congenital brain spectrum

Volume 9, Number 5 #{149}September, 1989 #{149}RadloGraphlcs 801

RadloGraphlcs Index terms:NEUROLOGIC IMAGING

#{149}BraInPEDIATRIC IMAGING

#{149}Neurologlc

CumulatIve Index terms:BraIn, abnormalItIesHoloprosencephalyBraIn, hernIaCorpus callosum,

abnormalItiesNervous system,

abnormalitIes

THIS EXHIBIT WAS DISPLAYED ATTHE 74TH SCIENTIFIC ASSEMBLYAND ANNUAL MEETING OF THE PA-DIOLOGICAL SOCIETY OF NORTHAMERICA. NOVEMBER 27-DECEM-BER 2. 1988, CHICAGO, ILLINOIS. ITWAS RECOMMENDED BY THE PEDI-ATRIC IMAGING PANEL AND WASACCEPTED FOR PUBLICATION AF-TER PEER REVIEW AND REVISIONON MARCH 21. 1989.

From the Department ofRadiology, Geisinger MedicalCenter, Danville, PA.

Address reprint requeststo L. L. Coleman, M.D., De-partment of Radiology, Gei-singer Medical Center, NorthAcademy Ave., Danville, PA17822.

Congenital central nervoussystem anomalies

Larry B. Poe, M.D.

Linda L. Coleman, M.D.

Faruq Mahmud, M.D.

Abstract: Magnetic resonance Imaging, because oflts multiplanarcapablilties and exquisite contrast differentiation, has risen above allother forms ofln vlvo Imaging for the classification and determination ofcongenital cenfral nervous system (CNS) anomalies. We briefly discusspertinent aspects 01CN$ embiyoiogy and, using a recently proposedclassification ofcentrai nervous system anomalies, present examples ofa spectrum ofabnormailties that one mayencounter in practice. Theseanomalies include: the Chiari malformations, encephaioceles,hoioprosencephaly, septooptic dysplasia, Dandy.Waiker variant,hydranencephaly, phakomatoses, schizencephaly, agyria orpachygyria,and dysgenesls of the corpus caiiosum.

Introduction

Recently, van den Knaap and Valk (71) proposed a modificationof the Volpe classification of congenital central nervous system(CNS) anomalies. This is a useful scheme to relate specific congenitalanomalies to the timing of common embryopathologicderangements. This classification is based on MRI appearances andis organized in terms of the timing of gestational disturbances. Theunderlying etiology, in over 60% of congenital CNS structuralanomalies, is unclear; but inherited factors account for 20%;chromosomal anomalies, 10%; and environmental factors, 10% (27).While the distinct disruptive events may be obscure, the greatsignificance of these lesions is not; one-third of all major anomaliesinvolve the central nervous system (78). Magnetic resonanceimaging, because of its multiplanar capabilities and exquisitecontrast differentiation, has created a new standard for examiningcongenital anomalies and provides anatomic information in vivo thatwas not attainable with earlier types of Imaging.

Table I

Specific Anomalies Presented inThis Discussion include: (after Timing of

van den Knoap (71 ) Gestational Defect

1 . Dorsal InductionA. Chiani malformations 4 weeksB. Encephaloceles 4 weeks

2. Ventral InductionA. Holoprosencephaly 5-6 weeksB. Septooptic dysplasia 6-7 weeksC. Dandy-Walker malformation 7-1 0 weeks

3. Neuronal Proliferation and HistogenesisA. Neurofibromatosis 5 weeks-6 monthsB. Tubenous sclerosis 5 weeks-6 monthsC. Primary hydranencephaly 3 months or later

4. MigrationA. Schizencephaly 2 monthsB. Agynia and pachygynia 3 monthsC. Gray maffer heterotopias 5 monthsD. Dysgenesis of corpus callosum 2-5 months

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I. Disorders of Dorsal Induction

The development of the human brain be-gins as the notochordal process and primitivemesoderm induce the neural plate. The neuralplate, through growth and invagination, even-tually gives rise to the neural tube and ulti-mately to the brain and spinal cord. This pro-cess, referred to an neurulation, occurs withinthe third and fourth weeks of gestation (70,72).Dorsal induction anomalies include all defectsof closure of the neural tube such as anen-cephaly, cephaloceles, spinal dysraphic statesand hydromyelia. The Chiani malformations areincluded in this category, although it is unclearhow they are related.

A. CHIARI I MALFORMATION

The hallmark of a Chiari I malformation isthe herniation of the cerebellar tonsils belowthe foramen magnum. Slight tonsillar ectopia iscommonly seen on midsagittal MR images innormal patients, but depending on how themeasurement is taken, only ectopia greaterthan 3-5 mm is considered to be significant(1,5). Symptoms in this disorder are related pri-manly to compression of the spinal cord or cere-bellum or to associated syningohydromyelia

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- ------ I Imalformatlon Ti weighted midsagiffal MRimage This scan exhibits tonsillar herniation; ante-nor angulation of the medulla at its junction with thespinal cord, a normal fourth ventricle and a normalmamillopontine distance.

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(55,78). Symptoms usually do not becomemanifest until early adulthood.

Caudal herniation of the cerebeliar tonsilsthrough the foramen magnum may occasion-ally extend as low as 03. This obliterates thecisterna magna. Syringohydromyelia of thecervical cord is seen in up to 75% of cases onMRI. The lower brain stem may have a slightanterior angulation at the level of the foramenmagnum. Despite caudal elongation of thetonsils, the fourth ventricle is normal in position,though it may be slightly distorted in shape(Figure 1). There is an association of the Chiani Imalformation with craniovertebral junction ab-normalities such as basilar impression, occipita-lization of the atlas, and Kippel-Feil deformity.Hydrocephalus is found in up to 44% of cases(17,26,27,55,56,69,78). Since brain stemchanges are variable, the mamillopontine dis-tance may be normal or reduced (26,56).

