The role of color and power Doppler ultrasound in the prenatal diagnosis of sirenomelia

Authors


Abstract

Sirenomelia is a rare congenital abnormality characterized by a variety of anomalies involving the lower limbs, severe oligohydramnios, bilateral renal agenesis, anorectal atresia and aberrant fetal vasculature. The prenatal diagnosis and differentiation from isolated bilateral renal agenesis is severely limited by the accompanying oligohydramnios that hinders visualization of fetal anatomy. We present three prospectively assessed cases of sirenomelia, two of which had bilateral renal agenesis, and all of which had a single umbilical artery derived from the aberrant vasculature that accompanies the syndrome. Copyright © 2004 ISUOG. Published by John Wiley & Sons, Ltd.

Introduction

Sirenomelia sequence is a rare (incidence of 1 : 60 000 births) lethal pattern of congenital anomalies characterized by a number of hallmark skeletal anomalies including fusion of the lower extremities or a single lower limb, bilateral renal agenesis or dysgenesis with absent or hypoplastic renal arteries, oligohydramnios and the presence of aberrant vasculature.

Prenatal sonographic detection of the lower limb anomalies associated with this condition is severely limited by the oligohydramnios that accompanies bilateral renal agenesis. However, assessment of the vascular pattern of the fetus will facilitate diagnosis as the presence or absence of fluid does not tend to limit color and power Doppler assessment of the fetal vasculature1, 2.

Case Reports

More than 20 000 routine obstetric scans were performed on a low-risk patient population over a 5-year period (January 1998–January 2003) in the authors' institution. Routine antenatal ultrasound scans, including fetal echocardiograms, were performed in all cases. All cases of severe oligohydramnios or anhydramnios were subjected to a targeted sonographic evaluation of the fetal vasculature umbilical cord with color /or power Doppler ultrasound. Abnormal vasculature was diagnosed when either an abnormal vessel was noted in the abdomen coursing anteriorly to the umbilicus without passing lateral to the urinary bladder, or when the aorta ended abruptly or did not divide into two iliac arteries.

Three cases of sirenomelia were detected prenatally, two of which underwent termination of pregnancy prior to 23 weeks of gestation, while one pregnancy proceeded to term after the couple received counseling. The presence of the abnormal fetal abdominal vasculature was determined prenatally using ultrasound. Final postmortem mapping of the fetal vascular patterns was obtained on pathological examination of the fetuses in two cases (Cases 1 and 2) and in a 1-day-old infant in the third case (Case 3).

The maternal histories, sonographic findings and outcomes are summarized in Table 1 and illustrated in Figures 1–4.

Figure 1.

Case 1: symmelia apus at 19 weeks of gestation. (a) Gray-scale ultrasound image showing anhydramnios with poor visualization of fetal anatomy. (b) Color flow image of the umbilical cord demonstrating a single umbilical artery. (c) Transverse ultrasound scan through the fetal upper abdomen demonstrating bilateral renal agenesis. (d) Color flow image through the fetal abdomen demonstrating aberrant origin of umbilical artery (vitelline vessel) traversing anteriorly to the umbilicus. Note the absent relationship between the vitelline vessel and the expected position of the fetal bladder. (e) Postabortal specimen demonstrating a single femur and hypoplastic tibia.

Figure 2.

Case 2: symmelia apus at 19 weeks of gestation. (a) Gray-scale ultrasound image showing a blind-ending single femur with complete anhydramnios. (b) Ultrasound image demonstrating poor visualization of normal fetal anatomy due to the anhydramnios. (c) Sagittal color flow views of the fetus demonstrating a large aberrant vessel extending anteriorly with a small hypoplastic abdominal aorta, which terminates abruptly just distal to the origin of the aberrant vitelline vessel. (d) Postabortal radiograph demonstrating a single, blind-ending femur and an abnormal, heart-shaped pelvic bone.

Figure 3.

