Conjoined Twins Conjoined twins are incompletely separated monozygotic i. A fetiform mass located within a basically nor- twins. They have long fascinated both medical profession and lay mal fetus public. Such events are rare and occur in 1/50,000 to 1/100,000 ii. Inclusion of a monozygotic diamniotic twin births and one in 400 sets of monozygotic twins. It is a complica- within the bearer is the best explanation tion of monochorionic twinning at 13 to 15 days after conception. 5. Duplicitas symmetros (symmetrical conjoined twins resulting from incomplete fission of the uniovum) a. Terata Katadidyma (twins joined at the lower part of GENETICS/BASIC DEFECTS the body and double above) 1. Conjoined twins i. Dicephalus (twins with 2 separate heads and necks a. Rare variants of monozygotic, monochorionic twins side by side with 1 body; lateral conjugation) b. Two theories of con-joined twin formation ii. Diprosopus (twins with 2 faces side by side, 1 i. Resulting from the secondary union of two orig- head, and 1 body) inally separate monovular embryonic discs iii. Ischiopagus (twins joined by the inferior mar- ii. Resulting from an incomplete separation of the gins of the coccyx and sacrum with 2 completely inner cell mass at around 13–15 days of gesta- separate spinal columns; caudal conjugation) tion of the monovular twins iv. Pygopagus (twins joined by posterior surfaces of c. Twins can be conjoined at any site from the cranium the coccyx and sacrum, back to back; posterior downward to the sacrococcygeal region conjugation) d. Approximately 60% are stillborn v. Craniothoracopagus e. Female predominance: approximately 3:1 female-to- vi. Ileothoracopagus male ratio b. Terata anadidyma (twins joined at the upper part of 2. Embryologic classification of conjoined twins the body and double below) a. Ventral (87%) i. Craniopagus (twins joined at the top of cranial i. Rostral (48%) vaults; cephalic conjugation) a) Cephalopagus 11% ii. Dipygus (twins with 1 head, 1 thorax, 1 b) Thoracopagus 19% abdomen, and double pelvis with or without 2 c) Omphalopagus 18% sets of external genitalia and up to 4 legs; lateral ii. Caudal (11%) (ischiopagus) conjugation) iii. Lateral (28%) (parapagus) iii. Syncephalus (twins joined by the face; the faces b. Dorsal (13%) turn laterally) i. Craniopagus (5%) c. Tera anakatadidyma (twins joined by the midportion ii. Rachipagus(2%) of the body) iii. Pygopagus (6%) i. Omphalopaus (twins joined from the umbilicus 3. Anatomic classifications of conjoined twins, based on to xiphoid cartilage; anterior conjugation) how the body axes of the twins are mutually oriented in ii. Xiphopagus (twins joined at xiphoid process) the embryonic disc iii. Rachipagus (twins joint by the vertebral column; a. Notochordal axes facing each other back to back) i. Ventro-ventral iv. Thoracopagus (twins joined at the chest wall; a) Thoracopagus anterior conjugation) b) Xiphopagus 6. Duplicatas asymmetros (asymmetrical conjoined twins c) Omphalopagus resulting from unequal and incomplete fission of the ii. Cranial ventro-vental (cephalothoracopagus) uniovum and unequal placental circulation to twins) iii. Caudal ventro-ventral (ischiopagus) a. Cephalic conjugation iv. Cranial end-to-end (craniopagus) i. Craniopagus parasiticus v. Caudal end-to-end (pygopagus) ii. Janus parasiticus b. Notochordal axes facing side-by-side iii. Epignathus heteropagus i. Dicephalus b. Anterior conjugation ii. Diprosopus i. Thoracopagus parasiticus 4. Anatomic classifications of conjoined twins, based on ii. Epigastrius how the subsequent events of migration, growth, and c. Posterior conjugation body folding result in different types of conjoined twins i. Ischiopagus parasiticus a. Dipygus ii. Pyopagus parasiticus b. Fetus-in-fetu iii. Sacral parasiticus 241 242 CONJOINED TWINS CLINICAL FEATURES iii. Temporal craniopagus iv. Occipital craniopagus 1. Thoracopagus twins b. Classification based on surgical and prognostic a. Represents 40% of conjoined twins purposes b. Conjoined at the thoracic region i. Partial type (brains separated by bone or dura c. Face to face with each brain having separate leptomeninges) d. Associated congenital heart defects ii. Complete type (presence of cerebral connection) i. Present in 75% of cases 6. Rachiopagus twins (fused upper spinal column; back to ii. Presence of varying degree of pericardial sac back) fusion 7. Pygodidymus twins (fused cephalothoracic region; dupli- iii. A conjoined heart with two ventricles and a vary- cate pelves and lower extremities) ing number of atria (most frequent abnormality) 8. Pygopagus twins/pygomelus twins (joined back-to-back iv. Ventricular septal defect in virtually all patients at the sacrum; additional limb or limbs at or near buttock) 2. Omphalopagus twins (fused umbilical region)/Xiphopagus 9. Iniopagus twins/craniopagus occipitalis twins (fused twins (fused xiphoid process of sternum) head, at parasitic occipital region) a. Constitutes one third of all types of conjoined twins 10. Epicomus twins/craniopagus parasiticus twins (smaller, b. The most readily separable conjoined twins since