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Keywords:

  • congenital anomaly;
  • fetal US;
  • renal agenesis;
  • renal ectopy

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES

Objective

To assess the usefulness of sonographic evaluation of compensatory hyperplasia of the contralateral kidney for the differential diagnosis of fetal unilateral empty renal fossa (ERF).

Methods

We retrospectively measured the ratio of the anteroposterior (AP) and transverse (TR) diameters of the contralateral kidney in 24 fetuses with unilateral ERF including 12 cases of unilateral renal agenesis, six cases of a unilateral pelvic kidney and six cases of a unilateral ectopic multicystic dysplastic kidney (MCDK). For the normal reference value, we calculated the AP : TR diameter ratios of both kidneys in 20 normal fetuses in the second and third trimesters. We calculated the accuracy of the AP : TR ratio to detect contralateral compensatory hyperplasia.

Results

The median (range) AP : TR diameter ratios of the normal kidneys were 0.84 (0.72–0.89) in the second trimester and 0.81 (0.65–0.89) in the third trimester. All cases of unilateral renal agenesis and ectopic MCDK showed the presence of compensatory hyperplasia, while no case of renal ectopy was seen with compensatory hyperplasia. The median (range) ratios for unilateral renal agenesis and ectopic MCDK were 1.0 (0.95–1.02) and 1.1 (1–1.25) in the second and third trimesters, respectively. The median ratios for a unilateral pelvic kidney were 0.8 (0.74–0.85) and 0.77 (0.74–0.84) in the second and third trimesters, respectively. Using 0.9 as the discriminating value, the sensitivity, specificity, and accuracy of the AP : TR diameter ratio for the prenatal diagnosis of compensatory renal hyperplasia was 100%.

Conclusion

Sonographic evaluation of compensatory hyperplasia of the contralateral kidney may be useful for the differential diagnosis of a fetal unilateral ERF. Prospective studies are warranted. Copyright © 2009 ISUOG. Published by John Wiley & Sons, Ltd.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES

When a kidney is not seen in its normal location on sonography, the situation is defined as an empty renal fossa (ERF). The prevalence of ERF in fetuses has been reported as being 2.9%1. The most common cause of ERF is either renal ectopia or agenesis2, 3. Patients with unilateral renal agenesis or renal ectopia are usually asymptomatic, but unilateral renal agenesis may appear as a finding in a fetal syndrome such as VACTERL association and its occurrence in patients with deletions in chromosome 22q11.2 is 10-fold greater than that in the general population4. Renal ectopy has an increased risk for complications such as hydronephrosis, infection and calculus formation. Early detection and the differential diagnoses of these anomalies are helpful for prenatal and postnatal management and for counseling parents. However, these anomalies are challenging in many cases, especially in the second trimester. A sonographic finding of unilateral ERF occurs in both renal ectopia and agenesis, and detection of an abnormal location of a kidney is important to distinguish between the two. However, a pelvic kidney, which is the most common type of ectopic kidney, is not easily detected on prenatal sonography due to the surrounding bowel loops and the limited resolution of this technique, especially during the second trimester.

It is well known that compensatory hyperplasia of the remaining kidney usually occurs after unilateral nephrectomy, and there have been several reports describing its mechanism and physiology in animals5–10. Kaufman et al.11 and Celsi et al.12 reported that compensatory renal hyperplasia is age-dependent, being more prevalent in young than in adult animals. There has also been an experimental report of compensatory renal hyperplasia that occurred in an ovine fetus13. Compensatory hyperplasia of one kidney may occur in human fetuses when the other kidney is absent or has no renal function, such as in cases of unilateral renal agenesis and ectopic multicystic dysplastic kidney (MCDK). Thus, compensatory hyperplasia of the contralateral kidney may be a useful sonographic finding with which to distinguish these anomalies from renal anomalies with two functioning kidneys, such as unilateral ectopic kidney.

Compensatory renal hyperplasia may manifest as an increase in renal length. However, the increase may not be remarkable, especially in the second trimester, and renal length measurement may not be accurate in many cases due to the lack of definition of the margins caused by the similarity between renal tissue and the adrenal gland and surrounding bowel loops. Hyperplasia also causes an increase in the anteroposterior (AP) diameter of the kidney, causing it to have a globular appearance on axial images. Compensatory renal hyperplasia has been evaluated by measurement of the renal AP diameter14. However, fetal renal size may vary according to parameters such as fetal age and size, while a ratio of diameters, such as the AP : transverse (TR) diameters, is less likely to vary.

