Normal Renal Echogenicity

Introduction

The differential diagnosis of normal sized or enlarged echogenic kidneys is quite large and diverse. In order to narrow the differential diagnosis, renal architecture, renal size, cortico-medullary differentiation, associated abnormalities and amniotic fluid volume must all be assessed.

Normal Renal Echogenicity

The normal echogenicity of the fetal kidney is equal to that of the liver (Figs. 1 and 2). Since this is a subjective assessment, it may have significant intra and interobserver variation. Generally, fetal renal echogenicity refers to the appearance of the entire kidney rather than to an isolated component of the kidney, i.e. the cortex, medulla or pyramids.

Fig 1. Normal renal echogenicity. Click for larger image.

Fig 2. Normal transverse image of kidneys at 38 weeks gestation. Click for larger image.

The primary ultrasound parameter that may significantly affect renal echogenicity is gain. Hence, the subjective assessment of renal echogenicity should only be made when the gain setting is appropriate¹.

The glomeruli and tubules in the renal cortex provide the interfaces that contribute to renal echogenicity. The absence of glomeruli in the medulla, as well as the radial arrangement of the tubules results in the lower echogenicity of the renal medulla and the normal corticomedullary differentiation¹.

Physiologic Echogenic Kidneys

Transient renal medullary echogenicity in neonates has been attributed to the deposition of protein casts within the tubules and is considered a normal variant2,3. The etiology for increased renal echogenicity in normal fetal kidneys has not been determined (Fig. 3).

Fig 3. Bilateral echogenic kidneys in a normal fetus. Click for larger image.

Echogenic Kidneys

Glomerulocystic kidney disease encompasses a number of renal abnormalities that have cystic glomeruli. It is also found after renal obstruction. The glomerulocystic diseases outlined on Table I affects both fetuses and neonate.

Table I. Glomerulocystic disease.

• Autosomal dominant polycystic disease
• Trisomy 13
• Trisomy 18
• Obstructive uropathy
• Short-rib polydactyly
• Meckel-Gruber syndrome
• Bardet Biedl syndrome
• Derived from: Bissler JJ et al. Pediatr Nephrol 2010;25:2049.

The differential diagnosis of neonatal echogenic kidneys (Table II) cannot be extrapolated to the fetus (Table III). In one study only 5% of neonate with echogenic kidneys had no evidence of renal disease⁴.

Table II. Etiology of echogenic kidneys in children.

• Renal dysplasia
• Acute glomerulonephritis
• Chronic glomerulonephritis
• Acute tubular necrosis
• Hemolytic-uremc syndrome
• Chronic renal failure
• Nephrotic syndrome
• Glycogen-storage disease
• Sickle cell anemia
• Polycystic renal disease
  • Infantile
  • Adult
• Derived from: Kransky AM et al. Pediatrics 1983;72:840; Kraus RA et al. Radiographics 1990;10:1009-1018.

Table III. Echogenic fetal kidneys.

• Physiologic variation
• Intrinsic renal disease
  • Autosomal dominant polycystic renal disease
  • Autosomal recessive polycystic renal disease
• Obstructive uropathy
• Aneuploidy
  • Trisomy 13
  • Trisomy 18
• Infection
  • Cytomegalovirus
  • Candida
• Overgrowth Syndromes
  • Beckwith-Wiedemann
  • Perlman
  • Simpson-Golabi-Behmael
• Meckel-Gruber
• Bardet-Biedl syndrome
• Renal vein thrombosis
• Congenital nephrotic syndrome

In acute renal disease, interstitial cellular infiltrates may result in increased renal echogenicity. With chronic renal disease, fibrosis and scarring give rise to increased echogenicity of the kidneys^1,4. Hence, renal echogenicity is a multifactorial response to various alterations in the structural development of the kidney. While diffuse renal echogenicity generally correlates with severe renal disease, it neither indicates the type of pathology or the severity of the disease⁵. The same caveats extend to echogenic fetal kidneys.

