Assessment of fetal adrenal gland and thymothoracic ratio in preterm premature membrane rupture: a prospective case-control study

Introduction

Premature preterm rupture of membranes (PPROM) is defined as the rupture of fetal membranes before 37 weeks of gestation [1]. It occurs in less than 1 % of all pregnancies [2]. Known triggers of PPROM include maternal factors (such as pregnancy-related hypertension, preeclampsia, and cervical shortening), fetal factors (such as congenital anomalies), and environmental factors (including heavy metal poisoning, micronutrient deficiencies, assisted reproductive techniques, and stress) [3], 4]. Various pathological events, e.g., subclinical or overt infections, inflammation, mechanical stress, and hemorrhage, can disrupt homeostatic processes, initiating a series of biochemical changes that result in PPROM [5].

The immunology of normal pregnancy is shaped by the interaction of maternal, placental, and fetal compartments. Within the fetal compartment, the thymus serves as the main component of the immune system, while the adrenal glands play a key role in hormonal regulation during stress. The thymus develops primarily from the third and fourth pharyngeal pouches of the endoderm during embryogenesis. It begins to develop around the ninth week of human gestation, grows until puberty, and subsequently undergoes atrophy. The size of the thymus can change in response to disease [6]. The thymus is a lymphoid organ containing mature T-lymphocytes that carry different surface T-cell antigen receptors. T-cells leave the thymus without encountering antigens and differentiate into memory cells after antigen stimulation, subsequently invading peripheral tissues [7].

The adrenal glands are crucial regulators of homeostasis and the endocrine stress response. Histologically, the adrenal gland is divided into the outer cortex and inner medulla beneath a shared capsule. The human adrenal gland appears as an adrenogonadal primordium 28–30 days after conception [8]. The adrenal gland consists of the cortex and medulla. The adrenal cortex produces steroid hormones, including glucocorticoids, mineralocorticoids, and adrenal androgens, while the medulla produces catecholamines, epinephrine, and norepinephrine [9].

This study aimed to examine the role of the fetal thymus, which serves as a source of T-cells, and the fetal adrenal gland, a key regulator of stress, in the pathogenesis of PPROM, a condition influenced by inflammation and various stress factors. This is the first study to jointly evaluate the relationship between PPROM and ultrasound measurements of the fetal adrenal medulla (length, width, depth, and volume) and thymus parameters such as the fetal thymus and thymothoracic ratio (TTR).

Materials and methods

Ultrasonographic examination

The patients were examined during 28–37 weeks of gestation, at the time of their PPROM diagnosis. Each patient was evaluated by the same maternal-fetal medicine specialist (B.B.Ö.) under the supervision of an experienced supervisor (D.S., professor of maternal-fetal medicine). The examinations were conducted using convex probes (2–5 MHz). Ultrasound examinations followed the guidelines and recommendations of the International Society of Ultrasound in Obstetrics and Gynecology [10].

Each patient underwent a single examination, and the data were recorded. Measurements were taken during fetal apnea periods, with the fetus in the dorsoposterior position. Fetal biometric measurements were performed first, followed by thymus measurements. For thymus measurements, a transverse section of the fetal thorax was used, and a three-vessel view was obtained. The thymus was visualized as a homogeneous structure in the anterior mediastinum. The intrathoracic mediastinal diameter was measured along a parallel line from the anterior border of the thoracic vertebra to the inner border of the sternum. The anterior-posterior thymus diameter was measured from behind the sternum to the transverse aortic arch. The TTR was defined as the ratio of the anterior-posterior thymus diameter to the thoracic mediastinal diameter (Figure 1) [11].

Figure 1:

TTR:The TTR was defined as the ratio of the anterior-posterior thymus diameter to the thoracic mediastinal diameter.

Adrenal gland measurements were taken in the dorsoanterior position. For the adrenal gland width and length, a transverse plane of the abdomen was used. In this plane, the adrenal glands were visualized on both sides of the vertebra, the adrenal gland closest to the probe was used for measurement and the length and width of the adrenal gland were measured (Figure 2). Depth measurements were taken in the sagittal plane (Figure 3). Adrenal volume was calculated using the formula, adrenal gland length × width × depth × 0.52 3 [12]. This formula was also applied to calculate adrenal medulla volume. All volumes were reported in cubic millimeters (mm³). In addition to the individual measurements of the thymus and adrenal medulla, we calculated the thymothoracic ratio (TTR) and the ratio of TTR to adrenal medulla volume (TTR/adrenal medulla volume).