The purpose of this study, therefore, was to evaluate the usefulness of sonographic evaluation of contralateral compensatory renal hyperplasia by measurement of the AP : TR diameter ratio for the differential diagnosis of fetal unilateral ERF.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES

This was a retrospective study of fetuses with unilateral ERF diagnosed prenatally at Seoul National University Hospital or Cheil General Hospital from 2002 to 2007. The research ethics board approved the study and the informed consent requirement for patients was waived. We searched the hospital databases and reviewed the prenatal ultrasound images, selecting cases that had images appropriate for the measurement of the AP and TR diameters of the contralateral kidney. The inclusion criteria for images were: symmetrical, fetus in prone position, axial image of the contralateral normal kidney, with pelvis clearly visualized; fetal spine symmetrically located near the midline of the image. Asymmetrical or oblique coronal images were excluded.

We identified 47 cases of unilateral ERF from our database. Twenty-four cases, including 12 cases of unilateral renal agenesis, six cases of unilateral pelvic kidney and six cases of unilateral ectopic MCDK satisfied the inclusion ciriteria and were included in this study. All cases were confirmed by postnatal sonography. Sixteen fetuses underwent sonographic examinations during both the second (20–24 weeks) and third (28–36 weeks) trimesters, seven fetuses underwent only a third-trimester sonographic examination and one fetus underwent sonography during only the second trimester. We divided the fetuses into two groups, one consisting of those cases with a single functioning kidney, which included subjects with agenesis or ectopic MCDK, and the other consisting of cases with two functioning kidneys, which included subjects with a pelvic kidney.

The original sonographic examinations were performed by four experienced sonologists using three kinds of ultrasound machines, a LOGIQ 500 and MR700 (GE Medical Systems, Milwaukee, WI, USA) and an HDI 5000 (Philips Medical Systems, Cleveland, OH, USA). One experienced radiologist (J.Y.C.) measured the AP and TR diameters on the digitalized images using a picture archiving and communication system (PACS) workstation (m-view, Marotec, Seoul, Korea). The maximum TR diameter of the kidney was measured first, followed by the perpendicular AP diameter, as shown in Figure 1. The mean value of three measurements was obtained and the ratio of these mean AP : TR diameters was calculated for each measurement.

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Figure 1. Ultrasound images showing measurement of the renal anteroposterior : transverse (AP : TR) diameter ratio in a 34-week fetus with a unilateral ectopic kidney. (a) Axial image of the fetal abdomen showing the empty left renal fossa. The AP : TR diameter ratio of the right kidney (RK) was calculated as 0.76. (b) Sagittal image of the fetal abdomen demonstrating the presence of the ectopic left kidney (arrow) at the superoposterior portion of the urinary bladder (BL).

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To establish normal reference values, we measured the AP : TR diameter ratios of bilateral kidneys in 20 normal fetuses in the second trimester (20–26 weeks) and in another 20 fetuses in the third trimester (28–36 weeks) using the same method. All fetuses were confirmed to be normal by a postnatal physical examination. We compared the ratio of the second- and third-trimester AP : TR diameters in fetuses with a unilateral ERF and in normal fetuses using the unpaired t-test. We calculated the accuracy of the AP : TR diameter ratio to determine contralateral compensatory hyperplasia in fetuses with unilateral ERF.

The prenatal ultrasound findings, postnatal radiological findings and medical records were reviewed to assess any accompanied anomalies in the fetuses with unilateral ERF kidneys.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES

The median AP : TR diameter ratios of the bilateral normal kidneys and the contralateral kidneys in fetuses with a unilateral ERF are summarized in Table 1. The overall mean AP : TR diameter ratios of the bilateral normal kidneys were 0.84 (0.72–0.89) for the second trimester and 0.81 (0.65–0.89) for the third trimester (Figure 2). There was no significant difference in the AP : TR diameter ratios between bilateral kidneys. The median AP : TR diameter ratios of the contralateral kidneys in the group with a single functioning kidney were 1.0 (0.95–1.02) and 1.10 (1.00–1.25) in the second and third trimesters, respectively. The median AP : TR diameter ratios in the group with two functioning kidneys were 0.80 (0.74–0.85) and 0.77 (0.74–0.84) in the second and third trimesters, respectively. The AP : TR diameter ratios in fetuses with renal agenesis and an ectopic MCDK were significantly higher (P < 0.05) than those in fetuses with ectopic kidneys for both the second and the third trimesters. There was no significant difference between the AP : TR diameter ratios of normal and ectopic kidneys.