B. CHIARI II MALFORMATION

The Chiani II malformation is a complex ofanomalies involving almost all parts of the neu-ral axis (56). The hallmark of this malformation isdysgenesis of the hindbrain manifested by acaudally displaced fourth ventricle and brainstem, with cerebellar tonsillar, and vermianherniation through the foramen magnum. Thisis almost always associated with meningomye-locele and obstructive hydrocephalus. Grossly,most of the changes in the posterior fossamight be ascribed to the limited space avail-able for development and growth, since theposterior fossa is small. This causes the devel-oping bone, dura and brain parenchyma tomold and to be molded by surrounding struc-tures (17,53,54,56,76). The tentonium is dysplas-tic; its free edge is very wide and U-shaped and

its caudal edge infenionly is inserted close to theforamen magnum. This allows the cerebellumto extend above the incisura, creating whathas been referred to as a “pseudomass” or“towering cerebellum” (Figure 2). Alternative-

Figure 2Chiarl II malformation Ti weight-ed MR image Note the toweringcerebellum above the dysplastictentonium (arrows). There are also

distortion of the cerebellar folioand hydrocephalus.

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Chiarl II malformation Ti weighted parasagiltal MRimages (A) Note the elongation of the fourth yen-tnicle; the low lying cerebellar tonsils and vermis,beaked tectum (arrow 1 ), and concave clivus; theabsence of the splenium of corpus callosum (arrow

2), the occipital lobe “stenogynia” (arrowheais),poorly identified cerebellar folia and nonvisualizationof the cerebral aqueduct. (B) Also, note the tempo-ropanietal herniation through the dysplasfic fenton-ium which flattens the superior portion of the core-bellum (arrow 3).

4A (Magnified for greater detail)

Figure 4Chiari II malformation Sequential Ti weightedtransverse MR images reveal petrous bone scallop-ing inferionly (A, short arrows). There is anterior and

lateral migration or “growth” of the cerebellumaround the brain stem obliterating the perimesence-phalic cistern infenionly (B, arrowheads). Note the“bullet” shaped cerebellum. There is also a “heartshaped brain stem secondry to tectal beaking (C,long arrow).

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ly, the posteroinfenior cerebrum may dropthrough the wide incisura, flattening the supe-nor portion of the cerebellum (26) (Figure 3).For this reason, the straight sinus tends to bemuch more vertically oriented and shorterthan normal. The cerebellum is hypoplasticand poorly differentiated, with a characteristiccraniocaudal elongation. This is associated

with poor visualization of the cerebellar foliaon sagittal images (76). The cerebellum grows,or migrates, anteriorly and laterally around thebrain stem creating a so-called “triple peak”on transaxial images (56). The cerebellopon-tine angle cisterns and the cisterna magna areobliterated, and there is petrous bone scallop-ing in 70-90% of cases (55,56) (Figure 4).

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The midbrain is stretched increasing themamillopontine distance (26). The fourth yen-tnicle is thin and elongated and, in some cases,completely obliterated as it herniates caudally(Figure 5). The herniation of the medulla poste-nor to the spinal cord creates a cervicomedul-lary kink in up to 70% (65) (Figure o). The cas-cade of herniating brain stem, fourth ventricle,

structures, not only at the fonamen magnum,but at the significantly smaller arch of Cl(53,54,56). Because of molding, the fonamenmagnum is enlarged in 75% of patients. Thecollicular plate becomes fused creating abeaked tectum that invaginates into the cene-bellum, and there is concave scalloping of theclivus in up to 80% of cases (55).

Figure 5Chiarl II malformation This Ti weighted midsagittal MR image of adifferent patient, reveals the beaked tectum, the nearly obliterated andelongated fourth ventricle, the large massa intermedia of the thalamuspartially obliterating the third ventricle, and characteristic craniocaudalelongation of the cerebellum. Despite the small posterior fossa, whichcreates an anterior compression of the medulla and cerebellum at thelevel of the foramen magnum, there is still a prominent prepontine cis-tern. The midbrain has been stretched considerably as manifested bythe increase in mamillopontine distance (arrows).

Figure 6Chiari II malformation This midsagittal image of the spine revealsthe herniation of the medulla posterior to the spinal cord creating the“cervicomedullary kink” (long arrow). Note also the syringohydro-myelia (arrowhead).

Figure 7Chiari II malformation This Ti weighted transverse MR image showsthe classic asymmetric enlargement of the lateral ventricles, designat-ed “colpocephaly.” A shunt enters from the left.

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Although in the usual case, the fourth ventni-cle will be greatly attenuated, in 5% of cases,the fourth ventricle becomes isolated with bal-loon-like dilatation, presumably secondary toadhesions (56). Over 90% of patients have hy-drocephalus, which may not become evidentuntil after the meningomyelocele has been re-paired. There is asymmetric enlargement ofthe lateral ventricles in a typical colpocepha-lic pattern (Figure 7). The third ventricle maybe only slightly enlarged, but it has a bicon-cave appearance created by enlargement ofthe massa intermedia of the thalami (Figure 8).The anterior horns of the lateral ventricles maybe pointed infeniorly (Figure 9), because ofprominent impressions from the caudate nu-clei. This appearance may also be seen in the40% of these patients in whom the septum pel-lucidum is absent (10,17,26,55,56,76). Hydro-cephalus is associated with a dysfunctionalaqueduct of Sylvius, but the aqueduct is notnecessarily mechanically obstructed. Nonvi-sualization of the cerebral aqueduct is presentin up to 70% of cases on MR imaging (55,56),and there is a wide prepontine cistern, as wellas a wide, supracenebeilan, CSF-containingspace that enlarges after ventricular shunting(53,76).

Figure 9Chiarl II malformation A coronal Ti weighted MRimage demonstrates inferior pointing of the frontalhorns despite the presence of a stretched septumpellucidum. There are prominent impressions pro-duced by the caudate nuclei (arrowheads).

The falx is always abnormal and may be fen-estrated on partially absent, leading to gyral in-tendigitation across the midline in 20-80% ofcases (55,56,76,78) (Figure 10).

