Anencephaly

What is the clinical setting when you will consider anencephaly?  

Factors that place a woman at increased risk include:

• Family history of neural tube defects.
• Women with increased risk of neural-tube defects are offered alpha-fetoprotein (AFP) testing as part of a diagnostic work-up. 
• Exposure to certain environmental agents: diabetes (hyperglycemia), hyperthermia, drugs and medications (anticonvulsants – valproic acid and carbamazepine), aminopterin, isotretinoin.
• Neural-tube defects as part of a genetic syndrome with known recurrence risk: Meckel-Gruber, Roberts-SC phocomelia, Jarco-Levin, and HARDE Syndromes.
• Individuals who belong to high-risk racial or ethnic groups and/or in high-risk geographic regions: United Kingdom > China, Egypt, India >U.S.

What is the useful imaging modalities in evaluating anencephaly? What is the utility of the procedure, including limitations and accuracy?   

• Ultrasound.
• Anencephaly can theoretically be diagnosed as early as 8 weeks; however, it can be missed in the first trimester. 
• There is 100% accuracy in the second trimester for this diagnosis by ultrasound. 
• One study showed sonography alone was 97% sensitive and 100% specific in diagnosing an open neural tube defects.  

What are the imaging findings of anencephalic pregnancy?   

• The fetus is evaluated by targeted ultrasound once the AFP levels are confirmed. 
• Anencephaly and other cranial defects are usually readily visualized and most spine defects can be detected as well. 

Finding indicative of anencephaly: Absence of the cranial vault and brain above the base of the skull and orbits.
Failure to obtain an adequate view of the bi parietal diameter in the second trimester should raise suspicion.
Hydramnios from impaired fetal swallowing commonly accompanies anencephaly but is typically a late finding. (85% after 25 weeks but in only 10% before this time).

Findings indicative of another neural tube defects:

• Lemon sign: frontal notching. Fetal head looks like a lemon.
• Banana sign: elongated cerebellum.
• Obliteration of the cisterna magna.
• Ventriculomegaly with the lateral ventricle width greater than 10 mm.
• Small bi parietal diameter.