Conjoined twins are a rare biological phenomenon resulting from the incomplete division of a single fertilized egg into two separate identical embryos. This incomplete fission results in two individuals physically joined at various points of the body, a condition estimated to occur in only 1 in 50,000 to 1 in 200,000 live births. The question of whether their mortality is shared depends entirely on the specific anatomy and the degree to which they share vital physiological systems. Outcomes range from simultaneous death to temporary asynchronous survival.
Classification and Shared Physiology
The medical prognosis for conjoined twins is determined by their classification, which is based on the primary site of connection. The presence of two independent hearts, brains, and separate primary circulatory systems is the fundamental determinant of whether two conjoined individuals can survive independently or will succumb together.
Common Classifications
The most common form is thoracopagus, where the twins are joined at the chest, often sharing a single heart. If the attachment extends to the chest and abdomen, it is termed thoraco-omphalopagus, frequently involving a shared heart, liver, and digestive tract. Twins joined solely at the abdomen, known as omphalopagus, typically have separate hearts and brains but may share a liver or parts of the gastrointestinal system. Other classifications include pygopagus (joined at the lower back) and craniopagus (joined at the skull).
Mechanisms Leading to Simultaneous Mortality
Simultaneous death is the most common outcome for conjoined twins who survive infancy, usually caused by the failure of a shared, non-redundant vital system. This occurs most frequently in twins who share a single heart or have a significantly merged circulatory system, such as in thoracopagus cases. A catastrophic failure in one twin, like severe heart failure or a massive systemic infection, immediately compromises the co-twin. The shared blood supply quickly distributes toxins or overwhelms the shared organ system.
If one twin’s heart stops or a major organ fails, the physiological collapse rapidly affects the other. The surviving twin’s heart attempts to circulate blood through the non-functioning vascular system of the deceased twin. This sudden increase in circulatory volume and loss of vascular resistance leads to a fatal drop in blood pressure (systemic hypotension) in the living twin. Without immediate medical intervention, the co-twin typically dies within minutes or hours due to circulatory shock and rapid sepsis from the decomposing body.
Factors Allowing for Asynchronous Death
The possibility of one twin surviving for a period after the death of the other, known as asynchronous death, relies on a high degree of physiological independence. This is more likely in cases like omphalopagus twins, where the heart and lungs are separate, and shared structures are limited to the liver or lower gastrointestinal tract. If one twin suffers a localized injury or illness that does not rapidly poison the shared blood supply, they may die without immediately causing the co-twin’s demise.
Even with separate vital organs, the surviving twin’s prognosis is grim without urgent intervention. The deceased twin’s body quickly becomes a source of blood-borne infection and toxins, which circulate through the shared system, necessitating immediate emergency separation to save the surviving individual. Historically, even twins with relatively minimal connections, like Chang and Eng Bunker, died within hours of each other, demonstrating the rapid cascade of physiological collapse caused by the shared circulation and the deceased twin’s body.
Medical Decisions During Terminal Illness
When one conjoined twin becomes terminally ill or dies, medical teams face challenging ethical and surgical dilemmas. If the twins share a vital, non-separable organ, the death of one signifies the inevitable death of the other, and care shifts to palliative support for both. If the twins possess separate vital systems, an emergency separation becomes a necessary intervention to save the surviving individual.
The decision to proceed with separation, especially when the weaker twin is still alive but terminally ill, is complex, often requiring the input of bioethics committees. The procedure is performed to save the life of the viable twin, even though it will hasten the death of the non-viable twin. This difficult medical decision acknowledges that allowing the terminal illness of one to run its course while conjoined will result in the death of both, making separation the only course of action to secure the survival of one person.