Conjoined twins

Conjoined twins are incompletely separated monozygotic twins. They have long fascinated both medical profession and lay public. Such events are rare and occur in 1/50,000 to 1/100,000 births and one in 400 sets of monozygotic twins. It is a complica- tion of monochorionic twinning at 13 to 15 days after conception.

GENETICS/BASIC DEFECTS

1. Conjoined twins

a. Rare variants of monozygotic, monochorionic twins b. Two theories of con-joined twin formation

i. Resulting from the secondary union of two orig- inally separate monovular embryonic discs ii. Resulting from an incomplete separation of the

inner cell mass at around 13–15 days of gesta- tion of the monovular twins

c. Twins can be conjoined at any site from the cranium downward to the sacrococcygeal region

d. Approximately 60% are stillborn

e. Female predominance: approximately 3:1 female-to- male ratio

2. Embryologic classification of conjoined twins a. Ventral (87%)

i. Rostral (48%)

a) Cephalopagus 11%

b) Thoracopagus 19%

c) Omphalopagus 18%

ii. Caudal (11%) (ischiopagus) iii. Lateral (28%) (parapagus) b. Dorsal (13%)

i. Craniopagus (5%) ii. Rachipagus(2%) iii. Pygopagus (6%)

3. Anatomic classifications of conjoined twins, based on how the body axes of the twins are mutually oriented in the embryonic disc

a. Notochordal axes facing each other i. Ventro-ventral

a) Thoracopagus b) Xiphopagus c) Omphalopagus

ii. Cranial ventro-vental (cephalothoracopagus) iii. Caudal ventro-ventral (ischiopagus)

iv. Cranial end-to-end (craniopagus) v. Caudal end-to-end (pygopagus) b. Notochordal axes facing side-by-side

i. Dicephalus ii. Diprosopus

4. Anatomic classifications of conjoined twins, based on how the subsequent events of migration, growth, and body folding result in different types of conjoined twins a. Dipygus

b. Fetus-in-fetu

i. A fetiform mass located within a basically nor- mal fetus

ii. Inclusion of a monozygotic diamniotic twin within the bearer is the best explanation

5. Duplicitas symmetros (symmetrical conjoined twins resulting from incomplete fission of the uniovum) a. Terata Katadidyma (twins joined at the lower part of

the body and double above)

i. Dicephalus (twins with 2 separate heads and necks side by side with 1 body; lateral conjugation) ii. Diprosopus (twins with 2 faces side by side, 1

head, and 1 body)

iii. Ischiopagus (twins joined by the inferior mar- gins of the coccyx and sacrum with 2 completely separate spinal columns; caudal conjugation) iv. Pygopagus (twins joined by posterior surfaces of

the coccyx and sacrum, back to back; posterior conjugation)

v. Craniothoracopagus vi. Ileothoracopagus

b. Terata anadidyma (twins joined at the upper part of the body and double below)

i. Craniopagus (twins joined at the top of cranial vaults; cephalic conjugation)

ii. Dipygus (twins with 1 head, 1 thorax, 1 abdomen, and double pelvis with or without 2 sets of external genitalia and up to 4 legs; lateral conjugation)

iii. Syncephalus (twins joined by the face; the faces turn laterally)

c. Tera anakatadidyma (twins joined by the midportion of the body)

i. Omphalopaus (twins joined from the umbilicus to xiphoid cartilage; anterior conjugation) ii. Xiphopagus (twins joined at xiphoid process) iii. Rachipagus (twins joint by the vertebral column;

back to back)

iv. Thoracopagus (twins joined at the chest wall;

anterior conjugation)

6. Duplicatas asymmetros (asymmetrical conjoined twins resulting from unequal and incomplete fission of the uniovum and unequal placental circulation to twins) a. Cephalic conjugation

i. Craniopagus parasiticus ii. Janus parasiticus iii. Epignathus heteropagus b. Anterior conjugation

i. Thoracopagus parasiticus ii. Epigastrius

c. Posterior conjugation

i. Ischiopagus parasiticus

ii. Pyopagus parasiticus

iii. Sacral parasiticus

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CLINICAL FEATURES

1. Thoracopagus twins

a. Represents 40% of conjoined twins b. Conjoined at the thoracic region c. Face to face

d. Associated congenital heart defects i. Present in 75% of cases

ii. Presence of varying degree of pericardial sac fusion

iii. A conjoined heart with two ventricles and a vary- ing number of atria (most frequent abnormality) iv. Ventricular septal defect in virtually all patients 2. Omphalopagus twins (fused umbilical region)/Xiphopagus

twins (fused xiphoid process of sternum)

a. Constitutes one third of all types of conjoined twins b. The most readily separable conjoined twins since

their union may involve only skin and portions of the liver, occasionally including portions of the sternum c. Most omphalopagus twins joined by a skin bridge