Autosomal Dominant Polycystic Kidney Disease (ADPKD)

This is the most common hereditary renal cystic disease with an incidence of 1/1000. It is usually manifested in adults between 35 and 40 years of age. Prenatal detection has been documented since 1982. However, the diagnosis is often made after 28 weeks' gestation. In general, prenatal ultrasound lacks both sensitivity and specificity. Approximately 22% of ADPKD is diagnosed prior to 10 years of age⁶. The initial nephron formation is normal. Overtime, cystic dilatation of the nephrons and collecting ducts occur. These pathologic findings have been detected in a 14 week fetus⁷. Occasionally, small prenatal cortical cysts will be detected sonographically. The size and number of cysts continue to increase into adulthood. The renal cysts are associated with enlarged echogenic kidneys and a loss of corticomedullary differentiation. The amniotic fluid volume is normal. The prenatal detection of ADPKD is frequently associated with maternal transmission and the onset of new mutations in the parent^8,9.

A family history is critical in the differentiation of ADPKD from other potential diagnoses that have echogenic kidneys. However, in a third of cases detected antenatally, the affected parent is not aware of the disease prior to pregnancy⁷.

Autosomal Recessive Polycystic Kidney Disease (ARPKD)

The incidence of ARPKD is approximately 1/40,000¹⁰. Since it is autosomal recessive the recurrence risk is 25%. Variable expression of ARPKD has been reported within families¹¹. The sonographic appearance of enlarged echogenic kidneys with a loss of corticomedullary differentiation is due to cystic dilatation of the collecting tubules¹²(Fig. 4). However, not all cases of ARPKD will have this classic appearance. Herman and Siegel¹³ have described a case of autosomal recessive polycystic kidney disease with enlarged kidneys and only pyramidal hyperechogenicity. A normal reniform shape is maintained with ARPKD.

Fig 4. Autosomal recessive polycystic kidneys (arrows). Click for larger image.

The earlier ARPKD presents, the worse the prognosis. Although there are case reports of 1st trimester diagnoses based upon renal enlargement and echogenicity, this is the exception rather than the rule¹⁴. A 20 week ultrasound examination may be entirely normal. The proportion of dilated renal tubules determines the time of ARPKD presentation, i.e. perinatal, neonatal, infantile or juvenile¹⁵. The perinatal type of ARPKD that can be diagnosed antenatally is associated with late 2nd and 3rd trimester oligohydramnios and early neonatal demise secondary to pulmonary hypoplasia or renal failure. However, there are case reports of fetuses with ARPKD and 3rd trimester oligohydramnios who have survived with normal renal function11. Perinatal ARPKD may have normal amniotic fluid¹⁵.

Transient nephromegaly can simulate infantile polycystic kidney disease in the immediate neonatal period¹⁶.

In those neonates who survive the perinatal period, tubular atrophy and interstitial fibrosis results in a reduction in kidney size. The subsequent development of cortical cysts may have the appearance of autosomal dominant polycystic renal disease¹⁷. ARPKD tends to stabilize in surviving patients with the 1st year of life being the period of highest risk. Autosomal recessive polycystic kidney disease is still a major cause of infant renal failure¹⁸.

Fetal Obstructive Uropathy

Increased renal echogenicity is a non-specific response to changes in the architectural integrity of the kidney. In cases of lower urinary tract obstruction the increased echogenicity has been shown to be due to a reduction in glomerular number, disordered growth and differentiation of the kidney, and an increase in interstitial fibrosis (Fig. 5). The onset of fibrosis indicates that renal damage is irreversible. Increased renal echogenicity with oligohydramnios is highly predictive of obstruction¹⁹. Normal amniotic fluid volume may still be present with renal fibrosis. Hence, a normal amniotic fluid volume does not exclude irreparable renal damage²⁰.

