Figure 6: Acephaly/ acardia illustrated with color engraving after injection of the vascular tree (Herholdt, 1830)
The prevailing pathogenetic theory is that this profound abnormality is the result of very early disorganization of extensive developmental fields, as a result of placental anastomoses between monozygous twins [concordance for gender has been 100%]; it can be thought of as an extreme example of twin to twin transfusion. Earlier ideas about the acardia being a primary defect have not gained much acceptance. In any case, the evidence is in favor of this being a sporadic event, since there has been no report of familial recurrence. Dean and Ceballos11 expressed the view that rapidly growing structures such as the branchial arches, maxilla, and mandible are rudimentary because of the severe loss of blood flow in early development; some acardiac/acephalic fetuses do, however, possess teeth (albeit sometimes in peculiar places and configurations).
Another theory proposed for the origin of this condition is the fertilization of the first polar body12, but this has not gained much attention.
Careful dissections since the early reports centuries ago have revealed the profound disorganization of multiple organ systems in acardiac fetuses. Only very recently have we been able to study the chromosomal constitution of these rare fetuses; there have been only few reports of aneuploidy, but not many affected fetuses have been studied. A review of 11 cases in which both the acardiac and the surviving twin had karyotypes performed, with two new cases, revealed:
g 4 normal females,
g one mosaic 46,XY/47,XY,+C,
g one mosaic 46,XX/47,XX, +minute, 47,XX,+ring,
g one mosaic 46,XY/47,XY,+G,
g one 47,XXY,
g one 45,X,
g one 70,XXX,+15,
g one 45,XX,t(4;21)del(4p),
g one mosaic 46,XX/47,XX,+11.
There was a case with 47,XXY in both the acardiac and the survivor, and one with 94,XXXXYY in the acardiac and 47,XXY in the survivor. All the other surviving phenotypically normal co-twins had normal karyotypes13. It would appear that profound chromosomal abnormalities may predispose to massive disruption of very early developmental fields, but such disruptions may also occur in the presence of normal karyotype.
It is important to document the karyotype of the surviving twin, since some chromosomal abnormalities may not produce obvious malformations, but still have important implications for the prognosis. Whenever discovery of the problem occurs, an effort should be made to obtain karyotypes of both twins (that is, prenatally or at delivery). Modern ultrasonographic examination of most pregnancies provides the hope of relatively early detection of an acardiac twin14-15.
The management of such cases is a thorny issue, since retention of a dead fetus presents a risk to the survivor. Either the development of heart failure, or the effects of products of necrosis, can injure or kill the normal co-twin. Selective delivery is difficult, but there have been reports of success 1,16. Earlier suggestions for medical management of fetal heart failure by maternal administration of oral digoxin17, and ligation of the acephalic fetus" umbilical cord18 are less aggressive and of uncertain efficacy, but were favored by those who published the cases and since then by others 19. Still, if the acardiac is quite small the option for conservative watch and wait approach may be sufficient20; such would probably not be the case in the event of prenatal detection of an acardiac as large as the present one (where a favorable outcome for the pump twin is surprising).
The problem of determining whether the acardiac twin is alive or dead has received some attention24. The tissues of the fetus are kept alive by their essentially parasitic relationship to the co-twin, whose heart perfuses both bodies. Yet the acardiac twin has no possibility of survival once that special relationship has ended; this is true even aside from the acardiac status, since complete acephaly also means both absence of most of the central nervous system (equivalent to anencephaly), and of the upper digestive tract.
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