Can a partial cranial vault be observed on prenatal ultrasound in a fetus with anencephaly?

Partial Cranial Vault in Anencephalic Fetuses on Ultrasound

Yes, a partial cranial vault can be observed on prenatal ultrasound in fetuses with anencephaly, as anencephaly is defined by partial or complete absence of skull bones rather than total absence in all cases.

Understanding Anencephaly's Anatomical Spectrum

Anencephaly represents a spectrum of cranial vault abnormalities rather than a uniform complete absence:

• Anencephaly involves partial or complete absence of the skull bones, usually with a remnant of brain tissue attached to the base of the skull 1
• The condition results from failure of anterior neural tube closure, leading to variable degrees of cranial vault deficiency 1
• Morphological studies demonstrate two distinct skeletal subtypes: one with posterior cranial fossa morphology close to normal, and another with a malformed and much smaller posterior cranial fossa 2

Ultrasound Detection and Timing Considerations

The appearance of anencephaly on ultrasound depends critically on gestational age:

• Ultrasound findings can appear normal until ossification has definitively failed to occur 3
• Reliable diagnosis is possible at 12-13 weeks gestation when the cranial vault should be forming 3
• Detection rate for anencephaly using maternal serum AFP screening is 95% or greater, with biparietal diameter (BPD) measurement ruling out anencephaly when present 1, 4
• First-trimester ultrasound at 12-13 weeks allows detailed fetal anatomy screening and reliable anencephaly diagnosis 3

Key Diagnostic Features

When evaluating for anencephaly, look for these specific ultrasound findings:

• Absence or severe deficiency of the cranial vault above the orbital rims and nuchal line 1
• Exposed, undifferentiated neural tissue may be visible as a remnant 1
• BPD measurement cannot be obtained in complete anencephaly, which itself serves as a diagnostic indicator 1, 4
• The degree of cranial vault absence varies, with some cases showing partial bone formation 1, 2

Clinical Implications

Understanding the variable presentation is essential for accurate counseling:

• Anencephaly is virtually always fatal before or shortly after birth, with 100% mortality 1, 5
• The condition occurs in 1-2 per 1000 births in the United States 1
• Early diagnosis at 12-13 weeks allows for timely management decisions and reduces complications from later pregnancy termination 3, 5
• Accurate gestational age determination using crown-rump length (CRL) measurement is crucial for proper timing of screening and diagnosis 4

Common Diagnostic Pitfalls

Avoid these errors when evaluating suspected anencephaly:

• Do not assume complete absence of all cranial structures is required for diagnosis - partial cranial vault can be present 1, 2
• Scanning too early (before 12 weeks) may miss the diagnosis as ossification has not yet failed definitively 3
• Failure to use transvaginal ultrasound in the first trimester may compromise detailed anatomical assessment 3
• When BPD cannot be measured in the second trimester, consider anencephaly as the primary differential diagnosis 1, 4