Until 1993, cerebral palsy was a mystery, known only as a birth defect of unknown origin. Then in 1993 a study (1) was carried out among 3,000 children who were conceived in a twin pair. The amazing result of this research was that wombtwin survivors were 108 times more likely to have cerebral palsy than singleton babies and 13 times more likely than twins whose co-twin survived.
Three out of four of the wombtwin survivors in the study were from identical twin sets. Over the next ten years, more and more studies were carried out which have repeatedly confirmed this strong link between cerebral palsy and the loss in utero of an identical twin.
In 1997 the notion that cerebral palsy was something to do with a vanished twin was made clear by Dr Peter Pharoah (2), who has made this a special area of study.
In a study carried out in Denmark in 2004 (3 ) it was found that as the rate of multiple births doubled in the eleven years from 1980 to 1990, so did the incidence of cerebral palsy. This study confirmed that babies born into a twin or multiple birth are four times more likely than singletons to have cerebral palsy.
In 1976 only 4.6 of the babies born with CP were born in a twin pair or group of multiples, but by 1990 that proportion had increased to 10%. I assume that this increase is due to the increased incidence of twin or multiple births conceived by artificial reproduction methods that began in 1980. Along with this revolution in reproductive medicine has come a marked increase in identical twinning and cerebral palsy.
The idea that people with cerebral palsy are mostly wombtwin survivors has come under close scrutiny for many years. Further research carried out in the UK (4) revealed a further clue: it was found that twin pairs of the same sex were most likely to have cerebral palsy. If these were wombtwin survivors, or lost their twin near birth, the chances were even higher. Most like-sex twins are identical (monozygotic) and it was beginning to look like cerebral palsy may after all be strongly linked to being an identical wombtwin survivor.
A study that seemed to cast doubt on all of this (5) was reported in 2003. Researchers took a group of 467 mothers who had an ultrasound scan during pregnancy. Of these 86 had children with cerebral palsy and had seen evidence of a vanished twin on ultrasound. Only 2 of the control group had seen evidence of a vanished twin on ultrasound. Now the strange result was that bleeding in early pregnancy, which is supposed to be a sign of a vanished twin, was reported by four times as many control mothers as mothers whose babies had cerebral palsy. A mystery indeed, until we consider that ultrasound scanning is something of a blunt instrument and bleeding in early pregnancy is not the only way to diagnose a vanished twin.
Prevalence of cerebral palsy around the world
The incidence of cerebral palsy around the world appears to be consistent in various countries at around .2% of births. (6) However, as cerebral palsy is hard to diagnose exactly, these figures are difficult to confirm. In Japan, where the twinning rate is very low at 0.3%, the incidence of cerebral palsy is about 0.15 of births (7), which is a low rate, but a significant proportion of the twins born in Japan- most of whom are identical (monozygotic)
This consistent result is another clue that cerebral palsy may be linked to being an identical wombtwin survivor. The rate of identical twinning is consistent throughout the world at about 4% whereas fraternal twinning varies very widely. It appears from these statistics that half of the wombtwin survivors in the world are identical wombtwin survivors, who have had a very particular womb experience.
Twin -twin transfusion
In a study made in 2002, it was suggested that twin to twin transfusion may be a probably cause of cerebral palsy. (8) Now twin-twin transfusion only happens in very particular circumstances in the womb. Depending on when the initial cell mass created at conception divides to form twins, each identical twin may have a separate placenta. This occurs in about 70% of identical (monozygotic) twins. If the placentas are not separate, shunting of blood back and forth may or may not occur. If shunting does occur, either, neither or both twins may be damaged.
The degree of damage may vary and there are several possible scenarios. If there is severe damage to both twins, they will both die. One twin may be severely damaged and die very soon while the other is perfectly normal and will survive. (9)
Many of these little identical twins are probably lost in the first four or five weeks of pregnancy, so even an ultrasound scan would not identify them. The fact remains however that children with cerebral palsy, like most identical wombtwin survivors, have a very hard time around birth. It seems that after all problems at birth may after all play an important part in whether a child develops cerebral palsy or not. (10) The mystery remains.
Althea Hayton
References and links
1 Judith K. Grether, Ph.D.; Karin B. Nelson, M.D.; and Susan K. Cummins, M.D. Twinning and cerebral Palsy: Experience in Four Northern California Counties, Births 1983 Through 1985 Paediatrics, December, 1993, Vol.92, Number 6, pp.854-858 link
2 Pharaoh PO, Cooke RW. A hypothesis for the aetiology of spastic cerebral palsy—the vanishing twin. Dev Med Child Neurol 1997;39:292296. link
3 Topp M, Huusom L D, Langhoff-Roos , Delhumeau C, Hutton J and Helen Dolk Multiple birth and cerebral palsy in Europe: a multicenter study Acta Obstetricia et Gynecologica Scandinavica Volume 83 Issue 6 p 548 - June 2004 link
4 Pharoah P O D, PriceT S, PlominR Cerebral palsy in twins: a national study Archives of Disease in Childhood Fetal and Neonatal Edition 2002; 87 :F122-F124 link
5 Newton R Casabonne D Johnson A Pharoah P A Case-control Study of Vanishing Twin as a Risk Factor for Cerebral Palsy Twin Research, 1 April 2003, vol. 6, no. 2, pp. 83-84(2) link
6 Boyle CA, Decoufle P, Yeargin-Allsopp M. Prevalence and health impact of developmental disabilities in US children. Pediatrics. 1994 Mar 93(3):399-403. link
7 Suzuki J Masatoshi I Incidence patterns of cerebral palsy in Shiga Prefecture, Japan, 19771991 Brain and Development Volume 24, Issue 1 , January 2002, p.39 link
8 Glinianaia SV, Pharoah PO, Wright C, Rankin JM; Fetal or infant death in twin pregnancy: neurodevelopmental consequence for the survivor. Arch Dis Child Fetal Neonatal Ed. 2002 Jan;86(1):9-15. link
9 Dr Peter Pharoah, personal communication.
10. Stelmach T, Pisarev H, Talvik T.J Ante- and perinatal factors for cerebral palsy: case-control study in estonia. Child Neurol. 2005 Aug;20(8):654-60. link
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