03 -Disorders of Diverticulation and Cleavage

Chapter Disorders of Diverticulation Disorders of Diverticulation and Cleavage Sulcation and Cellular Migration Disorders of Diverticulation and Cleavage: Holoprosencephalies and Related Disorders Alobar Holoprosencephaly Semilobar Holoprosencephaly Lobar Holoprosencephaly Septooptic Dysplasia Arrhinencephaly Disorders of Sulcation and Cellular Migration Lissencephaly Nonlissencephalic Cortical Dysplasias Heterotopias Schizencephaly Unilateral Megalencephaly DISORDERS OF DIVERTICULATION AND CLEAVAGE: HOLOPROSENCEPHALIES AND RELATED DISORDERS During the fourth gestational week, the neural tube forms three primary brain vesicles: the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon) During the fifth week the forebrain further divides into two secondary vesicles: the telencephalon and the diencephalon (see Fig 1-2) Anlage of the telencephalon and diencephalon separate by day 32; partial division of the telen cephalon into two cerebral hemispheres occurs by the end of the fifth fetal week Complete or partial failure in division of the developing cerebrum (prosencephalon) into hemispheres and lobes results in the holoprosencephalies.1 In holoprosencephaly, there is failure of lateral cleavage into distinct cerebral hemispheres and failure of transverse cleavage into diencephalon andtelencephalon Prosencephalic abnormalities are directly related to the mesenchymal tissue of the prechordal mesoderm This tissue is responsible for cleavage of the telencephalon and development of the midline facial structures.3 The majority of patients with moderate or severe forms of holoprosencephaly also have facial anomalies As stated by DeMyer and paraphrased by Harwood-Nash, “The face predicts the brain.” Holoprosencephaly is classically divided into three types by degree of brain cleavage (Table 3-1): Alobar holoprosencephaly (most severe) Semilobar holoprosencephaly (moderately severe) Lobar holoprosencephaly (mildest form) In fact, these disorders form a continuum with no sharp division between the different types 38 PART ONE Brain Development and Congenital Malformations Table 3-1 Holoprosencephalies Finding Alobar Semilobar Craniofacial anomalies Ventricles Septum pellucidum Falx cerebri Interhemispheric fissure someanteroinferior Severe Monoventricle Absent Absent Absent Variable Rudimentary occipital horns Absent Partial Partial Absent or mild Squared-off frontal horns Absent Well formed Present; Thalami, basal ganglia Fused Partially separated fusion Separated Alobar Holoprosencephaly A "holoprosencephalic" appearance of the ventricular system is normal in early fetal development At about weeks of gestation, the primitive ventricles appear as a single relatively undifferentiated cavity (see Fig 1-4) In the most severe form of holoprosencephaly, alobar holoprosencephaly, there is persistence of this primitive central monoventricle Pathology and imaging manifestations Alobar holoprosencephaly is characterized by nearly complete lack of ventricular and hemispheric cleavage The brain is basically an undifferentiated holosphere with a central monoventricle and fused thalami (Fig 3-1) Imaging studies show a completely unsegmented rim of brain that surrounds a largely undifferentiated central CSF-filled cavity (Fig 3-2) There is no interhemispheric fissure, falx cerebri, or corpus callosum (Figs 3-2, A, and 3-3, A to C) A large posterior midline cyst is often present Labor (Fig 3-3, D and E)5 This should be distinguished from callosal agenesis with dorsal third ventricular interhemispheric cyst (see Fig 2-26, A) Here, the interhemispheric fissure is complete, the falx is present, and the frontal horns have a bicornuate appearance6 (Fig 3-3, F) Associated abnormalities Severe craniofacial anomalies are seen in most cases of alobar holoprosencephaly These include cyclops with rudimentary displaced nose (ethmocephaly) and monkeylike head with defective nose and severe hypotelorism (cebocephaly).3 Reported extracranial abnormalities include polyclactyly, renal dysplasia, omphalocele, and hydrops.5 Most neonates with holoprosencephaly who die soon after birth have associated major congenital malformations Several chromosomal abnormalities have been reported with cyclopia or holoprosencephaly, most commonly trisomy13 Fig 3-1 Gross pathology of alobar hoIoprosencephaly A, Intact brain