ANENCEPHALY

The most common neural tube defect. US findings include absence of the cranial vault and cerebral hemispheres above the level of the orbits. The cerebral hemispheres may be replaced by an amorphous neurovascular mass. The condition is inevitably fatal. 

-> Brain stem and portions of midbrain are present
-> Neural tube closure failure between 3-4 weeks
-> Should be routinely diagnosed between 16-20 weeks EGA by US; Can be detected as early as 12 weeks
-> 3 criteria for diagnosis are absence of cranial vault, inability to measure BPD due to lack of reference plane, and large 'eyeglass-like orbits'
-> Vascular stroma covers the cranial defect
-> 50% of fetuses have associated spinal defects
-> Higher incidence of omphalocele and eventration
-> Characteristic fetal mov't is episodes of immobility alternating with excesive, uncoordinated activity 
-> Differential diagnosis: microcephaly, acrania 
-> Serum elevation of Alpha-fetoprotein

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