Fig. 2: Top: Acrania from failure of migration of the ectodermal mesenchyme. Bottomt: Acrania from amniotic band disruption.
Acrania occurs during the beginning of the fourth week of development when the anterior neuropore closes. The desmocranium (the mesenchymal primordium overlying the future cerebral hemisphere) which normally becomes the epidermis of the scalp, remains a membranous coverage. The normal migration of mesenchymal tissue under the calvarium ectoderm does not occur. Thus, the calvarial dermal bones of the skull, related musculature, and dura mater are absent. However, the cartilaginous skull bones form a normal skull base and foramen magnum. Philogenetically the origin of the viscerocranium and chondrocranium corresponds to the support used in the fishes. The considerable enlargement of the brain resulted in a herniation through these primitive cartilaginous bones. The protection of the growing brain could not be achieved by theses bones, and instead was provided by differentiation of the overlying skin into membranous bones. The development of the neurocranium is necessary to induce the formation of the brain. In its absence, the cerebral tissue fails to differentiate into two hemispheres. The cerebellum, brainstem and cranial nerves are normal, but the growth of the diencephalon, and ocular globe is abnormally small1.
The condition is identified by the absence of the calvarium. The cerebral hemispheres are surrounded by a thin membrane. The demonstration of abundant, albeit abnormal brain tissue on the sonogram helps differentiate cases of acrania from the more common anencephaly. Acrania and meroacrania may not be neural tube defects and should be differentiated from anencephaly.
The prenatal diagnosis and intrauterine monitoring of acrania are possible with ultrasound during pregnancy. The biochemical and cytological examination of amniotic fluid samples may suggest the presence of a neural tube defect, but a specific prenatal diagnosis of acrania is only possible by ultrasound scanning.
The associated anomalies seen in our cases of acrania are listed in Table 2. Similar pathologic findings have been described in the amniotic band syndrome2. Only two of our cases were due to amniotic bands.
The differential diagnosis includes anencephaly and large cephaloceles. In anencephaly, cerebral tissue is completely absent; while in cephaloceles, the cranial vault can always be detected and a part of the brain is intracranial. A distinction should also be made between acrania and conditions characterized by lack of mineralization of the skull bones such as hypophosphatasis and osteogenesis imperfecta Type II. In these skeletal dysplasias, the intracranial anatomy is normal, and the brain is surrounded by a thick layer of tissue representing soft tissues and unossified bone. Bowing, fractures or shortening of long bones are usually present.
Prognosis & management
Acrania is uniformly lethal2. Antenatal identification allows the clinician to make appropriate and timely management decisions.
1. Mannes EJ, Crelin ES, Hobbins JC, et al: Sonographic demonstration of fetal acrania. AJR 139: 181, 1982.
2.Romero R, Pilu G, Jeanty J, et al. Prenatal Diagnosis of Congenital Anomalies. Appleton & Lange, Norwalk, Connecticut, 1988. pp 75-76.
3. Kristoffersen K, Pedersen BN, Secher NJ, et al: Akrani og spina bifida diagnosticeret ved bestemmelse af alfa fotoprotein: 16. graviditesuge. Ugeskr Laeger 137: 1719,1975.
4. Vergani P, Ghidini A, Sirtori M, et al: antenatal diagnosis of fetal acrania. J Ultrasound Med 6: 715-717, 1987.
5. Moore KL: The Developing Human, 3rd ed. Philadelphia, W.B. Saunders, 1982, pp351-354.
6. Frezal J, Kelly J, Guillemot ML et al.: Anencephaly in France. Am J Hum Genet 16: 336,1964.
7. Giround A: Anencephaly. In: Vinken GW, Bruyn PW (des): Handbook of clinical neurology. Amsterdam, Elsevier/ North Holland Biochemical Press, 1977, Vol 30, pp 173-208.