Parasitic twin: the twin with no heart

An embryo that begins to split into identical twins and does not finish the process can end in several ways. In conjoined twinning, both twins develop fully and remain joined at some surface, each with its own heart driving its own circulation. Separating them means dividing whatever they share, and both may survive.

Heteropagus twinning is a different problem. One twin develops normally, and the other does not develop a functioning heart. Without a pump of its own it cannot sustain itself, so it survives only as long as it stays attached to its sibling's circulation. Anatomists call the complete twin the autosite and the incomplete one the parasite. The parasite may have recognisable parts: limbs, a face, sometimes bowel, but no independent life.

FIG 1conjoined vs parasiticoriginal diagram

It is rare. Published estimates put it at roughly one per million live births, which is why most paediatric surgeons never see a case.

The instinct on seeing a stable newborn is to wait, grow the baby, and operate when she is stronger. Heteropagus twinning inverts that reasoning.

The attached tissue is perfused by the autosite's heart, which means a newborn heart is pumping for two bodies. The parasite consumes oxygen and nutrients and returns nothing. Left in place, it imposes a continuous volume load on a circulation that has just made the transition to breathing air, and it draws off the supply the baby needs to grow. Waiting does not make the operation safer. It makes the patient weaker.

In this case the surgical team described the attachment as feeding off her blood supply and depriving her of what she needed to develop, and moved to remove it.

Appearance tells a surgeon almost nothing before cutting. What the parasite is joined to tells them everything.

The vessels running into parasitic tissue can be substantial, and in some cases the two share more than blood: bowel, bone, or a segment of another organ. Dividing a large feeding vessel without control causes rapid loss in a patient whose entire blood volume is a few hundred millilitres. Cutting through a shared structure without knowing it is shared can damage something the surviving child needs. This is why imaging and radiological consultation come before the incision, and why the team here worked with radiologists in planning.

The separation took about four hours. She recovered, fed at the breast, and was discharged. The hospital did not charge the family, who were farmers, for any part of the care.

Two lessons sit side by side here. Clinically, an attached parasitic twin is a haemodynamic burden rather than a cosmetic one, and delay works against the patient rather than for her. Practically, the scan that detects an anomaly only helps if what it shows is explained to the family. A finding that stays inside the imaging department changes nothing about how a birth is planned or how prepared the parents are when it happens.