11 Anencephaly

CASE 11

Clinical Presentation

Fetal MRI was requested following abnormal prenatal ultrasound in two different patients.

Figure 11A

Figure 11B

Figure 11C

Figure 11D

Radiologic Findings

Anencephaly and exencephaly are primarily morphologic diagnoses, typically recognized by fetal ultrasonography and better depicted by fetal MRI. A series of views from prenatal MRI study shows the typical features of anencephaly. There is total absence of the cerebrum and calvarium superior to the orbits and the misshaped skull base (Figs. 11A and 11B). The fetal MRI of exencephaly (Figs. 11C and 11D) demonstrates that the hemispheres are extruded from the open skull and floating into the amniotic fluid.

Diagnosis

Exencephaly/anencephaly sequence

Differential Diagnosis

Severe microcephaly: No defect observed on the skin, scalp, or calvarium; exceedingly small but complete brain.
Encephalocele: Basically a lesser degree of exencephaly, compatible with life. Brain-containing meningeal sac herniated through a skull defect, affecting more commonly the occipital lobes, the posterior fossa content (Chiari III), or the basal structures (frontal, nasal, sphenoidal encephaloceles).
Hydranencephaly: Secondary, presumably ischemic destruction of most of the forebrain, within a normal skull, whereas anencephaly is the destruction of an unprotected extruded brain tissue.
Amniotic band syndrome: Often associated with acrania, but secondary to entrapment by the fibrous bands. Other parts of the body are affected as well.
Cutis aplasia is a defect of skin, scalp, and skull, usually without any subjacent brain necrosis.

Discussion

Background

Anencephaly/exencephaly sequence is a type of neural tube defect related to myelomeningocele (Chiari II). A high serum a-feto protein may be the first clue during early pregnancy. Fetal ultrasound is confirmatory. There is a variable reported frequency of 0.5 to 10 per 1000 live births, with a strong female predominance. The incidence of anencephalic births is decreasing due to maternal folate administration and prenatal recognition.

Etiology

Anencephaly has been linked with folate deficiency and the methylene tetrahydrofolate reductase (MTHFR) unfavorable genotype or folate antagonists. Clomiphene citrate ovulation induction is another possible factor. Familial cases have been reported. Chromosomal abnormalities (aneuploidy) are present in ~2%, in particular trisomy 2p.