Fetal thymus size in uncomplicated twin and ... - Wiley Online Library

Ultrasound Obstet Gynecol 2010; 36: 302–307 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/uog.7578

Fetal thymus size in uncomplicated twin and singleton pregnancies F. GAMEZ*, J. DE LEON-LUIS*†, P. PINTADO*, R. PEREZ*, J. N. ROBINSON‡ E. ANTOLIN*, L. ORTIZ-QUINTANA* and J. SANTOLAYA-FORGAS† *Departments of Obstetrics and Gynecology, Hospital General Universitario Gregorio Maran˜ on, ´ Madrid, Spain and †The University of Tennessee Health Science Center, Memphis, TN and ‡Brigham and Women’s Hospital, Boston, MA, USA

K E Y W O R D S: fetal thymus; prenatal diagnosis; twins

ABSTRACT Objectives The main objective of this study was to determine whether fetal thymic measurements could be obtained in twins, with a secondary goal to determine whether thymic measurements from uncomplicated singleton and twin pregnancies are comparable. Methods The transverse diameter and perimeter of the fetal thymus were measured prospectively in 678 singleton and 56 twin pregnancies, and their relationships with gestational age were determined and compared between groups. Results Thymic measurements were possible in 757 (95.8%) of the 790 fetuses. Measurements were not possible in 19 of 678 singletons (2.8%) and in 14 of the 112 (12.5%) twins (P < 0.001). After construction of nomograms for the transverse diameter and perimeter of the fetal thymus, similar measurements were noted for singletons and twins. Conclusions These results suggest that sonographic measurements of the thymus are feasible in twin pregnancies and that, in uncomplicated pregnancies, these measurements are similar to those noted for singletons. These findings pave the way for future studies aimed at determining the clinical utility of thymic measurements in complicated singleton and twin pregnancies. Copyright  2010 ISUOG. Published by John Wiley & Sons, Ltd.

malignancy in autoimmune diseases, such as myasthenia gravis3 . Measurements of the size of the thymus can also be used to detect ‘thymic involution’ due to apoptosis of cortical thymocytes4 after the activation of the hypothalamic–pituitary–adrenal axis in response to acute respiratory distress syndrome, trauma, sepsis, malnutrition, physical stress or graft versus host disease5 – 7 . Several authors have already reported on the normal development of the fetal thymus in singleton pregnancies2,8 – 10 . Moreover, during the prenatal period, the size of the thymus has been correlated with T-cell output11 – 14 and the numbers of circulating CD41 and CD45RA1 na¨ıve T-cells15 , opening up the possibility of making fetal thymic measurements a promising non-invasive screening tool for the prenatal diagnosis of functional thymic insufficiency in response to activation of the stress pathway. This particular application is slowly becoming accepted in perinatal medicine16 – 20 . Although in recent years several investigators have addressed the potential implications of prenatal assessment of thymic size, no information has been provided on fetal thymic measurements in twin pregnancies, in spite of its potential for screening thymic responses in monochorionic twins affected by twin-to-twin transfusion syndrome or by severe growth discordance. The primary goal of this study was to determine whether sonographic measurement of the perimeter and transverse diameter of the thymus in uncomplicated twin pregnancies is feasible. The secondary goal was to determine if the fetal thymuses of uncomplicated singleton and twin pregnancies are similar in size.

INTRODUCTION In neonates determining the size of the thymus is useful for making a diagnosis of congenital absence or hypoplasia of the thymus1,2 or thymic hyperplasia or

PATIENTS AND METHODS This was a prospective cross-sectional observational study involving a total of 734 pregnant women. The study was

Correspondence to: Dr J. Santolaya-Forgas, UMDNJ-Robert Wood Johnson Medical School, Department of Obstetrics, Gynecology and Reproductive Sciences, 125 Paterson Street, New Brunswick, NJ 08901, USA (e-mail: [email protected]) Accepted: 11 December 2009

Copyright  2010 ISUOG. Published by John Wiley & Sons, Ltd.

