Adrenal gland size in fetuses with congenital heart disease

Introduction

Congenital heart diseases (CHD) are the most common major congenital malformation. In about one-third of all severe fetal anomalies, CHDs occur as the sole anomaly or in combination with other abnormalities [1]. For the purpose of this study, we apply the definition provided by Mitchell et al. [2], which describes CHD as “a gross structural abnormality of the heart or intrathoracic great vessels that is actually or potentially of functional significance” [2], thus excluding congenital arrhythmias or minor structural abnormalities like bicuspid aortic valves.

Approximately 8 of 1,000 live births have CHD [1], 3], whereas the incidence of major CHD is 2 of 1,000 birth [4]. The prenatal detection rate ranges from 30 to 60 % depending on the sonographer’s expertise, the gestational age at the time of the examination, the type of CHD and individual conditions like maternal obesity [5]. CHD have a great impact on morbidity and mortality of newborns.

Studies addressing the effect of CHD with outflow tract alterations on the fetal maturation describe an abnormal brain metabolism and brain growth, possibly due to inadequate cerebral perfusion [6]. The cerebral blood flow rate is reduced in children with brain perfusion via the ductus arteriosus and retrograde aorta ascendence leading to a poorer cerebral development [7].

The adrenal gland is an endocrine organ responsible for the synthesis of catecholamines in the medulla and steroid hormones (aldosterone, glucocorticoids, androgen precursors) in the cortex [8]. In fetuses, the adrenal gland as part of the metabolic stress system influences the prenatal homeostasis and has an impact on the initiation of parturition [9], 10]. The growth of the fetal adrenal gland is a highly balanced process involving cell proliferation, differentiation, apoptosis, hypertrophy and cell migration during the prenatal period [9]. It ensures an accurate maturation of the whole fetal organ system [9]. Studies have shown that alterations in the fetal adrenal gland can be associated with fetal pathologies like growth restriction or maternal diabetes [9], 10].

Up to now, the measurement of the adrenal gland is not part of routine prenatal ultrasound and is not yet standardized. However, there are some publications focusing on the fetal adrenal gland and its maturation with regard on several clinical issues like fetal growth restriction or gestational diabetes [10], [11], [12]. Methods of measuring the size and proportions differ from a two-dimensional analysis in the B-mode scan [12] to a three-dimensional approximation of the volume [11]. In our study, we refer to a ratio of the total adrenal gland width divided by the medulla width based on the B-mode scan. This adrenal gland ratio is relatively easy to assess during routine ultrasound without changing the probe. Additionally, it did not show any association to the gestational age and thus, is comparable throughout the pregnancy [12].

To the best of our knowledge, studies have not yet examined the link between CHD and the size of the fetal adrenal gland.

The aim of this study was to compare the adrenal gland size of fetuses with congenital heart diseases (CHD) and normal fetuses.

We propose that CHD can be linked to an enlarged adrenal gland in affected fetuses, caused by elevated metabolic stress levels due to hemodynamic changes.

Materials and methods

Study design

In this cross-sectional prospective study, we measured the size of the adrenal gland in 124 fetuses with and without CHD. The study was approved by our Institutional Review Board and designed according to the Declaration of Helsinki. Informed consent was obtained prior to the examination.

Examinations were performed during routine ultrasound in the Department of Gynecology and Obstetrics at the University Hospital Münster between 05/2014 and 09/2022. A total of 62 cases with CHD (CHD group_total: n=62) was matched with an equal number of unaffected fetuses based on the gestational age (control group_total, n=62). The gestational age based on the crown-rump length ranged from 20 + 0 to 39 + 3 weeks of gestation.

Exclusion criteria were maternal age below 18 years, multiple pregnancies, maternal diseases like infections or a preexisting or gestational diabetes mellitus, growth restricted fetuses and fetuses with complex multiple anomalies in addition to the heart disease.

To evaluate the data, the CHD affected fetuses were classified according to the hemodynamic relevance of their condition for the prenatal blood circulation, particularly the perfusion of the brain as a very intense fetal metabolic organ. The CHD group_total was subdivided into three clusters depending on the type of CHD and the direction of bloodstream in the preductal aortic arch.

