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Multi–Detector Row CT for Depicting Anatomic Features of Cephalothoracopagus Varieties: Revised Approach¹

¹ From the Departments of Clinical and Experimental Surgery (R.G., M.C., S.C., F.S.S., A.R.) and Public and Preventive Medicine, Section of Human Anatomy (V.E.), Seconda Università, Piazza Miraglia, 80100, Naples, Italy; Department of Radiology, Cardarelli Hospital, 80100, Naples, Italy (M.S.); and Department of Diagnostic Imaging, IRCCS Ospedale Casa Sollievo della Sofferenza, Foggia, Italy (G.G.). Address correspondence to R.G. (e-mail: roberto.grassi@unina2.it).

	Abstract

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Conjoined twins can be classified on the basis of the site of union; thus, three main types can be described: (a) ventral union, (b) dorsal union, and (c) rarer forms of union. Ventral union is characterized by the fusion of the two embryos on the ventral side (eg, the abdomen). Dorsal union twins are joined on the dorsal aspect (eg, the vertebral column or occipital bone). Ventral union twins include the group of crucipage twins (ventral midline structures at 90° to the dorsal midline structures), which show interesting features in the organization of the midline. Twins conjoined at the head and chest are called cephalothoracopagus twins. The cephalothoracopagus variety called "Janus" is characterized by the presence of two opposite faces, which are composite structures half of which belong to one twin and half to the other. A complete set of five variants of cephalothoracopagus is presented and, to the authors' knowledge, analyzed for the first time with multi–detector row helical computed tomography. This modality is an invaluable tool for obtaining high-resolution images of the brain, chest, abdomen, and spine and for demonstrating organ position, shared viscera, and limited vascular anatomy. In addition, data acquired in three-dimensional volumes can further be manipulated and then reconstructed. For this purpose, the authors developed dedicated software for three-dimensional reconstruction to analyze data from specimens preserved in formalin. The anatomic findings are discussed here for their embryologic value and to revise the classification of cephalothoracopagus twins. These data offer detailed information for accurate comprehension of imaging studies and for theoretical studies concerning the formation of several anatomic structures.

© RSNA, 2004

Index Terms: Twins, abnormalities, 856.12115, 856.12117, 856.879

	Introduction

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The existence of conjoined twins has raised many artistic and philosophic questions throughout history, as, for example, whether these twins possess one soul or two. The ancient pantheon of monstrous gods may be regarded as the oldest classification of human monstrosities, and their representation as the oldest record. The earliest record of conjoined twins was made in 945 AD in Constantinople. Known as the Armenian twins, the boys were conjoined from the waist to the abdomen. The attempt to separate them was unsuccessful and both died. In 1841, Adolph Guglielmo Otto gave a precise description of an impressive collection of conjoined twins in the museum of Vratislavia in his book Monstrorum Sexcentorum Descriptio Anatomica (1).

Conjoined twins are classified according to the site of union by using the suffix pagus (fixed). The most widely accepted classification follows Spencer (2) and is reported here with minimal variation.

Ventral Union (87% of Cases)
Rostral (48% of Cases)— Cephalopagus (11% of cases)— Twins are joined at the head and, depending on the variety, at the chest and the abdomen, resulting in a single deformed head with two complete bodies. This configuration is rare and nonviable. A subset of cephalopagus (1.7% of conjoined twins, one case out of 58 conjoined-twins cases, one case out of three million births) is represented by cephalothoracopagus twins. These twins are joined from the top of the head to the umbilicus, resulting in a single head, fused chests, and two lower bodies. In this case, the orientation of the twins determines the position of their faces. Four types can occur (Table): (a) symmetric, or janiceps, in which the two faces are well formed, face opposite directions, and are perpendicular to the plane passing through the two vertebral columns; (b) asymmetric, or iniop, in which one face is well shaped and the other is hypotrophic (from cyclopia to two pairs of ears), with the faces oriented oppositely and perpendicular to the plane passing through the two vertebral columns; (c) craniothoracopagus, or deradelphus, in which only one face is present in the single head, at the midline between the two vertebral columns. There is only one brain, and the hearts and gastrointestinal tracts are fused; (d) the fourth variety is discussed later.

Omphalopagus (18%–34% of cases)— Twins are joined at the lower chest, anywhere from the sternum to the waist. Omphalopagus twins are much more viable than the similarly joined thoracopagus twins because the heart is never involved. A subset of omphalopagus twins, xiphopagus twins, consists of twins joined at the xiphoid cartilage of the sternum. These twins rarely share vital organs other than the liver and are generally easy to separate. The original "Siamese" twins, Chang and Eng Bunker (1811–1874), were xiphopagus twins.

Caudal (11% of Cases)— Ischiopagus— Twins are joined end-to-end at the lower abdomen and pelvis (coccyx and sacrum, genitourinary system), with legs on either side of the shared waist. The viability of ischiopagus pairs depends on the number of shared vital organs.