shows a completely unsegmented holosphere B, Coronal cut section shows a central monoventricle (arrows) The thalami are fused, the falx is absent, and there is no interhemispheric fissure (From archives of the Armed Forces Institute of Pathology.) Fig 3-2 A and B, Axial noncontrast CT scans of alobar holoprosencephaly The basal ganglia are fused (B, arrows), and a large CSF-filled monoventricle fills most of the intracranial cavity The interhemispheric fissure and falx cerebri are absent Fig 3-3 A to E, Imaging studies in alobar holoprosencephaly Coronal ultrasound (A) and Tl-weighted MR (B) scans show a central horseshoe-shaped monovenricle (large arrows) with fused thalami centrally (open arrows) There is no falx or interhemispheric fissure The corpus callosum is absent C and D, Axial T1-weighted MR scans show the fused thalami (open arrows) and large central CSF-filled ventricular cavity that is almost completely undifferentiated A dorsal interhemispheric cyst is present (black arrows), also demonstrated on the sagittal Tl-weighted scan (E, arrows) (A to D, From archives of the Armed Forces Institute of Pathology.) 40 PART ONE Brain Development and Congenital Malformations Fig 3-3, cont’d F, Coronal gross specimen of callosal agenesis with an interhemispheric cyst (large arrows) shown for comparison Note the bicornuate appearance of the lateral ventricles (small arrows) The thalami are completely separated and the interhemispheric fissure is well developed (E, From archives of Armed Forces Institute of Pathology F, Courtesy E Tessa Hedley-Whyte.) Sernilobar Holoprosencephaly Pathology and imaging Semilobar holoprosencephaly is intermediate in severity There is partial but interrupted attempt at brain diverticulation A somewhat H-shaped monoventricle with partially developed occipital and temporal horns is common A rudimentary faIx cerebri and incompletely formed interhemispheric fissure are often seen, with partial or complete fusion of the basal ganglia (Fig 3-4) Associated abnormalities In general, facial anomalies are either absent or are milder than those associated with alobar holoprosencephaly Facial lesions commonly seen with various degrees of semilobar holoprosencephaly include hypotelorism, as well as median and lateral cleft lip.3 Fig 3-4 Axial (A) T1- and (B) T2-weighted MR scans in an infant with severe semilobar holoprosencephaly Primitive occipital horns are present (large black arrows) and a partial, rudimentary interhemispheric fissure is identified (small black arrows) The basal ganglia are fused (B, open arrows) Chapter Disorders of Diverticulation and Cleavage, Sulcation and Cellular Migration Lobar Holoprosencephaly Pathology and imaging In this form of holoprosencephaly there is nearly complete brain cleavage The ventricles appear well lobulated Absence of the septum pellucidum gives a squared-off or boxlike configuration to the frontal horns (Fig 3-5) Separation of the basal ganglia is seen A nearly completely formed interhemispheric fissure and falx cerebri are present, although their most anteroinferior aspects may be absent and the frontal lobes fused inferiorly across the midline (Figs 3-5 and 3-6) An azygous anterior cerebral artery may be present (Fig 3-7) 41 Rarely, the inferior frontal lobes are separated and the posterior frontal or parietal regions are continuous across the midline, so-called middle interhemispheric fusion This variant of holoprosencephaly is associated with other abnormalities such as neuronal migration anomalies, callosal dysgenesis, and hypoplastic anterior falx cerebri.1 Associated abnormalities The optic vesicles and olfactory bulbs may be hypoplastic Mild hypotelorism can be seen, but severe facial anomalies are rare Fig 3-5 Holoprosencephaly intermediate between mild semilobar and lobar types is demonstrated A well-formed third ventricle is present, seen on the coronal T1-weighted MR scan (A, Arrows) Complete separation of the basal ganglia is present, but the interhemispheric fissure is only partially formed Fusion of the frontal lobes is seen on coronal T1-weighted (B) and axial T2-weighted studies (C) 42 PART ONE Brain Development and Congenital Malformations Fig 3-6 Mild lobar holoprosencephaly is demonstrated on coronal (A and B) and axial (C) Tl-weighted scans Note the relatively well-formed falx cerebri and interhemispheric fissure (large black