A comparative study of thymic size in preterm neonates with clinical and with / without radiological evidence of respiratory distress syndrome (RDS) in relation to birth weights and gestational age

 

Mounisha K^*, Jeevika M U¹, Hari Kiran Reddy², Abilash Reddy, Guruprasad B Gowdar, Shivprasad S Goudar, Dhaded S, S Yogesh Kumar, Elizabeth McClure, Robert Goldenberg

 

¹Professor & HOD, ²Associate Professor, Department of Radio Diagnosis, JJMMC, Davanagere, Karnataka, INDIA.

 

Abstract              Background: Thymus size in neonates is represented by Cardiothymic -Thoracic ratio (CT/T) which can be measured on chest radiograph. Several pre and post natal factors have been shown to affect the thymus size in neonates. Respiratory Distress Syndrome(RDS) is an important cause of mortality in the preterm neonates and CT/T ratio in infants with RDS may be larger, owing to decrease in serum cortisol levels in them. We therefore intend to find out the thymus size in infants diagnosed with RDS. Aim: To determine thymic size in preterm neonates with clinical and radiological evidence of RDS and in neonates without radiological evidence of RDS and with birth weights and gestational age into consideration and categorising the sample into 3 groups. Materials and methods: The study was conducted with the help and guidance of project ( Project to Understand Research Preterm pregnancy Outcomes and Still births in South Asia PURPOSe) team in the NICU of Department of Paediatrics, JJMMC, Davanagere, Karnataka India, over a period of nine months in three hospitals named: 1. Bapuji Child Health Institute and Research Centre 2. Chigateri District General Hospital. 3. Women and Child Hospital. Premature (<37 weeks of gestation) neonates admitted in NICU with clinical evidence of RDS and with a chest radiograph taken within 24 hours suggestive of RDS or normal were included in the study. CT:T ratio was determined on chest radiograph. Results: Total 138 neonates were enrolled in the study. The mean CT:T ratio in the study of neonates with clinical and radiological evidence of RDS was 0.39 ±0.03. The mean CT:T ratio in the study of neonates with no radiological evidence of RDS was 0.32±0.02. As the birthweight and gestational age of the baby decreases, the CT/T ratio is increased and they were at high risk with less chances of survival. Conclusion: A large CT:T ratio on chest radiograph can be used as a marker for prognosis along with their birth-weights and gestational age. Pre-terms with a CT:T of more than 0.40, birth- weights <1000 grams and gestational age less than 32 weeks have a poorer prognosis in terms of survival.

Keywords: Thymus size, preterm, neonate, respiratory distress syndrome, chest radiograph

 

INTRODUCTION

The thymus is an important gland which is closely concerned with development of cell- mediated immunity and its responses. It continues to grow until puberty and then progressively reduces in size thereafter. Thymus size in neonates is represented by the cardiothymic-thoracic ratio (CT/T) as measured on chest radiograph. Thymic size which is measured by CT/T ratio in neonates with RDS may actually be larger, owing to a decrease in serum cortisol levels in those preterm neonates who develop RDS. This could also be explained by the decrease in lung volume that accompanies hyaline membrane disease(HMD). Increased cortisol levels causes increase in lymphocytes which also may be a reason for enlarged thymus.² We therefore undertook this study to calculate the thymic size in neonates clinically diagnosed with RDS and with or without radiological evidence of RDS to find out whether it is related to survival and prognosis. In addition, we aimed to find the relation between the CT/T ratio with birth weights and gestational age in assessing the severity of the disease.

 

MATERIALS AND METHODS

This retrospective observational study was conducted with the help and guidance of PROJECT ( Project to Understand Research Preterm pregnancy Out comes and Still births South Asia PURPOSe ) team on 138 neonates admitted with clinical evidence of RDS in the NICU of Department of Paediatrics in hospitals named

1. Bapuji Child Health Institute and Research Centre
2. Chigateri District General Hospital.
3. Women and child Hospital under JJMMC, Davanagere, Karnataka, India, over a period of nine months from June 2018 to March 2019,

DISCUSSION

Thymus involution at birth is related to prenatal stress and the grade of involution is related to the duration of illness.⁶ Stress involution appears to be mediated by activation of the hypothalamic-pituitary- adrenal axis with gluco-corticosteroids causing a thymocytolytic effect with lymphocyte apoptosis in the thymic cortex. ⁷RDS is an important cause of neonatal respiratory failure and deaths and prematurity is the single most important risk factor associated with RDS. A positive correlation was found between birth weights, gestational age and thymic size when measured on chest radiograph. In our study, CT/T ratio was slightly higher in neonates of birth-weights <1000 grams indicating high severity of disease with poor prognosis. However, in neonates with birth-weights >1000grams there was a lower CT:T value with better prognosis and good survival rate CT/T ratio was slightly lower with good prognosis in neonates with gestational age of >32 weeks with less severity of the disease. However, in neonates with GA of <32 weeks there was a higher CT:T value with high severity of the disease and bad prognosis. The higher CT:T ratio in neonates with severe RDS may be due to decrease in lung volume that is associated with Respiratory distress syndrome. Prolonged intrauterine stress (pre-eclampsia or chorio-amnionitis ) has been shown to cause involution of the thymus. This stress related involution of thymus appears to be mediated by the activation of hypothalamic-pituitary-adrenal axis(HPA).The activation of HPA leads to a rise in serum corticosteroids which has been shown to have thymocytolytic effect.⁸ Neonates who are ELBW and more preterm have bad prognosis and higher mortality rate. chest radiograph which is relatively cheap and easily available investigation can be done with in 24 hours of life and CT:T ratio should be measured which does not require much expertise and can even be used in small clinics for identification of high risk cases like neonates with larger CT:T ratio having poorer prognosis and for estimating severity. This can be helpful in decision making regarding early referral of the neonate to higher centre which helps reduction in mortality.

 

CONCLUSION

CT/T ratio when determined within 24 hours on chest radiograph after delivery appears to be a useful indicator to help predict severity and survival in preterm infants with respiratory distress based on their weights and gestational age. Chest radiograph being relatively cheap and easily available investigation, a larger CT:T value can be used as a prognostic marker. Pre-terms with a CT:T ratio of more than 0.40 should be closely monitored for any worsening of symptoms and they should be referred early to centres which are well equipped in managing RDS. This will help in significantly reducing the mortality associated with RDS and prematurity.

 

REFERENCES

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