In CHD group_1 (n=7) we included all heart diseases with altered anatomy in the four-chamber view with regular anterograde flow in the ascending preductal aortic arch, e.g. intracardiac tumors like rhabdomyoma or ventricular septal defects (VSD) not affecting the outflow tract.

CHD group_2 (n=39) included all heart diseases with an altered outflow tract but regular anterograde flow in the ascending aortic arch, e.g. ventricular septal defects (VSD) and atrial septal defects (ASD) affecting the outflow tract, aortic coarctation (ISTA) with ultrasound-confirmed anterograde flow in the ascending aorta, tetralogy of Fallot (TOF), right-sided aortic arch or aortic stenosis with anterograde flow.

In CHD group_3 (n=16) we clustered all anomalies with impact on the outflow tract to the extent of an altered brain perfusion via reverse flow in the preductal aortic arch from the ductus to the brain vessels, e.g. hypoplastic left heart (HLH), aortic atresia and aortic coarctation (ISTA) with ultrasound-confirmed retrograde flow in the ascending aorta.

In a second step, we analyzed all CHD cases with outflow tract anomalies by summing up CHD group_2 and CHD group_3 to CHD group_2 + 3 (n=55) and compared this group with its matched control group (control group_2 + 3, n=55).

The characteristics of the study groups are summarized in Table 1.

Table 1:

Description of the study groups.

Control group_total	Normal fetuses matched on the gestational age at examination
CHD group_1	CHD with normal outflow tract
CHD group_2	CHD with altered outflow tract and anterograde flow in the ascending aortic arch
CHD group_3	CHD with altered outflow tract and retrograde flow in the ascending aortic arch
Control group_2 + 3	Normal fetuses matched on the gestational age at examination for CHD group_2 + 3
CHD group_2 + 3	All CHD with outflow tract anomalies (CHD group_2 + CHD group_3)

1. CHD, congenital heart disease.

The data was collected by specialists in prenatal ultrasound using the IU22 and EPIQ 7 Philips Ultrasound System (Philips Medical Systems, Andover, MA, USA).

Fetal adrenal gland measurements were performed in accordance with Heese et al. [12] by measuring the total width and the medulla width in a standardized transversal B-mode plane. Using the stored data (View-point^®, General Electric, Wessling, Germany), we then calculated the cortex width (total width minus medulla width divided by two) and the adrenal gland ratio (total width divided by medulla width) [12]. Figure 1shows the adrenal gland cranial to the kidney in a sagittal section (A) as well as an overview image of an abdominal cross-section on the level of the fetal adrenal gland (B). Exemplary measurements with zoomed-in images of the control group, CHD group_2, and CHD group_3, along with the calculation of the adrenal gland ratio, are shown in Figure 2.

Figure 1:

Ultrasound images of the fetal adrenal gland. (A) Sagittal section through a fetal kidney with the adrenal gland positioned cranially (arrow-marked). (B) Cross-section of the fetal abdomen showing the fetal adrenal gland (arrow-marked). This view was used for the measurements.

Figure 2:

Measurements of the total adrenal gland width and the medulla width in transversal planes in the B-mode scan. (A) Control group: Fetal adrenal gland ratio=0.311 cm/0.09 cm=3.456. (B) CHD group_2: Fetal adrenal gland ratio=0.449 cm/0.079 cm=5.683. (C) CHD group_3: Fetal adrenal gland ratio=0.55 cm/0.114 cm=4.824.

Results

The adrenal gland size was measured in 124 cases (CHD group_total: n=62, control group_total: n=62). The CHD group_total was subdivided into three clusters: CHD group_1 (n=7), CHD group_2 (n=39) and CHD group_3 (n=16). In a second step, all cases with alterations of the outflow tract independent of the direction of blood flow in the preductal aortic arch were added up in CHD group_2 + 3 (n=55).