Figure 8Chiari II malformation A Ti weighted midsagiffalMR image reveals hindbnain abnormalities similar tothose seen in Figure 2. In addition, there is thinningof the corpus callosum, prominence of the massa in-termedia of the thalamus (short arrow), elevation ofthe hypothalamus (long arrow), and vertical onienta-tion of the straight sinus (arrowhead). The last find-ing indicates inferior insertion of the tentorium. Notethe caudal migration of the occipital lobe.

Figure 10Chiari II malformation Ti weighted coronal (A)and transverse (B) images reveal the dysplastic falxwith intendigitation of the gyni. (A ventricular shunt ispresent in the left lateral ventricle).

Figure 1 1Occipital encephaioceie This parasagittal MR im-age reveals brain panenchyma that has herniatedthrough a posterior calvanial defect. Although theprotrusion contains a large vessel, represented by alinear signal void, it is difficult to identify possibleventricular structures because of distortion.

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The Chiani Ii malformation is associated withother central nervous system anomalies, in-cluding dysgenesis of the corpus callosum, syr-ingohydnomyelia and excessive gyration of thecortex. Excessive gyration leads to either so-called “stenogyria” (normal cortex histologi-cally) or “polymicrogynia” (abnormal cortexhistologically) (76) (Figure 3). These abnormali-ties emphasize the involvement not only of thehindbrain but of other portions of the neuralaxis as well.

it has been stated that Chiani I and II malfor-mations are unrelated (55,78), yet from MR im-ages, there seems to be a spectrum of inter-mediate anomalies between Chiani I andChiari II (26,65,69).

C. ENCEPHALOCELES

Encephalocele refers to a protrusion ofmeninges and brain substance through a de-fect in the skull. Encephalocystomeningocelerefers to the protrusion of some portion of theventricles, as well as brain panenchyma. In theUnited States, encephaloceles are most com-monly occipital in location (70%), but theymay be panietal (10-20%), frontal (10%), onbasal (10%). An encephalocele may simply ne-suit from a cranial defect, on it may occur con-currently along with other more complex brainanomalies. The contents of an encephaloceleoften undergo significant notation and diston-tion (Figure 11). The so-called “Chiari Ill malfor-

mation” specifies a cenebellar herniationthrough a cervicooccipital defect(15,17,23,28,55). The exquisite contrast nesolu-tion of MRI should allow easy differentiation be-tween a CSF filled herniation and one filledwith brain parenchyma, especially at the skullbase.

ii. Disorders of Ventral induction

Ventral induction refers to the induction ofevents in the nostral end of the primitive brainthat are intimately tied to the development ofthe face. These inductive events lead to theformation of the prosencephalon (forebrain),mesencephalon (midbrain), and rhomben-cephalon (hindbrain). The forebrain becomesthe telencephalon which divides into two cer-ebral hemispheres and two lateral ventricular

systems. These inductive events also lead tothe development of the optic vesicies, the 01-factory bulbs, the third ventricle, hypothala-mus and thalami. These events occur betweenfive and ten weeks of gestation.

Disorders of ventral induction include the ho-loprosencephalies, septooptic dysplasia, andagenesis of the septum pellucidum.

Figure 12Aiobar holoprosencephaiy Nonsequential transverse CT scans re-veal a monoventnicle with anteriorly fused thalami (arrow). There isabsence of the interhemisphenic fissure, third ventricle and corpuscallosum. A crescent shaped anterior cerebral mantle represents theundivided prosencephalon, the posterior margin of which cannot beidentified on the CT scan. The monoventnicle is distorted by a largecompression dorsal cyst, the anterior borden of which is approximat-ed by the hippocampal fonnix (arrowheads).

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A. HOLOPROSENCEPHALY

Holopnosencephaly results when theforebrain (prosencephalon) fails to undergodiverticulation . The anterior telencephalonfails to divide into cerebral hemispheres and amonoventnicle results. The diencephalon failsto diverticulate into separate thalami, and theolfactory bulbs fail to develop. Thedevelopment of the face is intimately relatedto development of the brain, so a spectrum offacial defects occurs ranging from mildhypotelonism to large midline clefts andcyclopia. There are three forms ofholoprosencephaly, which are classified as tothe degree of brain cleavage present. Theseare alobar, semilobar and lobar(2 17,28,29,49,55,57) . Holoprosencephaly isoften associated with chromosomalabnormalities of which tnisomy 13 is the mostcommon (27).

Aloban holoprosencephaly, in which thereis no diverticulation of ventricles on cerebralmantle is the most severe form. In it, there is asingle holoventnicle, which may be horseshoeshaped in the coronal plane, and nodifferentiation of the ventricular systemoccurs. The posterior margin of the

holovententnicle is often distorted by anenlarged “dorsal cyst”, the origin of which ispoorly understood (17,29). There is mostcommonly a “pancake” variety of brainparenchyma anteriorly, the most posteriormargin of which is defined by a band of tissuethat is usually not identifiable on a CT scan(29). The cerebral mantle is pachygyric. Nothird ventricle, Sylvian fissure, opercular cortex,falx, intenhemisphenic fissure, corpus callosum,on tentonium exist. The fused thalami and basalganglia are seen as bumps at the base of theholoventnicle anteriorly in the midline (Figures12 and 13). The cerebellum and brain stemmay be normal in gross appearance, but theyare often associated with the presence ofcysts, on other structural or histologic defects.The internal cerebral veins, superior andinferior sagittal sinuses, straight sinuses, andvein of Galen are absent. Alobanholoprosencephaly can be distinguished frommassive hydrocephalus and hydrocephaly,because in massive hydnocephalus, unlikealoban holopnosencephaly, the falx andinterhemisphenic fissure are present, and inhydranencephaly, the thalami are not fused(2,17,28,29,49,55,57).

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Figure 13Aiobar boioprosencephaiy Nonsequential sonographic images, inthe same patient as in Figure 12, reveal the monoventnicle which is tu-bulan shaped anteriorly (A) and slightly horseshoe shaped more pos-teniorly (B). The fused thalami indent the holoventnicle infeniorly (B,arrow).

