The relationship between sonographic fetal thymus size and the components of the systemic fetal inflammatory response syndrome in women with preterm prelabour rupture of membranes

Authors


Sir,

We read with interest the article by El-Haieg et al. regarding the relationship between sonographic fetal thymus size and the components of the systemic fetal inflammatory response syndrome (FIRS) in women with preterm prelabour rupture of membranes (PPROM). The authors concluded that an association exists between fetal thymic involution and components of FIRS in women with PPROM. A small fetal thymus may be considered a reliable sonographic marker of fetal involvement in the inflammatory response.1 We know that the authors used the thymus perimeter. In our opinion, the perimeter of the thymus is difficult to define and its measurement takes more time. We believe that the transverse diameter of the thymus can be defined more consistently and is therefore more readily measurable because the interface between the thymus and the lung are well identified. The lateral margins of the thymus are also well defined. The author’s thymus detection ratio was 82.5%. In the study of Cho et al.,2 measurements of the transverse thymus diameter were possible in 94% fetuses. In our department, we measured the transversal thymic size on routine ultrasound examination from 20 to 38 weeks of gestation using an Aplio SSA-77A (Toshiba, Tokyo, Japan) ultrasound machine with a convex probe using ultrasound at 3.5–7 MHz. The thymus detection rate was 96% (252 of 262 fetuses). We enrolled 19 women between 24 and 34 weeks of gestation with PPROM in our study. We had no problems visualising the thymus adequately. Measurement was possible in all 19 fetuses. Measurement of the transversal diameter is easier than that of the perimeter because the interface with the lungs is well defined in all cases.

Finally, the authors concluded that 30 (54%) women had histological chorioamnionitis and 13 (23%) had funisitis. This means that 13 women had no histological signs of inflammation or they had only amnionitis. In our group of women with PPROM, ten (53%) women had no histological signs of inflammation, four (21%) had amnionitis alone, four (21%) had chorioamnionitis and one (5%) woman had funisitis. A small thymus transverse diameter (<5th percentile for gestation age according to Cho et al.2) was recorded 43% (8/19). A small thymus was found in three of four cases with amnionitis and in all cases of chorioamnionitis and funisitis. We therefore agree that a normal sized thymus might be useful in ruling out latent intrauterine infection. Moreover, in our opinion, a small transversal thymic diameter is associated with acute inflammatory changes in all placental tissue samples and not only with chorioamnionitis and funisitis.

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