Fig 5. Echogenic kidneys due to lower urinary tract obstruction. Click for larger image.

Aneuploidy

Fetuses with aneuploidy usually have multiple congenital malformations. As the number of detectable congenital anomalies increases, so does the likelihood that the fetus will be karyotypically abnormal. When three congenital defects are detected, approximately 50% of fetuses have a karyotypic abnormality²¹. While not a common anomaly, enlarged echogenic kidneys are present in approximately 30% of trisomy 13 fetuses^22,23 and to a lesser degree with trisomy 18^24,25. The presence of renal microcysts, predominantly in the cortex of fetuses with trisomy 18²⁶ and trisomy 13²⁷ may explain the echogenicity noted on the ultrasound examinations.

Congenital Infection

Multiple reflecting surfaces due to tubular dilatation results in the normal sized echogenic kidneys that have been associated with fetal candida²⁸ and cytomegalovirus²⁹ infection.

Overgrowth Syndromes

Beckwith-Wiedemann syndrome is characterized by visceromegaly, macroglossia, polyhydramnios and enlarged echogenic kidneys with a loss of corticomedullary differentiation (Fig 6). This overgrowth syndrome also has a variety of possible congenital anomalies, including omphalocele, hemihypertrophy and cardiac anomalies. Neonates with this syndrome also have a predilection for neuroblastomas and Wilms tumors. Significant mental delay with Beckwith-Wiedemann syndrome is due to uncorrected neonatal hypoglycemia and is, therefore, preventable. Since the genetics of Beckwith-Wiedemann syndrome are complex, fetal screening is not currently available.

Fig 6. Enlarged echogenic kidney with Beckwith-Wiedemann syndrome. Click for larger image.

Perlman syndrome is an autosomal recessive disease with macrosomia, ascites, facial dysmorphology, polyhydramnios and bilaterally enlarged echogenic kidneys. The prognosis with Perlman syndrome is poor; most neonates die from pulmonary hypoplasia due to the enlarged kidneys, and marked ascites that compresses the diaphragm. When survival extends beyond 1 year of age, developmental delay is inevitable. Infants with Perlman syndrome have a predisposition for Wilms tumor^30,31. The fetal stigmata are similar to both Beckwith- Wiedemann and Simpson-Golabi-Behmael syndromes (SGBS).

Simpson-Golabi-Behmael syndrome is an x-linked overgrowth syndrome that consequently affects males, while females are carriers. In addition to the visceromegaly and echogenic kidneys, affected males have a protruding jaw and macroglossia. Other anomalies include congenital heart defects and diaphragmatic hernia. Intelligence is generally normal³².

Meckel-Gruber Syndrome

This is an autosomal recessive disorder characterized by an occipital encephalocele (Fig. 7), postaxial polydactyly (Fig. 8), and echogenic significantly enlarged kidneys. Of this triad of anomalies, polydactyly is present in 55% of cases. 2nd trimester oligohydramnios is also characteristic and prohibits successful identification of polydactyly in all cases evaluated sonographically. Since the amniotic fluid volume is still normal and the hands are characteristically extended, an ultrasound examination between 11-14 weeks' gestation may provide a more definitive diagnosis than either a 2nd or 3rd trimester study³³.

Fig 7. Occipital encephalocele (arrow). Click for larger image.

Fig 8. Post-axial polydactyl (arrow). Click for larger image.

Bardet-Biedl Syndrome (BBS)

This syndrome is an autosomal recessive disease with large echogenic kidneys, polydactyly, syndactyly, mental delay and short stature. Molecular diagnosis is not yet available. The incidence of BBS is approximately 1/125,000. The diffuse enlarged echogenic kidneys do not have normal cotico-medullary differentiation. Renal involvement occurs in 30-100% of cases. By one year of age renal size returns to normal and distinct corticomedullary cysts appear. The digital abnormalities associated with this syndrome are detected in approximately 70% of cases^34,35.

