ECHOGENIC (HYPERECHOGENIC) BOWEL

 

  • Echogenic bowel is a well-defined, homogenous, hyperechogenic, lesion in the lower abdomen, which does not shadow (i.e. has no calcific areas), and usually resolves within a few weeks with no long term sequelae to the fetus (1,2).
  • Fetal bowel is considered hyperechoic if it is as echogenic as the adjacent bone (more echogenic than fetal liver).
  • Echogenic bowel in the third trimester is a relatively common finding with uncertain clinical significance
  • Although initially considered a normal variant in second-trimester fetuses, it has been described as a prenatal marker for cystic fibrosis, chromosomal aneuploidy and numerous other pathologies.
  • Prevalence = 0.2% to 1.8% (3).
  • This finding can be diffuse or focal.
  • Typically localized in the lower abdomen.
    • Nyberg and associates 1993 (2):
      • Hyperechoic bowel is present in 0.81% of all second trimester fetuses scanned, compared with 14.5% of fetuses with Down syndrome.
    • Bernacerraf an co-workers (11):
      • 0.6% incidence of hyperechoic bowel in the second trimester fetal population. Among 50 fetuses with hyperechoic bowel: 8(16%) were aneuploid (trisomy 21,6; Turners Syndrome,1; trisomy 13,1) and 8(16%) had severe IUGR
      • 12.5% of all second trimester fetuses with Down syndrome had hyperechoic bowel.
      • Hyperechogenic bowel therefore carried a 16-fold greater risk for Down syndrome.

 

ULTRASOUND

GRADING SYSTEM FOR ECHOGENIC BOWEL

 

Link to Ultrasound and Grading System for Echogenic Bowel

 

 

PATHO[HYSIOLOGY

 

The pathophysiology of increased bowel echogenicity is likely heterogeneous in nature. In most cases the increased echogenicity is thought to be due to:

         Abnormal, highly viscous meconium within the small bowel caused by obstruction (meconium ileus), poor bowel motility, or abnormal pancreatic enzymatic secretion (12).

         Alternatively, meconium peritonitis from a bowel perforation can cause subsequent bowel edema, which also makes the bowel appear echogenic (12) . If the meconium is confined to discrete sites of bowel perforation, the echogenicity appears as focal intra-abdominal calcifications (12) . Similar calcifications in the bowel have been associated with fetal infections, such as toxoplasmosis or cytomegalovirus (CMV), although the pathophysiology of this association is poorly understood.

         Focal areas of bowel echogenicity have also been attributed to areas of ischemia (12,13).

         Finally, bowel may appear echogenic because of the swallowing of blood, which is extremely echogenic (14).

 