arrows) The basal ganglia are separated by the third ventricle (open arrow) A small area of persistent fusion in the anteroinferior frontal lobe region is seen (small black Septooptic Dysplasia Pathology and imaging Septooptic dysplasia (d Morsier syndrome) can be considered a very milc form of lobar holoprosencephaly Absence or dysgen esis of the septum pellucidurn in conjunction witf optic nerve hypoplasia are the basic components (Fig 3-8, A) Imaging studies in these patients typicaII3 show squared-off frontal horns without a septum pel lucidum dividing the lateral ventricles (Fig 3-8, B) Associated abnormalities Two anatomic subsets of septooptic dysplasia have been identified One is associated with schizencephaly In this group the ventricles are normal, a remnant of the septum pellucidum is present, and the white matter of the optic radiations appears normal The clinical symptoms in this group are seizures and visual symptoms The second group does not have associated schizenceph aly, exhibits diffuse white matter hypoplasia wit ventriculomegaly, has complete absence of the sel turn, and typically presents clinically with symptom of hypothalamic-pituitary dysfunction Dysplasia or absence of the septum pellucidurn i also associated with other brain anomalies, includin aqueductal stenosis, Chiari 11 malformation, cephak celes, callosal agenesis, porencephaly, and hydraner cephaly.' Arhinencephaly Pathology and imaging In arhinencephaly the o factory bulbs and tracts are absent (Fig 3-9, A) Coro nal MR scans show olfactory sulci but no olfacto bulbs or tracts (Fig 3-9, B) Associated abnormalities Arhinencephaly is ac tually a spectrum of disorders Although isolated o factory aplasia can occur, it is usually part of a com plex cerebral and somatic malformation syndrome Most, although not all, cases of holoprosencephal are associated with absent olfactory bulbs and tracts Olfactory aplasia also occurs in some genetic condi tions such as Kallmann's syndrome (anosmia, hypo gonadism, and mental retardation).9 Chapter Disorders of Diverticulation and Cleavage, Sulcation and Cellular Migration Fig 3-7 Lobar holoprosencephaly with azygous anterior cerebral artery (ACA) A, Axial T2-weighted scan shows minimal fusion of the frontal lobe cortex (large arrows) The azygous ACA is indicated by the small arrows B, MR angiograrn shows the azygous ACA (open arrows) (Courtesy Joel Cureù.) Fig 3-8 Gross coronal gross pathology specimen (A) and coronal Tl-weighted MR scan (B) show characteristic findings of septooptic dysplasia The absent septum pellucidum with a squared-off appearance to the frontal horns is indicated by the small black arrows The optic nerves (open arrows) are mildly hypoplastic (A, Courtesy J Townsend.) 43 44 PART ONE Brain Development and Congenital Malformations Fig 3-9 A, Gross pathologic specimen of arhinencephaly The olfactory bulbs are absent B, Coronal Tl-weighted MR scan in a patient with arhinencephaly shows absence of the olfactory bulbs in their usual position below the olfactory sulci (arrows) (A, Courtesy Rubinstein Collection, Armed Forces Institute of Pathology.) SULCATION AND CELLULAR MIGRATION Beginning with the seventh gestational week, neuronal and glial precursors are generated in the germinal matrix that lines the lateral and third ventricles These young neurons then migrate along the radial glial fibers that extend from the ventricle to brain surface There is a direct correspondence between the site of cell proliferation within the germinal zone and location within the cerebral cortex.2 Disruption in the normal process of neuronal generation and cellular migration results in a spectrum of brain malformations.10 These are divided into several types, depending on the timing and severity of the arrest of neuronal migration.2 Lissfincephaly Etiology and pathology The developing brain of a 16 or 17 week fetus normally has a smooth, "agyric" appearance with shallow sylvian fissures and almost-no surface sulcation (Fig 3-10, A to C) Lissencephaly, or "smooth brain," refers to brains with absent or poor sulcation (Fig 3-10, D) Lissencephaly can be complete (synonymous with agyria) or incomplete, where a few shallow sulci are present In the latter case, it is synonymous with agyria-pachygyria, or nonlissencephalic cortical dysplasia (see subsequent discussion) Intrauterine infections can also result