Fetal echogenic bowel

DEFINITION AND DIAGNOSIS

Fetal echogenic bowel refers to increased echogenicity or brightness of the fetal bowel noted on second trimester sonographic examination (picture 1A-B). Hyperechogenicity can be diffuse or focal. Different criteria for the diagnosis of echogenic bowel have been suggested. The simplest criteria and the one that we use is echogenicity similar to or greater than that of adjacent bone, with the ultrasound gain set to the lowest point at which bone appears white [1]. Other authors have defined several grades of echogenicity, with the most severe form (grade 3) being as bright as bone, while grades 1 and 2 are mildly or moderately echogenic [2].

Echogenic bowel should be distinguished from meconium peritonitis, which is a sterile chemical peritonitis resulting from in utero small bowel perforation with extrusion of meconium. The area of meconium deposition, often referred to as a meconium "pseudocyst," becomes walled off by adhesions between bowel, omentum and peritoneum, which results in one or more cystic masses that often display calcifications (picture 2A-B). Sonographic findings include polyhydramnios, abdominal calcification, intraabdominal echogenic mass, fetal ascites, dilated bowel, and, sometimes, hydroceles or swollen vulvae [3,4]. Meconium peritonitis is associated with meconium ileus from cystic fibrosis in 8 to 40 percent of cases [4,5]. Intestinal obstruction from atresia, stenosis, volvulus, Intussusception, Meckel's diverticulum, imperforate anus, or peritoneal bands; fetal hepatitis, and unknown etiologies account for most of the remainder. Delivery should be in a center with pediatric surgical and intensive care facilities. Mortality is high.

TECHNICAL ISSUES

Transducer frequency is important when considering the diagnosis of echogenic bowel. Higher frequency transducers can make the differentiation between normal and abnormal bowel echogenicity difficult, leading to overdiagnosis of the latter. As an example, a study using both 8 and 5 MHz transducers sequentially on the same fetuses found the frequency of echogenic bowel was 31 and 3 percent, respectively [6]. For this reason, we will only diagnose echogenic bowel with a transducer frequency of 5 MHz or below. In addition, echogenicity similar to or greater than adjacent bone is a subjective determination, therefore prone to interobserver and intraobserver variability [7].

PREVALENCE AND NATURAL HISTORY

Physiologic midgut herniation complicates assessment of the bowel in the first trimester. (See "Ultrasound diagnosis of fetal abdominal wall defects", section on 'Embryology'.) The prevalence of fetal echogenic bowel in the second trimester is 0.5 to 1 percent[1,8]. In normal fetuses it usually resolves over a period of weeks, with no adverse sequelae. In the third trimester, the fetal bowel is commonly noted to be as echogenic as adjacent bone; however, this is not considered to be a clinically significant finding at this gestational age.

ETIOLOGY AND MANAGEMENT

Fetal echogenic bowel is a nonspecific sonographic finding. In the vast majority of cases, the fetus (including the bowel) is normal. However, various physiologic processes associated with disparate pathologic conditions can also cause the bowel to appear echogenic.