Twins

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Questions Received:

  1. I am doing a paper on conjoined twins and need to know how is the care for conjoined twins different from that of twins?

  2. I have to do a presentation on birth defects and one of them is parapagus. The baby had normal upper and lower limbs and apparently only one thorax, but the spinal cord was exposed all along the back as well as the brain. It had one head, but the head looked too wide with two faces. What is the embryological developmental cause, eg: incomplete folding of the neurotube?

Responses:

I am doing a paper on conjoined twins and need to know how is the care for conjoined twins different from that of twins?

14th February 2000

The care given to twins after birth has some differences compared with care given to single babies. This is because delivery and neonatal complications occur more frequently in twin births than singleton births, the most common being premature delivery and low birth weight (Thigpen, 1996). The birthing process is also potentially more difficult with twins, but vaginal delivery is often possible if the babies are in suitable positions. Monozygotic twins differ from dizygotic twins in several ways, and it is preferable if this distinction can be made soon after birth and the care adapted accordingly. The identification can be based on several types of information. For example, if the sexes differ, the twins are dizygotic, if they are the same sex and share a single chorion, they are monozygotic, while if there are two chorions they may be monozygotic or dizygotic and a blood test will be required to compare the blood groups of the twins. If the blood groups differ, then the twins are dizygotic. DNA comparisons are now possible if required.

The twins are examined for birth defects after birth. Monozygotic twins have a somewhat higher risk of having birth defects than dizygotic twins. Some monozygotic twins have a shared blood circulation before birth, and an imbalance in blood supply can result in one twin being appreciably larger than the other. There may be pressure deformities of the limbs due to crowding in the womb, but these usually disappear completely in a few weeks after birth. Looking after two newborn babies can be exhausting, so additional support will ensure that the parents of twins are able to cope. The weight and height difference between singletons and twins decrease significantly in the first three months and disappears during childhood. At birth, monozygotic pairs differ more in size than do dizygotic twins, but become more similar as they grow. In contrast, dizygotic twins differ more as they grow postnatally.

The care required by conjoined twins will depend on the type and degree of conjunction. (diagrams) Conjoined twins are genetically identical twins who develop from a single fertilised ovum. During pregnancy they share the same amniotic cavity, chorion, and placenta. They are always the same sex, and more commonly female. At some point after birth the possibility of surgical separation will have to be considered. This decision is complicated by ethical problems, for example the allocation of shared organs and in some instances the necessity for one twin to die to save the other. Some conjoined twins have progressed to adulthood successfully without separation, so separation may not be the best option in every case (Raffensperger, 1997).

Preparing for the surgical separation of conjoined twins involves multidisciplinary planning by nurses, pediatricians, radiologists, anaesthetists, and surgeons (Winch and Gonyea, 1994; Wirt et al, 1995; Doski et al, 1997; Wilcox et al, 1998). Detailed investigation of the twins will be required before separation is attempted. If possible, separation is best delayed for several months after birth to give a better chance of survival (O'Neill et al, 1988). However, emergency separation may have to be undertaken if there are life-threatening complications (Jaffray et al, 1999).

If too much tension is placed on the skin flaps used to close the separation sites, there can be problems with wound breakdown (Zubowicz and Ricketts, 1988). Recovery is improved if sufficient tissue (for example: skin) or a prosthetic mesh is available to cover the exposed areas. Techniques of tissue expansion have been developed to prepare a surplus of the tissues required to achieve closure before the operation takes place (Zuker, Filler, and Lalla, 1986; O'Neill et al, 1988; Zubowicz and Ricketts, 1988; Hilfiker et al, 1998). (An example of physiological tissue expansion occurs during pregnancy as the mother’s abdominal wall enlarges to accommodate the growing baby within.)

Clearly the birth of conjoined twins will give rise to difficult dilemmas for parents and hospital staff, and the care provided will need to take these into account (Jakobowski, Hagelgans, and Leson, 1989).

References

I have to do a presentation on birth defects and one of them is parapagus. The baby had normal upper and lower limbs and apparently only one thorax, but the spinal cord was exposed all along the back as well as the brain. It had one head, but the head looked too wide with two faces. You could think of it as having three sides; the exposed brain at the back, three quarters of one face and three quarters of another. I say 3/4 because the left eye of the one face was joined to the right eye of the other face looking like two half eyes stuck together.

The baby also had cleft lips jaws and palates. The faces looked flattened typical of that of anencephaly and the baby was male. What is the embryological developmental cause, eg: incomplete folding of the neurotube?

21 February 2005

From your description, it seems that there was partial duplication of the embryonic axis at the future head end of the embryo at the stage when the neural plate was forming, towards the end of the 3rd week of development. A possible sequence of events that may have occurred at that time is illustrated in Figure 1.

Normal and Abnormal Sequence Closures< Figure 1 - Showing Normal and Abnormal Sequence Closures

 

 

Clearly, the neural plate has subsequently failed to close, both in the brain region and the spinal cord region. Two faces have been created, but as you say there has been a degree of midline merging of facial structures, together with several cleft defects. The partial fusion of the two medially-placed eyes reminds me of the situation quite often seen in cyclopic babies – the single midline eye has duplication of corneas, lenses, and irises, and may have a partial internal septum. The two images below of a cyclopic baby show a partially-duplicated eye of this sort:

Cycloptic Baby I< Figure 2 - Cycloptic Baby I

 

 

 

 

Cycloptic Baby II< Figure 3 Cycloptic Baby II

 

 

 

My interpretation of this type of abnormality is that two optic vesicles have come into proximity at an early stage and become partially fused, with the constituent cells trying to make the best of the new circumstances. During normal development, the optic vesicles grow laterally away from each other and this interaction does not occur, but in the case you have described and in cyclopia the two optic vesicles have come into contact and partially merged. The degree of merging will determine whether or not they induce two lenses or a single lens from the overlying ectoderm.

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