Baseline characteristics of the study cohort are shown in Table 2. The gestational age at examination, the estimated weight at examination, the week of gestation at delivery and pH of the umbilical artery after birth were comparable between control group_total and CHD group_1, CHD group_2 and CHD group_3 as well as between control group_2 + 3 and CHD group_2 + 3. The analysis found a significant, but clinically irrelevant difference regarding the APGAR score after 5 min, which was lower in the CHD affected cases in both steps of the comparison (both p<0.001). The birth weight was comparable between control group_total and CHD group_1, CHD group_2 and CHD group_3. In CHD group_2 + 3, birth weight was significantly lower than in the control group_2 + 3 (p<0.036), which was, however, clinically irrelevant.

In the first step, we compared several indicators for the adrenal gland size (total width, cortex width, medulla width, adrenal gland ratio of total width divided by medulla width) between the control group_total and CHD group_1, CHD group_2 and CHD group_3. The Kruskal–Wallis-Test detected significant differences of the medulla width (p<0.001), the cortex width (p<0.018), the total width (p<0.027) and the adrenal gland ratio (p<0.001) between the groups. As we focused on the ratio, the post-hoc pairwise comparison test then identified significant differences of the adrenal gland ratio between the control group_total and CHD group_2 as well as control group_total and CHD group_3. Both comparisons showed a higher adrenal gland ratio in the CHD affected cases (adrenal gland ratio of control group_total: 3.000 (2.764; 3.357); CHD group_2: 3.645 (3.286; 4.341); CHD group_3: 3.956 (3.310; 4.773); both p<0.001). Exemplary measurements of the control group (1A), CHD group_2 (1B), and CHD group_3 (1C) and their calculation of the adrenal gland ratio are presented in Figure 2. The analysis is provided in Figure 3.

Figure 3:

Boxplots of the adrenal gland ratio of the control group_total, CHD group_1, CHD group_2 and CHD group_3.

We then compared CHD group_2 + 3 (alterations of the outflow tract independent of direction of blood flow in the preductal aortic arch) and its matched control group_2 + 3. The Mann–Whitney-U-Test identified significant differences of the medulla width and the adrenal gland ratio between the two groups (adrenal gland ratio of control group_2 + 3: 2.966 (2.750; 3.355); adrenal gland ratio of CHD group_2 + 3: 3.723 (3.286; 4.404); p<0.001; medulla width of control group_2 + 3: 0.114 (0.099; 0.148); medulla width of CHD group_2 + 3: 0.099 (0.0790; 0.114); p<0.001). The Boxplot in Figure 4 visualizes the differences in the adrenal gland ratio, which was higher in the CHD group_2 + 3. The analysis detected no significant differences of the total adrenal width and the cortex width between the two groups.

Figure 4:

Boxplots of the CHD group_2 + 3 and its matched control group_2 + 3.

Discussion

This study showed that, compared to a control group, the adrenal gland ratio (total width/medulla width) is higher in fetuses affected by CHD with an altered outflow tract and anterograde as well as retrograde flow in the ascending aortic arch. Adding up all CHD cases with outflow tract alterations in a second step of the analysis allowed the conclusion that fetuses with outflow tract anomalies generally show a higher adrenal gland ratio compared to unaffected fetuses.

The adrenal gland is the largest endocrine organ in fetuses. At birth, it has a comparable weight to the adult gland [13] and is, thus, 10 to 20 t larger in relation to body weight [14]. Its cortex consists of an outer definite zone, a transitional zone and an inner dominating fetal zone, which makes up 80–90 % of the cortex [15]. Cells in the fetal zone produce large amounts of dehydroepiandrosterone (DHEA) metabolites which are used by the placenta for estrogen production. This part of the cortex involutes rapidly after gestation and the adult zonation of the cortex (zona glomerulosa, zona fasciculata, zona reticularis) is forming [15], 16].

The medulla is built up of chromaffin cells that migrate into the center until the 18th week of gestation [17]. Their final encapsulation does not take place until the late fetal development [14].