ORIGINAL PAPER

Fetal thymus size in twin and singleton pregnancies approved by the Institutional Review Board of the Grego´ General University Hospital, Madrid, Spain. rio Maran˜ on All women were recruited consecutively between January 2007 and December 2008. All women had their pregnancy dated, and in the case of twins chorionicity established, during a first-trimester sonographic evaluation. The study population was selected from a cohort of women referred for prenatal ultrasound evaluation without signs or symptoms of preterm labor or rupture of membranes. Exclusion criteria were: (1) pregnancies resulting in neonates whose birth weight was below the 10th percentile or above the 90th percentile for gestational age according to institutionally-derived nomograms21 ; (2) twin pregnancies in which the difference between the sonographically estimated fetal weight of each twin was greater than 20%; and (3) fetuses with congenital abnormalities diagnosed in utero or after birth. All ultrasound evaluations were performed at between 19 and 38 weeks’ gestation using a transabdominal 4–8-MHz transducer (GE Logiq 9 or Voluson Expert, GE Medical Systems, Zipf, Austria). The thymus was measured after identification of the interface between the thymus and lungs at the level of the three-vessel view of the thorax during a fetal heart evaluation2,8,22 (Figure 1). All conventional fetal biometric measurements, including thoracic perimeter using the trace function of the ultrasound machine, were consistently evaluated. Fetal thymic measurements were obtained in duplicate by the same observer, allowing for a 5-min interval between examinations. Intra- and interobserver reproducibility of fetal thymic measurements at our institution have been previously reported9,23 . Clinical information was retrieved from maternal and neonatal medical records by one of the investigators (P.P.), who was unaware of the prenatal ultrasound findings.

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Data were analyzed using the SPSS software package Version 15.1 (SPSS Inc., Chicago, IL, USA) and its default settings. Differences in sonographic and perinatal outcome variables between singletons and twins were evaluated using the chi-square test and Student’s t-test, as appropriate. The relationships between the perimeter and transverse diameter of the thymus with gestational age were determined by linear regression modeling. The Kolmogorov–Smirnov test was used to confirm the normal distribution of the thymic measurements at different gestational ages. Relationships between thymic measurements and other fetal biometric parameters were also determined and the predicted changes with gestational age calculated from the regression equations. Z-scores ((measurement − mean)/SD) were then calculated for the assessment of model fit. Equations of the linear regression were used to calculate the mean and the 5th and 95th percentiles for each gestational age (percentiles = estimated mean ± 1.645 SD). P < 0.05 was considered statistically significant.

RESULTS A total of 734 women with 790 fetuses entered the study: 678 singleton fetuses (85.8%) and 112 twins fetuses (14.2%). Twenty-eight fetuses were the product of monochorionic and 84 of dichorionic twin pregnancies. No statistical differences were noted for mean gestational age at ultrasound evaluation between singleton and twin gestations. Our first aim was to determine the feasibility of prenatal thymic measurements in twin pregnancies. Thymic measurements were possible in 659 singletons (97.2%) and in 98 twins (87.5%). These proportions were significantly different (P < 0.001). The measurement (b)

Figure 1 Ultrasound image (a) and schematic diagram (b) illustrating measurements of the thymus. Evaluation of the thymic transverse diameter (A) was standardized by measurement of the diameter perpendicular to the line connecting the center of the sternum to the spine, with the calipers or trace function of the ultrasound machine placed at the interfaces between the thymus and lungs. The tracing of the perimeter of the thymus is shown (B), as is the fetal thoracic perimeter (C). Ao, aorta; Da, pulmonary artery (anteriorly) and ductus arteriosus (posteriorly); SVC, superior vena cava.


Ultrasound Obstet Gynecol 2010; 36: 302–307.

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Table 1 Comparison of sonographic measurements and perinatal outcome variables in singleton and twin pregnancies Parameter GA at ultrasound (weeks) BPD (mm) AC (mm) FL (mm) Thymic transverse diameter (mm) Thymic perimeter (mm) Thoracic perimeter (mm) GA at delivery (weeks) Birth weight (g) Newborn length (cm)

