Conjoined Twins
Anne Kennedy, MD
DIFFERENTIAL DIAGNOSIS
Common
Thoracopagus
Omphalopagus
Less Common
Pygopagus
Ischiopagus
Cephalopagus
Craniopagus
Rare but Important
Dicephalus
Fetus-in-Fetu
Parasitic Twin
ESSENTIAL INFORMATION
Key Differential Diagnosis Issues
Monochorionic
Single placenta
Twins same gender
Monoamniotic
No inter-twin membrane
Fetuses fused at some location
Contiguous skin covering at site of fusion is hallmark of this diagnosis
Conjoined twins with narrow or pliable area of fusion may vary in orientation to each other
May see fused umbilical cord
More than three cord vessels
Important to differentiate conjoined from monoamniotic twins given different prognosis
Monoamniotic twins are within the same sac but not fused
May have relatively fixed orientation if cord entanglement
Prognosis better for monoamniotic
No need for separation
Majority of conjoined twins die in utero or shortly after delivery
Formal fetal echocardiography mandatory in all conjoined twins
Complex congenital heart disease may preclude separation attempt
Fetal echocardiography easier than postnatal echo
Multiple acoustic windows through amniotic fluid
No interference from aerated lung
Helpful Clues for Common Diagnoses
Thoracopagus
Fetuses face each other
Fused from thoraces to umbilicus
Always some degree of cardiac fusion
Precludes separation in 75%
Common pericardial sac in 90%
Livers invariably fused
25% share biliary system
25% have common small intestine, usually duodenum
Omphalopagus
Fetuses face each other
Fusion of ventral abdominal cavities from umbilicus ± lower thorax
80% liver fusion, biliary anomalies common
30% have shared small intestine at distal ileum at level of Meckel’s diverticulum
Helpful Clues for Less Common Diagnoses
Pygopagus
Fetuses face away from each other
Joined at sacrum/buttocks
15% share urinary system with single bladder
One anus ± rectum, rest of bowel separate
May have spinal cord fusion
Ischiopagus
Joined from umbilicus to single pelvis
Fetuses face-to-face or end-to-end
Variable number kidneys/bladder
Cloacal malformation/anal atresia/vesicocolic fistulas occur
Internal and external genitalia may be shared
Variable number of extremities
Tetrapus = 4
Tripus = 3
Bipus = 2
Major challenges to separation are genitourinary and orthopedic
Genitourinary
Continence, adequate bladder/bowel emptying/sexual function
Orthopedic
Number of lower limbs
Pelvic reconstruction for prosthetic limb attachment
Cephalopagus
Craniopagus
Joined at skull anywhere except face or foramen magnum
Separability depends on degree of shared dural sinuses
Shared neural tissue precludes separation
Helpful Clues for Rare Diagnoses
Dicephalus
Two heads
Single torso
Usually single set of extremities
Inseparable
Fetus-in-Fetu
“Twin” forms a mass within an apparent singleton
Key difference between teratoma and fetus-in-fetu is presence of vertebral column
Mass is well-demarcated as contained within a membranous sac
Commonest in upper retroperitoneum
No malignant potential
Excision is curative
Parasitic Twin
Vestigial fetal parts or incomplete twin attached to fully developed co-twin
Intact twin known as autosite
Any vestigial parts may be seen
Other Essential Information
Nomenclature
Site of attachment + “pagus” (e.g., thoracopagus = twins joined at chest)
“Di” + site of duplication (e.g., dicephalus = twins with two heads on a single torso)
Suffix may be used to describe number of extremities
Ischiopagus tripus = twins joined at lower body with single pelvis and three lower extremities
Overall prognosis for conjoined twins is poor
40-60% stillborn
35% liveborn twins die in first 24 hours
Each set of conjoined twins is unique
Associated malformations in some may constitute lethal condition
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