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KEY POINTS

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KEY POINTS

  1. Many congenital anomalies of the brain do not derive from abnormal embryogenesis but are the consequence of destructive processes that may occur any time in gestation, particularly in the third trimester.

  2. Most of these destructive processes are the consequence of vascular accidents, hemorrhage, or occlusion. The etiology is often unknown, but they may derive from a variety of obstetric complications, such as placental insufficiency, coagulation disorders, drug consumption, and transplacental infections.

  3. Disruptive lesions of the fetal brain are clinically important because they may have severe consequences, but they frequently escape early detection.

  4. Intracranial hemorrhage is probably the most common and therefore the best known of all intrauterine disruptions of the fetal brain. The hemorrhage occurs usually into the lateral ventricles, and the sonographic pictures change with time. An echogenic collection is first seen, and in the following days it develops into a complex mass frequently complicated by severe ventriculomegaly.

  5. Prenatal stroke is considered the most important determinant of cystic destruction of the cortex that, depending on the time of occurrence and the severity, may result in a spectrum of conditions, including porencephaly (single or multiple cysts replacing brain parenchyma), schizencephaly (a gray matter–lined cleft in the cerebral mantle connecting the cavity of lateral ventricles to the subarachnoid space), and hydranencephaly (complete destruction of the cerebral hemispheres).

  6. Cerebellar lesions are discussed separately even though they also deal with intracranial hemorrhage. However, they deserve more focused attention.

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Intrauterine insults may lead to brain ischemia (stroke), which is a major contributor to the sonographic brain findings that we will discuss in this chapter. Prenatal stroke can be the result of an arterial ischemic event, a venous thrombosis, or hemorrhage. The end-stage lesion is a cavity in the brain tissue of variable size and location.1 The location of the cavity is predictable and stable depending on the vessel that was affected.2 For example, stroke affecting the middle cerebral artery (MCA) will result in porencephaly and that affecting both internal carotid arteries (ICAs) in hydranencephaly. There are several factors determining the propensity of the immature brain to undergo dissolution and eventually cavitation: (1) the high water content of the unmyelinated brain, (2) the relative paucity of myelinated fibers, and (3) deficient glial response. The first two factors result in dissolution of the brain, and the latter is responsible for the cavitation.

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PORENCEPHALY

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Definition

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Porencephaly is a collective term for a variety of cystic lesions of the brain. Some of these cavities communicate with the ventricular system, the subarachnoid space, or both. These defects have many similarities in etiopathogenesis with schizencephaly and hydranencephaly.1 It is the outcome of an insult such as ischemic stroke, infection, hemorrhage, or trauma occurring between the second trimester of pregnancy and the early postnatal period. This insult results in focal or multifocal areas of brain necroses, which subsequently undergo dissolution and cavity formation....

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