Singleton

Twin

P

28.1 ± 5.6 71.1 ± 14.7 230.6 ± 57.3 51.6 ± 12.5 24.22 ± 6.7

27.5 ± 5 68.5 ± 13.3 223 ± 48.8 49.7 ± 11.4 23.9 ± 6.3

0.2 0.03 0.03 0.05 0.2

67.9 ± 19.9 161.3 ± 35.3 39.1 ± 1.4 3238 ± 312 49.3 ± 1.7

66.3 ± 20 0.3 153.7 ± 34.9 0.09 36.2 ± 1.4 < 0.001 2351 ± 252 < 0.001 46.2 ± 2 < 0.001

Data are expressed as mean ± SD. Student’s t-test was used to make comparisons between groups. AC, abdominal circumference; BPD, biparietal diameter; FL, femur length; GA, gestational age. Table 2 Comparison of sonographic and perinatal outcome variables in twins according to chorionicity

Parameter GA at ultrasound (weeks) BPD (mm) AC (mm) FL (mm) Thymic diameter (mm) Thymic perimeter (mm) Thoracic perimeter (mm) Birth weight (g) Newborn length (cm)

Monochorionic (n = 28) 28.1 ± 4.7 71 ± 12.7 229.1 ± 45.9 52.2 ± 10.8 24 ± 5.6 68.7 ± 16.9 160.4 ± 34.3 2339 ± 181 45.3 ± 1.7

Dichorionic (n = 84) 27.5 ± 5.1 67.7 ± 13.5 219.1 ± 49.1 49.6 ± 11.5 23.2 ± 6.5 65.5 ± 20.7 150.3 ± 33.7 2353 ± 254 46.1 ± 1.9

P

0.6 0.3 0.2 0.2 0.6 0.5 0.3 0.1 0.1

Data are expressed as mean ± SD. Student’s t-test was used to compare study variables depending on chorionicity. AC, abdominal circumference; BPD, biparietal diameter; FL, femur length; GA, gestational age.


Thymic transverse diameter (mm)

(a)

50

40

30

20

10

0 15

20 25 30 35 Gestational age at ultrasound (weeks)

40

15

20 25 30 35 Gestational age at ultrasound (weeks)

40

(b) 120

Thymic perimeter (mm)

of the presenting twin (Twin A) could not be obtained on five occasions and that of the non-presenting twin (Twin B) on nine occasions (non-significant difference). We used logistic regression analysis to determine if gestational age contributed to the failure to measure the fetal thymus in singleton and twin pregnancies and found that, starting at 19 weeks’ gestation, the odds for failure increased by a weekly factor of 1.12 in singleton and 7.97 in twin pregnancies (P < 0.001). Therefore, both twinning and gestational age are predictive of failure to measure the fetal thymus. Our second aim was to determine whether the size of the thymus is comparable in singleton and twin fetuses. Table 1 shows summary statistics of the sonographic measurements of the thymus in singletons and twins and Table 2 those for monochorionic and dichorionic twins. After confirmation of the normal distribution of the samples (Kolmogorov–Smirnov P > 0.05), fetal thymic measurements in singletons and twins were noted to be statistically similar. Significant differences between groups were noted for the biparietal diameter, abdominal circumference, femur length, gestational age at delivery

100 80 60 40 20 0

Figure 2 Transverse diameter (a) and perimeter (b) of the thymus against gestational age in singletons ( ) and twins ( ). The mean and 5th and 95th percentiles for singleton fetuses are shown, having been calculated using the respective linear regression equations shown in Table 3 for (a) and Table 4 for (b). Analyses of these data points suggest that the size of the thymus is similar in singleton and twins fetuses.

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and newborn biometric measures. No statistical differences were noted for fetal biparietal diameter, abdominal circumference, femur length, thoracic perimeter, thymic transverse diameter, thymic perimeter or neonatal weight and height between Twin A and Twin B even after subclassification into monochorionic (n = 28) or dichorionic (n = 84) twin pregnancies. Figure 2 depicts the change in fetal thymus size in singletons and twins with gestational age. Analysis of the data points for the transverse diameter and perimeter of the thymus does not suggest any difference between singleton and twin pregnancies. The transverse diameter and perimeter of the thymus and the fetal thoracic perimeter were similar for Twin A and Twin B. The results of linear regression analysis between the sonographic measurements of the thymus and gestational age and other common fetal biometric variables in singleton fetuses are shown in Tables 3 and 4. Tables 5


