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KEY POINTS
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 during the third trimester.
Most of these destructive processes are the consequence of vascular accidents, hemorrhages, or occlusion. Although in many cases the etiology is unknown, evidence is rapidly growing regarding possible genetic conditions implicated in these disorders. Other etiologies to be considered are placental insufficiency, coagulation disorders, drug consumption, and transplacental infections.
Fetal ischemic stroke and intracranial hemorrhage are clinically important because they may have severe consequences, but they frequently escape detection.
Prenatal stroke is considered the most important determinant of cystic destruction of the cortex, depending upon the time of occurrence, and the severity may result in a spectrum of conditions that include 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).
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 picture changes with time. An echogenic collection is first seen, and in the following days it develops into a complex mass frequently complicated by severe ventriculomegaly.
Cerebellar lesions are discussed separately since the prenatal differentiation between stroke and hemorrhage, although problematic, has similar effects.
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As occurs in the newborn, the fetal brain is particularly susceptible not only to homeostatic changes, but is also influenced by genetic and epigenetic factors that can lead to hypoxia, ischemia, and hemorrhage. These events usually remain undiagnosed during pregnancy, since the majority of cases occur after the second-trimester US examination.
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FETAL ISCHEMIC STROKE
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In this section we will follow the definitions proposed by the National Institute of Neurologic Disorders and Stroke,1 where perinatal stroke is defined as a “group of heterogeneous conditions in which there is a focal disruption of cerebral blood flow secondary to arterial or cerebral venous thrombosis or embolization occurring between 20 weeks of fetal life through the 28th postnatal day, and confirmed by neuroimaging or neuropathological studies.” The same group proposed to use the term fetal ischemic stroke (FIS) when “diagnosed before birth by using fetal imaging methods or in stillbirths on the basis of neuropathologic examination.”1
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The incidence of FIS is not known due to the lack of reports based on population studies. Overall incidence of ischemic perinatal stroke (IPS) has been reported to range between 1 in 2300 to 1 in 5000 births1 and accounts for 30% of children affected with hemiplegic cerebral palsy (CP) who were born at term or late preterm gestations.2,3 Perinatal arterial ischemic ...