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2010-03-31-16 Answer to the case the week #265 © Miheeva

 Answer to the case of the week #265

April 1, 2010 - April 15, 2010

Natalya Miheeva, MD.


The following images represent an interesting case of the Parasitic conjoined twins, atypical parasitic ischiopagus conjoined twins.

Synonym: Duplicata incompleta, Asymmetric conjoined twins, heteropagus.



Asymmetric or parasitic conjoined twins are a rare anomaly of monochorionic monoamniotic twins. It consists of an incomplete twin attached to the fully developed body of the co-twin.


Case report


A 25-year-old woman, G2P2 presented to our center at 18 weeks of pregnancy for a routine ultrasound examination. The woman had no family history of congenital anomalies and had not taken any medication during her pregnancy. Ultrasound examination at 12 weeks was normal. Our examination at 18 weeks revealed a heterogenous external mass located above the pubis. It resembled a lower limb.

The host fetus had a single head, thorax, heart and abdomen of a normal appearance. Two upper and two lower extremities with no abnormalities. However, it had anomalies of the spine, an incomplete bladder duplication.  The right kidney was ectopic in the pelvis and there was a single umbilical artery.
Parents opted for the termination of the pregnancy based on the ultrasound findings.The fetus was a male of 420 grams. 

A pathomorphologic study revealed a parasitic fetus at the area of the pubis.

  • The outer part of the parasite consisted of the two structures. Skin-covered suprapubic soft tissue mass (15 mm in diameter), connected by a connective tissue stem (5 mm in diameter) to the abnormally formed lower limb with rudimentary tibia and two merged sirenomelic feet.  The foot had 8 toes and the left hand had pre-axial polydactyly, type 1.
  • The inner part of the parasite consisted of a bone-cartilage mass connected to the pelvis, precisely to the L3-S1 laminae and the right ilium of the host fetus. It divided the urinary bladder base in two. The left kidney, which was normally located connected to the left half of the bladder via left ureter. The ectopic right kidney was located in the pelvis and it was connected to the right half of the bladder via the right ureter. Two rudimentary adrenals which belonged to the parasite were found by the left testicle.
  • The X-ray of the main fetus revealed multiple segmentation abnormalities of the cervical and thoracic vertebra with accessory Th10 hemivertebra and absent L5 transverse process. The external part of the parasite had rudimentary tibia, femur, and 8 metatarsals.
  • The histological exam of the host fetus demonstrated an accessory penile cavernous body and incomplete duplication of the bladder with duplicated mucosa and smooth muscle and common serosal layer. The internal component of the fetus-parasite consisted of the rudimentary pelvis and cartilaginous primordials of the axial skeleton.


Images 1,2: Image 1 shows a heterogenous external mass located at the pubic area, note the sirenomely. Image 2 shows a bladder duplication and the single umbilical artery.




Images 3,4: Image 3 shows a 3D image of the external mass, parasitic twin. Image 4 shows a X-ray image of the parasite, note the rudimentary femur and metatarsal bones.




Images 5,6: Image 5 shows a 3D image of the spine with absent L5 transverse process, indicated by arrow. Image 6 shows a X-ray, note the abnormalities of the cervical spine, dysosthosis of the Th5, hemivertebra at the level of the Th10 and absent transverse process of the L5. 




Videos 1,2: Video 1 shows an external mass located at the pubic region of the fetus, note the sirenomely. Video 2 shows the external mass and the color Doppler imaging. 



Images 7,8: Images were taken during the pathologic study. Images show the bone-cartilage mass of the parasite which was connected to the pelvis of the fetus at the level of the L3-S1 and the right ilium.


Images 9, 10: Image 9 shows the fetus after pregnancy termination, note the parasitic twin at the pubic region. Image 10 shows a detail of the parasitic twin, micromelia is nicely demonstrated.




Image 11: Image 11 shows the fetal abdomen and thoracic cavity.






Conjoined twins are incompletely separated monozygotic twins. Asymmetric conjoined twins are twins in which one of the twins, parasite, is smaller and less formed than the other, autosite. The parasitic twin is dependent on the autosite twin.




Conjoined twins are a rare condition, with an estimated occurrence of 0.05-0.1:10,000 births(1). The asymmetric type is the rarest form and accounts for 1–2% of conjoined twins (2).




Conjoined twins occur sporadically. All the pathologic events are related to monozygosity with incomplete division of the embryonic disk 13 days after fertilization (3).