Fetal thymus size in twin and singleton pregnancies

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Table 3 Results of linear regression analysis between transverse diameter of the fetal thymus (TD-thymus) and other prenatal measurements in singleton pregnancies Variable

Regression equation: TD-thymus (mm) = −5.36 + (1.05 × GA) −3.62 + (0.39 × BPD) −0.02 + (0.10 × AC) −0.56 + (0.45 × FL) −2.52 + (0.31 × ThymP) −1.85 + (0.16 × ThorP)

Gestational age (GA) (weeks) Biparietal diameter (BPD) (mm) Abdominal circumference (AC) (mm) Femur length (FL) (mm) ThymP (mm) ThorP (mm)

SD

r2

P

4.01 4.13 4.10 4.18 2.88 4.16

0.89 0.79 0.90 0.78 0.93 0.83

< 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001

ThorP, thoracic perimeter; ThymP, thymic perimeter. Table 4 Results of linear regression analysis between perimeter of the fetal thymus (ThymP) and other prenatal measurements in singleton pregnancies Variable Gestational age (GA) (weeks) Biparietal diameter (BPD) (mm) Abdominal circumference (AC) (mm) Femur length (FL) (mm) TD-thymus (mm) ThorP (mm)

Regression equation: ThymP (mm) = −15.78 + (2.95 × GA) −9.71 + (1.09 × BPD) 0.20 + (0.28 × AC) 1.68 + (1.27 × FL) 1.95 + (2.73 × TD-thymus) −6.88 + (0.46 × ThorP)

SD

r2

P

13.05 13.42 13.23 13.43 8.43 12.34

0.85 0.76 0.71 0.78 0.92 0.78

< 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001

TD-thymus, transverse thymic diameter; ThorP, thoracic perimeter.

and 6 show the predicted mean and 5th and 95th percentiles of the thymic diameter and perimeter, derived from the regression equations calculated, in singleton fetuses for each week of gestation from 19 to 38 weeks. The SD from the linear regression analysis was used as a constant because there was little evidence of the SD being gestational-age-dependent. Figure 3 shows measurements of fetal thymus size across gestational age for presenting and non-presenting twins.

DISCUSSION The main emphasis of this study was to determine if fetal thymic measurements could be obtained in twins, with a secondary goal of determining if these measurements were similar to those obtained from singleton pregnancies. These questions are important because they could pave the way for future investigations concerning the clinical utility of prenatal thymic measurements in pathological singleton and twin pregnancies. Results from our feasibility analysis suggest that fetal thymic measurements between 19 and 38 weeks’ gestation can be obtained in almost 96% of pregnancies. The failure rate to obtain thymic measurements was 12.5% in twins and 2.8% in singleton pregnancies, with failure more likely to occur towards the end of pregnancy. Our findings also suggest that the perimeter and transverse diameters of the fetal thymus in uncomplicated singleton and twin pregnancies are similar and are not affected by twin order or chorionicity. Specific fetal nomograms are useful for discriminating normal from abnormal measurements at a particular point in development, as well as for defining patterns of change over time. Sonography is widely used in the


Table 5 Nomogram for fetal thymus transverse diameter according to gestational age (GA) in singleton pregnancies Predicted thymus transverse diameter (mm)

GA (weeks)

n

5th percentile

Mean

95th percentile

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

23 40 38 26 27 35 36 23 22 30 44 44 47 41 35 37 38 29 21 23

7.8 8.8 9.9 10.9 12.0 13.0 14.1 15.1 16.2 17.2 18.3 19.3 20.4 21.4 22.5 23.5 24.6 25.6 26.7 27.7

14.3 15.4 16.5 17.5 18.6 19.6 20.7 21.7 22.8 23.8 24.9 25.9 27.0 28.0 29.1 30.1 31.2 32.2 33.3 34.3

20.9 22.0 23.0 24.1 25.1 26.2 27.2 28.3 29.3 30.4 31.4 32.5 33.5 34.6 35.7 36.7 37.8 38.8 39.9 40.9

Predicted mean and 5th and 95th percentiles for the fetal thymus transverse diameter at gestational age intervals were calculated using a constant SD of the estimate due to lack of evidence for SD being gestational-age dependent (SD = 4.01, from Table 3).

evaluation of fetal biometry and growth in singleton and twin pregnancies24,25 . Our findings concerning the sonographic changes in thymic measurements over time in singleton pregnancies are in agreement with those reported by Cho et al.8 and only slightly lower than


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GA (weeks)

n

5th percentile

Mean

95th percentile

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

23 40 38 26 27 35 36 23 22 30 44 44 47 41 35 37 38 29 21 23

18.8 21.8 24.7 27.7 30.6 33.6 36.6 39.5 42.5 45.4 48.4 51.3 54.3 57.2 60.2 63.1 66.0 69.0 71.9 74.9

40.3 43.2 46.2 49.1 52.1 55.0 58.0 60.9 63.9 66.8 69.8 72.7 75.7 78.6 81.6 84.5 87.5 90.4 93.4 96.3

61.8 64.7 67.6 70.6 73.5 76.5 79.4 82.4 85.3 88.3 91.2 94.2 97.1 100.1 103.0 106.0 108.9 111.9 114.9 117.8

Predicted mean and 5th and 95th percentiles for fetal thymic perimeter at gestational age intervals were calculated using a constant SD of the estimate due to lack of evidence for SD being gestational age-dependent (SD = 13.05, from Table 4).

Thymic transverse diameter (mm)

Predicted thymus perimeter (mm)

(a)

40

30

20

10

0 20

25 30 Gestational age at ultrasound (weeks)

35

20

25 30 Gestational age at ultrasound (weeks)

35

(b) 120 100 Thymic perimeter (mm)

Table 6 Nomogram for fetal thymic perimeter according to gestational age (GA) for singleton pregnancies

80 60 40 20

those reported by Zalel et al.2 . Our results indicate that both the thymic transverse diameter and thymic perimeter are similar in uncomplicated singleton and twin pregnancies and that they are not affected by twin order or chorionicity. Therefore, we propose using nomograms from uncomplicated singleton pregnancies to discriminate normal from abnormal measurements in twins from 19 weeks’ gestation, as well as for defining patterns of change over time in pathological pregnancies. Thymic measurements were possible in 97% of singleton pregnancies and in 87.5% of twin pregnancies, a statistically significant difference (P < 0.001). To determine if twinning or gestational age at ultrasound evaluation were significant factors for failure to obtain thymic measurements we used logistic regression analysis and found that the odds ratio for failure in singletons was 1.12 per week interval and that it was 7.97 per week interval in twin pregnancies (P < 0.001). Therefore, the failure rate is gestational-age-dependent, increasing as the pregnancy advances. The primary limitation of our study is that thymic size in twin pregnancies with discordant fetal measurements was not investigated. A second limitation relates to the fact that discordant growth in monochorionic twins can be detected during the first half of pregnancy26 and that the nomograms that we have constructed start at 19 weeks’ gestation. Therefore, the effect of gestational age prior


0

Figure 3 Transverse diameter (a) and perimeter (b) of the thymus against gestational age in twins, showing the mean and 5th and 95th percentiles calculated from linear regression equations. Presenting ( ) and non-presenting ( ) twins are shown; analyses of these data points suggest that the size of the thymus does not differ between the two fetuses.

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to 19 weeks on our ability to measure the fetal thymus is unknown. Because of these limitations future studies at earlier gestational ages and in pregnancies affected by twin-to-twin transfusion syndrome or severe growth discordance are required before the overall relevance of thymic measurements in perinatal medicine can be determined. In conclusion, our findings confirm that the fetal thymus can be measured prenatally in twin pregnancies and suggest that the size of the thymus is similar in uncomplicated singleton and twin pregnancies between 19 and 38 weeks’ gestation. This study provides new reference ranges for the thymic transverse diameter and perimeter during the prenatal period. Further studies are required to determine whether these nomograms are useful for the detection of potential states of thymic involution in pathological singleton and twin pregnancies.


Fetal thymus size in twin and singleton pregnancies

ACKNOWLEDGMENTS This work was supported in part by the Spanish Fondo de Investigaciones Sanitarias (FIS# PI081712). The authors are grateful to Ms Becky Uhlman for editorial assistance and Mr David DeLeon for assistance with graphics.

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