Anencephaly: Selected Medical Aspects

Anencephaly: Selected Medical Aspects Anencephaly is generally defined as the congenital absence of skull, scalp and forebrain (cerebral hemispheres). In most cases, the cause is unknown; whatever the teratogenic factor, it must operate very early in embryogenesis, around the time of closure of the cephalic end of the neural tube (3-1/2 weeks of gestation). [1] Animal models and early human fetuses demonstrate that the exposed anterior position of the neural tube initially proliferates, as though attempting to form a forebrain, resulting in disorganized mass of primitive glial, neural, and vascular tissue arising from the upper end of the brainstem. This stage in the evolution of anencephaly is known as exencephaly. During gestation, this tissue typically degenerates, leaving by term only a remnant of varying amount called the cerebrovasculosa. Although the term "anencephaly" literally means "no brain," the actual amount of nervous system tissue compatible with that diagnosis can vary anywhere from only a few grams up to a normal full-term brain weight. [2] Anencephaly is classically subdivided into two forms: holo-anencephaly (complete absence of forebrain and cranium) and mero-anencephaly, in which "the cranium and the brain are present in rudimentary form." [3] Others employ the related terms "holoacrania" and "meroacrania," referring to the degree of bone absence. The brainstem in anencephaly can display a spectrum of involvement anywhere from relatively normal to totally absent, with the congenital defect extending all the way into the spinal canal (craniorachischisis). There may also be associated malformations of other organ systems. Differential Diagnosis In the great majority of cases, the diagnosis of anencephaly is very obvious, and there is little, if any, chance of mistaking it for another condition. Nevertheless, not all cases are so straightforward. If anencephaly were clearly distinct from all other congenital brain malformations, it should be possible to give an operational definition of it that includes all cases of anencephaly and excludes all cases of everything else, yet such a definition has not been offered by anyone so far. In their textbook on anencephaly, Lemire and colleagues stated: "An almost incomprehensible array of synonyms and classifications of anencephaly exists in the literature; many include entities now considered to be pathogenetically unrelated to the anencephaly spectrum." [4] Another expert on congenital neuropathology, Josef Warkany, wrote: As a congenital malformation that cannot be overlooked, [anencephaly] exemplifies the problems and difficulties of teratologic research in man. The terminology is confusing. I use the terms "anencephaly" for partial or total absence of the brain, "exencephaly" for exposure of the brain...and "pseudencephaly" for massive "area cerebrovasculosa" imitating the shape of the brain that it replaces. [5] Although "exencephaly" is usually considered to be a mere stage in the evolution of anencephaly, his definition implies that what is exposed may be something more than mere cerebrovasculosa, and that exencephaly is not necessarily restricted to early fetuses. Clearly, mutually exclusive operational definitions of anencephaly and exencephaly cannot be given, because the two conditions differ only in terms of the amount of exposed tissue, and therefore represent but two overlapping regions of a contiuous spectrum. On the other hand, if exencephaly is to be included within the general category of anencephaly, as most authors conceive it, we must either abandon the notion of anencephaly as "brain absence" (even "forebrain absence") or provide mutually exclusive operational definitions for "brain" and "exencephalic tissue." The latter distinction is based on the degree of neural organization present, and is difficult, if not impossible, to determine by gross visual inspection. …