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Figure 2. Anteroposterior : transverse (AP : TR) diameter ratios of bilateral normal fetal kidneys measured during the second and third trimesters. ▴, right kidney; □, left kidney.

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Table 1. Anteroposterior : transverse (AP : TR) diameter ratios of bilateral normal kidneys and contralateral kidneys in fetuses with a unilateral empty renal fossa
 AP : TR diameter ratio (median (range))
 Second trimesterThird trimester
  1. MCDK, multicystic dysplastic kidney.

Right kidney in normal fetus (n = 20)0.84 (0.72–0.88)0.80 (0.65–0.89)
Left kidney in normal fetus (n = 20)0.85 (0.75–0.89)0.82 (0.67–0.87)
Renal agenesis + ectopic MCDK (n = 18)1.00 (0.95–1.02)1.10 (1.00–1.25)
Renal ectopy (n = 6)0.80 (0.74–0.85)0.77 (0.74–0.84)

All cases of unilateral renal agenesis (Figure 3) and ectopic MCDK (Figure 4) showed compensatory hyperplasia (i.e. a AP : TR diameter ratio significantly different from normal), while no case of renal ectopy (Figure 1) showed compensatory hyperplasia. Using 0.9 as the AP : TR diameter ratio cut-off gave 100% sensitivity, specificity and accuracy of the AP : TR diameter ratio for a prenatal diagnosis of compensatory renal hyperplasia, which may suggest the absence of the opposite functioning kidney, such in agenesis and ectopic MCDK (Figure 5).

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Figure 3. Ultrasound images showing measurement of the renal anteroposterior : transverse (AP : TR) diameter ratio in a 35-week fetus with unilateral renal agenesis. (a) Axial image of the fetal abdomen showing the empty right renal fossa. The AP diameter of the left kidney was increased and the AP : TR diameter ratio was calculated as 1.07. (b) Sagittal image of the left kidney (arrow) demonstrating an increased AP diameter and mildly prominent renal cortex compared with a normal fetal kidney.

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Figure 4. Ultrasound images showing measurement of the renal anteroposterior : transverse (AP : TR) diameter ratio in a 35-week fetus with a unilateral ectopic multicystic dysplastic kidney. (a) Axial image of the fetal abdomen showing the empty left renal fossa. The AP diameter of the right kidney (RK) was increased and the AP : TR diameter ratio was calculated as 1.02. (b) Coronal image of the fetal abdomen demonstrating the presence of a small left kidney (arrow) with cystic appearance at the superoposterior portion of the urinary bladder (BL).

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Figure 5. Anteroposterior : transverse (AP : TR) diameter ratios of contralateral kidneys in fetuses with unilateral renal agenesis (♦), ectopic kidney (▴) and ectopic multicystic dysplastic kidney (MCDK) (□). The AP : TR diameter ratios in fetuses with renal agenesis and an ectopic MCDK were elevated compared with normal controls (see Figure 2), unlike the ratios in fetuses with an ectopic kidney.

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The mean maternal age for the 24 cases with a unilateral ERF was 31 (range, 22–38) years. There were 13 male and 11 female fetuses. Twenty fetuses had no other abnormality as seen on prenatal sonography. One fetus with renal agenesis had a small cystic lesion at the posterior upper portion of the urinary bladder, which was confirmed as a seminal vesicle cyst on postnatal imaging. One fetus with renal agenesis and one fetus with an ectopic kidney had a single umbilical artery. Complex heart anomalies, including a double outlet right ventricle, ventricular septal defect and severe pulmonary stenosis, were detected in one fetus with a unilateral ectopic MCDK.