The exact pathogenesis of  heteropagus twinning is uncertain and may be due to an ischemic event at an early gestation or an incomplete cleavage of the inner cell mass of the blastocyte (4).



  1. Free monozygotic twins
  2. Complete equal and symmetric conjoined twins
  3. Incomplete equal and symmetric conjoined twins (partial duplication)
  4. Unequal and asymmetric conjoined twins (heteropagus or parasitic conjoined twins) (5).

Classification of the parasitic conjoined twins is based on one of the 8 anatomic sites in which intact or symmetrical conjoined twins are united according to the Spencer"s classification (6,7,8).

Embryonic aspect  Type                         
 Rostral  Cephalopagus
 Caudal  Ischiopagus
 Lateral  Parapagus
         Dicephalus: 2 heads
         Janus: 2 faces
 Dorsal  Craniopagus


















Sonographic findings

  • Inseparable fetal bodies and skin contours
  • No change in the relative positions of the fetuses in particular during fetal motion
  • Shared organs are observed.

Hence the site and extent of twin fusion are variable, a careful ultrasound evaluation may help to identify parasitic conjoined twins in utero.
Parasitic conjoined twins usually consist of externally attached supernumerary limbs, sometimes viscera, but very rarely with a functional heart or brain (6).

Differential diagnosis

Differential diagnosis include parasitic twin, fetus in fetu (internal parasite) and teratoma. Presence of rudimentary lower limbs with foot in our case suggested the parasitic conjoined twin.


The prognosis depends on the site and extent of twin fusion (7).


Delivery at a tertiary center is highly recommended for optimal neonatal intensive care and pediatric surgical intervention.


In pygopagus, the supernumerary limb may be attached to the sacrum, usually dorsal, and not always in the midline. Some of them have sensory and motor innervation and a few have vertebrae. The vertebral column above the lumbosacral level is not significantly abnormal, and the intestine may be protruding or open into the dorsal defect (6).
Rachipagus parasites (most consisting only of supernumerary limbs) show vertebral conjuction above the sacrum with neural or bony connections to the autosite (6,9).
Parapagus shows vestigial structures united to the lateral aspect of the head and the entire trunk along with six limbs.
Finally, in typical ischiopagus, duplicated legs are usually located on the ventral aspect of the pelvis (often with diastasis pubis). The genitalia (often double) are between the normal and the parasitic legs (6,10).

This data support that the current case is a parasitic ischiopagus. In an atypical ischiopagus conjoined twin, Spencer considered that invariable involvement of the cloaca suggests that allantois is the primary site of union. The conjoined ilea indicates that the caudal portion of the yolk sac is shared (11). Cloacal anomalies present in our case give further support for ischiopagus parasitic twinning.


1. Edmonds LD, Layde PM. Conjoined twins in the United States, 1970–1977. Teratology 1982;25 : 301–308.
2. Pei-Lin Chen, Kyuran AC. Prenatal MRI of Heteropagus Twins AJR 2003; 181:1676-1678.
3. Filly RA, Goldstein RB, Callen PW. Monochorionic twinning: sonographic assessment. AJR 1990; 154:459 –469.
4. Curry EK, Schraibman V. Epigastric heteropagus twinning. J Pediatr Surg2001; 36:E11 –E15.
5. Potter AN, Craig JM. Patology of the Fetus and Infant,3rd. ed. Chicago,Year Book Medical Publishers,1975, 207-37.
6. Spencer R. Parasitic conjoined twins: External, internal (fetuses in fetu and teratomas), and detached (acardiacs). Clin Anat. 2001;14:428–444.
7. Spencer R. Theoretical and analytical embryology of conjoined twins: Part I: Embryogenesis. Clin Anat. 2000;13:36–53. 
8. Spencer R. Theoretical and analytical embryology of conjoined twins: Part II: Adjustments to union. Clin Anat. 2000;13:97–120. 
9. Spencer R. Rachipagus conjoined twins: They really do occur! Teratology. 1995;52:346–356.
10. Chadha R, Dhar A, Bagga D, et al.  An unusual form of caudal duplication (dipygus). J Pediatr Surg. 1993;28:728–730. 
11. Spencer R. Minimally united ischiopagus twins: Infraumbilical union with cloacal anomalies. J Pediatr Surg. 1996;31:1538–1545. 


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