Introduction
Classification of conjoined twins
Type of fusion | Major clinical types/incidence | Definition/maximum site of fusion | Subtypes | Remarks |
---|---|---|---|---|
Ventral fusion | Cephalopagus 11% | Twins are fused from the head down to the umbilicus. These twins are usually nonviable | 1. Symmetric (two well-formed faces on opposite sides of the head) 2. Asymmetric (one well-formed face and one hypotrophic face on opposite sides) 3. Deradelphous (one midline face) 4. Deradelphous diprosopous (one midline face with duplication of some facial features) | Usually, they have one brain with possible duplication of other structures such as the cerebellum Usually, they share the upper gastrointestinal tracts down to the site of Meckel’s diverticulum, while the large bowels and rectums are separate |
Thoracopagus 47–71% | Twins are essentially fused at the thorax. Usually, fusion extends down to the umbilicus (thoraco-omphalopagus) | 1. Separate hearts and pericardium 2. Separate hearts but common pericardium 3. Fused atria and separate ventricles 4. Fused atria and ventricles | The fusion involves the sternum, diaphragm, upper abdominal wall and liver Sometimes, there is upper gastrointestinal tract fusion down to the site of Meckel’s diverticulum Major cardiac anomalies are nearly always present | |
Omphalopagus 20% | Twins are essentially fused at the abdomen. Hepatic fusion is invariable with no cardiac sharing | There may be variable degrees of small bowel sharing Twins with a single duodenum have a higher incidence of a shared biliary system | ||
Ischiopagus 6–11% | This term describes twins who are essentially fused at the pelvis, usually also with lower abdominal fusion | Commonly, the fusion is ventral (face to face); less commonly, the fusion may be “end to end” with 180° inclination angle between the body axes of both twins There may be hypodevelopment of the lower extremities with missing lower limbs Ischiopagus twins may be further subclassified according to the number of lower limbs as tetrapus, tripus and bipus with four, three or two lower limbs, respectively) | Each twin has anterior pubic diastasis (open pelvic ring like in exstrophy) Facing each other, each pelvis constitutes a hemicircle that are joined together by two cartilaginous joints between opposite pubic bones forming a common pelvic cavity The external genitalia and anus are always shared They may have shared or separate urinary bladders Fusion may extend up to the diaphragm with variable hepatic and gastrointestinal fusion | |
Dorsal fusion | Craniopagus 2% | Twins are fused at any segment of the meninges, bony cranium and skull except foramen magnum. They have separate faces and brain Twins do not have any shared thoracic, abdominal or pelvic organs | 1. Total craniopagus (there are large cranial unions with two brains) 2. Partial craniopagus (there are smaller unions limited to the dural or there is leptomeningeal fusion) Both are further subclassified into angular or vertical | Cortical fusion and shared cerebral venous sinuses may be seen in up to one third of cases Majority have associated cardiovascular, gastrointestinal, genitourinary, craniofacial, and neurologic abnormalities |
Pygopagus 18–28% | Twins are dorsally fused mostly at the sacrococcygeal and perineal regions | Twins usually share a common anus with or without a common rectum The genitourinary tract is less frequently involved The spinal cords are usually separate with or without communication of both thecal sacs The degree of spinal cord fusion determines the possibility of separation | ||
Rachipagus Less than 1% (rarest type) | Twins are fused dorsally above the sacrum | The union may involve the occiput and vertebral column in variable degrees One twin may be parasitic | ||
Lateral fusion: (side-by-side fusion) | Parapagus 28% | Twins have shared umbilicus, abdomen, and pelvis. The shared pelvis usually has a single symphysis pubis and one or two sacra | 1. Dithoracic parapagus: the union is limited to the abdomen and pelvis 2. Dicephalic parapagus: the union involves the entire trunk. They have single heart with cardiac anomalies 3. Diprosopic parapagus: one trunk and one head with two faces. two faces are on the same side of the head | They usually have associated genitourinary and anorectal anomalies Usually, there is underdevelopment of the upper and lower extremities with missing limbs. Parapagus twins may have two, three, or four arms, and only two or three legs |
Determination of spatial relationship and selection of postnatal imaging studies
General imaging guidelines | • Twins should have the same orientation during all imaging investigations • CT is performed by using multislice thin section machines • The dose of contrast agent is calculated according to the combined weight by using pump injector (contrast dose and iodine concentration may vary by institution) • Each twin should be injected on a separate day. Twins with limited pelvic fusion (as pygopagus twins) may undergo CT with simultaneous contrast injection • The images are transmitted into the workstation for additional post-processing and multiplanar reconstruction • MRI usually needs general anesthesia (General anesthesia is better than deep sedation) • Two anesthesia teams should be available throughout the study • A phased array surface coil is used to cover both twins during MRI examination |
Twins with fusion of the cranium and vertebral column | Imaging aims for evaluation of the central nervous system • Non-contrast CT with 3-D reconstruction of the bony calvarium and vertebral column • CT cerebral arteriography and venography • MRI standard sequences in addition to high resolution, 3-D heavily T2-WI volumetric sequences of the brain and spinal cord • Non-contrast MR arteriography and venography • Dynamic post-contrast MR angiography of the cerebral circulation • Functional MRI and Diffusion tensor imaging |
Twins with thoracic fusion` | Imaging aims for evaluation of the cardiac and pulmonary systems • Dynamic post-contrast study in the arterial and venous phases • ECG-gated post-contrast CT for cardiac anomalies • 3-D reconstruction of the thoracic cage |
Twins with abdominal fusion | Imaging aims for evaluation of the hepatobiliary and gastrointestinal systems and kidneys • Post-contrast study in arterial and venous phases for demonstration of shared arteries, or veins and demonstration of the surgical plane of hepatic separation • MRCP and HIDA scan: to rule out biliary fusion if suspected • Upper gastrointestinal series and follow through |
Twins with fusion of the pelvis and perineum | Imaging aims for evaluation of the urogenital systems, distal gastrointestinal system and perineum • Post contrast study in arterial and venous phases with acquisition of multiple delayed phases to demonstrate urologic anomalies especially of the urinary bladder and ureters • 3-D reconstruction of the bony pelvis • High resolution T2-weighted image in axial and coronal planes with and without fat suppression for demonstration of the associated genital, urologic anomalies, and pelvic floor anatomy • Distal loopogram (if colostomy is present) for demonstration of anorectal anomalies (e.g., rectourethral fistula) • Micturating cystourethrography demonstrate of reflux and other associated renal anomalies especially if there is urinary tract dilatation (detected by ultrasound) |