Prevalence of hyponatremia at diagnosis and factors associated with the longitudinal variation in serum sodium levels in infants with cystic fibrosis.

OBJECTIVE

To determine the prevalence of hyponatremia at diagnosis in patients with cystic fibrosis and identify the factors associated with changes in serum sodium concentration over time.

STUDY DESIGN

This longitudinal study investigated whether variations in serum sodium concentration were associated with age, diet, infection status, and climate/temperature. Multivariate analysis was performed using the random-effects model for longitudinal data.

RESULTS

Hyponatremia at diagnosis was observed in 19 of the 20 patients (95%). Factors identified as associated with variations in serum sodium concentration were diet (P = .008) and climate/temperature (P = .005). Intake of solid foods appeared to greatly increase the serum sodium concentration (increase of 5 mEq/L after introduction of solid foods); however, a confounding factor between diet and age cannot be definitively ruled out. Climate/temperature contributed in an inverse way; a 1°C-increase in ambient temperature was associated with a 0.5-mEq/L decrease in serum sodium concentration.

CONCLUSION

Infants with cystic fibrosis who feed on breast milk or formula and live in a high-temperature environment are at increased risk for hyponatremia, even when receiving a higher salt intake in accordance with recommendations.