CAUSES OF ECHOGENIC BOWEL

  • Fetal aneuploidy especially Trisomy 21 and less frequently trisomy 18 or 13, Turner's syndrome and triploidy. The cause of echogenic bowel in aneuploidy is less clear. It is thought to be due to decreased bowel motility with increased water absorption from the meconium. There appears to be decreased microvillar enzymes activity in the amniotic fluid of aneuploid fetuses (2). The association of echogenic bowel with aneuploidy, particularly trisomy 21, has been demonstrated in several studies (15-18).
    • A review of 11 published series of fetuses with echogenic bowel in which information was available about aneuploidy found that 11.8% had chromosomal abnormalities (12).
    • The risk of aneuploidy in a fetus with echogenic bowel is highest in the presence of additional sonographic abnormalities;
    • Echogenic bowel may be the only sonographically detected abnormality in an aneuploid fetus (18-20).
    • The relative risk of Down syndrome for fetuses with hyperechoic bowel has been reported to be 5.5 times the a priori risk (21). This association has been postulated to be caused by poor bowel motility resulting in increased water absorption and thickened meconium (22).
  • Cystic fibrosis associated with thick inspissated meconium (7-9). Meconium is more likely to be inspissated in the second trimester when the bowel lumen is narrow. This usually resolves as a result of enlarging small bowel and fetal swallowing of amniotic fluid. Echogenic bowel has been reported to be found on ultrasound in 50% to 78% of fetuses affected with CF (23,24).
    • The association of echogenic bowel with fetuses affected with CF is thought to be caused by changes in the consistency of meconium in the small intestine as a result of abnormalities in pancreatic enzyme secretion. This can result in detectible sonographic findings, such as diffuse echogenic bowel, focal echogenic bowel with calcifications, a hyperechoic mass, or bowel dilation (12,24,25).
    • These findings may appear as early as the second trimester (24,25).
    • CF has been reported to affect 0.8% to 13.3% of fetuses with echogenic bowel (20, 26-29, 34), markedly higher than the rate of CF expected in a white population in which the carrier frequency is 1 in 25.
    • As with any screening marker, echogenic bowel is most predictive of CF in populations at highest risk for CF. High-risk populations, however, are those that are most likely to be screened routinely for CF. There is some evidence that the detection of echogenic bowel in populations at low-risk for this disease does not increase the risk of CF when compared with the background risk.
  • Small bowel obstruction proximally (especially duodenal atresia) can produce hyperechogenic bowel by reducing the meconium fluid content (2,5).
  • Hirschsprung's disease (increased frequency in fetuses with Down syndrome) could produce hyperechogenic bowel due to hypoperistalsis.
  • Bowel atresia (6-8). Echogenic bowel is thought to be due to decreased amniotic fluid content of the meconium.
  • Intra-amniotic hemorrhage (1,11). Echogenic bowel is probably due to swallowed blood products resulting in a hypercellular meconium, probably with small clots within the bowel lumen. This is thought to be caused by fetal swallowing of blood, which is very echogenic.
    • One series found that 22% of fetuses with echogenic bowel had evidence of heme pigment in amniotic fluid (14).
    • In another series, 3.1% of amniotic fluid was grossly contaminated with blood (29).
    • Petrikovsky et al (30) examined 28 fetuses before and 12 hours after intrauterine transfusion, a procedure that commonly introduces blood into the amniotic cavity by postpuncture bleeding. Although none of the fetuses had echogenic bowel before intrauterine transfusion, 25% of these fetuses had evidence of bowel echogenicity within 12 hours of the bleeding episode and 18% still had evidence of echogenicity 2 weeks later (30).
    • In general, pregnancies with evidence of intra-amniotic bleeding but without additional anomalies have a good prognosis (18).
  • Oligohydramnios. Echogenic bowel is also thought to be due to decreased amniotic fluid content of meconium (2,5).
  • IUGR - Most fetuses with IUGR do not have echogenic bowel. The suggested mechanism is bowel ischemia due to hemodynamic redistribution and subsequent mesenteric ischemia is therefore questionable. Intrauterine fetal growth restriction has been estimated to complicate 4% to 18% of pregnancies with echogenic bowel, even in the setting of a normal karyotype (12,19,20,25). The association of echogenic bowel with IUGR may be caused in part by ischemia from redistribution of blood flow away from the gut (3).
    • The presence of IUGR or elevated maternal serum alpha-fetoprotein in the second trimester in association with echogenic bowel seems to be associated with a particularly poor fetal prognosis.
    • In one series, all six fetuses with both echogenic bowel and elevated maternal serum alpha-fetoprotein were growth restricted: four died in utero, one of two live-born infants died during the neonatal period, and the single survivor developed necrotizing enterocolitis requiring surgery (31) . This poor prognosis has been confirmed in other studies (18).
  • Poor perinatal outcome due to uteroplaental insufficiency, especially in cases in which the maternal serum alpha-fetoprotein concentration is elevated (2,5,6,9). As alpha-fetoprotein is a pure fetal product, cases with raised levels are likely to have experienced significant feto-maternal bleeding. This group may represent a subset of fetuses with severe placental damage.
  • Other less common associations.
    • Cytomegalovirus.
    • Toxoplasmosis.
    • Parvovirus.
    • Thalassemia The high frequency of echogenic bowel in first and second trimester fetuses suggests bowel wall edema, due to severe anemia and hypoxia may be the cause (10).

The association of congenital infections with echogenic bowel has been reported to be from 0% to 10% (20).

         The most commonly detected infectious agent is CMV

         Simon-Bouy et al (29) prospectively checked maternal rubella, toxoplasmosis, and CMV serologies (IgG and IgM) in 682 cases of fetal echogenic bowel. When seroconversion was observed, CMV polymerase chain reaction testing was performed in amniotic fluid. Parvovirus B19 polymerase chain reaction was also performed in all cases.

         A total of 19 viral infections were diagnosed, which represented 2.8% of fetuses: 15 (2.2%) CMV and 4 (0.6%) parvovirus.

         In 11 of the fetuses with CMV, echogenic bowel was the only sonographic abnormality noted.

         All four of the fetuses with parvovirus had associated abnormalities. It is unclear how a viral infection results in the echogenic appearance of the bowel. It may be caused by direct intestinal damage from inflammation or meconium peritonitis or indirectly by ascites, anemia, or growth restriction (12,32).

