How do you define a proencephalic cyst?
This is harder than you think it would be. Some authors use the term very broadly to encompass essentially all brain clefts / cysts (and thus include schizencephaly), whilst others reserve the term for an ecephaloclastic cleft (most common usage). The necessity of a cyst communicating with the ventricular system and / or subarachnoid space to earn the designation porencephaly is also a point of contention. Some authors use the term for cysts whether or not communication exists. Others reserve the term of cysts that communicate with at least one space, in which case they divide porencephalic cysts into internal (communicating with the ventricle) or external (communicating with the subarachnoid space). Still others require communication with both.
Assuming the narrower more common definition of porencephaly (i.e encephaloclastic process), what is the main distinguishing feature of schizencephaly?
Schizencephalic clefts are lined by abnormal grey matter, whereas porencephalic cysts are lined by white matter.
A moderate size cyst in the right parieto-occipito-temporal junction measuring 5 x 4 x 6 cm has a small communication with the occipital horn of the right lateral ventricle. It follows CSF signal in all sequences, with no diffusion restriction or enhancement. It is not lined by a grey matter and is associated with a small amount of adjacent FLAIR hyperintensity. It exerts mild mass effect on the adjacent parenchyma with slight remodelling of the overlying calvarium noted.