Lateral (28% of Cases)— Parapagus— Twins are joined side-to-side at the lower body (pelvis and variable trunk) and face the same direction. Because they have separate heads and upper bodies, they are also called dicephalus twins.

Dorsal Union (13% of Cases)
Craniopagus (2%–5% of Cases)— Twins are joined at the cranial vault and often share brain tissue and/or circulation. This is a rare form of conjoined twinning; one occurs in 2.5 million births. Most pairs are viable if left conjoined. The twins can be joined at the top or the side of the cranial vault (vertical craniopagus), in which case they lie on the same axis but are oriented in opposite directions, laterally or frontally.

Rachipagus (2%)— Twins are joined at the vertebral column.

Pygopagus or Ileopagus (6%)— Twins are joined back-to-back at the hips and sacrum.

Rarer Forms
Dipygus or Caudal Duplication Syndrome— Duplication of the inferior half of the body, below the lumbar vertebrae. These infants often have two pairs of legs, two pelvises, and two sets of genitals.

Diprosopus Simple— Duplication of facial structures on a single head. Humans with this condition are usually stillborn, but cattle and sheep can live for some time.

Parasitic Twin— Parasitic or asymmetric conjoined twinning can vary from a single extra leg to a complete second body that is fully dependent on the first.

Fetus in Fetu— One twin is encapsulated in a cyst or tumor inside the other.

Other Forms— The existence of nonidentical conjoined twins (eg, different sexes or skin color) and conjoined triplets is still dubious.

The subset of cephalothoracopagus twins is particularly interesting given its features at the midline (3). To our knowledge, no previous reports detailed the variants of cephalothoracopagus twins with multi–detector row helical computed tomography (CT) and three-dimensional (3D) reconstructed images. Thus, four-section multi-detector CT and 3D visualization techniques have been used here to give further insights into this rare type of conjoined twinning.

In the present study, CT findings offer a revisited appraisal of Spencer’s classification and provide details for future studies.

	Materials and Methods

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The twins under study came from the Museum of Anatomy of the Second University of Naples. Specimens were collected in the early decades of the 20th century and preserved in formalin. Five specimens of cephalothoracopagus twins were scanned with a four-section multi-detector CT scanner (Aquilion super 4; Toshiba, Tokyo, Japan). Images were acquired from head to feet with the following scan parameters: collimation, 4 x 0.5 mm; 120 kVp; 350 mAs; matrix, 512 x 512; pitch, 2. Three-dimensional volume rendering was performed on a commercially available workstation (J-Vision; Tiani, Vienna, Austria). Dedicated software named "Schadel Plastik 1" and "Schadel Plastik 2" was produced with the cooperation of a software company (REM, Baronissi Salerno, Italy) to allow reconstruction of all anatomic data and the simultaneous 3D evaluation of whole-body structures by assignment of distinct colors to the various fetal tissues, whose densities were sometimes similar and difficult to differentiate after the effects of the formalin, which was imbibed by tissue.

	Results

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The internal organs were well represented at CT. Brain regions showed some artifacts (regions of vacuolization), possibly due to the long fixation time.

The five cases are described.

Deradelphous Diprosopous Cephalothoracopagus
The twins are joined at the head and chest (Movie 1a). Structures of the splanchnocranium show an incomplete division at the midline, resulting in three eyes (one eye is shared) and two nasal and oral cavities. At the bottom of the central, shared ocular cavity, two separate optical foramina for the ocular nerves can be traced (Movie 1b). The inferior part of occipital bone is severely deformed and shows two great foramina. The two vertebral columns are completely independent and diverge inferiorly. The arms and legs of the twins are well formed and independent (Movie 1c). The CT scan does not allow distinction of visceral structures, owing to the prolonged fixation time of the specimen.

Deradelphous Cephalothoracopagus
The twins are joined at the head and chest (Movie 2a). The ocular cavities are well shaped. The nasal cavities show an initial attempt at duplication: The nasal septum is duplicated, but there is a shared middle nasal cavity (Movie 2b). The oral cavities are also complex and deformed because of the presence of a septum (Movie 2c).

Iniop Cephalothoracopagus
The twins are joined at the head and chest (Movie 3a). The fusion of the cranial parts is incomplete. In fact, the single head shows a composite face on one side. On the other side, only two ears are present, whereas the midline structures are missing. This phenomenon is called "concentration of the facial structures" and consists of the hypotrophy of one face when compared to a perfect Janus. The face is well shaped but is a composite structure shared by the twins. The sphenoidal bone represents the center of mass of the single head and is a deformed structure shared by the twins. Two occipital bones are evident and inferiorly articulate with two completely independent vertebral columns that diverge inferiorly. The two sets of arms and legs are well formed and independent. In this case, the hyperdensity of the specimen made the "transparent view" impossible to achieve. The CT scan also depicts the surface of some thoracic and abdominal structures (eg, heart, thoracic aorta, diaphragm, liver, peritoneal folds, bowel loops) (Movie 3b).

