Conjoined Twins: Developmental Malformations and ... - SonoWorld

Conjoined Twins: Developmental Malformations and Clinical Implications
Pediatric and Developmental Pathology
DOI: 10.1007/sl0024-003-4038-x
© 2003 Society for Pediatric Pathology
Conjoined Twins: Developmental Malformations and
Clinical Implications
Rowena Spencer. The Johns Hopkins Press, Baltimore, MD, 2003. 476
pages, $85.00 (hardback).
J. Bruce Beckwith
Department of Pathology and Human Anatomy, Loma Linda University, Loma Linda, CA, USA
Online publication: 6 June 2003
Correspondence to: J. Bruce Beckwith — Email: beckwithbrowning(a),earthlink.nct
This book represents many years of dedicated study by a respected pediatric surgeon with a passionate
interest in conjoined twinning. Her extensive clinical experience and unparalleled familiarity with 20th
century case reports led me to expect a superb treatise on clinical and anatomical aspects of this subject.
It was disappointing to find the book to be centered primarily upon her controversial theory of
embryogenesis. Embryological speculation occupies many pages that could have been devoted to topics
relevant to the expertise of the author and the needs of her clinical colleagues. Many challenging
technical and ethical problems involved in the management of conjoined patients are ignored, and most of
the others are given only brief attention. Illustrations, an indispensable feature of any treatise on
conjoined twinning, are relatively sparse and unhelpful, with virtually no illustrations of internal
anatomy. Most of the figures are diagrams based upon her embryological speculations, or drawings and
photos of external features. The surgeon, pathologist, or radiologist puzzling over the anatomical
peculiarities of a case of conjoined twins will likely be disappointed by the relative paucity of detailed
anatomical description and useful illustrations in this book.
A positive feature is the bibliography, which cites a substantial portion of the 20th century literature in
English on conjoined twinning in humans, along with occasional references in German and French
(including numerous misprints), and a few from previous centuries. This list includes many references
from years not covered by Medline searches. Also welcome are tables listing certain features of cases in
some of her categories of twinning.
http://link.springer-ny.com/!ink/service/journals/10024/contents/03/4038/paper/body.html (1 of 5) [9/9/2003 11:23:52 AM]

Conjoined Twins: Developmental Malformations and Clinical Implications
The embryological theory of the author is that all conjoined twins result from fusion of separate germ
discs. This is presented with an evangelistic zeal that betrays a lack of objectivity, with few qualifications
or expressions of doubt. She assumes that all patterns and degrees of twinning are due to the same
mechanism, and that fusion and fission are the only two mechanisms requiring consideration, so that
disproof of one proves the other. Concepts that might apply to some cases are applied universally. For
example, on page 416, the author expresses without qualification the opinion that teratomas are parasitic
conjoined twins.
Her discussion of the theoretical embryology of "parapagus" twinning (pages 246-247) provides a
particularly disturbing example of fundamental convictions based upon unconvincing evidence. She
dismisses fission as the mechanism of rostral duplications in the following words:
Bifurcation of a single notochord has been proposed (Machin and Sperber, 1991), but as
the notochord probably elongates by proliferation at the caudal aspect (O'Rahilly, 1998),
bifurcation during growth should result in caudal, not cranial duplication [italics mine].
The implied support of a leading authority on embryology lends an aura of respectability to this
statement. However, the only reference provided is to a personal communication, and the adverb
"probably" seems to be given the status of certainty. That statement constitutes one of two chief
arguments offered as proof that "parapagus" twinning results from fusion of originally separate germ
discs. The other argument is presented in the following words (p. 246):
In a review of the entire spectrum of parapagus—it might seem logical to assume that they
arise from two separate, nearly parallel notochords on one embryonic disc, very close
together caudally but with varying degrees of separation rostrally. However, secondary fusion
of two separate embryonic discs is indicated by the indisputable evidence of duplication of
some portion of the umbilicus, umbilical cord, or placenta in a few parapagus [italics mine].
The reasons why these few "indisputable" cases so conclusively prove her point are not explained
further. No details are provided concerning these cases, and I searched in vain for reference to where
they were published. On page 254, it is mentioned that these cases comprised 2% (i.e., 6) of her 305 pairs
of "parapagus" twins. On page 246, it is mentioned that three of them were characterized by "an
additional umbilical vessel," without noting whether these were arteries or veins, or how their number
was documented. Accessory umbilical vessels can result from several developmental mechanisms other
than fusion of separate embryonic discs. These six "indisputable" cases, plus the aforementioned
reference to a statement by Dr. O'Rahilly, are considered sufficient to justify the following sweeping
generalization (p. 246):
All things considered, then, it is postulated that all conjoined twins, including the
parapagus, arise from two individual notochords on two originally separate embryonic
discs [italics mine].
http://link.springer-ny.com/link/service/journals/10024/contents/03/4038/paper/body.html (2 of 5) [9/9/2003 11:23:52 AM]

