Synonyms: Leiomyomatous hamartoma, mesenchymal hamartoma of the kidney, renal hamartoma, fibroma of the kidney.

Definition: Solid tumor of the kidney composed of immature mesen­chymal cells and renal components.

Prevalence: Rare, about 120 cases reported in the neonatal period with 13 cases described in-utero.

Etiology: Unknown.

Pathogenesis: Suggested to arise from the metanephric blastema or from secondary mesenchyme.

Associated anomalies: In about 14%, including genitourinary and gastrointestinal anomalies, hydrocephalus, polydactyly.

Differential diagnosis: Wilms" tumor.

Prognosis: Favorable.

Recurrence risk: Not known to be increased.

Management: Postnatal nephrectomy.

MESH Mesonephroma-diagnosis, -embryology, -etiology, -pathology; Neoplasms, Embryonal and mixed embryo­logy; -pathology  ICD9 753.3 CDC 753.360 MIM 267000 BDE 2241 POS 3648

Address correspondence to: Andreas Rempen, MD, University   Clinic of Obstetrics and Gynecology, Josef-Schneider-Str.4, D-8700 Würzburg, Germany, Ph 49-931-201-3621, Fax 49-931-201-3406 ¶Pathology Institute, §Pediatric Clinic, ºPediatric Surgery

Introduction

Congenital abnormalities of the kidney frequently detected by prenatal sonography are commonly cystic lesions such as hydronephrosis or cystic kidneys^1,2. Solid tumors in the renal fossa, however, are more difficult to distinguish from the neighboring viscera, which may exhibit a quite similar echogenicity. This report describes the prenatal appearance of a congenital mesoblastic nephroma including Doppler ultrasound studies. The condition was accompanied by polyhydramnios, which serves as an indicative sign of the lesion.

Case report

The 34-year-old primigravida was referred at 26 weeks" gestation because of polyhydramnios. Obstetric history was otherwise unremarkable.  The ultrasound examination confirmed gross polyhydramnios and a single female fetus. A solid tumor was located in the abdomen along the left side of the spine crossing the midline and expanding ventrally (fig. 1).

Fig. 1: Left-sided solid renal tumor with accompanying polyhydramnios at 27 weeks. A small stomach (black arrow) and bladder (b) are visible.

The mass measured 36 x 48 x 53 mm. The tumor appeared to be lobated and its internal echoes exhibited an inhomogeneous pattern with echogenic and echo-poor areas (fig. 2).

Fig. 2: The mass is inhomogeneous with well-defined and gently undulating borders.

It was surrounded by a gently undulated and well-defined margin. The shape of the tumor resembled a grossly enlarged kidney, but echo-structures of renal pelvis were not discernible. Color Doppler showed a diffuse vascularization predominantly at the periphery of the tumor. Pulsed-wave Doppler studies within the organ revealed elevated diastolic signals with decreased pulsatility indices as opposed to values in unaffected kidneys (fig. 3).

Fig. 3: Pulsed-wave Doppler ultrasound of arterial vessel within the renal tumor showing a low pulsatility index (PI=1.28).

The left kidney appeared normal. The urinary bladder was filled but distorted by the tumor. The stomach was very small and seemed to be compressed, but other fetal malformations were not present. Fetal biometry was within the normal range.

Persistent uterine contractions prompted tocolysis with b- mimetics and induction of lung maturation with steroids. Repeated transabdominal drainage of amniotic fluid was necessary to relieve maternal abdominal pain and dyspnea due to increasing polyhydramnios. Culture from amniotic cells revealed a normal karyotype (46,XX).

At 29 weeks the fetus showed an appropriate growth, but the tumor had also increased in size, now measuring 50 x 55 x 71 mm. Doppler ultrasound revealed normal resistance and pulsatility indices in the fetal aorta and umbilical artery. No signs of heart failure were present.

Three days later the membranes ruptured followed by late decelerations of fetal heart rate. Since the cervix was closed, a cesarean section was performed, and a premature girl weighing 1,690g was delivered with an Apgar score of 2, 4 and 6 at 1, 5 and 10 minutes, respectively. The child was immediately intubated. The maturity index according to Farr was estimated to be 29 weeks. A bulky firm mass was palpated on the left side of the infant"s abdomen. Severe respiratory distress was treated with multiple doses of surfactant.

Surgery

On the third postnatal day, surgery was performed. A 110g solid tumor was extirpated. Two weeks later, sigmoid perforation with consecutive peritonitis due to necrotizing enterocolitis necessitated a second operation with resection of a small part of the sigmoid and a temporary colostomy. The infant developed bronchopulmonary dysplasia and was treated with steroids. The girl was discharged at the age of 11 weeks and is now 18 months old and in good condition.

Pathology

On pathological examination the tumor appeared trabeculated with a yellowish cut-surface and no apparent areas of necrosis or hemorrhage (fig. 4).

Fig. 4: Gross section of congenital mesoblastic nephroma with a trabeculated surface similar to a leiomyoma. The ureter is seen on the right side.

The histological findings corresponded to congenital mesoblastic nephroma (fig. 5).

Fig. 5: Histology of congenital mesoblastic nephroma showing (top) proliferation of spindle cells with entrapment of normal immature tubules and glomeruli (HE, x240) and (bottom) monomorph appearance and few mitoses of the spindle cells (HE, x600).

In the marginal area of the mass, angiomatous structures were seen. There were only small parts of renal parenchyma left at the periphery. The tumor expanded to the renal pelvis, which was compressed to a small slit. There was no invasion to adjacent structures.