Postnatal sonography of 23 neonates revealed the same renal anomalies that were detected on prenatal sonography. Associated findings detected on prenatal sonography were also visible on postnatal imaging studies, and several additional findings were detected during the postnatal period. One fetus with unilateral renal agenesis with a single umbilical artery as seen on prenatal sonography had a trachea–esophageal fistula and an imperforate anus as depicted on postnatal images, and a diagnosis of VACTERL association was made. Termination of pregnancy was performed at week 24 in the fetus with unilateral ectopic MCDK agenesis and complex heart anomalies. The other 22 neonates showed a good outcome, with no chromosomal abnormalities or other congenital syndromes.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES

To determine the extent of renal compensatory hyperplasia on prenatal sonographic images, standard charts of the normal kidney according to fetal growth are necessary. There have been several reports describing the measurement of fetal kidneys15–20. Chitty and Altman19 published charts of the length and AP and TR diameters of normal fetal kidneys according to gestational age. Although they did not measure the AP : TR diameter ratio for each fetus, the calculated ratios as determined from the mean AP and TR diameters were approximately 1.0 between 22 and 38 gestational weeks. Konje et al.20 also performed a cross-sectional study of differences in fetal renal size. Calculated ratios from the mean AP and TR diameters ranged from 0.85 to 0.92 between 22 and 38 gestational weeks. In our study, the median AP : TR diameter ratio in the control group of subjects was approximately 0.83 at the second trimester and 0.80 at the third trimester. The difference of the AP : TR diameter ratios among these three studies cannot be fully explained, but we believe that the findings in our study may be closer to the true AP : TR diameter ratios as we measured the AP : TR diameter ratio for each fetus individually.

Although there have been several reports about the mechanism and physiology of compensatory renal hyperplasia5–10, there has been no report about the sonographic measurement or estimation of compensatory hyperplasia in fetal kidneys. Compensatory hyperplasia is usually determined when the length of the contralateral kidney is larger than the normal range in patients who have undergone a unilateral nephrectomy. We are usually not concerned with an increase of the AP diameter and the globular appearance of the kidney, although this may be detected in almost all cases and is a typical finding in cases of compensatory renal hyperplasia. The greater increase in the renal AP diameter compared with the TR diameter in fetuses with compensatory hyperplasia might result from the existence of the renal hilum: the anterior and posterior parts of a fetal kidney have renal parenchyma and hypertrophy occurs in both directions, while the medial part has no parenchyma due to the presence of the renal hilum and hypertrophy in a medial direction cannot occur.

During fetal life, the increase in renal length due to compensatory hyperplasia may be not remarkable, especially during the second trimester, and renal length measurement may not be accurate in many cases. Measurement of the AP and TR diameters of the fetal kidney is usually more accurate as the outer margin of the fetal kidney is more clearly defined on axial images. Konje et al.20 compared renal size in fetuses that were appropriate and small-for-gestational age (SGA). They reported that the AP and TR renal diameters were somewhat smaller in the SGA compared with normal fetuses and the differences manifested as early as 26 gestational weeks. The renal lengths in the SGA fetuses were not significantly smaller than those in the normal fetuses in the study. These results may support the idea that longitudinal measurement of the kidney does not always correlate with the AP and TR diameters.

Although we have reported the measurement of the AP : TR diameter ratio in fetuses with a unilateral ERF, we believe it is not necessary to obtain an exact measurement of the contralateral kidney in clinical practice. The globular appearance of this kidney due to compensatory hyperplasia may be readily observed on sonography, which suggests the absence of function of the opposite kidney such as in cases of agenesis or an ectopic MCDK. The finding of an apparently normal contralateral kidney, without compensatory hyperplasia, in a fetus with a unilateral ERF suggests that the other functioning kidney is located elsewhere, and the possibility of an ectopic kidney should be investigated.

There are several limitations to this study. It was retrospective, analyzing images obtained by different examiners using different equipment, and so reproducibility could not be assessed. The numbers of cases, especially cases with an ectopic kidney, were insufficient for statistical power. In addition, we had insufficient numbers in the normal control group to obtain a standard chart of the AP : TR diameter ratios of fetal kidneys according to gestational age.

In summary, fetal compensatory renal hyperplasia may be detected accurately by the finding of an increased AP : TR diameter ratio as measured on axial sonographic images. The finding may suggest absence of the opposite kidney, or presence of a non-functioning kidney, such as in cases of agenesis or ectopic MCDK. Sonographic evaluation of compensatory hyperplasia of the contralateral kidney may be useful in the differential diagnosis of fetal unilateral ERF.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES

This study was supported by the research fund of the Radiological Research Foundation of Korea (2008-05).

REFERENCES

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES