The Acardiac Anomaly K U R T BENIRSCHKE AND VIRGINIA DES ROCHES HARPER Department of Patholm, University of California,San Diego, La Jolla, California 92093
ABSTRACT Karyotype analysis of a premature human acardiac twin disclosed normal chromosomes. A review of previous cytogenetic, placental and animal studies suggests that chromosomal errors are not the cause of the acardiac anomaly. Rather, they point to the placental vascular anastomoses as the principal pathogenetic event. The acardiac fetus represents easily the most severe congenital anomaly. Generally, it is one of monozygotic C‘identical”) twins whose co-twin is almost normally developed and often survives. The pathogenesis of this unusual malformation is in dispute, but little doubt exists that intrauterine growth is achieved by the perfusion afforded from the normal co-twin via large blood vessel anastomoses in the placenta. Schatz (19001, who observed numerous such anomalous twins, suggested that the reversal of circulation through large artery-to-artery and vein-tovein anastomoses was the principal event leading to this defective development. He hypothesized that the frequently present omphalocele enhanced the possibility of acardiac malformations by partially obstructing blood flow. Others have sought different pathogenetic agents, such as unequal splitting of embryos during the twinning event and, most recently, malsegregation of chromosomes with resulting aneuploidy. This paper describes a typical example of a n “holoacardius” twin with detailed cytogenetic study, and discusses the pertinent findings from the literature. The normality of chromosome number and structure indicates that genetic disturbances are not the likely causes of this anomaly, a conclusion which is further enhanced by the findings of artiodactyle acardiacs. Case report This first pregnancy of a 19-year-old white woman ended in spontaneous premature delivery of a 1,240 g male and the anomalous twin. The externally normal twin was severely asphyxiated a t birth and died within two hours, never having established spontaneous respiration or perfusion of his extremities. Autopsy was performed a t another hospital TERATOLOGY, 15: 311-316.
following embalming. The following weights were recorded and compared with expected weights (Potter and Craig, ’75): Heart and lungs 50 g (36 g), liver 85 g (60 g), thymus 7 g (4 g), kidneys 12 g (12 g), brain 180 g (180 g). The ear lobes were thought to be low and mandible adjudged to by hypoplastic, the palate was closed. The heart was enlarged and the left ventricular myocardial thickness exceeded that of the right by 1 mm when measured near the valves. The aortic outflow tract was considered obstructed by hypertrophied muscle bundles but slides are not available. The acardiac weighed 1,210 g and was male, with diminutive external genitalia (fig. 1). The right arm was rudimentary, the left absent, and the congested legs were normally formed. X-ray examination disclosed a malformed skull, scoliosis of the spine and hypoplasia of sacrum and both humeri. A normal number of hypoplastic ribs was present. External ears, orbits and mouth were hypoplastic and deformed. Despite the presence of palpebral fissures, there were no eyes. The nares were occluded. On dissection and microscopic examination the following organs and tissues were identified: Brain with focal calcification, tentoria, choroid plexus, spinal cord with dilated central canal, tongue, larynx, bronchi, submandibular glands, stomach, small intestine with Meckel’s diverticulum and small omphalocele, colon with sigmoidal atresia, anal opening, adrenal glands with marked congestion and focal calcification of fetal zones, hypoplastic kidneys with four rows of glomeruli and slightly distended collecting tubules, ureters and bladder, undescended congested testes with normal complement of spermatogonia, right umbilical artery, umbilical vein. Despite nuReceived Aug. 24, ’76. Accepted Jan. 21, ’77.
311
312
KURT BENIRSCHKE AND VIRGINIA DES ROCHES HARPER TABLE 1 Chromosome number
Number of metaphases analyzed
Fig. 1 Gross appearance of acardiac. Note face, right arm rudiment, congested normal legs. Ruler is 15 em.
merous sections, no heart, liver, lung, spleen or pancreas were identified. The intestine had normal innervation and was filled with unstained detritus. The skull was markedly deformed. The hard palate, while closed anteriorly, had a central defect through which probes could be inserted into the deformed nasal cavity (fig. 2). There was massive subcutaneous edema, particularly in the neck region, with large fluid filled spaces occupying the subcutaneous adipose tissue. Skeletal muscle of the abdominal wall and in the legs was well developed. The placenta weighed 410 g and had monoamnionic, monochorionic membrane relationship. The umbilical cord of the normal twin was 30 cm long and had normal vessels, that of the acardiac was 15 cm long, thin and possessed one artery near the fetus with an
45
46
41
3
49
2
atrophic second artery near the placenta, a vein with small mural thrombus and remnants of allantoic and omphalomesenteric ducts. The cords arose next to one another from the margin of the placenta, a t which site two arteries and two veins communicated with each other through large anastomoses. Slight chorionitis was present and the villous architecture displayed immaturity. Several pieces of skin from the acardiac were explanted into Leighton tubes and processed by the fibrous tissue culture technique described by Basrur et al. (‘63). Outgrowth was rapid and subcultured into Falcon flasks from which chromosome preparations were made after one hour terminal colcemid exposure, hypotonic swelling with KC1 and acetic acid/methanol fixation by standard procedures. Giesma banding of air-dried slides was accomplished by the method of Sumner et al. (‘71). Fifty-four apparently complete, randomly selected metaphases were photographed and karyotypes constructed following the Paris nomenclature (‘75). The results are tabulated in table 1 and a representative karyotype is shown in fig. 3. No evidence of chromosomal error was detected, the sex chromosomes were XY. In the few cells with a diploid number other than 46 we showed random loss of different chromosomes upon karyotyping. DISCUSSION
Acardiac fetuses have aroused interest for a century since Schatz (1900) described a substantial number of specimens and suggested a complex taxonomy. Their phenotype and size vary greatly but all have in common absent or rudimentary (hemicardiac) hearts and anastomotic vascular supply to a twin. This usually entirely normal twin accomplishes the circulation for the acardiac and occasionally suffers cardiac hypertrophy for that reason. In the acardiac, the circulation is reversed; through the usually single umbilical artery blood enters and it leaves the body through the umbilical vein. Moreover, the “arterial” blood that enters is presumably under lower than normal fetal pressures for optimal
313
THE ACARDIAC ANOMALY
Fig. 2 Basal view of skull with foramen magnum and central palatine cleft. TABLE 2
Acardiac twins karvotwed species
1963 Man 1966Man 1967 Man
1973 Man
Man 1976 Man 1967 Cattle 1969Cattle
1972 Sheep
Donor
Normal male Normal male 46,XY (lymphocyte) Normal female ' 46,XX (27 lymphocytes) 47,XX+minute (1lymphocyte) 46,XX (9fibroblasts) Normal male 46,XY (38lymphocytes) 46,XY (40 lymphocytes) Normal male 46,XY (30 lymphocytes) Premature male, died Normal male 60,XY (373 lymphocytes) Normal female 60,XX (lymphocytes) Normal male twins 70%54,XY (lymphycytes) 18%53,XY 12%
ABSTRACT Karyotype analysis of a premature human acardiac twin disclosed normal chromosomes. A review of previous cytogenetic, placental and animal studies suggests that chromosomal errors are not the cause of the acardiac anomaly. Rather, they point to the placental vascular anastomoses as the principal pathogenetic event. The acardiac fetus represents easily the most severe congenital anomaly. Generally, it is one of monozygotic C‘identical”) twins whose co-twin is almost normally developed and often survives. The pathogenesis of this unusual malformation is in dispute, but little doubt exists that intrauterine growth is achieved by the perfusion afforded from the normal co-twin via large blood vessel anastomoses in the placenta. Schatz (19001, who observed numerous such anomalous twins, suggested that the reversal of circulation through large artery-to-artery and vein-tovein anastomoses was the principal event leading to this defective development. He hypothesized that the frequently present omphalocele enhanced the possibility of acardiac malformations by partially obstructing blood flow. Others have sought different pathogenetic agents, such as unequal splitting of embryos during the twinning event and, most recently, malsegregation of chromosomes with resulting aneuploidy. This paper describes a typical example of a n “holoacardius” twin with detailed cytogenetic study, and discusses the pertinent findings from the literature. The normality of chromosome number and structure indicates that genetic disturbances are not the likely causes of this anomaly, a conclusion which is further enhanced by the findings of artiodactyle acardiacs. Case report This first pregnancy of a 19-year-old white woman ended in spontaneous premature delivery of a 1,240 g male and the anomalous twin. The externally normal twin was severely asphyxiated a t birth and died within two hours, never having established spontaneous respiration or perfusion of his extremities. Autopsy was performed a t another hospital TERATOLOGY, 15: 311-316.
following embalming. The following weights were recorded and compared with expected weights (Potter and Craig, ’75): Heart and lungs 50 g (36 g), liver 85 g (60 g), thymus 7 g (4 g), kidneys 12 g (12 g), brain 180 g (180 g). The ear lobes were thought to be low and mandible adjudged to by hypoplastic, the palate was closed. The heart was enlarged and the left ventricular myocardial thickness exceeded that of the right by 1 mm when measured near the valves. The aortic outflow tract was considered obstructed by hypertrophied muscle bundles but slides are not available. The acardiac weighed 1,210 g and was male, with diminutive external genitalia (fig. 1). The right arm was rudimentary, the left absent, and the congested legs were normally formed. X-ray examination disclosed a malformed skull, scoliosis of the spine and hypoplasia of sacrum and both humeri. A normal number of hypoplastic ribs was present. External ears, orbits and mouth were hypoplastic and deformed. Despite the presence of palpebral fissures, there were no eyes. The nares were occluded. On dissection and microscopic examination the following organs and tissues were identified: Brain with focal calcification, tentoria, choroid plexus, spinal cord with dilated central canal, tongue, larynx, bronchi, submandibular glands, stomach, small intestine with Meckel’s diverticulum and small omphalocele, colon with sigmoidal atresia, anal opening, adrenal glands with marked congestion and focal calcification of fetal zones, hypoplastic kidneys with four rows of glomeruli and slightly distended collecting tubules, ureters and bladder, undescended congested testes with normal complement of spermatogonia, right umbilical artery, umbilical vein. Despite nuReceived Aug. 24, ’76. Accepted Jan. 21, ’77.
311
312
KURT BENIRSCHKE AND VIRGINIA DES ROCHES HARPER TABLE 1 Chromosome number
Number of metaphases analyzed
Fig. 1 Gross appearance of acardiac. Note face, right arm rudiment, congested normal legs. Ruler is 15 em.
merous sections, no heart, liver, lung, spleen or pancreas were identified. The intestine had normal innervation and was filled with unstained detritus. The skull was markedly deformed. The hard palate, while closed anteriorly, had a central defect through which probes could be inserted into the deformed nasal cavity (fig. 2). There was massive subcutaneous edema, particularly in the neck region, with large fluid filled spaces occupying the subcutaneous adipose tissue. Skeletal muscle of the abdominal wall and in the legs was well developed. The placenta weighed 410 g and had monoamnionic, monochorionic membrane relationship. The umbilical cord of the normal twin was 30 cm long and had normal vessels, that of the acardiac was 15 cm long, thin and possessed one artery near the fetus with an
45
46
41
3
49
2
atrophic second artery near the placenta, a vein with small mural thrombus and remnants of allantoic and omphalomesenteric ducts. The cords arose next to one another from the margin of the placenta, a t which site two arteries and two veins communicated with each other through large anastomoses. Slight chorionitis was present and the villous architecture displayed immaturity. Several pieces of skin from the acardiac were explanted into Leighton tubes and processed by the fibrous tissue culture technique described by Basrur et al. (‘63). Outgrowth was rapid and subcultured into Falcon flasks from which chromosome preparations were made after one hour terminal colcemid exposure, hypotonic swelling with KC1 and acetic acid/methanol fixation by standard procedures. Giesma banding of air-dried slides was accomplished by the method of Sumner et al. (‘71). Fifty-four apparently complete, randomly selected metaphases were photographed and karyotypes constructed following the Paris nomenclature (‘75). The results are tabulated in table 1 and a representative karyotype is shown in fig. 3. No evidence of chromosomal error was detected, the sex chromosomes were XY. In the few cells with a diploid number other than 46 we showed random loss of different chromosomes upon karyotyping. DISCUSSION
Acardiac fetuses have aroused interest for a century since Schatz (1900) described a substantial number of specimens and suggested a complex taxonomy. Their phenotype and size vary greatly but all have in common absent or rudimentary (hemicardiac) hearts and anastomotic vascular supply to a twin. This usually entirely normal twin accomplishes the circulation for the acardiac and occasionally suffers cardiac hypertrophy for that reason. In the acardiac, the circulation is reversed; through the usually single umbilical artery blood enters and it leaves the body through the umbilical vein. Moreover, the “arterial” blood that enters is presumably under lower than normal fetal pressures for optimal
313
THE ACARDIAC ANOMALY
Fig. 2 Basal view of skull with foramen magnum and central palatine cleft. TABLE 2
Acardiac twins karvotwed species
1963 Man 1966Man 1967 Man
1973 Man
Man 1976 Man 1967 Cattle 1969Cattle
1972 Sheep
Donor
Normal male Normal male 46,XY (lymphocyte) Normal female ' 46,XX (27 lymphocytes) 47,XX+minute (1lymphocyte) 46,XX (9fibroblasts) Normal male 46,XY (38lymphocytes) 46,XY (40 lymphocytes) Normal male 46,XY (30 lymphocytes) Premature male, died Normal male 60,XY (373 lymphocytes) Normal female 60,XX (lymphocytes) Normal male twins 70%54,XY (lymphycytes) 18%53,XY 12%