that contains liver and bowel d. Conjoined liver in 81%

e. Conjoined sternal cartilage in 26%

f. Conjoined diaphragm in 17%

g. Conjoined genitourinary tract in 3%

h. Malformations of the abdominal wall (usually omphalocele) in at least one of the twins in 33%

i. Congenital heart defects in at least one of the twins in 25%

i. Ventricular septal defects ii. Tetralogy of Fallot

j. Concordant congenital heart defects in only one out of 9 sets of twins

3. Pygopagus twins

a. Constitutes 19% of conjoined twins

b. Conjoined at sacrum (buttocks and lower spine) c. Most commonly back-to-back (face away from each

other)

d. May share part of the sacral spinal canal e. May share common rectum and anus

f. Often with fused genitalia 4. Ischiopagus twins

a. Constitutes 6% of conjoined twins b. Conjoined back-to-back at the coccyx

c. Often with a common large pelvic ring formed by the union of the two pelvic girdles

d. May have 4 legs (ischiopagus tetrapus) e. May have three legs (ischiopagus tripus)

f. Frequently share the lower gastrointestinal tract i. Intestines joined at the terminal ileum ii. Emptying into a single colon

g. May have a single bladder and urethra h. Displaced anus

i. Common vaginal anomalies

j. Common rectovaginal communications 5. Craniopagus twins (fused at the cranial vault) (2%)

a. Classification according to the area of junction i. Frontal craniopagus

ii. Parietal craniopagus (most common)

iii. Temporal craniopagus iv. Occipital craniopagus

b. Classification based on surgical and prognostic purposes

i. Partial type (brains separated by bone or dura with each brain having separate leptomeninges) ii. Complete type (presence of cerebral connection) 6. Rachiopagus twins (fused upper spinal column; back to

back)

7. Pygodidymus twins (fused cephalothoracic region; dupli- cate pelves and lower extremities)

8. Pygopagus twins/pygomelus twins (joined back-to-back at the sacrum; additional limb or limbs at or near buttock) 9. Iniopagus twins/craniopagus occipitalis twins (fused

head, at parasitic occipital region)

10. Epicomus twins/craniopagus parasiticus twins (smaller, parasitic twin joined to larger autosite at occiput) 11. Monocephalus twins (single head with 2 bodies) 12. Diprosopus twins (single body with 2 faces) 13. Dicephalus twins (symmetric body with 2 heads) 14. Dipygus parasiticus twins (head and thorax completely

merged; pelvis and lower extremities duplicated) 15. Cephalopagus conjoined twins

a. The rarest type of conjoined twins

b. Fused from the top of the head to the umbilicus c. Presence of two faces on the opposite sides of the

head with one face usually being rudimentary d. Separation of the lower abdomen

e. With 4 arms and 4 legs

f. Prognosis dismal dependent on the following factors i. Presence of associated anomalies

ii. Degree of fusion of the intracranial, intrathoracic and/or intra-abdominal structures

iii. Extent of venous connections 16. Epigastric heteropagus twins

a. A rare type of conjoined twinning

b. Resulting from an ischemic atrophy of one fetus at an early stage of gestation

c. Pelvis and lower limbs of the ischemic fetus (incom- plete parasitic twin; heteropagus) attached to the epi- gastrium of the well-developed fetus (the autosite) 17. Fetus in fetu

a. The parasites embodied in the autosite, usually within cranial, thoracic, and abdominal cavities dur- ing the developmental process of the asymmetrical conjoined twins

b. Most likely arise from inclusion of a monochorionic, diamniotic, monozygotic twin within the bearer due to anastomoses of vitillene circulation

18. Prognosis

a. A high mortality rate

i. Nearly 40% are stillborn ii. 1/3 die within 24 hours of birth

iii. No prospect of survival when complex cardiac union is present

b. Causes of death

i. Severely abnormal conjoined heart

ii. Pulmonary hypoplasia due to distortion of fused

thoracic cages

19. Examples of historically famous conjoined twins a. So-called Biddenden Maids (1100–1134) in

England

i. Probably pyopagus twins

ii. Their famous image imprinted on the “cakes”

iii. Walks with their arms around each other b. Chang and Eng Bunker (1811–1874) from Bangkok

and settled in the USA (“Siamese twins”) i. Later married to twin girls

ii. Fathered 22 children

c. Blazˇek sisters (1878–1922) from Bohemia i. 2 heads

ii. 4 arms iii. 4 legs

iv. Partially fused torso

v. Combined reproductive organs

vi. Delivered an infant through a single vagina but the gestation had occurred in the uterus of one of the twins

DIAGNOSTIC INVESTIGATIONS

1. Prenatal echocardiography

a. Presence and extent of cardiac conjunction b. Associated cardiac defects

2. Prenatal radiography

3. Prenatal ultrasonography (including transvaginal two- dimensional sonography), especially three-dimensional sonographic examinations (The surface-rendered image of the conjoined twin and its demonstration on an axially rotating cine loop facilitates explanation of the precise nature of the abnormalities, especially in the case of cephalothoracopagus conjoining)

