Fractional Urate Excretion in Nonedematous Hyponatremia

• Etiologic categorization of nonedematous hyponatremia [ Time Frame: 2 years ] [ Designated as safety issue: No ]
  We will attempt to categorize the etiology of nonedematous hyponatremia by the following parameters in decreasing order of importance, total and extracellular water determinations, fractional urate excretion, plasma renin and aldosterone levels, urinary sodium concentration and urine osmolality. We will also determine total and extracellular water in a group suspected of having renal salt wasting by virtue of having increased fractional urate excretion and normal serum sodium concentrations.
  
  

The investigators have demonstrated that a normal FEurate in a nonedematous hyponatremic patient is highly consistent with the diagnosis of reset osmostat (RO). Since as much as 36% of patients with SIADH have RO, the investigators will evaluate these patients by either noting dilute urines in spontaneously excreted urines or after a modified water-loading test. The investigators have found that a normal FEurate in a nonedematous hyponatremic patient is highly consistent with RO.

The investigators intend to treat euvolemic patients with hyponatremia with tolvaptan, the V2 ADH receptor blocker, to determine the effectiveness of this form of therapy in a group of patients in whom correction of hyponatremia has been difficult to achieve by usual methods.

Nonedematous hyponatremic patients with serum sodium < 135 mmol/l will be recruited from Winthrop-University Hospital and from our outpatient practice.

It is anticipated that the present studies will provide valuable information on the relative prevalence of SIADH and C/RSW in patients with nonedematous hyponatremia. One possible drawback to these studies is our inability to determine total and extracellular water volumes in patient who are admitted to the neuro/neurosurgical ICU where the acute illnesses require parenteral therapy that will create a nonsteady state situation where total and extracellular water volumes cannot be accurately determined. This is an important possible omission because volume studies in this population of studies have indicated more that two thirds of patients having decreased volumes that were consistent with C/RSW, yet the medical literature states that C/RSW is rare. Studies in hyponatremics elsewhere in the hospital should shed important light on the methods to differentiate SIADH from C/RSW, data which can assist us in differentiating both syndromes in the neuro/neurosurgical ICU, where the investigators intend to perform FEurates.

It is anticipated that volume studies in patients with Alzheimer's disease with normal serum sodium and increased FEurate will demonstrated decreased volumes and confirm our earlier observations that many of these patients are renal salt wasters. It would be interesting to test whether volume repletion with saline will improve them mentally and physically.