Meroanencephaly: pathology and prenatal diagnosis.

Meroanencephaly is a rare form of anencephaly characterized by malformed cranial bones and a median cranial defect, through which protrudes abnormal tissue, called the area cerebrovasculosa. Area cerebrovasculosa denotes abnormal spongy, vascular tissue admixed with glial tissue ranging from a thin membrane to a large pseudoencephalic mass simulating cerebral tissue, that is composed of connective tissue, hemorrhagic vascular channels, glial nodules, and disorganized choroid plexuses. There are three types of anencephaly: (1) meroanencephaly, where there is rudimentary brain tissue and partial formation of the cranium; (2) holoanencephaly, the most common type, in which the brain is completely absent, and (3) craniorachischisis, the most severe, where area cerebrovasculosa and area medullovasculosa fill both cranial defects and the spinal column. In meroanencephaly, there is a median defect present, through which protrudes the area cerebrovasculosa. We present 3 cases of meroanencephaly diagnosed prenatally, along with histopathologic analysis. One case showed ectopic glial tissue in the lung and adrenal medullary hyperplasia. Two cases were diagnosed in the early second trimester by ultrasound scanning. Meroanencephaly may be mistaken for encephalocele both at the bedside exam and sonographically, and should be included in the differential for protruding anterior cranial masses.