Case 3: symmelia dipus at 18 weeks of gestation. (a) Gray-scale ultrasound image demonstrating two normal femora with soft tissue fusion centrally. This appearance is confirmed on the postnatal image on day 1. (b) Gray-scale and postnatal images demonstrating fusion of the feet at the heels with a relatively normal amount of amniotic fluid around the feet from the single functioning kidney. (c) Color flow image demonstrating the abnormal vitelline vessel arising from the abdominal aorta and coursing anteriorly into the umbilical cord. (d) Power Doppler image in a sagittal plane demonstrating a normal caliber aorta (Ao), which bifurcates into two normal common iliac arteries. The aberrant vitelline artery (Vit) originates anterior to the aorta and traverses the abdominal cavity and exits the fetal abdomen at the umbilicus as the single umbilical artery. The caliber of the vitelline artery is only slightly smaller than the aorta. (e) Infant on day 1. The absent Potters facies is due to the small amount of amniotic fluid that was present throughout the pregnancy from the one poorly functioning kidney. Note the soft tissue fusion of the upper legs and the absent anus.

Figure 4.

Vascular patterns associated with sirenomelia in a normal fetus and in Cases 1–3. IMA, inferior mesenteric artery; LIA, left iliac artery; LUA, left umbilical artery; RIA, right iliac artery; RUA, right umbilical artery; SMA, superior mesenteric artery.

Table 1. Maternal history, outcome and sonographic findings in three fetuses with sirenomelia
ParameterCase 1Case 2Case 3
  1. CNS, central nervous system; GA, gestational age; GI, gastrointestinal; GUT, genitourinary tract; TOP, termination of pregnancy.

Maternal historyNormal, no drugs/diabetesNormal, no drugs/diabetesNormal, no drugs/diabetes
Maternal age (years)213327
GA of initial scan (weeks)191918
ClassificationSymmelia apus (no feet)Symmelia apus (no feet)Symmelia dipus (two feet)
OutcomeTOPTOPRefused TOP; demise from renal insufficiency aged 10 days
Vertebral anomaliesSacral dysplasiaSacral agenesisNone
Pelvic anomaliesSingle pelvic bone lying horizontally in pelvis None
GI anomaliesBlind-ending colon, anorectal atresiaBlind-ending colon, anorectal atresiaHypoplastic colon, imperforate anus, cloaca
GUT anomaliesRenal agenesisRenal agenesisSmall, poorly functioning unilateral kidney
Genital anomaliesNo external genitaliaNo external genitaliaNo external genitalia (cloaca)
Skeletal anomaliesSingle blind-ending femur, no tibiae or fibulae, no feet, heart-shaped pelvic boneSingle blind-ending femur, no tibiae or fibulae, no feetTwo femora (fusion of soft tissues), two tibiae (fusion of soft tissues), no fibulae, two feet
CNS anomaliesNoneNoneNone
Heart anomaliesNoneNoneNone
Amniotic fluidAnhydramniosAnhydramniosNormal at 18 weeks; decreased from 20 weeks until term
Vascular anomalies 
 Celiac arteryNormal from aortaHypoplastic arising from vitelline vesselNormal from aorta
 Superior mesenteric arteryNormal from aortaNot seen—? technicalNormal from aorta
 Inferior mesenteric arteryAbsentAbsentHypoplastic but present
 Renal arteriesAbsentAbsentHypoplastic—patent left renal artery
 Iliac arteriesSingle left iliac artery as a continuation of aortaSingle left iliac artery arising as branch of vitelline vesselTwo iliac arteries from distal aorta
 AortaSingle non-branching vessel terminating as left iliac arterySmall hypoplastic artery (only patent for 1 cm distal to origin)Well formed—slightly smaller than vitelline vessel; single small renal artery arises from left side
 Vitelline vesselLarge dominant vessel, no branchesSingle midline vessel coursing down to pelvis; celiac artery arises from this vessel; a single left iliac artery arises from its distal portionLarge artery arises from aorta just distal to origin of celiac artery and superior mesenteric artery; coursed through abdominal cavity on a short mesentery to umbilicus; no branches
 Umbilical cordTwo-vessel cordTwo-vessel cordTwo-vessel cord

Discussion

The pattern of sonographic anomalies that characterize sirenomelia, especially the single or fused lower limbs, often go undetected prenatally as ultrasonographic exploration of the fetus is hindered by the severe oligohydramnios or anhydramnios that occurs as a result of the bilateral renal agenesis3.

