Despite the regular births of Siamese twins in South Africa, medical experts still can’t explain the syndrome, writes Rowan Callaghan
THE birth of Siamese twins Monde and Nomonde last week seems yet another in a long list since Mpho and Mphonyane Mathibela first stole South African hearts some eight years ago. Reports of Siamese twins come in from the region regularly, giving the impression that southern Africa has a higher rate of these births than anywhere else.
Yet medical experts here cannot pinpoint the reasons for this apparent increase, and can only speculate on what causes such births. Possible explanations range from a higher rate of consanguity (intermarriage) to natural phenomena. And experts are divided on whether the number of Siamese twins in southern Africa is increasing or higher than other parts of the world.
According to Professor Sydney Cywes, a retired University of Cape Town paediatrics professor, accurate statistics are hard to come by as only a small number of all Siamese twins survive. He says 50 to 60% are stillborn. Another factor that makes it difficult to measure is the number of unreported aborted foetuses. “But the incidence does appear to be higher in South Africa.”
Cywes says a possible explanations is that the problem occurs when family members intermarry, as breeding in a limited gene pool increases the likelihood of genetic disorders. This would explain why there seem to be more Siamese twins born in developing countries.
But Professor Denis Viljoen of Cape Town University’s Human Genetics Department has a totally different opinion. “It’s just a numbers game,” he says simply. One in every 100 000 births worldwide is a set of Siamese twins. In South Africa and in other developing countries, says Viljoen, the birth rate is just higher than in Western countries, giving the impression of more Siamese births.
What is higher in Third World countries, he says, is the frequency of non-identical twins, but these are never born joined. The frequency of identical twins is the same throughout the world. And, Viljoen argues, it’s an abnormality of the twinning processs, the incomplete separation of identical twins, not intermarriage, that causes the Siamese syndrome.
Identical twins are produced from one ovum fertilised by one sperm, which subsequently splits into two. With ordinary identical twins, the division takes place a short time after the egg is fertilised and the two embryos develop separately. In Siamese twins, the splitting is delayed at this crucial stage and they start to develop their various features together. The reason why the fertilised egg splits in the first place is still unknown, says Viljoen.
“Siamese twins, like all identical twins, are the same sex and have the same genetic code,” he says. Connected twins can be joined at the head (craniopagus), thorax (thoracopagus), posterior (pygopagus) or naval area (ischiopagus). There are also janicepes twins who are merged together and are always stillborn.
Viljoen did admit that, despite the random nature of these births, there are times when there is a spate of Siamese twins. This, however, could be just a chance event.
Siamese twins first gained worldwide attention when the original Burmese twins, Eng and Chang Bunker, were born at the turn of the century. The two were adopted by an American couple, taken to America and displayed at various circuses.
South Africa’s most famous twins, Mpho and Mphonyane, made headlines when doctors announced they planned to separate them. As they were joined at the brain, one had to be sacrificed. The operation was hailed as a breakthrough, but the surviving twin has not come out of the operation unscathed and has suffered from brain damage.
This result is not uncommon. In a survey conducted on 31 sets of Siamese births in South Africa in the early Eighties, 15 sets were stillborn while successful separation was achieved on only four sets of twins.