in a smooth lissencephalic-appearing brain (see Chapter 16) Imaging findings Imaging studies in childre with so-called type I lissencephaly show colpocephaly and a thickened cortex with broad, flat gyri smooth gray-white matter interface, and straight oblique or shallow sylvian fissures, giving a figure eigh appearanc11 (Fig 3-11) If intrauterine infection has resulted in lissencephaly, parenchymal calcification can be present as well (Fig 3-12) A second type of lissencephaly, type II, has been described as an agyric, severely disorganized unlayered cortex10 with poor 12 corticomedullary demarcation MR studies in patients with type II lissencephaly show thickened cortex that has a polymicrogyric appearance associated with hypomyelination of the underlying white matter A third type of lissencephaly, the cerebrocerebellar type, occurs without a figure eight configuration and has microcephaly, moderately thick cortices, enlarged ventricles, and hypoplastic cerebellum andbrain stem11 Chapter Disorders of Diverticulation and Cleavage, Sulcation and Cellular Migration Fig 3-10 A to C, Sixteen week aborted fetus The brain normally has a smooth, agyric appearance at the this stage of development D, Coronal gross pathology section of an agyric (lissencephalic) brain Note virtually complete adsence of sulcation and gyration (Courtesy Rubinstein Collection, Armed Force Institude of Pathology Fig 3-11 Contrast-enhanced axial CT scan of lissencephaly The sylvian fissures are shallow (arrows), giving the brain a figure eight appearance The virtual absence of surface sulcation is characteristic for lissencephaly 45 46 PART ONE Brain Development and Congenital Malformations Fig 3-12 Axial noncontrast CT scans through the midventricular (A) and basal ganglionic (B) levels in a newborn with intrauterine TORCH Note the small, agyric brain with shallow sylvian fissures (A, open arrows) and figure eight appearance Dystrophic calcification in the basal ganglia and subcortical white matter (black arrows) is also present Associated abnormalities Miller-Dieker syndrome is associated with type I lissencephaly Type II lissencephaly is associated with Walker-Warburg syndrome These patients have ocular malformations, cephaloceles, and profound congenital hypotonia13 (Fig 3-13; see Fig 2-18, A to C) Fukuyama's congenital muscular dystrophy (cerebro-oculo-muscular syndrome) is probably part of this spectrum as well.10, 11 Fukuyama's congenital muscular dystrophy may also have migrational malformations (pachygyria/polymicrogyria) and delayed myelination.14 Nonlissencephalic Cortical Dysplasias The agyria-pachygyria complex has recently been reclassified into a more general category, nonlissencephalic cortical dysplasia, because the terms pachygyria and polymicrogyria actually are histologic descriptions and can be difficult to distinguish on MR examination.15 The cortical dysplasias can be either diffuse or focal, unilateral or bilateral Pathology and imaging Gross pathologic and imaging appearance in the nonlissencephalic cortical dysplasias varies from a diffusely thickened, abnormal cortex that has an irregular, "bumpy" gyral (polymicrogyria) and relative paucity of underlying white matter (Figs 3-14 and 3-15) to more focal areas of thickened, flattened cortex (pachygyria) (Figs 3-16 and 3-17) Approximately one quarter of patients with nonlissencephalic cortical dysplasias Fig 3-13 A For legend, see next page Chapter Disorders of Diverticulation and Cleavage, Sulcation and Cellular Migration Fig 3-13, cont'd Sagittal (A) and coronal (B) Tl-weighted MR scans in a patient with Walker-Warburg syndrome, sphenoethmoidal cephalocele (large arrows), and lissencephaly The right hemisphere has a disorganized appearance, with thickened, nearly agyric cortex (small arrows) and poor gray-white matter demarcation (Courtesy Joel Cureù ) Fig 3-15 Axial T2-weighted scan demonstrating polymicrogyria (arrows) with a relative paucity of underlying white matter 47 Fig 3-14 Gross pathology specimen, lateral view, demonstrating foci of multiple small, “bumpy” gyri along the sylvian fissure characteristic of polymicrogyria (arrows) Fig 3-16 Gross pathology specimen of nonlissencephalic cortical dysplasia Some comparatively more normal sulcation and gyration is present in the frontal lobes, but both sylvian fissures are shallow and the parietooccipital cortex is almost completely agyric (Courtesy YakovIev Collection, Armed Forces Institute of Pathology.) 