Conjoined Twins: Developmental Malformations and Clinical Implications
Dr. Spencer's speculations on the fusion-fission controversy resurrect a debate that was popular during
the 18th and early 19th centuries, when embryology was primarily the domain of philosophers. During
the latter part of the 19th century, this question was subjected to scientific investigation, especially in the
great embryological laboratories of Germany. It was soon recognized that the original philosophical
concepts, designated in German as the "Spaltungstheorie" and "Verwachsungstheorie" ("fission" and
"fusion" theory), were too simplistic. Though the evidence was more favorable to the "fission" theory
(i.e., incomplete separation of monozygotic twins), it was also apparent that the induction and
organization of body axes was more complex than these terms implied. Fusion versus fission was
rephrased in terms more relevant to evolving concepts of developmental mechanics. The monographs of
Schwalbe [1] and Hiibner [2] provide detailed overviews of experimental studies into the developmental
mechanisms of conjoined twinning in the decades preceding the First World War. Hiibner's monograph
[2], published in 1911, includes an appropriate epitaph for the simplistic fusion-fission controversy (p.
792):
"Verschmelzung 1 oder "Spaltung 1 , dies beiden Worte, die einst so heftige Discussionen
verursachten, sind heute blosse Worte ['Fusion' or 'splitting,' these words that once
provoked such violent controversy, are now empty words].
In 1923, Newman's analytical review of the embryology of twins [3] added several new arguments
refuting fusion as the mechanism for most cases of conjoined twinning, and that concept received little
support in the following decades. The experimental embryology of conjoined twinning has now been
extended to the molecular level. One model involves the production of axial duplications in xenopus
larvae by injection of components of the Wnt/beta-catenin cascade at the 2 to 8-celled stage [4 5].
Comparable results are reported in zebrafish [6]. The formation and development of two-headed embryos
and other axial duplication anomalies can be observed continuously in vivo. These and numerous other
experimental models developed over more than a century provide substantial evidence inconsistent with
Spencer's fusion theory of "parapagus" twinning. The existence of that literature is briefly mentioned in
Spencer's chapter 2, but is dismissed on the basis that animal models do not necessarily apply to humans.
Perhaps experimental embryology would not have been dismissed so casually if it had provided more
support for her speculations. Armchair embryology may suffice for politicians and theologians, but is a
poor substitute for descriptive and experimental science.
Chapter 9 is devoted to the "rachipagus" pattern, a novel category of conjunction introduced by Dr.
Spencer in 1995 [7]. She considers this to be the pattern providing the most convincing proof of her
developmental theory. Twins joined back-to-back, with fusion of spinal columns above the sacrum, were
not believed to exist until Dr. Spencer discovered a suggestive case report in a French regional medical
journal [8]. That report (which I have not seen) was a radiographic study of 7-month fetuses said to be
joined back-to-back. No photographs or anatomical details are provided. The radiograph, as reproduced
on page 333 and in Spencer's 1995 article, depicts two individuals that appear to have posterior fusion of
skulls, two faces directed in opposite directions, and apparent coalescence of vertebrae T6 to L3. The
spinal columns diverged in the lumbar and cervical regions. The reproduced image is small and
http://link.springer-ny.com/link/service/journals/10024/contents/03/4038/paper/body.html (3 of 5) [9/9/2003 11:23:52 AM]