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In semilobar holoprosencephaly, there isbeginning formation of the occipital ortemporal horns or both, and there is somedevelopment of the interhemisphenic fissureand falx posteriorly. The venous sinuses may bemore developed than in alobanholoprosencephaly (17,29,49,55).

In lobar holoprosencephaly, there is nearlycomplete cleavage of the forebrain with ashallow anterior interhemisphenic fissure.Histologically, there is some fusion of the twohemispheres anteriorly. The septum pellucidumis absent, and the corpus callosum may bedysgenetic on normal. There may behypoplasia of the optic vesicles and olfactorybulbs (17,29,49,55).

Septooptic dysplasia is a rare anteriormidline anomaly that is often considered to bea mild form of loban holoprosencephaly, sinceit results from abnormal ventral induction anddiverticulation (28,48,55) . Clinical findingsinclude optic nerve hypoplasia, hypotelonism,hypopituitanism (with diabetes insipidus in50%), seizures, nystagmus, and hypotonia.There is no consistent relationship between theclinical findings and the findings on CT, and

septooptic dyspiasia may occur with noaccompanying nadiologic changes (74). Whenradiologic changes are present they includeabsence of the septum pellucidum. isolatedabsence of the septum peilucidum has beensaid to be a normal variant, but in a review of2,000 cases examined by MRI, Barkovich foundnone in which it occurred as an isolated entity.Absence of the septum peliucidum should,therefore, alert one to the possibility of otherstructural defects (10). Also, in septoopticdysplasia, the frontal horns are usually dilated,appear to be squared off dorsally, and arepointed inferiorly on coronal imaging. Theoptic nerves and chiasm may be small, andthe optic chiasm may be distorted becausethe decussation may be rotated 90#{176}to thevertical (Figures 14 and 15). There is oftendilatation of the chiasmatic and suprasellarcisterns with dysgenesis of the hypothalamusas well as dilatation of the anterior recesses ofthe third ventricle. The fornix may be lowerthan normal, and in patients with diabetesinsipidus, there may be enlargement of thepituitary stalk (17,27,28,29,44,48,55).

Figure 14Septooptic dyspiasia This Ti weighted midsagittalMR image reveals severe hypoplasia of the optic chi-asm, hypothalamus, lamina tenminalis, and anteriorcommissune. There is dilatation of the anterior re-cess of the third ventricle. The pituitary gland issmall, and the infundibulum is not identifiable on this3 mm section.

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Figure 15Septooptic dyspiasia Nonconti-guous coronal Ti weighted MR im-ages reveal absence of the septumpellucidum and squared off frontalhorns which have inferior points(A, arrowheads). The optic �hiasmis not identifiable in its expectedlocation above the diminutive pitu-itary gland (B, arrow).

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B. THE DANDY-WALKER MALFORMATION

The Dandy-Walker syndrome is a hindbnainmalformation that is characterized by variablehypopiasia of the vermis, and cystic dilatationof the fourth ventricle, which communicateswith a posterior fossa cyst. In a true Dandy-Walker cyst, the fonamen of Magendie is oblit-enated, and no communication exists be-tween the Dandy-Walker fourth ventricle cystand the subanachnoid space. Some of the su-penior vermis is usually present; it is superiorlydisplaced toward the tentonium by the retno-cenebellan cyst. This cyst also displaces anteni-only and laterally the cenebellar hemispheres,which may be hypoplastic. The posterior fossais enlarged, and there is elevation of the insen-

tion of the tentonium (35,37,60). Hydnocephalusis variable at birth, but usually develops overtime (37). The posterior medullary velum formsthe cyst wall (78).

The Dandy-Walker variant occurs muchmore commonly than the true Dandy-Walkercyst (Figure 16). In the “variant” condition, thefourth ventricle is smaller and better formed,the retrocerebeilar cyst is smaller, and theremay be communication between the fourthventricle and the subarachnoid space througha patent foramen of Magendie(17,35,37,55,60,78).

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Over 60% of cases of Dandy-Walker cystor variant will have other associated congeni-tal, central nervous system anomalies includ-ing agenesis of the corpus collosum and holo-prosencephaly (37). A well known complica-

Figure 16Dandy-Walker variant (A) ThisTi weighted midsagittal MR imagereveals a grossly dilated fourthventricle that communicates with aretrocerebellan cyst. There is hypo-piasia of the inferior vermis and thesuperior vermis is displaced intothe incisura. The third ventricle isdilated (arrow). There is completeabsence of the corpus collosum.(B) This Ti weighted transverseMR image reveals the enlargedfourth ventricle communicatingwith the retnocenebellar cyst. Thecerebellar hemispheres are dis-placed anteriorly and laterally bythe cyst.

tion of the Dandy-Walker cyst is trapping ofthe cyst above the tentorium, which causesthe cyst to assume a characteristic “keyhole”configuration as it extends above the incisura(75).

ill. Disorders of Histogenesis

Histogenesis refers to the organization ofcells into tissues. Disorders of histogenesis in-dude neoplasms and vascular malformations.The phakomatoses are also related to abnon-mal histogenesis and include neurofibromato-sis and tubenous sclerosis.

A. NEUROFIBROMATOSIS

Neurofibromatosis is the most common ofthe phakomatoses and may occur either spon-taneously or from an autosomal dominant in-henitance with variable expressivity. In the cen-tral nervous system, this disorder affects themesodermal and ectodermal tissues. Tradition-ally, neurofibromatosis has been divided intocentral and peripheral types with many casespresenting findings of both groups. Recently,the NIH Concensus Development Conferencehas classified the disorder as neunofibromatosisI (von Recklinghausen’s disease) and neunofi-bromatosis 2 (bilateral acoustic neuromas).

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hAFigure 17Neurofibromatosis (A) This Ti weighted trans-verse MR image reveals bilaterally enlarged pre-chiasmatic portions of the optic nerves (arrows).There is also an off-midline signal void in the vermi-an region which suggests an enlarged dysplasticvessel, perhaps secondary to a vascular malforma-tion (arrowhead). (B) A midsagittal image in thesame patient reveals an enlarged optic chiasm (an-now). There is a typical “caput medusa” of a venousangioma identified in the cerebellum (arrowhead).