48 PART ONE Brain Development and Congenital Malformations Fig 3-17 Axial (A) and coronal (B) T1-weighted MR scans of nonlissencephalic cortical dysplasia Note the shallow sylvian fissures and thickened, nearly agyric-appearing cortex (arrows) (C) T2-weighted axial scan shows the paucity of normal white matter and the thickened cortex with smooth gray-white interface Laminar heterotopias If diffuse arrest of neuronal migration occurs, a layer of neurons is interposed between the ventricle and cortex Alternating layers of gray and white matter are seen in this "band," or "laminar," form of heterotopia (Fig 3-18).16 The abnormally located gray matter is isointense with cortical gray matter on all imaging sequences (Fig 3-19) have abnormalities of the underlying white matter, usually seen as foci of increased T2 signal, that suggest the presence of glioSiS.15 Heterotopias Gray matter heterotopias are collections of otherwise normal neurons in abnormal locations secondary to arrest of neuronal migration along the radial glial fibers.10 Heterotopias can be bandlike (laminar) or nodular, focal, or diffuse Nodular heterotopias These abnormalities of cellular migration can also be either diffuse or focal A striking form of nodular heterotopia occurs when multiple small foci of gray matter are seen in the subependymal region (Figs 3-20 and 3-21) The differential diagnosis is subependymal nodules (SENs) seen with tuberous sclerosis (TS) In TS the nodules are irregularly shaped and often calcified on CT studies On MR studies, SENs are not precisely isointense with cortical gray matter and occasionally show enhancement after contrast administration In contrast, heterotopias parallel cortex in signal on all 17 MR imaging sequences and not enhance Chapter Disoders of diverticulation and Cleavage, Sulcation and Cellular Migration 49 Fig 3-18 Gross pathology specimen, coronal section, demonstrating laminar (band) heterotopic gray matter (large black arrows) The layers of gray matter alternate with the white matter, giving a laminated or "target" appearance to the brain (From archives of the Armed Forces Institute of Pathology.) 50 PART ONE Brain Development and Congenital Malformations Fig 3-20 Axial gross pathology specimen of diffuse subependymal nodular heterotopia The lateral ventricles appear studded with multiple foci of heterotopic gray matter (arrows) (Courtesy Okazaki H, Scheithauer B: Slide Atlas of Neuropathology, Gower Medical Publishing Company) Fig 3-21 Nodular subependymal heterotopic gray matter (arrows) is nicely seen on axial noncontrast CT scan (A) and (B) T1-and (C) T2-weighted MR studies (Courtesy R Ethier.) Chapter Disorders of Diverticulation and Cleavage, Sulcation and Cellular Migration Pathology and imaging Heterotopias that are more focal have a variable appearance They can deform the cerebal hemispheres and incedent adjacent ventricles (Fig 3-22) Again, the signal or MR imaging is isointense with gray matter on all sequences No enhancement is seen following contrast administration When masses of heterotopic gray matter are located more peripherally near the cortex, they may be associated with prominent anomalous cortical draining veins (Figs 3-23 and 3-24) 52 PART ONE Brain Development and Congenital Malformations Fig 3-24 A, Long TR/short TE axial MR scan shows a mass of heterotopic gray matter (white arrows) with prominent cortical draining veins (black arrow) B, Internal carotid angiogram, lateral view, venous phase, shows the anomalous cortical draining veins (arrows) Occasionally part or all of a cerebral hemisphere can be completely dysplastic, with absence of normal sulcation and sylvian fissure (Fig 3-25) In extreme cases, no lateral ventricle can be identified and the hemisphere consists entirely of dysplastic, disorganized masses of gray matter with hypoplastic 17a white matter (Fig 3-26) These cases can resemble an intracranial mass, especially if the ipsilateral ventricle is absent or severely deformed Biopsy of tissue from these dysplastic hemispheres may lead to the erroneous diagnosis of gangliocytoma (see Chapter 14) Fig 3-25 Axial gross pathology specimen of dysplastic cerebral hemisphere No identifiable frontal horn of the lateral ventricle can be seen, and the frontal lobe is composed of