You are in: eMedicine Specialties > Radiology > PEDIATRICS HydranencephalyArticle Last Updated: Feb 18, 2008AUTHOR AND EDITOR INFORMATIONSection 1 of 10
  Author: Andrew L Wagner, MD, Assistant Professor of Radiology, Instructional Faculty, University of Virginia School of Medicine; Director of Neuroradiology, Department of Radiology, Rockingham Memorial Hospital Andrew L Wagner is a member of the following medical societies: American College of Radiology, American Roentgen Ray Society, American Society of Neuroradiology, and Radiological Society of North America Coauthor(s): Dennis Rohrer, MD, Consulting Staff, Department of Radiology, Rockingham Memorial Hospital Editors: Charles M Glasier, MD, Professor, Departments of Radiology and Pediatrics, University of Arkansas for Medical Sciences; Chief, Magnetic Resonance Imaging, Vice-Chief, Pediatric Radiology, Arkansas Children's Hospital; Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; Marta Hernanz-Schulman, MD, FAAP, Professor, Radiology, Radiological Sciences, and Pediatrics, Director, Department of Pediatric Radiology, Radiologist-in-Chief, Director, Department of Diagnostic Imaging, Vanderbilt University Medical Center, Vanderbilt Children's Hospital; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; Lawrence M Davis, MD, Assistant Professor of Diagnostic Imaging (Clinical), Department of Diagnostic Imaging, Brown Medical School Author and Editor Disclosure Synonyms and related keywords: hydrocephalus, anencephaly, absent cerebral cortex, absent basal ganglia, Fowler-type hydranencephaly, macrocrania, microcrania, alobar holoprosencephaly INTRODUCTIONSection 2 of 10
  BackgroundThe word hydranencephaly is a fusion of hydrocephalus and anencephaly, but the condition actually represents a distinct disorder. The condition is characterized by near-total or total absence of the cerebral cortex and basal ganglia. The thalami, pons, cerebral peduncles, and cerebellum are usually present, as may be a small amount of tissue from the occipital, frontal, and temporal lobes.1 PathophysiologyAlthough hydranencephaly is not a well-defined entity, it is considered to be the result of a destructive process or lesion in a previously normal brain. The changes resulting in hydranencephaly can occur at any time from the 11th or 12th week of gestation through the first postnatal year. Hydranencephaly usually occurs sporadically, although Witters and colleagues reported that a rare, specific form of the disorder, Fowler-type hydranencephaly, appears to have an autosomal recessive inheritance.2 The most commonly suspected causes of the nongenetic form of hydranencephaly are in utero vascular accidents and infections, such as those caused by Toxoplasma gondii, cytomegalovirus, and herpes simplex, which can cause necrotizing vasculitis and encephalitis.3, 4 Placental abnormalities, diffuse hypoxic-ischemic brain necrosis, the toxic effects of drugs, and thromboplastic material embolized from a deceased co-twin also have been implicated as causative factors.5, 6, 7 Moreover, the incidence of hydranencephaly is increased in the fetuses of mothers who smoke. Knowledge about possible etiologies of hydranencephaly comes from various observations and experiments.8 Studies in sheep and monkeys have demonstrated that bilateral ligation of the carotid arteries results in destruction of the cerebral hemispheres, with relative preservation of the portions of the brain supplied by the posterior circulation, giving an appearance similar to that of hydranencephaly.9 See also the following related eMedicine topic: Brain Death in Children Toxoplasmosis, CNS FrequencyUnited StatesHydranencephaly occurs in less than 1 in 10,000 births. InternationalHydranencephaly occurs in less than 1 in 10,000 births. Mortality/MorbidityHydranencephaly is associated with a markedly reduced life expectancy, with the condition often resulting in stillbirth or a lifespan of less than 1 year. However, there are reports of prolonged survival. With aggressive care, patients can live into their third decade.12
RaceNo known race predilection exists. SexNo known sex predilection exists. AgeHydranencephaly is primarily a disease of the fetus; encephaloclastic encephalomalacia can occur in cases of severe perinatal insult.14 AnatomyHydranencephaly is a loss of the cerebral tissues supplied by the anterior circulation (that is, the internal carotid arteries), with preservation of tissue supplied by the posterior circulation (specifically, the vertebrobasilar system). The portions of the brain supplied by the anterior and posterior circulations can be variable. However, the posterior circulation typically supplies the posterior fossa structures, occipital lobes, and thalami, whereas the anterior circulation supplies the temporal, parietal and frontal lobes. The thalami typically receive their blood supply from posterior circulation perforating vessels, which is why the thalami are preserved in cases of hydranencephaly. The anterior or posterior circulation may supply the basal ganglia, and feeding vessels may originate from both circulations. In addition, portions of the posterior and medial parietal lobes can have a blood supply from the posterior circulation, although they are usually fed by the middle cerebral artery (MCA) and anterior cerebral artery (ACA), which are anterior branches. Therefore, hydranencephaly is anatomically characterized by a loss of the cerebral mantle, with variable preservation of small remnants of the occipital and temporal lobes, as well as, at all times, small, basilar portions of the frontal lobes. The thalami and posterior fossa are preserved, as is the brainstem (although it is usually atrophic at histologic evaluation). Clinical DetailsAt birth, children with hydranencephaly may appear healthy, but developmental delay becomes apparent within a few weeks. Common manifestations of the condition include