The most dominant vascular feature of sirenomelia is the presence of an aberrant umbilical artery derived from a persistent vitelline artery1. The vascular steal theory describes this feature as the pathogenetic mechanism that causes sirenomelia, attributing the lower limb abnormalities to hypoperfusion secondary to blood shunting into the dominant vitelline vessels and away from the absent or hypoplastic iliac arteries4.

Two separate systems of classification of sirenomelia have been proposed, one based on the number of feet and the other according to the fused bones (Table 2).

Table 2. Classification of sirenomelia according to the number of feet12 and fused bones22
Classification systemLimbsFeet
Number of feet
 Symmelia apusFused, one femur and tibiaNone
 Symmelia unipusTwo femoraOne foot or partial fusion of both feet
 Symmelia dipusFusion of both limbsNormal (no fusion)
 BipodiaTwo femoraBilateral fusion of the feet with non-fused distinct toes
Fused bones
 Type IPaired femora, tibiae and fibulae 
 Type IISingle fused fibula 
 Type IIIFibula absent 
 Type IVFemora partly fused, single fibula 
 Type VFemora partly fused, fibula absent 
 Type VISingle femur and tibia 
 Type VIISingle femur, absent tibia and fibula 

There are a few documented cases in which the diagnosis of sirenomelia was made during the first trimester using combined two- and three-dimensional sonography5, 6. This is made possible because oligohydramnios is usually less severe in the first trimester on account of the smaller contribution of fetal urinary excretion to the amount of amniotic fluid, allowing clearer lower limb imaging by ultrasound. Visualization of lower extremity anomalies consistent with sirenomelia sequence has been noted as early as 9 weeks of gestation7. However, because oligohydramnios itself is usually an important feature of sirenomelia in the second trimester, sonographic evaluation is often very limited and may be delayed past the time when transvaginal ultrasound is most useful8. For this reason, other reliable methods of second- and third-trimester diagnosis have been sought. One technique that has been employed is the artificial instillation of amniotic fluid. This invasive technique allows for a more accurate sonographic evaluation of fetal anatomy and behavior in addition to an assessment of fetal urodynamics in severe oligohydramnios9, 10.

Similarly, the use of magnetic resonance imaging (MRI) to complement sonographic findings can be useful in the diagnosis of sirenomelia. MRI is an ideal adjunct to ultrasound in such cases because the fetal motion that hinders MRI image quality is minimal in the presence of oligohydramnios11.

Another approach to the prenatal diagnosis of sirenomelia involves intimate knowledge of the vascular anomalies associated with the condition. Normally, by the third week of embyrogenesis, paired segmental aortic arteries, originating from the ventral surface of the dorsal aortae, communicate with an elaborate anastomotic vascular network in the mesoderm along both sides of the yolk sac12, 13. These paired vessels are the vitelline arteries, which normally coalesce into three major ventral branches of the fused dorsal aorta by day 26. Proximal portions of these primary trunks will persist as the celiac, superior mesenteric and inferior mesenteric arteries, which respectively supply the foregut, midgut and hindgut and their derivatives4, 12. In sirenomelia, the celiac and superior mesenteric arteries are usually present and appropriately sized, but the inferior mesenteric artery is represented as a slender branch from the aorta or from the persistent vitelline artery if it is present at all. This variability is adequately demonstrated in the comparison of Cases 1 and 2, where the inferior mesenteric artery is absent, to Case 3, in which the inferior mesenteric artery is hypoplastic. The aberrant derivative of the vitelline artery complex is the major abdominal artery that is characteristic of sirenomelia and is the blood shunt described in the vascular steal theory. This artery arises from the high abdominal aorta and runs a variable course through the abdomen to the umbilicus where it exists as the umbilical artery carrying blood from the fetus to the placenta4.

