Conjoined twins are identical twins physically connected at birth. This unique condition arises from a single fertilized egg that undergoes an unusual splitting process during gestation. Occurring in approximately 1 in 50,000 to 1 in 200,000 live births, the condition captures significant medical and public interest due to its complexities.
Understanding Conjoined Twins
Conjoined twins originate from a single fertilized egg, making them identical twins, meaning they are always the same sex. Unlike typical identical twins where the embryo fully separates, conjoined twins result from an incomplete division. This incomplete separation typically occurs between 8 and 12 days after fertilization, when embryonic layers begin to develop.
If the embryo splits later than about 13 to 15 days after conception, the separation process halts before completion. These twins share a single placenta and amniotic sac, known as monochorionic monoamniotic. While the precise reason for this incomplete splitting remains unknown, theories suggest either a delayed or incomplete fission of the embryonic axis, or, less commonly, a fusion of two initially separate embryos.
Classifying Conjoined Twins
Conjoined twins are categorized based on their primary point of anatomical fusion, influencing shared organs and prognosis.
Thoracopagus
These twins are joined at the chest, often sharing the sternum, diaphragm, and a portion of the abdominal wall. They frequently have shared cardiac tissue and may also share the liver, making separation complex.
Omphalopagus
These twins are connected at the abdomen, from the lower thorax to the groin. They often share a liver and gastrointestinal organs. While shared vascular supply is common, shared cardiac musculature is less frequent.
Pygopagus
These twins are joined dorsally at the sacrum and coccyx. They commonly share a conjoined spinal cord and often a single lower rectum and anus.
Ischiopagus
These twins are joined primarily at the pelvis. Their fusion can be ventral (face-to-face) or dorsal (back-to-back). They frequently share a single bladder and uterus, and may have two to four lower limbs.
Craniopagus
These twins are fused at the head, though their bodies remain separate. This is a rare type, accounting for about 2% of cases, and often involves shared large dural sinuses and vascular structures. Surgical separation is exceptionally challenging due to the potential for shared brain tissue.
Surgical Separation
The possibility of surgical separation for conjoined twins depends on the extent of shared organs and vital structures. When twins share a heart or brain, separation is often deemed too risky or impossible, as it could lead to life-threatening complications for one or both. A multidisciplinary team, including pediatric surgeons, anesthesiologists, and nurses, plans the procedure.
Imaging techniques such as ultrasonography, MRI, and echocardiograms are used to precisely map the shared anatomy and any concurrent malformations. This understanding guides the team in determining the feasibility and timing. While some separations may occur emergently, many are planned when the twins are older, typically between 3 months and 3 years of age, allowing them to grow stronger.
Surgical separation is complex with risks, including infection, significant blood loss, and damage to shared organs. Outcomes are individualized; some separations result in both twins surviving, while others may lead to the loss of one or both. Reconstructive surgeries are often necessary after separation to repair any affected organs, skin, or limbs.
Outlook and Considerations
The prognosis for conjoined twins varies widely, influenced by the type of conjunction and surgical separation possibility. Approximately half of conjoined twin pregnancies result in stillbirth, and one-third of live births may not survive beyond 24 hours due to organ failure or complications. Of those surviving beyond 24 hours, about 25% are candidates for separation surgery. Approximately 60% of twins who undergo separation surgery survive. Advances in medical imaging, surgical techniques, and critical care have improved survival rates.
For conjoined twins, whether separated or remaining conjoined, long-term care and specialized medical support are often necessary. This support can include ongoing physical and occupational therapy. The journey for conjoined twins and their families requires continuous medical attention and adaptation, focusing on enhancing quality of life.