Porencephaly signifies a detect in brain structure, having the appearance of a cystlike cavity, which may communicate with the ventricular system or which may be separated from it by a thin membrane. Frequently, such defects are unaccompanied by clinical symptoms or signs during life and are usually first identified at autopsy or accidently at operation. The condition probably arises (1) as a true developmental aplasia or hypoplasia, or (2) as an acquired defect due to trauma, to encephalitis, or to circulatory disturbances, such as thrombosis or embolism, producing ischemic necrosis and resorption of brain tissue. According to Yakovlev and Wadsworth1 true porencephaly results from arrest of development of the wall of the secondary cerebral vesicle during fetal life, with formation of a virtual cleft in the cerebral mantle. Such clefts are usually bilateral and symmetrical and do not conform to areas of specific vascular supply. On the other hand, the acquired types are usually single and unilateral, with borders usually showing evidence of old necrotic foci, gliosis, connective scar tissue, and meningeal thickening. The cyst is filled with fluid, which is usually colorless, and is covered by a thin layer of pia-arachnoid. The acquired types may occur anywhere in the cerebrum but are most commonly found in the area supplied by the middle cerebral artery.
In porencephalic cysts which communicate with the ventricular system the diagnosis may be made by pneumoencephalography. Noncommunicating cysts occasionally may be identified unexpectedly by ventriculography carried out for an entirely different diagnostic purpose. Such a. . .