Figure 18Neurofibromatosis This T2 weighted transverseMR image of the same patient as in Figure i 7, re-veals abnormal signal intensity bilaterally in the re-gion of the optic tracts and lateral geniculate bodies(arrows). This abnormal signal extends into the op-tic radiations.

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Optic nerve pilocytic astrocytoma (glioma)is one manifestation of central neurofibroma-tosis. It is the most common tumor in the cen-tral nervous system, being found in up to 30%of neunofibromatosis patients (12,31,45,55). It iscommonly bilateral, and in up to 25%, extendsinto the optic chiasm, optic tracts and radia-tions (Figures 17 and 18). Approximately 75%of these lesions are identified in the first de-cade of life. The majority of those discovered

on MRI or CT scanning are in asymptomaticpatients (45). Apparently, when the gliomas in-volve only the optic nerves, their signal intensi-ty is the same as that of adjacent brain on T2weighted images. MRI is particularly useful infollowing the intracanalicular portions of theoptic nerves. Once the tumor involves the chi-asm and other visual pathways, markedly in-creased signal intensity is identified on 12weighted imaging (1231,39,59).

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Most schwannomas in children are causedby neunofibromatosis. These may be solitary,but they are often bilateral on multiple. Cranialnerves ill-Xll may be affected, but most com-monly the eighth nerve and next most com-monly the fifth nerve are involved (Figure 19).Forty to eighty percent of eighth nerveschwannomas are bilateral (13,31). The signalintensity of schwannomas is equal to on lessthan that of gray matter on TI weighted imag-

ing and greater than that of gray matter on T2weighted imaging. Patients with neurofibroma-tosis are also at greaten than normal risk of soli-tary on multiple meningiomas. Both schwanno-mas and meningiomas tend to occur in ayounger age group in those patients with neun-ofibromatosis than in those without, and intra-ventricular meningioma is more common inthese patients than in the general population.

Figure 19Neurofibromatosis (A) A Ti weighted coronal MRimage at the level of the internal auditory canals re-veals bilateral acoustic schwannomas (arrow-heads). The lesion on the patient’s right has alarge intracanalicular component, and the lesion onthe left compresses the brain stem and extends exo-phyticaily into the cerebellopontine angle. (B) in theexpected position of the left fifth nerve, there is anodular mass which is suggestive of a schwannoma(arrow i ). (C) A more anterior image suggests aleft third nerve schwannoma (arrow 2), and a massin the region of Meckel’s cave, encasing the verticalportion of the right internal carotid artery which isalso likely to be a fifth nerve lesion (arrows 3). Anyone of these lesions could possibly be a meninglo-ma.

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Figure 20Neurofibromatosis This Tiweighted sagittal MR image of thethoracic spine reveals an intramed-ullary astrocytoma with cysticcomponents (arrow). in the lowercervical spine, there is a subtle enplaque meningioma (arrowhead).

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Other tumors found in these patients withneunofibromatosis include brain stem gliomas,pilocytic astrocytomas of the hypothalamus,ependymomas, and anachnoid cysts. A classicmesodermal change, which may be seen onMR1, includes hypoplasia of the sphenoidwings; this may create a temporal lobe ence-phalocele, with or without exophthalmus. Vas-cular malformations are commonly found, andneurofibromatosis patients are at increasedrisk of spinal column abnormalities, includinggliomas, meningiomas, lateral meningocelesand neurofibromas (13) (Figure 20).

Multiple focal areas of abnormally in-creased signal intensity are found in the major-ity of neurofibromatosis patients on T2 weight-ed images. These are free of mass effect andcommonly involve the globus pallidus, thala-mus, centrum semiovale, internal capsule,brain stem, and cerebellum (Figure 21). The

Figure 22Piexiform neurofibroma This Ti weighted coronalMR image reveals extension of a dumbbell lesionthrough a neural fonamen (arrow) into the soft tis-sues of the neck.

exact nature of these lesions is unclean. Theymay represent dysplastic hamartomas andsome cases may represent gliomatosis or dys-myelination(1213,39). It is also possible thatsome of these lesions may be secondary toprevious, asymptomatic occlusions of smallvessels (36). TI weighted MR images usually re-veal no abnormalities in these locations.

Dysplastic lesions within the brain, includ-ing neunonal migrational abnormalities anddural ectasia (notably of the internal auditorycanals) may also be seen (66).

A plexiform neurofibroma is pathognomonicof neurofibromatosis; it usually grows along thenerve of origin in a sheet-like extension withcompression of adjacent structures (14) (Fig-une 22). The signal intensity of these lesions isgreater than that of normal neural tissues on T2weighted images (63).

Figure 21Neurofibromatosis This T2weighted transverse MR imageshows multiple focal areas of ab-normally high signal intensity in thebasal ganglia, thalami, the posteri-on limb of the internal capsule andother areas, including the regionsof the lateral geniculate bodies.These may represent areas of glio-matosis, dysplastic hamartomas,atypical glial cells, or possibly pre-viously unrecognized vascular in-suits.

weighted F...� image reveals a calcified age reveals the low intensity of the subcon-subependymal nodule in the anterior tical hamartomas on short TR, short TE (Tihorn ofthe lateral ventricle on the left, weighted) images (arrows).a lange calcified subcortical lesion(hamartoma) on the right and at leasttwo areas of abnormally high signalintensity in the subcortical white mat-ten (arrows).

Poe etal. Congen/ta/ central nervous system anoma/ies


B. TUBEROUS SCLEROSIS

Tuberous sclerosis is an autosomal domi-nant neunoectodermal disorder with a highrate of spontaneous mutation. It is character-ized by a classic clinical triad of adenoma Se-baceum, seizures and mental retardation. Thecomplete triad is seldom seen at the time ofpresentation, however. Other classical findingsinclude subungual fibnomas, cardiac nhabdo-myomas, and renal angiomyolipomas. Hamar-tomas of the brain are present in every case.