Discussion

Prevalence

Congenital mesoblastic nephroma, although rare, is the most common renal neoplasm in the first months of life. About 120 cases in the neonatal period have been reported, and 13 cases seen in utero have been found in the literature^3-15 (Table 1).

Review of the literature.

Pathogenesis

The histogenesis of congenital mesoblastic nephroma is controversial. It is believed either to arise from the metanephric blastema¹⁶ or to originate from secondary mesenchyme¹⁷.

Pathology

This predominantly mesenchymal, often circumscribed tumor is a usually benign neoplasm distinct from Wilms" tumor. The weight ranges from 21 to 1,889g^11,18 and diameters between 0.8 and 14 cm¹¹. On cut-surface it is whorled, firm, rubbery and yellowish- gray but may show pseudocystic areas due to necrosis and hemorrhage. Histological features consist of bundles of spindle cells resembling fibroblasts and smooth muscle cells, incorporating scattered tubules and glomeruli, focal clusters of vascular structures, sporadic calcifications and islands of hematopoetic components. Increased mitotic figures may occur. Unlike Wilms" tumor, it does not have a capsule. It is contiguous with normal kidney tissue and can infiltrate the adjacent tissue.

Antenatal diagnosis

Table 1 comprises twelve reported cases that have been studied with ultrasound during pregnancy^3-11,13-15 along with our current observation. On prenatal sonography, congenital mesoblastic nephroma usually presents as a paravertebral abdominal non-cystic unilateral mass often with a low-level echogenicity and an inhomogeneous echo pattern. Highly echodense regions can occur. Some small echo-free areas may represent hemorrhage and necrotic tissue, as reported in cases observed postnatally^19,20. Mesoblastic nephroma can partly show a lobation with linear demarcations indenting the surface and interlobar grooves as described in normal kidneys²¹.

A well-defined border around the tumor may represent the interface between the lesion and the adjacent tissue, because histologically it has no capsule. The tumor can also show an indistinct outline⁸. The space-occupying process may distort the shape of the bladder and stomach, and the dimensions of the fetal trunk may be disproportionally large^9,13, depending on the size of the expanding tumor.

Doppler

In our case, Doppler demonstrated a diffuse vascularization with elevated diastolic frequencies and decreased pulsatility indices, suggesting a reduced vascular resistance within the tumor. This corresponds well with reported results of angiography which disclosed congenital mesoblastic nephroma as a hypervascular mass²⁰. Given large arteriovenous shunts, cardiac failure with consecutive hydrops may occur prenatally²². Thus, cardiac insufficiency has been observed in a newborn with congenital meso­blastic nephroma²³.

Associated anomalies

All pregnancies in Table 1 developed polyhydramnios, which can serve therefore as an indicative symptom. Out of 14 other case reports of congenital meso­blastic nephroma in neonates with references to pregnancy complications, polyhydramnios occurred in ten²⁴. Since amniotic fluid volume is regulated mainly by fetal swallowing and voiding²⁵, it is suggested that polyhydramnios resulted from excessive fetal urine production or decreased fluid absorption.

Increased renal blood flow (that was likely in our case) and impaired renal concentrating ability²⁶ may be reasons for fetal polyuria¹⁵. Displacement and compression of the viscera by the huge renal tumor may cause mechanical obstruction of the intestine, interfering with amniotic fluid absorption.

Differential diagnosis

Sonography cannot provide a clear distinction between congenital mesoblastic nephroma and nephroblastoma (Wilms" tumor) because both are essentially histological diagnoses. Although Wilm"s tumor was observed in the neonatal period²⁷, we are unaware of a report with prenatal detection.

In diffuse nephromatosis²⁸, both kidneys are involved, which can show acoustic shadowing due to calcification. Infantile polycystic kidney disease  can be recognized by nonvisualization of the bladder, oligohydramnios and bilaterally enlarged echogenic kidneys. Adult polycystic kidney disease can be presumed in the presence of family history. Kidney enlargement in other inherited disorders such as Meckel syndrome is usually bilateral.

In case of cystic areas, unilateral enlarged dysplastic kidneys can usually be diagnosed by their primarily sonolucent appearance, with cysts diffusely disseminated within the organ. Solid tumors such as neuroblastoma of adrenal gland or extrathoracic pulmonary sequestration  can usually be separated from the normal-appearing kidney.

Obstetric management

Polyhydramnios may lead to uterine contractions, premature rupture of membranes and preterm delivery. Furthermore, it can cause abdominal and rib pain or dyspnea due to overdistension of the uterus. Suppression of uterine contractions may be obtained by tocolytic agents like b-mimetic drugs. In case of gross polyhydramnios, therapeutic amniocentesis may additionally be used. Because amniotic fluid will usually reaccumulate, serial punctures are commonly necessary.

The visualization of a renal mass should lead to a careful sonographic search for accompanying abnormalities¹⁸ and to serial controls to document growth of the tumor and to monitor fetal development. In case of cardiac compromise of the fetus, early delivery may be necessary to prevent fetal demise. Although most prenatally detected fetuses with congenital mesoblastic nephroma were delivered transabdominally because of coexisting complications (Table 1), in our opinion, the lesion per se is not an indication for primary cesarean section in an otherwise not compromised fetus unless dystocia due to an extreme tumor size is present.

Therapy and prognosis

The therapy of choice is resection of the tumor in the neonatal period^16,18. Although most patients can be cured by nephrectomy alone, follow-up is mandatory, as the condition can recur.

References

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