Patients who developed mild postoperative hyponatremia following total joint arthroplasty had largely comparable 90-day outcomes to patients who remained normonatremic, according to a retrospective single-center study. Although postoperative hyponatremia was associated with higher odds of medical and overall complications, those associations were no longer observed after acute kidney injury was excluded from composite outcomes.

Alexander J. Acuña, MD, of Rush University Medical Center, and colleagues reviewed 4,381 patients who underwent inpatient primary or revision total hip or knee arthroplasty at a US tertiary care center from 2010 to 2023. Patients were categorized according to preoperative and postoperative sodium status:

• Group 1 (normonatremic/normonatremic): 3,441 patients

• Group 2 (normonatremic/hyponatremic): 830 patients

• Group 3 (hyponatremic/hyponatremic): 110 patients

Postoperative hyponatremia, defined as a serum sodium level below 135 mEq/L, occurred in 21% of patients. Mild hyponatremia was defined as sodium levels of 130 to 134 mEq/L, and sodium measurements were obtained on postoperative day 1.

After adjustment for demographic, comorbidity, and procedural factors, patients who developed postoperative hyponatremia had 1.75 times the odds of medical complications and 1.63 times the odds of overall complications compared with patients who remained normonatremic. However, the association was largely driven by AKI, with postoperative hyponatremia associated with nearly twice the odds of acute kidney injury (AKI). No adjusted differences were observed for other individual medical complications.

When AKI was excluded as an outcome, postoperative hyponatremia was no longer associated with medical or overall complications.

The researchers also found no adjusted differences in surgical outcomes, including periprosthetic joint infection, surgical site infection, wound dehiscence, delayed wound healing, periprosthetic fracture, or unplanned revision procedures. Rates of unplanned readmission and emergency department visits were similarly comparable between patients with new postoperative hyponatremia and normonatremic patients.

Patients with persistent hyponatremia had higher adjusted odds of emergency department visits, although the subgroup was small.

In severity analyses, sodium values of 132 and 133 mEq/L were associated with higher odds of overall complications, while sodium values of 134 mEq/L were associated with higher odds of medical complications. However, these associations were no longer observed once AKI was excluded from the analysis.

The researchers additionally evaluated mildly hyponatremic patients who developed postoperative AKI and found no statistically significant increases in overall complications, readmissions, or emergency department visits, although the subgroup analysis was limited by small event numbers.

The findings add to mixed prior evidence regarding the clinical significance of postoperative hyponatremia following total joint arthroplasty. Previous studies have reported associations with prolonged hospitalization, medical complications, and nonhome discharge, while others have found no meaningful differences in postoperative outcomes.

Despite the largely reassuring findings, several unresolved questions remain. The current study was unable to determine whether sodium normalization before discharge influenced outcomes. The researchers noted prior work suggesting that patients whose sodium normalized before discharge may paradoxically have experienced higher complication rates, although those findings were based on a small subgroup analysis.

Several limitations warrant caution in interpreting the results. The study was retrospective and conducted at a single institution, limiting generalizability. The analysis included only inpatient arthroplasty cases and excluded same-day discharge procedures, staged bilateral arthroplasty, and patients with hypernatremia. The researchers also could not assess medication use, perioperative hydration protocols, baseline functional status, or the underlying etiology of hyponatremia.

In addition, sodium measurements were limited to postoperative day 1, preventing assessment of subsequent sodium correction before discharge or later postoperative sodium trends.

The findings may have implications for postoperative laboratory monitoring and inpatient management strategies as outpatient total joint arthroplasty becomes increasingly common. Prior studies cited by the researchers found that many postoperative hyponatremia cases required no intervention or only conservative management.

“In the largest institutional series to date, patients with postoperative hyponatremia largely had comparable 90-day outcomes to patients who were normonatremic postoperatively,” the researchers wrote. They added that “asymptomatic mildly hyponatremic patients may be safely discharged without the additional costs associated with hyponatremia management.”

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Source: JBJS Open Access