Renal Vein Thrombosis

Neonatal renal vein thrombosis has an incidence of approximately 0.5/1000 admissions to neonatal intensive care units³⁶. A renal vein thrombosis may be either unilateral or bilateral.

Neonatal and perinatal renal vein thrombosis is characteristically associated with diseases that can result in perinatal stress. Maternal conditions include diabetes mellitus, antiphospholipid antibodies, systemic lupus erythematous, and sickle cell disease. Congenital cytomegalovirus, non-immune hydrops, meconium peritonitis, and fetal distress have also been associated with renal vein thrombosis³⁷.

In the neonatal period, the classic triad for renal vein thrombosis includes an enlarged echogenic kidney, hematuria and thrombocytopenia. When antenatal renal vein thrombosis is unilateral, the diagnosis is based upon, not only nephromegaly and renal echogenicity, but also streaking or diffuse calcifications within the kidney. If renal vein thrombosis is bilateral, autosomal dominant or autosomal recessive polycystic renal disease, as well the other diagnoses outlined on Table III must be considered.

During the first week after the onset of renal vein thrombosis, there is an absence of renal venous flow, resulting in generalized renal enlargement. There is also increased renal echogenicity that is more prominent in the cortex. Echogenic streaking may be detected that follow the intralobular vessels. During the second week after renal vein thrombosis, there is a greater degree of renal echogenicity that obscures architectural landmarks. Corticomedullary differentiation is lost due to venous congestion and edema. In the third and subsequent weeks there is generally rapid venous collateralization or recanalization of the thrombis. In the most favorable cases, there is gradual renal recovery, with a return to a normal renal size and echogenicity over the first month after the thrombosis. However, renal failure and atrophy may also occur^36,37,38,39.

Congenital Nephrotic Syndrome

Congenital nephrotic syndrome of the Finnish type is an autosomal recessive disease with severe proteinuria and secondary hypoalbuminemia. The reported incidence is 1/10,000 livebirths. The kidneys are enlarged and contain diffuse punctuate echogenicities consistent with microcysts due to dilatation of the proximal convoluted tubules. The placenta is always enlarged, weighing > 25% of the birthweight⁴⁰. The in-utero proteinuria can result in non-immune hydrops.

In all types of congenital nephrotic syndrome, the maternal serum alpha fetoprotein level is markedly elevated. The severe proteinuria characteristic in nephrotic syndrome results in an increased excretion of alpha fetoprotein into the amniotic fluid, from which it enters the maternal circulation⁴¹.

The gene responsible for congenital nephrosis of the Finnish type is NPHS1. This is an autosomal recessive disease with about half of reported cases occurring in Finland. A high incidence of NPHS1 has also been reported among Mennonites in Lancaster County, Pennsylvania. For prenatal diagnosis in families with NPHS1, 1st trimester chorionic villous sampling can be performed.

90% of the protein loss with congenital Finnish nephrosis is albumin. Because of the heavy protein loss, hypogammaglobulinemia also occurs with a secondary increased risk for infection. The only treatment is renal transplantation. However, infants frequently die from infection, electrolyte imbalance, malnutrition, or renal failure before renal transplantation is possible^40,41. Recurrence of nephrosis in the transplanted kidney may occur because of antibodies against the nephrin molecule⁴².

Fetal nephrotic syndrome may be secondary to congenital infection, i.e. cytomegalovirus, toxoplasmosis, or syphilis⁴⁰.

Pierson syndrome is an autosomal recessive disease with congenital nephrotic syndrome due to diffuse mesangial sclerosis. Neonatal demise is secondary to renal failure. Mark and co-workers⁴³ have reported a case with echogenic kidneys detected by 15 weeks' gestation.

Placentomegaly and hydrops may occur due to hypoproteinemia secondary to proteinuria. In other cases of Pierson syndrome without hydrops, fetal renal failure results in reduced protein excretion.