There are several characteristic centralnervous system findings in tubenous sclerosis, in-cluding subependymal nodules, cortical ha-martomas, white matter abnormalities, giantcell astnocytomas, and ventricular dilatation(13,52).

The multiple subependymal nodules are, infact, hamartomas composed of giant cells.They have MR relaxation times similar to that ofwhite matter if they are not calcified. Calcifi-cation of these lesions is better identified byCT than by MRI. These hamartomas (tubers)may inconsistently be seen as nodules that dis-tort the cortical architecture, on they may beidentified in the subcortical and deep whitematter where they may be calcified or have acystic component (Figure 23). These lesionsare frequently and characteristically of highsignal intensity on T2 weighted MR imaging. OnTI weighted imaging, their signal intensitiesare equal to or less than that of white matter(13,32,40,43,50,52,64) (Figure 24). This finding of

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multiple confluent and focal areas of abnor-mally high signal intensity on T2 weighted im-aging is characteristic of tuberous sclerosis.Hamartomas are postulated to have this ele-vated signal on T2 weighted imaging becauseof associated fibrillany gliosis on demyeiination(13,40,50,52,64). Clusters of hetenotopic giant

Figure 25Tuberous sclerosis (A) At this level on a trans-verse T2 weighted MR image, cortical and subcorti-cal areas of abnormal signal intensity are identifiedin the left frontal region, both temporal lobes, andboth cerebellan hemispheres (arrows). (B) At thishigher level, a large number of hyperintense lesionsare again identified. Note the lesions of high signalintensity radiating from the subcortical region towardthe ventricles (arrows), and the signal voids of cal-cified subependymal nodules. (C) At a still higherlevel in the same patient, more confluent subcorticalwhite matter abnormalities are identified.

cells within the white matter may be orientedin a radial distribution and seen to extend fromthe subcortical region to the ventricle (13)(Figure 25). It is possible that some of these le-sions may be secondary to small vessel occlu-sive disease (36) on rarely small gliomas.

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Subependymal nodules (Figure 26) maydegenerate into giant cell astnocytomas in upto 10% of tuberous sclerosis patients. Theseare located most frequently at the foramen ofMonro where they cause obstructive hydro-cephalus. The conversion of a subependymalnodule into a giant cell astrocytoma may beheralded by an increase in MR signal intensityon T2 weighted images, just as these lesionsare identified on CT by their contrast enhance-ment (13). Ventricular dilatation may also de-velop secondary to “dysplasia” (52).

Although the number of subependymalnodules and the presence of ventricular dila-tation are unrelated to mental status, there isa suggestion that the number of peripheral le-sions, identified on MRI, is related to the clinicalseverity of the disease (13,40).

C. HYDRANENCEPHALY

Hydranencephaly may be an agenetic orencephaloclastic central nervous system disor-den manifested by replacement of the cere-bnal hemispheres by a thin membranous sacfilled with cerebrospinal fluid and necrotic de-bnis (19,30,34). The sac represents the relativelyintact leptomeninges and atrophic glial cells(Figure 27A).

Figure 27AHydranencephaiy (A) This parasagittal image re-veals a fluid filled supratentonial space with relativepreservation of the posterior fossa structures andportions of the occipital lobe. (The supratentorialspace is filled with necrotic debris.)

Figure 26Tuberous sclerosis This Ti weighted transverseMR image of a 5 month old male infant reveals sub-ependymal nodules probably representing noncalci-fied hamartomas. There is also an area of abnormal-ly low signal intensity in the periventnicular whitematter on the left (arrow). (Some periventnicularhigh intensity artifact is present.)

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It has been generally believed that this en-tity reflects infarction of the supraclinoid inter-nal carotid arteries, with on without concomi-tant encephalitis. There are many cases inwhich these arteries are patent, however, em-phasizing the diversity of events that may cul-minate in the final morphologic picture (34).The vertebral artery system is relatively intactin hydranencephaly. As a result, there is usuallysignificant preservation of posterior fossa struc-tunes and of the inferior, posterior portions ofthe temporal and occipital lobes (Figure 27A).The brain stem may be atrophic histologically.Characteristically, small round unfused thalam-Ic remnants are seen (25,61) (Figure 27B).There are a multitude of agents reported to beresponsible for hydnanencephaly including ma-

Figure 27B-D(B) This transverse image reveals the charactenis-tic rounded, unfused thalamic masses (arrow-heads). (C) On a more superior image, portions ofthe occipital lobe are identified outlining the posteni-or aspect of the falx. (D) This transverse CT scan, inthe same patient, better defines the faix, whichhelps to differentiate the fluid filled supratentonialspace of hydranencephaly from the dorsal cyst ofholoprosencephaly.

ternal syphilis, cytomegalovinus, Herpes sim-plex, toxoplasmosis, and ionizing radiation(18,29,27). In primary hydnanencephaly, thenoxious event apparently affects the brain af-ten normal ventral induction and diverticula-tion (3-6 months); whereas, encephaloclastichydnanencephaly occurs later. Its occurrenceat 3-6 months on later would account for thepresence of the falx in hydranencephaly,which indicates prior cleavage of the telence-phalic vesicles into cerebral hemispheres, andit is also consistent with the presence of someresidual cerebral cortex (27,61) (Figures 27 Cand D). There may be a spectrum of destruc-tion ranging from hydranencephaly to multi-cystic encephalomalacia (68) (Figure 28).

ncephalociastic hydranencephaiy vs muiticystic encephaiomaiaciaSagiffal and coronal Ti weighted MR images reveal resorption of thesupratentorial parenchyma with a few scattered septations, preserva-tion of the posterior fossa structures, and presence of the falx. Thefindings are similar to those seen in Figure i 2. This child had enceph-alitis as an infant, and serial CT scans (not shown) demonstrated pro-gressive cystic destruction to this final endpoint. This suggests that aspectrum of destructive processes between encephaloclastic hydran-encephaly and multicystic encephalomalacia may exist.

