Clinically important hyponatremia is relatively uncommon. This disorder manifests with symptoms attributable to cerebral edema that include anorexia, nausea and vomiting, confusion, slurred speech, lethargy, weakness, agitation, headache, and seizures1. Symptoms are generally seen in an acute setting (developing over forty-eight hours or less) with serum sodium levels at or below 125 mEq/L (125 mmol/L) or in the chronic setting with levels at or below 110 mEq/L (110 mmol/L)1. If the disorder is left untreated or is corrected too rapidly, permanent neurologic sequelae can result. Surgical stress is recognized as a common cause of hyponatremia, most frequently in association with transplant, abdominal, cardiovascular, and orthopaedic trauma surgery2,3. This electrolyte disorder is more commonly present in elderly female patients, in those with a lower body weight, and in those taking certain medications, such as thiazide diuretics and selective serotonin reuptake inhibitors2.
Hyponatremia has been reported as a complication after scoliosis procedures4,5 and adult lumbar spine fusions6. To our knowledge, there has been only one report of symptomatic hyponatremia after total hip arthroplasty7 and one after total knee arthroplasty8. We are also aware of one report of tramadol-induced hyponatremia following unicompartmental knee arthroplasty9, although asymptomatic hyponatremia occurs in patients undergoing lower extremity arthroplasty. In a review of 408 consecutive patients who had a lower extremity joint arthroplasty, twenty-one patients (5.1%) had asymptomatic hyponatremia (a serum sodium level of =130 mmol/L) postoperatively10.
We report three cases of clinically symptomatic hyponatremia, treated over a five-year period, that presented as severe postoperative confusion in patients who had had an elective total hip or total knee arthroplasty. Two of three patients were informed that information concerning their care would be submitted for publication. The third patient recently died.
Postoperative confusion may occur after orthopaedic procedures such as lower extremity total joint arthroplasty, especially in elderly patients. Contributing factors commonly include the use of intravenous or oral narcotic analgesics, the use of phenothiazine antiemetic agents, and, rarely, fat embolism syndrome. The three cases described in the present report represent another important cause of postoperative confusion that orthopaedic surgeons should consider in the differential diagnosis of postoperative confusion. All three patients were women, two were more than eighty years old, and two were taking thiazide diuretics. Upon review of the cases, the amount of intravenous fluid given intraoperatively or postoperatively was not considered to be excessive by our anesthesiologists.
The etiology of postoperative symptomatic hyponatremia in patients who have undergone lower extremity arthroplasty may be multifactorial. Many of these patients may have subclinical hyponatremia preoperatively because of the chronic use of thiazides or other diuretics. In a study of 408 consecutive patients who underwent total hip or knee arthroplasty and were screened for hyponatremia (defined as a serum sodium level of =130 mmol/L), fifty-one patients (12.5%) were taking a thiazide diuretic perioperatively and eight of those patients had development of asymptomatic postoperative hyponatremia10. The causes of hyponatremia are many. The stress of major surgery may increase the secretion of antidiuretic hormone11. There may be a dilution of intravascular sodium when excessive hypotonic saline solution is given intravenously. The authors of one study proposed that there also may be a translational mechanism for hyponatremia, with an extracellular shift of free water to the vascular system in the postoperative period12. Elderly female patients may be at higher risk for this complication because of an impaired ability to maintain fluid homeostasis through water excretion and because of the smaller average intravascular volume in women compared with men1,2,11.
On the basis of our experience and that of others8,10, we recommend preoperative electrolyte measurement for patients undergoing lower extremity joint arthroplasty, especially if they have risk factors such as older age and the use of thiazides or other diuretics. Perioperative discontinuation of thiazide diuretics may be considered in consultation with the patient's internist, and fluid balance should be carefully monitored. We now recommend intravenous fluid maintenance with normal saline solution for most patients. Patients are encouraged to drink beverages that contain sodium and to avoid drinking electrolyte-free water, and we discontinue intravenous fluids when the patient's oral intake exceeds 400 mL per eight hours. Symptomatic hyponatremia should be considered and the serum sodium concentration should be measured in any orthopaedic patient experiencing postoperative confusion. Rapid diagnosis and appropriate treatment of symptomatic hyponatremia, in consultation with an internist or nephrologist, are necessary to prevent serious neurologic sequelae in these patients. 