Thymic size at birth in preterm infants with severe respiratory distress syndrome can be used to predict the likelihood of survival: A retrospective cohort study

Objective. To determine whether the thymic size in preterm infants with severe respiratory distress syndrome (RDS) can be used to predict survival. We also set out to determine which antenatal and postnatal factors have an influence on, or correlation with, thymic size. Methods. A retrospective study was conducted on 55 consecutive preterm infants who were ventilated for RDS. A chest X-ray (CXR) was taken within the first 24 hours, and the cardiothymic/thoracic ratio (CT/T ratio) calculated. This ratio was then correlated with outcome, as well as antenatal maternal and postnatal factors. Results. Of the 49 infants included in the study (6 were excluded), 15 died and 34 survived. There was a statistically significant correlation between the CT/T ratio and survival (p=0.029). In those infants above 1 030 g, the CT/T ratio was more significant (p=0.038) than birth weight in predicting survival. The severity of RDS did not influence the CT/T ratio. The only maternal and postnatal factors influencing CT/T ratio were the presence of pre-eclamptic toxaemia (PET) and birth by caesarean section (CS), but these factors did not influence likelihood of survival. Factors found to be not associated with thymic size were antenatal steroid administration, maternal HIV status, clinical chorio-amnionitis, gender, gestational age (small or appropriate weight) and lymphocyte count. Conclusions. A small thymus measured in the first 24 hours can be used to predict likelihood of survival in infants weighing more than 1 030 g, but not in smaller infants. Prenatal stress associated with PET and indication for CS may cause the thymus to shrink.