The adrenal gland ratio is a useful marker for the evaluation of the adrenal gland size at different stages of the pregnancy, as it was proven independent of the week of examination in previous studies [10], 12]. Previous studies have investigated the fetal adrenal gland size with regard to various factors. Garcia-Flores et al. [11] found a larger fetal adrenal gland in pregnancies with gestational diabetes with a relation to the birth weight and to maternal biochemical markers of diabetes in the third trimester [11]. Hetkamp et al. [10] also described an enlarged fetal adrenal gland based on the dominance of the cortex in fetuses of pregnancies with gestational diabetes [10]. Other studies proved an association of an enlarged fetal adrenal gland size with preterm labor stating that the fetal zone enlargement is highly effective to identify women at risk for preterm labor within one week [18], 19]. Moreover, the fetal adrenal gland and the cortex width were found to be enlarged in growth restricted fetuses [12].

Focusing on CHD in fetuses, there is consistent data about relevant effects in fetuses. Previous studies have suggested an association between CHD and alterations of the central nervous system, such as prenatal structural brain anomalies, a reduction of brain volume and altered blood circulation of the brain [20]. Analyses of the brain biometry showed an enlarged posterior ventricle in CHD fetuses, which was more distinct in fetuses with a retrograde flow in the aortic arch [21]. One possible explanation for the association between CHD and alterations of the central nervous system is the abnormal cerebral perfusion. This explanation is supported by the fact, that the occurrence of brain lesions dominate in fetuses with left-heart alterations [20].

A regular blood circulation ensures a preferred supply of the brain with oxygenated blood from the placenta via ductus venosus, foramen ovale, left atrium and ventricle and the ascending aortic arch to the fetal brain vessels [22]. This blood contains an oxygen saturation of 65 %, which is considerably higher than the blood saturation of 55 % in the blood that passes from the right heart via the truncus pulmonalis and ductus arteriosus into the descending aorta [22].

By affecting the functionality of the fetal heart, CHD can cause changes in blood circulation. Especially alterations of the left heart can, thus, affect the oxygen supply of the brain, exposing the fetus to a higher metabolic stress level. For example, aortic atresia causes a mixture of the higher and lower saturated blood in the right ventricle which is then transported to the brain via the ductus arteriosus and retrograde through the preductal aortic arch. The resulting supply of the brain vessels with lower saturated blood of approximately 53–55 % oxygenation [22] causes higher levels of metabolic stress in affected fetuses compared to clinically unremarkable fetuses.

Although no general conclusions can be drawn regarding changes in Doppler sonographic measurements of cerebral perfusion in CHD, some studies demonstrated an altered blood circulation of the fetal brain in fetuses with CHD [20]. Berg et al. [23] found a significant lower pulsatility index of the middle cerebral artery (MCA-PI) and cerebroplacental ratio in fetuses with HLH, whereas these findings could not be detected in fetuses with TOF or severe aortic stenosis [23]. Others detected a significant lower MCA-PI in fetuses with transposition of the great arteries [24].

Our findings detected a higher proportion of the cortex in fetal adrenal glands in these affected fetuses. Understanding the important endocrine role of the adrenal gland in the stress system, we assume that the altered cerebral perfusion is a stressor causing a hypertrophy of the adrenal gland cortex.

By using a standardized measurement in a B-mode image, we ensured that the approach of this study is replicable and suitable for routine ultrasound examinations. The intra- and interobserver variability of this measurement method was tested “excellent” [12]. An alternative method described in the literature – a multiplanar three-dimensional measurement to assess the volume of the adrenal gland – is a lot more time-consuming and depending on the examiner’s expertise and only shows “acceptable” inter- and intraobserver variability [11].

Limitations of our study can be seen in the size of the cohort, especially the relatively small number of cases in the CHD subgroups. Still, the proportion of cases with outflow tract anomalies is comparably high. Furthermore, we did not measure the cerebral blood flow to assess the brain circulation. This study descriptively demonstrated the relationship between CHD and measurements of the adrenal gland, but further studies are needed to explore the underlying mechanisms.

Conclusions

As 2 of 1,000 newborns are affected by major CHD, the understanding of its metabolic effects is of high clinical relevance. In future, the knowledge of alterations in fetal adrenal gland proportions could be used as a diagnostic tool in the prenatal prognosis of fetuses with CHD affecting the outflow tract. Further studies with bigger cohorts and cerebral measurements are needed to clarify the mechanism and establish reference values for the fetal adrenal gland.