Hyponatremia and CNS pathology

Hyponatremia ([Na+] < 130 mEq/L) is the most common electrolyte abnormality in hospitalized patients and is a relatively common complication after subarchnoid hemorrhage (SAH) with an incidence of 10% to 34%. Occurring, in general, several days after the event, the hyponatremia is caused by either SIADH or cerebral “salt wasting”. Patients who are euvolemic or mildly hypervolemic with excess free water have SIADH. Contrast this with the patients who are hypovolemic (volume contract), hyponatremic, and have high urine sodium concentrations, thus suffering from a depletion of both free water and sodium; these patients are suffering from cerebral “salt wasting”. SIADH, as the name implies, occurs because of abnormal and excessive secretion of antidiuretic hormone; whereas, cerebral “salt wasting” occurs likely due to a release of natriuretic peptide from the brain. While this is a subtle and sometimes difficult distinction to make clinically, this distinction is very important as the treatment is different. Patients suffering from SIADH require free water restriction versus those with cerebral “salt wasting” who need both volume repletion and sodium administration. SIADH is also commonly associated with head injury, neurosurgery, brain abscess/tumor, meningitis, encephalitis, Guillain-Barre syndrome, and hydrocephalus.