How does the premature brain respond to ischemic injury?
Periventricular leukomalacia (PVL) is characterized by ischemia in an end arterial distribution (in the watershed regions of white matter that surround the ventricles). US may demonstrate an increase in periventricular echogenicity soon after the initial insult. Cystic change in these regions can be seen later in the subacute stage, as the white matter begins to be resorbed. MRI demonstrates abnormal periventricular signal intensity and volume loss. Before 28 weeks of gestation, the developing brain does not display a leukomalacic response, and only volume loss results. This volume loss may take the form of dilated ventricles and expanded extra-axial spaces or porencephalic cysts (expanded cystlike dilations from the lateral ventricles that may reach the cortex). A porencephalic cyst can be differentiated from schizencephaly by noting that, in the former entity, there is no gray matter lining the cyst cavity.