. The patients were divided into two groups according to clinical and laboratory characteristics: syndrome of inappropriate ADH secretion (SIADH) and cerebral salt wasting (CSW). Diagnostic criteria for SIADH were 1) plasma Na concentration < 130 mEq/L, 2) urine Na concentration > 20 mEq/L and 3) absence of clinically dehydration or edema. Diagnostic criteria for CWS were used as 1) plasma Na concentration < 130 mEq/L, 2) urine Na concentration > 120 mEq/L and 3) polyuria (urinary output > 2 ml/ kg/h) and 4) presence of clinical hypovolemia findings. The patients with renal, adrenal, thyroid or heart insufficiencies and who were given diuretics or mannitol were not included. In the CSW group, urine Na and osmolality were higher (p<0.001) whereas there were no significantly differences regarding initial and minimum plasma Na concentrations between two groups. Plasma osmolality in the CSW group [286 (268-296) mOsm/kg] was found to be higher than that in SIADH group [251 (233-264) mOsm/ kg] (p<0.001).While plasma osmolality was low (< 270 mOsm/kg) in all patients with SIADH, it was detected near to or within normal limits in CSW patients.These results show that the patients with CSW have higher plasma and urine osmolality than those with SIADH, and plasma osmolality in CSW syndrome remains within normal limits despite hyponatremia. In a patient with hyponatremia caused by cerebral damage, low plasma osmolality consistent with hyponatremia suggests a diagnosis of SIADH, and a higher value than expected is in favor of CSW syndrome. Higher osmolality also indicates volume loss and requirement of fluid replacement in CSW. We consider that the measurement of plasma and urine osmolalities besides other clinical and laboratory data will be useful tool in differentiation between the syndromes of SIADH and CSW. © 2010 OMÜ Tüm Haklari Saklidir.