OUTCOME

 

 

Outcome of 682 cases of echogenic bowel

Outcome

N (%)

Termination of pregnancy (N)

IU

Fetal Demise (N)

Neonatal

death (N)

Normal healthy newborn

447 (65.5)

Chromosomal abnormality

29 (4.3)

21

2

0

     Trisomy 21

17 (2.5)

15

1

0

     Other

12 (1.8)

6

1

0

          Severe

7 (1)

6

1

0

          Less severe (a)

5 (0.7)

0

0

0

Cystic fibrosis

21 (3.1)

16

0

0

Infectious diseases

19 (2.8)

12

1

0

     Cytomegalovirus

15 (2.2)

12

1

0

     Parvovirus

4 (0.6)

0

0

0

     Toxoplasmosis

0 (0)

     Rubella

0 (0)

Intrauterine Growth Restriction

28 (4.1)

0

2

0

Intra-amniotic bleeding

21 (3.1)

NS

NS

NS

Unexplained IUFD

13 (1.9)

13

Associated structural abnormalities

47 (6.9)

17

2

3

     Gastrointestinal abnormalities

20 (2.9)

3

1

0

     Multiple visceral abnormalities

12 (1.8)

4

1

1

     Cardiac abnormalities

5 (0.7)

2

0

1

     Other

10 (1.5)

8

0

1

      (a) - Less severe includes Klinefelter syndrome, Robertsonian translocation, and mosaic trisomy X.

Simon-Bouy B, Satre V, Ferec C, Malinge MC, Girodon E, Denamur E, et al. Hyperechogenic fetal bowel: a large French Collaborative Study of 682 cases. Am J Med Genet 2003;121A(3):20913.

The largest series in the literature is a prospective collaborative series of 680 cases of fetuses with echogenic bowel referred for prenatal diagnosis . In the above series:

         hyperechoic bowel was observed in 0.1% of second-trimester ultrasounds.

         Chromosome abnormalities were observed in 29 (4.3%) cases, of which 24 (3.5%) were severe, including 17 (2.5%) of which were Down syndrome.

         In 11 (65%) of the cases with Down syndrome, echogenic bowel was the only sonographic abnormality noted.

         The risk of a severe chromosomal abnormality (including Down syndrome) when echogenic bowel was isolated or seen in conjunction with other abnormalities was 1.6% and 1.9%, respectively.

 

 

Table of Echogenic Bowel and Pregnancy Outcome

Ref

GA (wks)

Poor perinatal outcome (%)

Chromosomal defects (%)

Cystic fibrosis(%)

Infection(%)

IUGR (%)*

Perinatal

death(%)**

1

15-34

40

3

13

0

21

14

2

15-24

47

25

0

1

18

8

3

14-24

42

16

0

0

19

12

4

15-21

34

6

0

6

13

17

5

14-34

39

4

1

4

7

13

6

16-20

14

6

5

0

0

0

7

14-23

46

6

3

6

7

8

8

16-22

28

5

5

0

11

11

9

15-23

29

11

0

10

7

8

* Excluding chromosomally abnormal fetuses.

** Excluding chromosomally abnormal fetuses and terminations.

 

 

It is still controversial whether isolated echogenic bowel justifies invasive testing irrespective of whether associated findings are present. Strocher and co-workers (9) found the prevalence of chromosome defects is at least five-fold higher than that expected on the basis of maternal age. They suggest that even for a 20-year old with a background prevalence of 1:1295 for trisomy 21, the adjusted risk is 1:260.

 

         One analysis of nine studies found that 34% of fetuses with echogenic bowel have a poor perinatal outcome, however the most important prognostic factor is whether or not there are associated fetal abnormalities (20).

         The prognosis seems to be particularly poor in the setting of early IUGR and elevated maternal serum alpha-fetoprotein. Fetuses with echogenic bowel as an isolated finding seem to have a much better prognosis. This is because in most cases the finding of isolated echogenic bowel is associated with a normal fetus.

         One study, which examined 175 fetuses in 171 pregnancies complicated by isolated echogenic bowel, found that only 6.3% of fetuses were affected with CF (five fetuses), aneuploidy (five fetuses), and CMV (one fetus) (34).

         In a larger series, 447 (65.5%) of 682 cases of echogenic bowel resulted in the birth of a normal healthy newborn (see previous table) . In the normal fetus, the finding of echogenic bowel usually resolves over a period of weeks with no adverse sequelae (33). Despite this, there does seem to be an increased risk of unexplained intrauterine fetal demise in fetuses with unexplained echogenic bowel, with a rate of 1.9% reported in one series (29).



 

REFERENCES

 

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