The patterns of vascular anomalies vary in all cases of sirenomelia; this is best explained by variability in the time, space and degree of severity of the teratogenic insults14. The consistent finding in all cases of sirenomelia is the presence of aberrant vasculature. A review of several documented cases4, 13, 15–20 reveals a number of frequent vascular features, some of which are demonstrated in our three cases (Figure 4). The appearance of the hypoplastic aorta, distal to the point of origin of the persistent vitelline artery, is variable. It may be hypoplastic (Case 1) or absent, or may end abruptly (Case 2). It may have a normal caliber and bifurcate into the right and left common iliac arteries (Case 3) or give rise to a single iliac artery (Case 1). When present and patent, the aorta commonly runs parallel to the aberrant artery in the abdomen and then enters the pelvis. Normal bifurcation into the right and left common iliac arteries supplying the lower limbs (Case 3) is uncommon. The common iliac arteries may give off vessels to the lower extremity18, but they usually end abruptly and if present they are usually undeveloped. When the distal aorta is non-patent, the iliac artery/arteries, if present, originate from the vitelline artery (Case 2). In some cases the aberrant artery may entirely replace the aorta at its point of formation and then give off aortic-like vessels distally as in Case 2. Similarly, there is variation in the presence and nature of branching blood vessels. The lateral branches of the dorsal aorta that normally originate distal to the origin of the persistent vitelline artery are usually absent in sirenomelia. When present, these arteries may originate from either the aorta or the anomalous artery and may be hypoplastic. The renal arteries are almost always absent, a feature that can be demonstrated with color Doppler imaging and, in the setting of second-trimester severe oligohydramnios, can be an accurate indication of renal agenesis21. However, a slender right renal artery arising from the abdominal aorta was present in Case 3. Despite the size of the vessel it was sufficient to supply a single dysplastic kidney. With this minimal renal function and the presence of a cloaca to facilitate urinary output, there was relatively normal amniotic fluid at 18 weeks; however, amniotic fluid volume did decrease after 20 weeks, but was never completely absent. The adrenal arteries are documented more frequently than the renal arteries, presumably because they develop from lateral branches adjacent to the origin of the superior mesenteric artery, a site that is proximal to the area from which the renal arteries arise12.

In sirenomelia, the development of the distal abdominal aorta and iliac arteries parallels the development of the pelvis and lower limbs. In Case 3, normal bifurcation of the aorta occurs and the lower body abnormalities are less pronounced than in Cases 1 and 2. Case 3 presents with simelia dipus alone versus simelia apus and a host of other skeletal anomalies in Cases 1 and 2.

Lastly, one of the most consistent vascular features of sirenomelia is the presence of a single umbilical artery, which represents the continuation of the persistent vitelline artery4, 15.

The prenatal use of color Doppler imaging in the presence of oligohydramnios is three-fold. First, it can be used to confirm the presence of bilateral renal agenesis when gray-scale images fail to detect the presence of renal tissue, especially when the adrenal glands take on a discoid configuration. DeVore22 described a method of predicting the location of the renal arteries throughout gestation based on their expected distance above the aortic bifurcation into the iliac arteries. Second, the caliber and patency of the aorta can be assessed in addition to the presence of normal umbilical arteries in fetuses with bilateral renal agenesis not associated with sirenomelia. Third, it can detect both the origin of the aberrant vitelline artery in the fetal abdomen and the absence of the normal intimate relationship between the umbilical arteries and the sidewalls of the bladder.

The ability to image the persistent vitelline artery in the abdomen or its continuation as a single umbilical artery using color-flow imaging has been documented1, 2. However, single umbilical artery is not a finding exclusive to sirenomelia23 nor is the persistent vitelline artery necessarily indicative of sirenomelia24. Visualization of vascular anomalies in the context of a pattern of other anomalies is a reasonable approach to prenatal diagnosis of sirenomelia.

Given the limitations that decreased or absent amniotic fluid imposes on the visualization of fetal movement and anatomical evaluation of the fetus, the ability to utilize color-flow mapping of the fetal renal and umbilical vessels is obviously beneficial. We suggest that any mid- or late-trimester pregnancy that is accompanied by oligohydramnios (in the absence of premature rupture of membranes) should be assessed with color-flow imaging in an attempt to map the fetal vasculature. Prenatal color and power Doppler imaging is a non-invasive investigation that can identify the existence of the aberrant abdominal vasculature and absent renal arteries that are associated with sirenomelia. The diagnosis of sirenomelia sequence is best made by the combination of the above findings and lower extremity abnormalities.

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