acrania
description
the terms acrania and exencephaly refer to the same condition, whereby there is congenital absence of the bony calvarium, resulting in exposure of the underlying cerebrum. the condition arises from incomplete rostral neurulation, specifically defective closure of the cephalic neuropore, so that migration of mesenchymal elements beneath the calvarial ectoderm is impaired, resulting in a failure of development of the calvarium, related muscles and dura. it is likely to represent the embryologic precursor of the more common anencephaly and is differentiated pathologically from anencephaly by the increased mass of cerebral tissue. clinically, however, both conditions are incompatible with life. pathologically, the exposed neural tissue is covered by a highly vascular layer of epithelium. two relatively equivalent cerebral hemispheric remnants are present, with a reddish mass of disorganised tissues, remnants of deep cerebral neural elements, blood vessels, fibrous tissues, and fluid-filled spaces.
diagnosis
there is no calvarium present. there will be a spectrum of forebrain distortion and disintegration, so that the cerebral and ventricular anatomy may appear normal, or there may simply be a residual brain mass; cerebral convolutions that are similar to sulcal configurations may be distinguishable. facial structures and the base of the skull are preserved in exencephaly, as in anencephaly. these parts of the cranium are recognizable sonographically if fetal position allows. first-trimester diagnosis of exencephaly and other anomalies is clearly possible, especially with the help of vaginal and other high-resolution ultrasonographic probes. however, the fetal cranium is not completely calcified before a gestational age of 10 to 11 weeks so caution should be exercised. beyond the eleventh week, it is usually possible to observe echogenic images corresponding to calcification of the bones of the cranium. it is occasionally seen in conjunction with spina bifida, and associated anomalies have been observed in the gastrointestinal and genitourinary systems.
differential diagnosis
a large encephalocele may present diagnostic difficulty; however, the complete absence of calvarium and lack of intracranial cerebral tissue indicates exencephaly. the presence of amniotic bands has been associated with a multiple deformation syndrome that may include exencephaly. the existence of a fibrous band between the edge of the fetal defect and the placenta would be pathognomonic of amniotic band syndrome. the limb-body-wall complex is a complicated fetal malformation with the essential features of neural tube defects, body-wall disruption, and limb abnormalities. in this condition exencephaly will be associated with facial clefts, limb defects, and scoliosis. in osteogenesis imperfecta type ii and hypophosphatasia, lack of cranial bone mineralisation may simulate acrania; however, intracranial anatomy will remain normal.
sonographic features
absence of the cranial vault.
cerebral tissue present with variable anatomic distortion.
‘pseudo-sulcal’ appearance of brain.
associated syndromes
references
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- denath fm, romano w, solcz m, donnelly d ultrasonographc findings of exencephaly in pentalogy of cantrell: case report and review of the literaturej clin ultrasound 22: 351-354
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- harrington bj, horger eo, edwards jga counselling dilemma involving anencephaly, acrania and amniotic bands genet couns 3: 183-186
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- wilkins-haug l, freedman wprogression of exencephaly to anencephaly in the human fetus – an ultrasound perspective prenat diagn 11: 227-233
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- bronshtein m, ornoy a acrania: anencephaly resulting from secondary degeneration of a closed neural tube: two cases in the same family j clin ultrasound 19: 230-234
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