Poeetai. Congenita/ centra/ nervous system anoma/ies


IV. Disorders of Migration

Successive waves of primitive neurobiastsmigrate from the germinal matrix to form thecerebral cortex and deep nuclei of the brainbetween two and four months of gestation(70-7277). Schizencephaly, polymicrogyria,pachygyria, lissencephaly, and simple graymatter heterotopias are all interrelated disor-dens that have in common abnormal migrationof neuroblasts from the germinal matrix. This ne-suits in abnormal arrangement and thicknessof the layered proliferative zones of the brain(6,58,77).

Schizencephaly is characterized by cleftsthat extend from the subarachnoid space tothe subependyma of the ventricles. A workingtheory of the development of schizencephalysuggests that there is a segmental failure ofgrowth of the neuroblasts of the affectedarea, leading to a holohemisphenic cleft.

These clefts are defined by a piai-ependymalseam that may be fused (Type I) (Figure 29) oropen and separate and filled with cerebrospi-nal fluid (Type II) (Figure 30). These clefts arecharacteristically lined by heterotopic graymatter, which differentiates this entity fromporencephaly. These clefts are usually bilater-al and symmetric, but may be asymmetric orunilateral (6,7,11,58,73,77). They usually occurclose to the central or Sylvian fissures and maybe vertically or horizontally oriented. Theymay, therefore, escape detection if studied inonly one imaging plane (6). There is often as-sociated absence of the septum peliucidumand part of the corpus callosum, especially inthe open cleft variety (10,17). Care must betaken to identify the contiguity of the cleftwith the ventricle, since reportedly large areasof heterotopic gray matter with an anomalousvessel may mimic this condition on MRI �9,).

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Figure 29Type I Schizencephaiy (A) A transverse CT scan after contrast en-hancement, in a child with a seizure disorder and congenital hemipa-nesis reveals a lobulated area of moderate attenuation similar to that ofthe gray maffer, in the left centrum semiovale (arrow). (B&C) Contig-uous transverse proton density MR images in the same patient, and ata similar level, show the lobulated mass on the left to have the samesignal intensity as the gray matter, suggesting heterotopia. A closelipped cleft is defined by the signal void of an anomalous vessel (B,arrow). The cleft does not definitely communicate with the ventricleon these images. The adjacent area of low signal intensity is a biopsysite (B, arrowhead). There are scattered areas of abnormally high sig-nal intensity in the deep white matter probably representing gliosis.(D) A Ti weighted coronal MR image suggests that the closed cleftcommunicates with the left lateral ventricle (arrowheads). The hetero-topic gray matter lining the cleft distorts the ventricle.

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Poe etal. Congenital central nervous system anomalies


Figure 30Type Ii Schizencephaiy (A&B) These noncontiguous transverse Tiweighted MR images reveal bilateral, slightly asymmetric, clefts in thefrontopanietal regions. They are “open lipped” and appear to commu-nicate with the enlarged ventricular atria. The clefts are lined by het-erotopic gray maffer (A, arrows). On the more superior image (B) aprimitive limiting membrane is identified, but it appears discontinuous(arrowhead). This membrane is likely composed of the primitiveependyma and pia which are fused. (C) This coronal Ti weighted MRimage reveals the cleft on the patient’s night side. A large area of pa-chygynic cortex is noted on the opposite side (arrow). There is alsoabsence of the septum peilucidum, with inferior pointing of the frontalhorns.

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Agynia and pachygnia (lissencephaly) referto a fiat, smooth brain surface caused by theabsence of cortical gyni. This is the most severeof the neuronal migrational disorders. The etio-logic insult occurs at about eight weeks ofgestation. Specifically, pachygynia refers tomultiple areas of broad, flat, shallow gyni. A to-tally agynic brain is a rarity, and most cases dis-play mixed areas of agynia and pachygynia;the difference between the two entities beingmerely one of severity. In these conditions, thecortex is abnormally thick, and there is failureof operculization of the insular cortex. As a re-suIt, the Sylvian fissures are shallow, creating aso-called “figure of 8” deformity. The temporallobes tend to be less involved than other por-

tions of the brain. The complement of whitematter is significantly thinned, leading to theabsence of intendigitation at the gray-whitematter junction and hypoplasia of the whitematter tracts including the corticospinal tract(Figure 31). The decreased development ofthe corticospinal tract may be recognized inmidsagittal imaging by the decrease in thesize of the brain stem. Hetenotopias are corn-rnonly recognized within the thinned whitematter, especially near the corners of the lat-eral ventricles (6,16,24,58,77).

Bankovich has described a thin band ofhigh signal intensity on 12 weighted imagingwithin the thickened cortex which is believedto represent an area of laminar necrosis (6).

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Figure 31Agyria and pachygyria This T2 weighted coronalMR image reveals thickened agynic cortex in the pan-etal lobe of the right cerebral hemisphere. Note thethinned subcortical white maffer, with failure of inter-digitation of white matter fibers. In the left hemi-sphere, there is a large area of pachygyric cortex(arrow) superiorly with a more normal appearanceof folded cortex and subcortical white matter infeni-only.

Polymicnogynia is characterized by exces-sive thickness and by excessively numerousconvolutions of the cerebral cortex. In this ie-sion, the cortex is histologically abnormal. Thedisorder is commonly found at necropsy asso-ciated with more generalized anomalies suchas the Chiani II malformation on schizencephaly.The numerous tiny gyri may fuse, paradoxicallygiving an impression of smooth cortex (6,58).Stenogynia, by contrast, is also characterizedby an excessive number of gyni, but in steno-gynia they are histologically normal. Stenogyniais also reported to occur in Chiani II malfonma-tions (76).

