1. General characteristics of fetal small bowel volvulus
Since Crisera et al[1] first reported fetal midgut volvulus in 1999, more and more cases of intestinal volvulus were confirmed by prenatal diagnosis, but most of them are case reports until now. However, the etiology of fetal small bowel volvulus still unclear. It has been reported that it is associated with malrotation[2]. If the relative position of the pedicle of SMA and SMV was used as the criterion for judging malrotation, no signs of malrotation were found in our group. It is suggested that fetal small bowel volvulus is not closely related to malrotation, which is consistent with previous reports[4,5]. Studies have reported that fetal intestinal volvulus is closely related to cystic fibrosis[4]. However, cystic fibrosis is mainly occured in Caucasian populations, which is rare in Chinese populations.
Fetal intestinal volvulus was mostly diagnosed in the third trimester in previous literatures. In recent years, the intestinal volvulus was gradually detected at an earlier gestational age by prenatal ultrasound. Among 8 cases of fetal intestinal volvulus reported by Sciarrone et al, the earliest diagnosis was made in 20 weeks of gestation[4]. In Bartholmot’s study, there were two peaks of diagnosis: the first around 27 weeks of gestation, and the other around 32 weeks[6]. In our case series, the earliest diagnosis was made in 22 weeks of gestation, with an average of (29.7 ± 4.9) weeks of gestation. And 3 cases were found in the second trimester. Besides, about 54.5% (6/11) of pregant women in this situation reported a decrease in fetal movements or abnormal changes of fetal heart monitoring, which is corresponded to some researches[6,7]. Therefore, prenatal ultrasound examination should pay more attention to whether fetuses of these pregnant women have signs of small bowel volvulus.
It has been reported that the intestinal volvulus is more frequent in men. Whether there is gender difference in fetal volvulus is unknown, which is rarely mentioned in previous literatures. In 8 cases of fetal volvulus reported by Sciarrone et al, there were 3 female fetuses and 5 male fetuses[4]. In our study, we found female fetuses were more prone to intestinal volvulus, the ratio of male to female was 4: 7, which may be related to the small sample.
2. Ultrasonographic features of fetal small bowel volvulus
Fetal small bowel volvulus can show different features on ultrasound according to the degree and the duration of it. The dilatation of small bowel is the most suggestive ultrasound feature in prenatal diagnosis of intestinal volvulus[4,6]. All cases in our study have found intestinal dilatation first on ultrasound. The dilated loop of small bowel is usually located in the proximal segment of the torsion, within a fluid‐filled level. With the progress of the disease, there is a horizontal level separating
an anechoic part superiorly and an echoic part inferiorly. In Bartholmot’s study, this sign was present in 38.5% of his cases, which was seen as a sign of intestinal distress and evidence of a loop without peristalsis[6]. The echogenic material below probably corresponds to meconium, with more or less blood content.
The whirlpool sign is the most direct prenatal ultrasound sign of diagnosis of intestinal volvulus[4,8-10], which resulting from a spiral involving the mesenteric vessels and the small bowel around the mesenteric artery or its branches. In our group of cases, the visualization of the whirlpool sign in ultrasound thus defines a positive diagnosis of intestinal volvulus. In addition, the moderate to hyperechoic mass in the center of whirlpool sign is the aggregation of multiple intestinal walls at the origination of torsion. During the follow-up, the mass of one case was no longer visible, accompanied by the disappearance of dilated bowel peristalsis and blood signal. It was considered that the necrosis of the intestinal wall at the torsion leads to the decrease of the mass echo. Another important sign of small bowel volvulus is the coffee bean sign[8,11]. If the volvulus lasts for a long time with a severe degree, the volvulus segment of the intestine may have ischemic necrosis, leading to a local intestinal adhesion. We did not observe this sign in our series, which may be related to the early prenatal treatment in most cases at the first time of diagnosis.
Fetal small bowel volvulus can also accompany with gastric dilatation, ascites, polyhydramnios, abdominal pseudocyst, peritoneal calcification etc. Fukushima et al has reported a case of fetal volvulus with only gastric dilatation at 30 weeks of gestation[12]. In our group, there were 5 cases showed gastric dilatation and four showed polyhydramnios in the first diagnosis or follow-up. It was considered that the gastrointestinal obstruction was caused by intestinal volvulus. Some researchers regard intestinal dilatation, ascites, pseudocyst and peritoneal calcifications as signs of meconium peritonitis[13,14]. However, no signs of peritoneal calcifications were found in 9 cases. Among 6 cases with ascites in our group, there were 4 cases had no evidence of perforation and meconium peritonitis during neonatal operation. Therefore, the ascites is more likely due to the continuous aggravation of torsion, leading to fluid leakage caused by the increase of the internal pressure. At the same time, persistent massive ascites is related to fetal anemia and cardiopulmonary compression leading to fetal ischemic distress[15].
In our group, 4 cases of intestinal volvulus regressed intrauterine spontaneously within 6 to 7 weeks after the first diagnosis, and the disappearance of whirlpool sign was earlier than intestinal dilatation in one case. In Bartholmot's study, 3 cases of intestinal volvulus showed the complete disappearance of the whirlpool sign in the follow-up ultrasound performed 2 weeks after the diagnosis[6]. However, due to the lack of other prenatal data, the possibility of a false-positive ultrasound diagnosis cannot be completely ruled out.
3. Prenatal treatment and prognosis of fetal small bowel volvulus
The prognosis of fetal small bowel volvulus depends on many factors, including gestational age at diagnosis, degree of volvulus, length of necrotic bowel, severity of complications and birth weight. The prognosis is generally well if effective measures can be taken in time.
In the past, fetal intestinal volvulus was often observed in the third trimester accompanied by perforation, meconium peritonitis or other serious complications, which need an emergency delivery and neonatal operation after diagnosis[16-18]. But in recent years, the number of fetal intestinal volvulus found in the second trimester has increased gradually. We should balance the risk of volvulus against the survival possibility of premature infants before intervening. And we think the ultrasonic features of intestine volvulus are of great value for prenatal intervention. In Sciarone’s study, except for 2 cases received immediate delivery due to ascites and the disappearance of intestinal peristalsis, the rest were followed up by ultrasound every 0.5-3 weeks, and all the fetuses had favorable prognosis finally[4]. 13 cases reported by Bartholmot et al were followed up by ultrasound until delivering except 2 cases induced labor due to cystic fibrosis[6].
In our study, for those gestational age greater than 32 weeks, considering the higher survival rate of newborns after delivery, once the signs of deterioration of intestinal volvulus were found, such as the disappearance of peristalsis and blood signal of dilated loop, massive ascites, abnormal parameters of blood flow, abnormal FHR monitoring and so on, termination of pregnancy may be more conducive to fetal prognosis. If the gestational age is early than 30 weeks, in order to extend the intrauterine time as far as possible, combining with the wishes of the mother, doctors can monitor the fetas by ultrasound every 0.5-1 week, so as to reduce the survival risk of premature delivery.