Definition: Acrania include absent calvarium and scalp above the orbits,intact skull and normal development of the face including the eyes, nose, and mouth, distortions of the brain parenchyma with loss of landmarks, the brain tissue being covered by an incomplete membranous structure,and deformity of the cerebellum and brain stem[1].
Etiology: the cranial end of the neural tube closes by 24 days and the caudal by 25-26 days. Then, the neural tube is covered dorsally by mesenchyme that forms the vertebral arches and skull. Closure of the vertebral arches is completed at 11 weeks of gestation. Defective closure of the neural tube results in neural tube defects (NTDs). Depending on the point of the defect NTDs may affect the brain (acrania, anencephaly, encephalocele) or spinal cord (spina bifida) [2].
Pathogenesis: neural tube defects develop during the third to fourth week of gestation and are due to a combination of genetic and environmental causes (multifactorial). The genetic causes are largely unknown. Two polymorphisms of the folate dependent enzyme 5,10-Methylenetetrahydrofolate reductrase (MTHFR), MTHFR C677T and MTHFR A1298C, are associated with an increased risk for NTDs. Environmental causes include diabetes mellitus and the antiepileptic drug valproate.[2]
Diagnosis and sonographic findings: the prenatal diagnosis of acrania is is easily made by ultrasound .
Differential diagnosis: different forms of congenital cranial defects, including acrania, anencephaly, acalvaria, and acrania.
Prognosis: It is a clinical entity which is incompatible with life [6].
Management: because of the poor prognosis, the decision to interrumpt the pregnancy is discussed with the couple.
The mechanism of action of folic acid in preventing NTDs is not known. Women who have children with NTDs are not overtly folate deficient. However, the rapidly dividing cells of the neural tube probably require a large amount of folate for DNA synthesis [2].
Bibliography
[1] Ali Sharif and Yihua Zhou.Fetal MRI Characteristics of Exencephaly: A Case Report and Literature Review , Case Reports in Radiology.Volume 2016 (2016), Article ID 9801267, 4 pages.doi.org/10.1155/2016/9801267
[2]Copp AJ, Greene ND. Genetics and development of neural tube defects.
[3 ] A. J. Nawale, S. A.Merchant, S. S. R. Koteyar, and P.Masand, “Exencephaly: a rare case diagnosed on antenatal ultrasound,” Bombay Hospital Journal, vol. 42, pp. 520–521, 2000.
[4] C.-P. Chen, T.-Y. Chang, Y.-H. Lin, andW.Wang, “Prenatal sonographic diagnosis of acrania associated with amniotic bands,”
Journal of Clinical Ultrasound, vol. 32, no. 5, pp. 256–260, 2004.
[5 ] C. Evans, T. Marton, S. Rutter, D. O. Anumba, E. H. Whitby, and M. C. Cohen, “Cranial vault defects: the description of three cases that illustrate a spectrum of anomalies,” Pediatric and Developmental Pathology, vol. 12, no. 2, pp. 96–102, 2009.
[6] Hendricks SK, Cyr DR, Nyberg DA, Raabe R, Mack LA. Exencephaly—clinical and ultrasound correlation to Anencephaly. Obstetrics and Gynaecology. 1988;72:p898–901