Positional plagiocephaly refers to a calvarial deformation that results from external pressure after birth when an infant is consistently placed in the same position for rest and sleep.

It should not be confused with Plagiocephaly refers to a type of craniosynostosis in which there is an asymmetric coronal and/or lambdoid sutures premature closure,

The treatment for positional plagiocephaly and craniosynostosis is completely different with the latter treated surgically while the former treated usually medically.  

Benign external hydrocephalus is benign enlargement of subarachnoid spaces seen in infancy. It usually involves the frontal lobe subarachnoid spaces.