Conjoined Twins: Developmental Malformations and Clinical Implications
somewhat fuzzy. Though it appears impressive, it conceivably could be misleading. "Janiform"
cephalothoracopagus twins, connected ventrally from umbilicus to vertex with two composite faces
oriented in opposite directions, might yield a similar appearance if the orientation of the radiograph was
perpendicular to the midfacial axis, with the two spinal columns partially superimposed, and each pelvis
was rotated approximately 90 degrees to face the same direction as one of the faces.
A few bizarre cases reported in Table 9-1 and in a 1996 article by Spencer et al. [9] are offered as
possible examples of rachipagus twinning, but none provides convincing proof of her interpretation.
Taruffi's monumental review of the world literature up to the early 1880's [10] disclosed no anatomically
confirmed case similar to that depicted in the French radiological paper, and a Medline search reveals no
recent reports of similar cases. Furthermore, I have not found a convincing description of this type of
twinning in another vertebrate species. Before adding the rachipagus pattern to the list of established
patterns of conjoined twins, I would like to see more anatomical evidence than the aforementioned single
radiograph.
The designation of rachipagus is not my only concern about the classification scheme and nomenclature
proposed by Dr. Spencer, which is based primarily upon her embryological theory. It adds new areas of
confusion and controversy without resolving old ones. Simplification is achieved by lumping the most
difficult and controversial patterns of twinning together into a single "atypical" category. Novel words
are introduced and older terms are given new meanings. While insisting upon precision in the application
of some terms, she is imprecise in others. For example, "omphalopagus" has long been used for ventral
supraumbilical conjunction confined to the umbilicus or cord, but Dr. Spencer extends the use of this
term to all cases in the thoracopagus series with separate hearts, regardless of the degree of thoracic
conjunction. Cardiac anatomy is so dominant a consideration in the surgical evaluation of conjoined
twins that there is no obvious need to incorporate it into the name of the twinning pattern at the cost of
anatomical precision.
Creation of the "parapagus" category is, in my view, a particularly unfortunate one. This designation is
proposed for a spectrum of patterns traditionally viewed as axial duplications, most often observed in the
anterior (or rostral) end of the axis. The "parapagus" designation reflects Spencer's fusion theory, which
seems unlikely to achieve wide acceptance. Existing designations for rostral and caudal axial
duplications accurately describe the anatomy of this pattern, and are consistent with the findings of
experimental and descriptive embryology.
Without belaboring all my reservations about this classification scheme, I will simply offer the opinion
that problems involving conjoined twins are not likely to be resolved by new taxonomies. In the
molecular era, new paradigms might lead to entirely different ways of thinking about twinning, but until
then I plan to muddle along with the familiar imperfections of existing terminologies rather than
accepting this new one.
My affection and respect for the author, with whom I have enjoyed a cordial correspondence for many
years, makes it especially painful to express so many negative comments about a book that represents so
http://link.springer-ny.com/link/service/joumals/10024/contents/03/4038/paper/body.html (4 of 5) [9/9/2003 11:23:52 AM]

Conjoined Twins: Developmental Malformations and Clinical Implications
much time and effort, and is so dear to her heart. My disappointment in this book reflects the height of
my expectations. If only she had left embryology to embryologists and focused upon clinical anatomy,
she could have created a splendid work of permanent value. In spite of its disappointing features, it is the
most extensive monograph on this topic in the English language, and contains information not easily
found elsewhere. It therefore deserves a place on the shelves of those who deal with these amazing and
eternally challenging patients.
References
1. Schwalbe E (1907) Morphologie der Missbildungen des Menschen und der Tiere, Teil II: Die Doppelbildungen. Gustav
Fischer, Jena, Germany
2. Hiibner H (1911) Die Doppelbildungen des Menschen und der Tiere. Ergebnisse der Allgemeinen Pathologie und
Pathologischen Anatomie des Menschen und der Tiere 15:649-796 [PubMedl
3. Newman HH (1923) The Physiology of Twinning. University of Chicago Press, Chicago
4. Molenaar M, van de Wetering M, Oosterwegel M, et al. (1996) XTcf-3 transcription factor mediates beta-catenininduced
axis formation in Xenopus embryos. Cell 86:391-399 [PubMedl
5. McMahon AP, Moon RT (1989) Ectopic expression of the proto-oncogene int-l in xenopus embryos leads to
duplication of the embryonic axis. Cell 58:1075-1084 [PubMedl
6. Kelly GM, Erezyilmaz DF, Moon RT (1995) Induction of a secondary embryonic axis in zebrafish following the
overexpression of beta-catenin. Mech Dev 53:261-273 [PubMedl
7. Spencer R (1995) Rachipagus conjoined twins: they really do occur! Teratology 52:346-356 [PubMedl
8. Betoulieres P, Caderas de Kerlau J, Gevaudand L, et al. (1960) Etude radiologique de squelette d'un monstre double
janicephale-rachipage. Montpellier Med J 58:30-39 [PubMedl
9. Spencer R, Chuaqui B, Garcia CJ (1996) Anencephaly, dorsal hypermelia, and duplication of the vertebral column: a
rare type of rachipagus conjoined twins. Teratology 53:253-260 [PubMedl
10. Taruffi C (1881-1894) Storia dell Teratologia, 8 volumes. Regia Tipografica, Bologna, Italy
http://link.springer-ny.com/link/service/joumals/10024/contents/03/4038/paper/body.html (5 of 5) [9/9/2003 11:23:52 AM]