dysplastic, disorganized heterotopic gray matter No sylvian fissure is present (From archives of Armed Forces Institute of Pathology.) Schizencephaly Pathology Schizencephaly (split brain) is a gray matter-lined cerebrospinal fluid-filled cleft that extends from the ependymal surface of the brain through the white matter to the pia (Fig 3-27) Two types are recognized: type I, or closed-lip schizencephaly, in which the cleft walls are in apposition, and type II, or open lip schizencephaly, in which the walls are separated In either instance the cleft is lined by heterotopic gray matter The clefts can be unilateral or bilateral, symmetric or asymmetric.18 Imaging CT scans of closed-lip schizencephaly may show only a slight outpouching, or "nipple," at the ependymal surface of the cleft (Fig 3-28) The full thickness cleft, or pial-epenclyrnal seam, may be difficult to defect on CT scan but is easily discernible Chapter Disorders of Diverticulation and Cleavage, Sulcation and Cellular Migration Fig 3-26 A, CT scan without contrast enhancement was interpreted as showing a “1eft hemisphere mass” (arrows) Biopsy was called "gangliocytoma." The patient was lost to follow-up Axial (B) and coronal (C) T2-weighted MR scans performed 12 years later show a completely disorganized hemisphere composed primarily of dysplastic gray matter (arrows) There is no evidence for neoplasm Fig 3-27 Coronal gross brain specimen demonstrates bilaterally symmetrical schizencephalic clefts (large arrows) Note abnormal heterotopic gray matter (small arrows) that lines the full-thickness clefts (Courtesy E Ross.) 53 54 PART ONE Brain Development and Congenital Malformations Fig 3-28 Axial CT scan without contrast enhancement shows bilateral closed-lip schizencephaly Note the outpouchings, or "nipples," of CSF (arrows) and the gray matter-lined pial-ependymal seams (arrowheads) Fig 3-29 Coronal T2-weighted MR scan of unilateral closed-lip schizencephaly The pial-ependymal seam (arrows) is easily identified Fig 3-30 Coronal Tl-weighted MR scan with unilateral,open-lip schizencephaly The full-thickness cleft extends from the ventricle to the surface of the brain and is lined with gray matter (arrows) Fig 3-31 Coronal Tl-weighted MR scan shows a very large unilateral open-lip schizencephaly Gray matter (arrows) lines the cleft (Courtesy P Van Tassel and Joel Cureù ) on MR studies (Fig 3-29) Open-lip schizencephaly has a larger, more apparent gray matter-lined CSF cleft (Fig 3-30) Occasionally the schizencephalic clefts are very large (Figs 3-31 and 3-32) The imaging differential diagnosis of schizencephaly is porencephalic cyst Because porencephalic cyst result from insults to otherwise normally developed brain, the CSF space is lined by gliotic white matter, not dysplastic heterotopic cortex (Fig 3-33) Chapter Disorders of Diverticulation and Cleavage, Sulcation and Cellular Migration Fig 3-31, cont'd Axial T2-weighted MR scan shows a very large unilateral open-lip schizencephaly Gray matter (arrows) lines the cleft (Courtesy P Van Tassel and Joel Cureù ) Fig 3-32 Coronal T1- (A) and coronal and axial T2-weighted (B and C) MR scans show large bilateral open-lip schizencephalic clefts that are lined by abnormal gray matter (arrows) (Courtesy P Van Tassel and J Cureù ) 55 56 PART ONE Brain Development and Congenital Malformations Fig 3-33 Porencephalic cyst A, Axial CT scan shows a wedge-shaped CSF cleft that extends from the cortex toward the ventricle (arrowheads) B, Axial long TR/ short TE and, C, T2-weighted scans show the cleft is lined by gliotic white matter (arrows), not cortex Contrast with the unilateral schizencephalic clefts shown in Figs 3-29 and 3-30 Unilateral Megalencephaly Pathology Unilateral megalencephaly is a term that describes hamartomatous overgrowth of part or all of one cerebral hemisphere associated with localized neuronal migrational anomalies of various severity.19 The hemisphere is enlarged compared to the opposite normal side Heterotopic gray matter is common and sulcation is frequently abnormal The ipsilateral ventricle is often enlarged (Fig 3-34) and the white matter may be either hyperplastic (Fig 3-35) or hypoplastic (Fig 3-36).2 Ultrasonography typically shows the enlarged ventricle and midline shift Homogeneous parenchymal echogenicity with decreased sulcation of the affected hemisphere can also be seen.19 Fig 