Figure 2:

The length and width of the adrenal gland and adrenal medulla.

Figure 3:

Depth measurements were taken in the sagittal plane of the adrenal gland and adrenal medulla.

Results

This study included a total of 90 patients, consisting of 45 patients with PPROM and 45 controls. Table 1 presents the clinical and demographic data, biochemical results, gestational age, delivery data, neonatal outcomes, first- and fifth-minute Apgar scores, and NICU admission requirements for both the PPROM and control groups. The PPROM and control groups were similar in terms of age, gravidity, parity, gestational age at the time of measurement, and first-minute Apgar scores (p > 0.05). However, there was a statistically significant difference between the PPROM and control groups regarding WBC count, CRP levels, gestational age at delivery, NICU admission, and fifth-minute Apgar scores (p < 0.05).

The ultrasound measurements reported in Table 2 were analyzed in a sequential, clearly defined order as follows: (1) fetal adrenal gland dimensions (length, width, depth, and calculated volume); (2) adrenal medulla dimensions (length, width, depth, and calculated volume); (3) fetal thymus size (length and width) and the thymothoracic ratio (TTR). All measurements were taken in the same standardized scanning plane and summarized in Table 2. There were no significant differences between the two groups in the measurements of the adrenal gland (width, length, depth, or volume) (p > 0.05). However, in comparison to the control group, the length, depth, and volume measurements of the adrenal medulla were significantly smaller in patients with PPROM (15.0 mm [8.4–19.0] vs. 11.95 mm [1.85–23.2], p = 0.002; 9.7 mm [5.4–12.3] vs. 7.7 mm [1.2–15.0], p = 0.002; and 217.54 [73.4–1,017.4] vs. 123.86 [25.6–608.13], p = 0.001, respectively). In addition, the fetal thymus width, length, and TTR were significantly smaller in the PPROM group compared to the control group (13.85 [4.6–20.6] vs. 10.7 [4.1–20.3], p < 0.001; 27.7 [16.1–37.6] vs. 22.5 (11.1–38.6), p = 0.000; and 0.39 [0.14–0.56] vs. 0.31 [0.15–0.57], p = 0.002, respectively). The TTR/adrenal medulla volume ratio was significantly greater in the PPROM group compared to the control group (0.0017 [0.00–0.01] vs. 0.0022 [0.00–0.02], p = 0.007).

To investigate the role of the significant adrenal medulla and TTR parameters in predicting NICU admission, a ROC analysis was performed. The cut-off value of the TTR was determined to be 0.372 mm, at which it had a sensitivity 86 % and a specificity of 62 % (area under the curve [AUC] = 0.750; p = 0.001), while the cut-off value of the adrenal medulla was 189.78 mm, with 75 % sensitivity and 70 % specificity (AUC = 0.797; p = 0.004) (Figure 4).

Figure 4:

Receiver operating characteristic (ROC) curve of the thymic-thoracic ratio and adrenal medulla for predicting neonatal intensive care unit admission.

Spearman’s correlation test revealed a moderate negative correlation between WBC and CRP levels and the TTR (Table 3), but no correlation was found between these levels and adrenal medulla volume.

Discussion

In this study, we investigated the fetal thymus and adrenal medulla, which are linked to inflammation and stress factors in the etiology of PPROM. We found that the fetal thymus and adrenal medulla were smaller in PPROM cases compared to controls, although the total size of the adrenal gland remained unchanged. The smaller adrenal medulla volume and TTR values in patients with PPROM were identified as predictors of NICU admission.

Subclinical or overt infections, inflammation, mechanical stress, and bleeding can disrupt homeostatic processes, leading to a series of biochemical changes that culminate in PPROM. Infection and the associated inflammation are the most commonly identifiable risk factors for PPROM [5]. The immune response of the host to bacterial invasion of the endocervix or fetal membranes leads to the production of multiple inflammatory mediators, which can weaken localized fetal membranes, eventually resulting in PPROM [13].

The normal immune system of pregnancy is shaped through the collaboration of fetal, maternal, and placental compartments [14]. In addition to the maternal immune system, the functional state of the fetal immune compartments is also expected to play a key role.