Iniop Cephalothoracopagus
The twins are joined at the head and chest (Movie 4a). The single head shows a composite face on one side. On the other side, the face is hypotrophic, consisting of two ears only. On this side, a single empty ocular cavity (ciclopy) is also evident (Movie 4b). At the neurocranial level, the ocular cavities are well shaped. The nasal cavities show an initial attempt at duplication: The nasal septum is duplicated, but there is a shared middle nasal cavity. The oral cavities are complex and deformed owing to the presence of a septum (Movie 4c).

Symmetric Cephalothoracopagus (Janiceps)
The twins are joined at the head and chest (Movie 5a). The fusion of the cranial parts is symmetric. The single head shows two composite faces, one on each side. This might be due to the enlargement of the cranial vault (Movie 5b). On one side, the two eyes of the face are contained in a single ocular cavity, giving it a cyclopic aspect. It is important to note that each face is well shaped but is shared by the twins and is therefore a composite structure belonging half to one twin and half to the other. The sphenoidal bone represents the center of mass of the single head and is a deformed structure shared by the twins. A single sella turcica communicates with the pharyngeal cavity. The two occipital bones are independent and inferiorly articulate with two completely independent vertebral columns that diverge inferiorly. The arms and the legs of the twins are well formed and independent (Movie 5c).

	Discussion

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According to the "fission" hypothesis, monozygotic, or identical, twins form when a single fertilized egg splits into two embryos; conjoined twins are monozygotic multiples that do not fully separate from each other (2,4,5). The phenomenon occurs at the blastula stage, between the 13th and 15th day after fertilization. At this time, the cells of the inner mass are totipotent and are capable of forming two normal individuals if they split equally, resulting in identical monozygotic twins. When incomplete separation occurs, conjoined twins result. Moreover, incomplete division of the embryo is associated with inhibition of complete differentiation of various organ systems, thus resulting in shared, incompletely developed organs. It has recently been proposed that conjoined twins result rather from secondary fusion of two separate embryos (6). This hypothesis acknowledges that the intact ectoderm of an embryo will not fuse and that the embryonic disks could unite only at locations in which the ectoderm is normally absent or normally programmed to fuse or break down (6).

Like all monozygotic twins, conjoined twins are always the same sex, both male or both female. It is estimated that 70% of conjoined twins are female. The further development of conjoined twins largely depends on the original situation of the two notochords. Recently, we have shown interesting morphologic data concerning a rare and fascinating configuration of conjoined twinning called janiceps (cephalothoracopagus symmetricus) (3). In fact, the twins are frontally joined from the head to the lower part of the abdomen. Each of the resulting two lateral faces is thus composed of two "hemifaces," one hemiface from each twin (2,7–12). Because of the difference between median posterior structures (such as the vertebral columns) and median anterior ones (such as the faces), placed on perpendicular planes, the study of structures that usually cross the midline, such as nerve decussationes, is of special interest. While the median posterior line is determined by the notochord, the median anterior line is formed by the fusion of the two sides of the body. The existence of two different symmetry axes makes this malformation a valuable tool for studying the midline structures. In particular, since many data are now available regarding the mechanisms that drive the formation of the optic commissure in animal models (13,14), janiceps twinning may be used to test these mechanisms in human beings.

This study consists of a number of preserved specimens collected in the early decades of the 20th century. Nowadays, apart from a few case reports published in the literature (15–20), such a collection of cephalothoracopagus twins will probably be unavailable because of the use of screening ultrasonography (US) in the prenatal period and the current laws regarding the interruption of pregnancy during the first 3 months. However, in this context, prenatal detection of fetal conjoined twinning anomalies with US can provide essential findings for early management planning (21–23).

In vivo, conjoined twins can be imaged with a combination of plain radiography, US, echocardiography, CT, and magnetic resonance (MR) imaging. Each modality depicts specific features and helps identify potential structural abnormailities in conjoined twins (24). MR imaging offers optimal anatomic details and yields additional information with respect to the other imaging modalities (25). Nevertheless, compared with US, MR imaging has lower sensitivity for assessing cardiac abnormalities because MR images are not gated for fetal cardiac motion. CT offers better lung and hollow viscus details, notwithstanding the use of x rays.

In the present study, the high CT scan resolution of the specimens is due to the thin acquisition thickness and the resulting isotropic voxel. In addition, dedicated software was created to differentiate and enhance body structures altered by the protracted exposure to formalin. Therefore, specific software is now available for further museum specimen studies, which may suggest additional considerations, implications, and details in theoretical, embryologic, morphologic, and classification studies. Moreover, interpreting these new images poses a unique and fascinating challenge to the radiologist who is studying rare abnormalities, such as those described here, and suggests new diagnostic elements and revisions of previously reported classifications.

	Footnotes

 
Abbreviations: 3D = three-dimensional.

	References

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This Article Abstract MPEG movies All Versions of this Article: e21v1 24/5/e21    most recent Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Grassi, R. Articles by Rotondo, A. Search for Related Content PubMed PubMed Citation Articles by Grassi, R. Articles by Rotondo, A.