The absence of cranial vault (acrania) with disorganized herniated brain tissue (exencephaly) is part of the acrania-exencephaly-anencephaly sequence. The key finding is the lack of an ossified cranial vault. Note that the posterior fossa "cyst" labeled in the images is likely reflective of abnormal brain parenchyma. 

This is usually an isolated finding but may occasionally be associated with chromosomal abnormalities. The prognosis is universally dismal; babies born with this condition do not tend to survive longer than a few minutes after delivery.

It is important to note that once a woman has had a pregnancy with this condition, she is at markedly increased risk of neural tube defects (i.e., anencephaly, encephalocele and meningomyelocele) for subsequent pregnancies, and double dosage of periconceptional folic acid supplementation is recommended.