Our study was motivated by the idea that the fetal response to inflammation and the stress accompanying PPROM may be insufficient. Previous studies in the literature have measured fetal thymus size in various conditions involving inflammation. For example, fetal thymus size was found to be larger in pregnancies complicated by HIV infection [15],while it was significantly smaller in a study involving 32 pregnant women with rheumatological diseases,where a negative correlation between thymus size and inflammatory mediators was observed [16]. Furthermore, studies measuring the TTR in the prediction of small-for-gestational-age (SGA) neonates found that TTR was larger in SGA fetuses [17]. Another study on autoimmune diseases showed that TTR was significantly smaller in maternal autoimmune diseases and was effective in predicting NICU admission [18]. In our study, fetal thymus width, length, and TTR were also found to be significantly smaller in patients with PPROM, and the TTR was a significant predictor of NICU admission. These findings suggest that the insufficient thymus size may indicate an inadequate T-cell production capacity, contributing to preterm birth through a compromised immune response to inflammation. Potential confounding factors such as maternal infection and prenatal corticosteroid use may contribute to variability by affecting fetal thymus and adrenal measurements, which may contribute to variability in the observed results.

In this study, the response of the fetal adrenal gland to the stress induced by PPROM was also evaluated. Although no differences were found in the total adrenal gland size, the adrenal medulla, which secretes catecholamines during stress, was significantly smaller in patients with PPROM compared to controls. The size of the adrenal medulla was also predictive of NICU admission. While the literature contains studies on adrenal gland size, research on the adrenal medulla is limited. For instance, a large multicenter prospective cohort study of 1,697 women investigating adrenal gland and fetal zone parameters did not find these parameters predictive of preterm birth [19]. In contrast, a study on growth-restricted fetuses found that the adrenal gland size was larger [20]. In another study, Zhang et al. evaluated using magnetic resonance imaging (MRI) examinations in pregnancies between 23 and 40 weeks of gestation and found no change in adrenal gland size [21]. However, a study on SGA fetuses reported that adrenal gland volume was larger, and it predicted SGA [22]. A recent study conducted in 2024 investigating preterm birth risk before 32 weeks using MRI did not find any changes in adrenal volume [23]. Goetze et al., who examined the role of adrenal gland changes in predicting late preterm birth, observed shorter measurements among patients [24]. However, in another study aiming to predict changes in the adrenal gland due to stress in fetuses with intrauterine growth retardation, the ratio of the adrenal gland to the abdominal circumference was examined, but no significant difference was observed [25]. Our study revealed no changes in total adrenal gland size, but the adrenal medulla was significantly smaller in PPROM cases, suggesting an insufficient stress response as a potential contributing factor.

While no change was observed in the adrenal gland in our study, unlike the studies mentioned above, the adrenal medulla was found to be smaller in PPROM cases, and the size of the adrenal medulla was also found to predict NICU admission. This was due to the adrenal medulla, which was inadequate to resist stress, as well as the inadequate secretion of catecholamines, just as the thymus size was inadequate for the inflammatory response.

This study is a novel and innovative investigation into the potential role of the fetal thymus and adrenal medulla—organs critical in immune response and stress regulation—in the pathogenesis of PPROM. Using ultrasonography, the dimensions of these fetal organs were assessed with standardized and reliable measurement techniques, and their relationship with clinical outcomes was thoroughly analyzed. The study employed a comparative design between PPROM cases and healthy pregnancies, highlighting significant differences in organ sizes and their potential clinical implications’ this context, the sizes of the thymus and adrenal medulla may serve as predictive ultrasonographic measurement for NICU admission, offering valuable insights for clinical practice.

This study demonstrates that fetal thymus and adrenal medulla dimensions are significantly reduced in cases of PPROM. Specifically, fetal adrenal medulla volume and thymic-thoracic ratio (TTR) were found to be significantly lower in the PPROM group. Additionally, moderate negative correlations were observed between inflammatory markers (WBC and CRP) and TTR. These findings suggest that alterations in fetal immune and stress response systems may play a role in the pathogenesis of PPROM.

The findings are limited to a single-center design, suggesting that the results obtained are limited to broader populations. Multicenter studies are required to validate the findings and increase generalizability.

In conclusion, changes in fetal thymus and adrenal medulla size may serve as potential ultrasonographic measurement for the diagnosis and prediction of neonatal complications associated with PPROM. However, further large-scale, multicenter studies are necessary to validate and clarify the clinical utility of these parameters. Such research could enhance our understanding of the fetal immune and stress response involvement in PPROM and contribute to the development of novel diagnostic and management strategies.