4. CAT scan and MRI of the abdomen and the chest a. Anatomy of the heart

b. Anatomy of the livers

c. Anatomy of the genitourinary systems

5. Gastrointestinal contrast studies to demonstrate the pres- ence and level of conjunction of the intestinal tract 6. Ventrally fused conjoined twins

a. Prenatal radiography/ultrasonography i. Fetal body parts on the same level ii. Constant relative fetal position iii. Fetal extremities in unusual proximity

iv. Face-to-face fetal position

v. Bibreech, less commonly bicephalic presentation vi. Hyperextension of one or both cervical spines b. Prenatal ultrasonography

i. Nonseparable continuous external skin contour ii. Single heart sound by Doppler

iii. Solitary large liver and heart iv. Multiple shared omenta

v. Solitary umbilical cord with >3 vessels 7. Cephalothoracopagus

a. Prenatal radiographic criteria

i. Both fetal heads at the same level ii. Backward fusion of the cervical spines

iii. A narrow space between lower cervical and upper thoracic spines

iv. No change in fetal relative positions after mater- nal movement

b. Prenatal sonographic criteria

i. Fusion of the skulls, face, thorax, and upper abdomen

ii. Fetal body parts at the same level iii. Constant relative fetal motion

iv. Fetal extremities in unusual positions v. Breech position

vi. Hyperextension of both cervical spines vii. Nonseparable external skin contour

viii. A solitary umbilical cord with multiple vessels ix. Polyhydramnios

x. Two actively beating hearts xi. Two sets of pelves, limbs and spine

GENETIC COUNSELING

1. Recurrence risk

a. Patient’s sib: not higher according to family study b. Patient’s offspring: report of delivery of a healthy

male infant to the pygopagus Blazˇek sisters 2. Prenatal diagnosis

a. Radiography

b. 2D/3D ultrasonography: prenatal diagnosis made as early as 10–12 weeks’ gestation

c. Transcervical embryoscopy for first trimester embry- onic evaluation of conjoined twins after a missed abortion

3. Management

a. Early prenatal diagnosis: highly desirable, given the extremely poor prognosis of some types of con- joined twins

b. Psychological and prognosis counseling c. Accurate preoperative investigation d. A team approach

e. Previous experience

f. Meticulous operative and postoperative management g. Substantial mortality rate related to the underlying

conditions

h. High likelihood of success if major associated anom- alies are absent

i. Options of obstetrical management

i. Continue the vaginal delivery and deliver the twins intact

ii. Deliver the twins vaginally after intrauterine sep- aration or a destructive procedure

iii. Cesarean section and deliver the twins intact iv. Cesarean section and deliver the twins after

intrauterine separation or destruction j. Anesthetic management for separating operations

i. Extensive cross circulation a) Through a liver bridge

b) Common cerebral venous sinus ii. Mechanical problems

a) One anesthetist required for each infant b) A third anesthetist to look after intravenous

infusions and monitors

c) A fourth anesthetist to look after massive blood loss, circulatory collapse or other cata- strophic occurrence

iii. Anticipate complex congenital heart defects that

was not diagnosed preoperatively

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Fig. 1. The conjoined twins are joined at the level of abdomen from umbilicus to the xiphoid cartilage (xiphoomphalopagus). This type of conjoined twins is the one most amenable to successful surgical correc- tion because the incidence of complex anatomical anomalies is low. Left twin was successfully separated from the right twin who succumbed shortly after surgery to multiple congenital anomalies including exstro- phy of the cloaca, left Bochdalek hernia, hypoplastic kidney, hypoplas- tic lungs, imperforate anus, and a large sacral meningomyelocele.

Fig. 2. Prenatal ultrasound (A) detected the above conjoined twins with a shared liver (CAB) and separate hearts, stomach, pelvis, and extremities. One fetus was noted to have a meningomyelocele (M).

The magnified view (B) shows part of the shared liver and meningomyelocele.

Fig. 3. These twins are thoraco-omphalopagus, connected at the tho- rax and upper abdomen. The heart showed complex anomalies with a common atrium and a single ventricle. Therefore, separation of the twins was not attempted.

Fig. 4. The radiograph of the twins in Fig. 3 showing the connection at the thorax and the upper abdomen.

Fig. 5. Dicephalic conjoined twins. Two separate heads, two separate necks, and only one body are evident. The twins shared a common pericardium with complex cardiac anomalies, a common aorta at the level of iliac arteries, a common small intestine and other GI tract dis- tally, a common bladder and urethra drained from a single kidney from each twin, and single normal female genitalia with normally placed fallopian tubes and ovaries. Surgical separation of the twins was deemed impossible and was not attempted.

Fig. 6. The radiograph of the above-conjoined twins showing separate heads, vertebrae, and upper GI tract. There is one pericardium sac and a fused liver.

Fig. 7. A stillbirth with dicephalic conjoined twins.

Fig. 8. A set of dicephalic conjoined twin embryos at 6–7 weeks of gestation.