irritability, abnormal muscle tone, seizures, and increasing head size (as a result of hydrocephalus). Swallowing difficulties lead to malnutrition, aspiration, and pneumonia. Transillumination of the skull has been used to document the large fluid collection in these patients, but its importance and use have decreased because of the availability of cross-sectional imaging. Preferred ExaminationIn utero, hydranencephaly is frequently diagnosed with ultrasonography. Magnetic resonance imaging (MRI) is probably the best modality for the overall evaluation of the anomaly and for the documentation of cortical remnants.15, 16 Limitations of TechniquesPostnatally, cranial ultrasonography can detect the absence of cerebral tissue, differentiated from alobar holoprosencephaly by the cleaved thalami and the presence of the falx cerebri. Because the brain beneath the fontanelle is clearly visible, absence of any cortical remnant at this level is helpful in the important differentiation of hydranencephaly from severe hydrocephalus. However, if the fontanelle is small or if appropriate high-frequency transducers are not available, it is possible in severe hydrocephalus to overlook a thin rim of cortical mantle. On computed tomography (CT) scans, cortical tissue below the parietal bony convexity may be overlooked. In these cases, thin sections and overlapped coronal reconstructions may be helpful. In severe hydrocephalus, MRI can reliably depict the remaining cerebral cortical rim.15, 16 DIFFERENTIALSSection 3 of 10
Holoprosencephaly Other Problems to Be ConsideredAnencephaly RADIOGRAPHSection 4 of 10
False Positives/NegativesCT SCANSection 5 of 10
FindingsIn hydranencephaly, CT scanning demonstrates an absence of most of the supratentorial structures, with preservation of the falx, thalami, and various amounts of the occipital lobes and basal ganglia. Macrocrania or microcrania may be present, or the head circumference may be normal. Degree of ConfidenceHydranencephaly and severe hydrocephalus may appear similar on CT scans because in both entities the falx is present and the thalami are unfused.16 The key to distinguishing hydrocephalus from hydranencephaly is the presence of a thin rim of residual cerebral cortical tissue in hydrocephalus that is not present in hydranencephaly. Thin sections and overlapped coronal reconstructions may be helpful in detecting this rim. In addition, the third ventricle, which is absent in hydranencephaly, is identifiable in hydrocephalus. The brainstem is seen in hydranencephaly and hydrocephalus. False Positives/NegativesNo normal variants mimic hydranencephaly. MRISection 6 of 10
FindingsMRI findings are similar to CT scan findings, although the improved soft-tissue contrast achieved with MRI allows for more confident identification of the falx and any residual supratentorial brain tissue. If fetal ultrasonographic findings are equivocal, fetal MRI is useful for the accurate prenatal diagnosis of hydranencephaly.15, 16 Degree of ConfidenceSee CT Scan. ULTRASOUNDSection 7 of 10
FindingsMost cases of hydranencephaly can be detected with prenatal ultrasonography, although fetal MRI may be necessary to confirm the diagnosis.17 If ultrasonography is performed prior to the etiologic insult, the initial study may be normal. When fetal ultrasonography is performed early in gestation, hydrocephalus and alobar holoprosencephaly can be difficult to distinguish from hydranencephaly. Hydranencephaly appears as a supratentorial fluid collection that replaces the cerebral hemispheres, with preserved, nonfused thalami and minimal (if any) preserved cerebral cortical tissue, usually in the occipital area.18 There is no uniform rim of preserved cerebral cortical tissue, as seen in fetal hydrocephalus. In cases caused by a massive intracranial hemorrhage, blood may initially be visualized as an echogenic mass in the supratentorial tissue.9, 10 On sequential scans, blood evolves into an anechoic fluid collection that replaces the frontal and parietal lobes. In the early stages of hydranencephaly, the brain may appear heterogeneous, with multiple, small cystic areas seen; this appearance may mistakenly be attributed to intracranial teratoma. Degree of ConfidenceDifferentiating hydranencephaly from hydrocephalus and alobar holoprosencephaly in the prenatal period can be challenging. The presence of a falx and of unfused thalami, as well as the absence of fused cortical tissue, excludes the diagnosis of alobar holoprosencephaly. Differentiation from hydrocephalus is somewhat more difficult, because the rim of peripheral cerebral cortical tissue that is diagnostic of fetal hydrocephalus may be difficult or impossible to visualize with prenatal ultrasonography. In difficult cases, prenatal MRI can be used to establish the correct diagnosis. Distinguishing between hydranencephaly and alobar holoprosencephaly can be difficult with fetal ultrasonography. These 2 entities should not be confused on high-resolution postnatal images or fetal MRIs. Alobar holoprosencephaly is characterized by the presence of a pancake-shaped mass of fused frontal lobe tissue, fusion of the thalami, and a large dorsal cyst. In hydranencephaly, there is no fusion of cerebral hemispheric tissue; indeed, little normal supratentorial tissue remains. The presence of a normal falx and the absence of thalamic fusion help to exclude holoprosencephaly.19 False Positives/NegativesNo normal variants mimic hydranencephaly. INTERVENTIONSection 8 of 10
Intervention is not known to improve the outcome of children with hydranencephaly. However, if the patient's head circumference is increasing because of concomitant hydrocephalus, shunting may be needed for clinical management. Medical/Legal Pitfalls
Resource Center Medical Malpractice and Legal Issues Special Concerns
MULTIMEDIASection 9 of 10
REFERENCESSection 10 of 10
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