Recurrent syndrome of inappropriate antidiuretic hormone secretion due to tolterodine in an elderly male patient.

Hyponatremia is defined as serum sodium of <135 mmol/L and equates with a low serum osmolality once translocational hyponatremia and pseudohyponatremia are ruled out. True hyponatremia develops when normal urine-diluting mechanisms are disturbed. In elderly patients, this complication is not uncommon, especially in nursing homes and assisted living facilities.