Porencephaly is a term that describes a fluid-filled cavity in open communication with the lateral ventricle (van der Knaap et al., 2006). The term porencephaly is often used interchangeably with porencephalic cyst, schizencephaly, cystic brain degeneration, and congenital brain clefts. Porencephaly was first described in 1859 as a cavity or cleft of the cerebral cortex (Heschl, 1859). These lesions may or may not communicate with the ventricular and subarachnoid systems. Two major subgroups are described: developmental porencephaly, which includes schizencephaly and congenital midline porencephaly and congenital encephaloclastic porencephaly (Hall, 2006).

Developmental porencephaly represents a primary failure of neuronal development and migration. Synonyms include true porencephaly, schizencephaly, and congenital porencephaly. Congenital midline porencephaly is a more recently described malformation, consisting of the triad of a midline parietal scalp anomaly (such as alopecia or cephalocele), hydrocephalus, and a midline intracranial cyst (Yokota and Matsukado, 1979; Vintzileos et al., 1987). While this malformation most likely represents a form of porencephaly, some authors consider it a variant of holoprosencephaly (Vintzileos et al., 1987).

In contrast, congenital encephaloclastic (disruptive) porencephaly results from cortical destruction due to an external insult in an otherwise normally developed brain. Synonyms include pseudoporencephaly, false porencephaly, and cystic brain degeneration. This destruction results in an intracerebral cystic cavity containing cerebrospinal fluid, and such a cyst may be single or multiple (Figure 21-1) (Hall, 2006). Congenital encephaloclastic porencephaly may have many different causes; of these, hemorrhagic infarction due to fetal venous congestion or occlusion is considered to be the most common (Dekaban, 1965; Cantu and LeMay, 1967; Nixon et al., 1974).

The known risk factors for acquired porencephaly are conditions that cause thrombophilia (such as Factor V Leiden and protein C deficiency), increased bleeding (perinatal alloimmune thrombocytopenia, von Willebrand disease), vasculopathy following in utero exposure to cocaine (Dominguez et al., 1991), infection with Coxsackie virus or cytomegalovirus (Tominaga et al., 1996; Chalhub et al., 1977), and trauma, resulting from ventricular puncture (Lorber and Grainger, 1963), amniocentesis (Eller and Kuller, 1994), or chorionic villus sampling (Sharma and Phadke, 1991) (Table 21-1). Familial porencephaly, consistent with both autosomal dominant and recessive patterns of inheritance, has also been reported (Berg et al., 1983; Sensi et al., 1990; Haverkamp et al., 1995). Mutations in the COL4A1 gene are increasingly being recognized as an important cause of blood vessel rupture in the fetus (van der Knaap et al., 2006).

Porencephaly is an extremely rare condition with an unknown incidence.

Porencephaly has been successfully diagnosed prenatally using both sonography and magnetic resonance imaging (Lithuania et al., 1989; Komarniski et al., 1990; Meizner and Elchalal, 1996; Levine et al., 1997; Pilu et al., 1997). The sonographic appearance is that of a fluid-filled space in the normal brain parenchyma. The cyst is more commonly unilateral but may be bilateral (Figures 21-1 and 21-2). When cysts are multiple they are frequently symmetric in appearance (Klingensmith and Cioffi-Ragan, 1986). Loss of cerebral tissue is often easily visible on coronal scans. Color Doppler sonography may be helpful in delineating a particular vascular abnormality associated with the cystic lesion (Suchet, 1994). Communication with the lateral ventricles or subarachnoid space is often visible. The ipsilateral ventricle is usually enlarged to compensate for the smaller brain mass. The diagnosis of porencephaly should be considered whenever marked asymmetric ventriculomegaly is found (Chervenak et al., 1983; Toma et al., 1990).