parasitic twin joined to larger autosite at occiput) their union may involve only skin and portions of the 11. Monocephalus twins (single head with 2 bodies) liver, occasionally including portions of the sternum 12. Diprosopus twins (single body with 2 faces) c. Most omphalopagus twins joined by a skin bridge 13. Dicephalus twins (symmetric body with 2 heads) that contains liver and bowel 14. Dipygus parasiticus twins (head and thorax completely d. Conjoined liver in 81% merged; pelvis and lower extremities duplicated) e. Conjoined sternal cartilage in 26% 15. Cephalopagus conjoined twins f. Conjoined diaphragm in 17% a. The rarest type of conjoined twins g. Conjoined genitourinary tract in 3% b. Fused from the top of the head to the umbilicus h. Malformations of the abdominal wall (usually c. Presence of two faces on the opposite sides of the omphalocele) in at least one of the twins in 33% head with one face usually being rudimentary i. Congenital heart defects in at least one of the twins d. Separation of the lower abdomen in 25% e. With 4 arms and 4 legs i. Ventricular septal defects f. Prognosis dismal dependent on the following factors ii. Tetralogy of Fallot i. Presence of associated anomalies j. Concordant congenital heart defects in only one out ii. Degree of fusion of the intracranial, intrathoracic of 9 sets of twins and/or intra-abdominal structures 3. Pygopagus twins iii. Extent of venous connections a. Constitutes 19% of conjoined twins 16. Epigastric heteropagus twins b. Conjoined at sacrum (buttocks and lower spine) a. A rare type of conjoined twinning c. Most commonly back-to-back (face away from each b. Resulting from an ischemic atrophy of one fetus at an other) early stage of gestation d. May share part of the sacral spinal canal c. Pelvis and lower limbs of the ischemic fetus (incom- e. May share common rectum and anus plete parasitic twin; heteropagus) attached to the epi- f. Often with fused genitalia gastrium of the well-developed fetus (the autosite) 4. Ischiopagus twins 17. Fetus in fetu a. Constitutes 6% of conjoined twins a. The parasites embodied in the autosite, usually b. Conjoined back-to-back at the coccyx within cranial, thoracic, and abdominal cavities dur- c. Often with a common large pelvic ring formed by the ing the developmental process of the asymmetrical union of the two pelvic girdles conjoined twins d. May have 4 legs (ischiopagus tetrapus) b. Most likely arise from inclusion of a monochorionic, e. May have three legs (ischiopagus tripus) diamniotic, monozygotic twin within the bearer due f. Frequently share the lower gastrointestinal tract to anastomoses of vitillene circulation i. Intestines joined at the terminal ileum 18. Prognosis ii. Emptying into a single colon a. A high mortality rate g. May have a single bladder and urethra i. Nearly 40% are stillborn h. Displaced anus ii. 1/3 die within 24 hours of birth i. Common vaginal anomalies iii. No prospect of survival when complex cardiac j. Common rectovaginal communications union is present 5. Craniopagus twins (fused at the cranial vault) (2%) b. Causes of death a. Classification according to the area of junction i. Severely abnormal conjoined heart i. Frontal craniopagus ii. Pulmonary hypoplasia due to distortion of fused ii. Parietal craniopagus (most common) thoracic cages CONJOINED TWINS 243 19. Examples of historically famous conjoined twins b. Prenatal sonographic criteria a. So-called Biddenden Maids (1100–1134) in i. Fusion of the skulls, face, thorax, and upper England abdomen i. Probably pyopagus twins ii. Fetal body parts at the same level ii. Their famous image imprinted on the “cakes” iii. Constant relative fetal motion iii. Walks with their arms around each other iv. Fetal extremities in unusual positions b. Chang and Eng Bunker (1811–1874) from Bangkok v. Breech position and settled in the USA (“Siamese twins”) vi. Hyperextension of both cervical spines i. Later married to twin girls vii. Nonseparable external skin contour ii. Fathered 22 children viii. A solitary umbilical cord with multiple vessels c. Blazˇek sisters (1878–1922) from Bohemia ix. Polyhydramnios i. 2 heads x. Two actively beating hearts ii. 4 arms xi. Two sets of pelves, limbs and spine iii. 4 legs iv. Partially fused torso GENETIC COUNSELING v. Combined reproductive organs vi. Delivered an infant through a single vagina but 1. Recurrence risk the gestation had occurred in the uterus of one of a. Patient’s sib: not higher according to family study the twins b. Patient’s offspring: report of delivery of a healthy male infant to the pygopagus Blazˇek sisters 2. Prenatal diagnosis DIAGNOSTIC INVESTIGATIONS a. Radiography 1. Prenatal echocardiography b. 2D/3D ultrasonography: prenatal diagnosis made as a. Presence and extent of cardiac conjunction early as 10–12 weeks’ gestation b. Associated cardiac defects c. Transcervical embryoscopy for first trimester embry- 2. Prenatal radiography onic evaluation of conjoined twins after a missed 3. Prenatal ultrasonography (including transvaginal two- abortion dimensional sonography), especially three-dimensional 3.