The best explanation for her mental status changes is her hypercalcemia. The etiologies for mental status changes are often long – especially in the elderly.While renal failure and subsequent encephalopathy is a possibility, calcium levels above 15 are often symptomatic.
Hypercalcemia causes a nephrogenic diabetes insipidus. The hypercalcemia downregulates the vasopressin receptors in the collecting duct of the nephron. This results in a loss of free water. The loss of free water from the nepron results in further dehydration and increasing levels of calcium creating a vicious feedback loop that can only be ameliorated with volume expansion. Thus, while the patient has renal failure, the renal failure is almost universally the result of hypercalcemia and dehydation. The astute physician will recognize that patients who have renal failure and hypercalcemia need volume expansion with isotonic crystalloid. The temptation is to withhold IV fluids because of concern for the kidneys' ability to tolerate the volume or heart failure. Most of these patients are very, very dehydrated.