3-34 Gross pathology of unilateral megalencephaly The left hemisphere is enlarged, with hyperplastic white matter The ipsilateral lateral ventricle is enlarged (arrows) (Courtesy B Horten.) Fig 3-35 A, Axial postcontrast CT scan in a patient with unilateral megalencephaly Note enlargement of the right cerebral hemisphere and lateral ventricle (white arrows) The white matter (open arrows) appears hyperplastic B, Coronal T1-weighted MR scan nicely shows the enlarged right lateral ventricle (black arrow) and prominent white matter (open arrows) C, Axial T2-weighted scan shows the enlarged, distorted lateral ventricle The prominent, dysplastic-appearing white matter (arrows) is striking 58 PART ONE Brain Development and Congenital Malformations Fig 3-36 Axial T2-weighted MR scan in a patient withlong-standing seizures and right-sided unilateral megalencephaly Note enlargement of the ipsilateral occipital horn (open arrow) In this particular case, the white matter in theparietooccipital region is hypoplastic and the gray matter around the sylvian fissure and insula is thickened with abnormal sulcation (white arrows) REFERENCES Byrd SE, Osborn RE, Radkowski MA et al: Disorders of midline structures: holoprosencephaly, absence of corpus callosurn, and Chiari malformations, Sem US, CT, MR 9:201-215, 1988 Barkovich AJ: Congenital malformations of the brain In Pediatric Neuroimaging, pp 77-147, New York, Raven Press, 1990 McGahan JP, Nyberg DA, Mack LA: Sonography of facial features of alobar and semilobar holoprosencephaly, AJR 154:143-148, 1990 DeMyer W: Classification of cerebral malformations, Birth Defects 7:78-93, 1971 Nyberg DA, Mack LA, Bronster A et a]: Holoprosencephaly: prenatal sonographic diagnosis, AJNR 8:871-878, 1987 Young JN, Oakes WJ, Hatten HP Jr: Dorsal third ventricular cyst: an entity distinct from holoprosencephaly, J Neurosurg 77:556-561, 1992 6a Barkovich AJ, Quint DJ: Middle interhemispheric fusion: an unusual variant of holoprosencephaly, AJNR 14:431-440, 1993 Barkovich AJ, Fram EK, Norman D: Septo-optic dysplasia: MR imaging, Radiology 171:189-192, 1989 Barkovich AJ, Norman D: Absence of the septum pellucidum: a useful sign in the diagnosis of congenital brain malformations, AJNR 9:1107-1114, 1988 Louis DN, Arriagada PV, Hyman BT, Hedley-Whyte T: Olfactory dysgenesis in hypoplasia: a variant in the arhinenencephaly spectrum? Neuroradiology 42:179-182, 1992 10 Barkovich AJ, Gressens P, Evrard P: Formation, maturation, and disorders of brain neocortex, AJNR 13:423-446, 1992 11 Dietrich RB, Demos D, Kocit et a]: Lissencephaly: MR and CT appearances with different subtypes, Radiol 185(suppl):123, 1992 12 Kimura S, Kobayashi T, Sasaki Y et al: Congenital polyneuropathy in Walker-Warburg syndrome, Neuropediatrics 23:14-17, 1992 13 Rhodes RE, Hatten HP Jr, Ellington KS: Walker-Warburg syndrome, AJNR 13:123-126, 1992 14 Aihara M, Tanabe Y, Kato K: Serial MRI in Fukuyama type congenital muscular dystrophy, Neuroradiology 34:396-398, 1992 15 Barkovich AJ, Kjos BO: Nonlissencephalic cortical dysplasias: correlation of imaging findings with clinical deficits, AJNR 13:95-103, 1992 16 Barkovich AJ, Jackson DE Jr, Boyer RS: Band heterotopias a newly recognized neuronal migration anomaly, Radiol 171:455-458, 1989 17 Barkovich AJ, Chuang SH, Norman D: MR of neuronal migration anomalies, AJNR 8:1009-1017, 1987 17a Castillo M, Kwock L, Scatliff J et al: Proton MR spectroscopic characteristics of a presumed giant subcortical heterotopia, AJNR 14:426-429, 1993 18 Barkovich AJ, Kjos BO: Schizencephaly: correlation of clinical findings with MR characteristics, AJNR 13:85-94, 1992 19 Babyn P, Chuang S, Daneman A, Withers C: Sonographic recognition of unilateral megalencephaly, J Ultrasound Med 11:563-566, 1992  ... moderately thick cortices, enlarged ventricles, and hypoplastic cerebellum andbrain stem11 Chapter Disorders of Diverticulation and Cleavage, Sulcation and Cellular Migration Fig 3-10 A to C, Sixteen... arrest of neuronal migration occurs, a layer of neurons is interposed between the ventricle and cortex Alternating layers of gray and white matter are seen in this "band," or "laminar," form of heterotopia... sequences and not enhance Chapter Disoders of diverticulation and Cleavage, Sulcation and Cellular Migration 49 Fig 3-18 Gross pathology specimen, coronal section, demonstrating laminar (band) heterotopic