The news that the conjoined twins born last Monday at the National Maternity Hospital, Holles Street, Dublin, are not suitable for separation is a severe blow to their parents and extended family.
While some conjoined twins have been separated in the past, the success of the surgical procedure depends on where the twins are joined and to what extent they share vital organs such as the brain, heart and liver. Unfortunately more than half are stillborn, and one in three survives for only a few days after birth.
Conjoined twins are identical. They occur when a single fertilised egg splits into two embryos. However, if this split is delayed for more than 12 days post-conception, the embryos do not fully divide and the twins may share body parts. Like all monozygotic (identical) twins, conjoined twins are always the same sex. About 70 per cent of conjoined twins are female.
The birth of conjoined twins is a rare event, occurring in between one in 50,000 and one in 100,000 births. About 28 per cent of conjoined twins are joined at both the chest and abdomen. Some 6 per cent are connected at the head; this is referred to as craniopagus.
Conjoined twins became known as Siamese twins following the birth of Chang and Eng Bunker who were born in Siam (now Thailand) in the early 1800s. They travelled the world as celebrities before eventually settling in the US.
The first set of conjoined twins were recorded in Constantinople in AD945.
The first surgical separation of conjoined twins took place in 1953. Had the recently born twins been suitable for separation they would most likely have been referred to Great Ormond Street Hospital, London.
Here the pioneering work of Prof Lewis Spitz and Prof Ed Kiely, which started over 25 years ago, has made the hospital one of the few centres in the world with the expertise to carry out this technically demanding surgery.
On January 30th, 2003, the team at the hospital successfully separated Zainag and Jannat Rahman. These twins shared a liver (referred to as omphalopagus) in a way that allowed the surgeons to give both an independent existence.
The many ethical dilemmas in separating conjoined twins were highlighted in July 2003 following the separation of adult twins Laleh and Ladan Bijani in Singapore. Despite a marathon 52-hour operation to separate the Iranian women, both died from sustained haemorrhage.