V. Dysgenesis of the Corpus Caliosum

The corpus callosum is the largest whitematter commissune connecting the two cere-bnal hemispheres. Its embryogenesis is corn-plex, but complete callosal presence appearsto be dependent upon the successful closureof the anterior neunopone and the induction ofthe precursors of the commissural plate (i.e.,the massa commissunalis) from the rostral wailof the telecephalon (the primitive lamina ten-minalis) (8,33,42,70,72). Since the corpus callo-sum is developed in the region of the commis-sunal plate by 12 to 13 weeks of gestation, in-suIts to its precursors must occur earlier than 12weeks for absence to be complete. This dison-den is included with migrational disturbancesby van den Knaap and Valk presumably be-

cause, in a sense, dysgenesis of the corpuscallosum results not only from abnormal induc-tion but from migration of cells and their axonsaway from the midline on paramidline regions.it is also very commonly associated with neur-oblast migrational disorders (schizencephaly,lissencephaly, etc.) Not all authors would clas-sify dysgenesis of the corpus callosum in this way.

Normally, axons from the cerebral hemi-spheres enter the commissural plate regionand cross to the opposite side. Failure of de-velopment of the corpus caliosum preventsthe crossing of the axonal fibers. As a result,they continue along the medial walls of thelateral ventricles as bundles that terminaterandomly in the occipital and temporal lobes.These are called the bundles of Probst (42). Aconstellation of findings results from these ba-sic defects, and the deformities found in thisentity vary depending on whether caliosal ab-sence is partial on complete.

With complete dysgenesis, the absence ofthe corpus callosum can be easily identified inall planes of imaging. On midsagittal imaging,it creates circumferentially radiating gyri per-pendiculan to the third ventricle in a “sunburst”pattern. There is failure of normal conver-gence of the calcanine and panietooccipitalsuici. The cingulate gyrus is not identifiable onmidsagittal imaging because it has rotated in-feniorly and laterally (everted) (Figures 32 and33). One may also note elevated internal cer-ebral veins and “wandering” anterior cerebralartery branches (3,8,33,42).

FDysgenesis of the corpus callo-sum A parasagittal MR image in thesame patient as in Figure 32 reveals afenestrated fornix (arrow) and aneverted cingulate gynus, bulging intothe lateral ventricle anteriorly (arrow-head) off the midline.

Poe etal. Congenita/ central nervous system anoma/ies


I_. �32Dysgenesis of the corpus caiio-sum This Ti weighted midsagittalMR image reveals complete absenceof the corpus callosum, and absenceof the cingulate gyrus in the midsagit-tal plane. The cingulate and pericallo-sal sulci are also absent. The adjacentcortical gyri are radiating in a “sun-burst” paffern perpendicular to the di-lated third ventricle (arrows). The cal-carine and parietooccipital sulci fail toconverge and are not distinct.

Coronal and axial imaging reveals that thelateral ventricles are widely separated andmay be infeniorly pointed. The Probst-bundie-cingulate gyrus-fornix complex medially, in-dents the lateral ventricles creating a con-cave “Viking helmet” deformity (33) (Figure34). This is best seen anteriorly where the fibersare thickest. The thalami are widely spacedsecondary to a dilated third ventricle, which isinterposed between the lateral ventricles andis in continuity with the interhemisphenic fissure(Figure 35). A cyst, which may be the dilatedthird ventricle or an arachnoid cyst, is found inthe interhemisphenic fissure in up to 30% ofcases (67). The development of the limbic sys-tern is intimately related to the developmentof the corpus callosum and, therefore, associ-ated malformations include focal narrowing ofthe cingulum; hypoplasia of the fornix and sep-turn pellucidum; and hypoplasia of the hippo-campal formations, creating patulous tempo-ral horns with a “keyhole” deformity (Figure36). The anterior commissune is usually hypo-

Figure 34Dysgenesis of the corpus caiio-sum This Ti weighted coronalMR image reveals a typical “Vikinghelmet” deformity of the lateralventricles. The medial concavity iscreated by the impression of theinferionly and laterally notated cm-gulate gyri and the Probst bun-die-fonnix complex (arrow). Theinterhemisphenic fissure extendsto the dilated and elevated thirdventricle.

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Figure 35Dysgenesis of the corpus cailo-sum This T2 weighted transverseMR image, in a patient with almostcomplete absence of the corpuscailosum, reveals the Probst bun-dies lining the medial aspects ofthe lateral ventricles (arrows).There is preferential enlargementof the occipital horns of the lateralventricles which are separated.The third ventricle is in a high posi-tion interposed between the lateralventricles.

plastic, but rarely may be enlarged (3,8,4255).Colpocephaly is present owing to the lack ofsupport of the splenium of the corpus callo-sum.

Although complete absence of the corpuscaliosum, may occur in isolation, associatedanomalies occur in 80-90% of cases (3,8). Inthose patients with central nervous systemanomalies, the corpus callosum is absent in upto 50% (8). These associated anomalies in-dude Dandy-Walker syndrome (Figure 16),Chiari I and Chiani II malformations, neuronalmigrational disorders, basal encephaloceles,holoprosencephaly, midline central nervoussystem lipomas (Figure 31), posterior fossacysts, facial clefts, and various metabolic dis-orders (8,20,2242).

Dysgenesis of the corpus callo-sum This proton density trans-verse MR image reveals the socalled “keyhole deformity” of thetemporal horns secondary to hypo-plasia of the hippocampal forma-tion (arrow).

Figure 37Dysgenesis of the corpus callo-sum This Ti weighted midsagit-tal MR image reveals the absenceof the corpus cailosum posteriorly,with a very bright posterior nodulein the midline above the cerebel-lum, representing a lipoma. Mostcentral nervous system lipomasoccur within the intenhemisphenicfissure adjacent to a partially ab-sent corpus callosum. Occasional-ly, a lipoma may reside within thequadnigeminal plate cistern as inthis case.

Magnetic resonance imaging has be- and, therefore, will likely be encountered income the imaging procedure of choice in the even the smallest imaging practice. We haveevaluation of congenital central nervous sys- reviewed the recent literature and presentedtem anomalies. Even though a given anomaly representative cases to illustrate the scope ofmay itself be rare, central nervous system these pathologic conditions.anomalies on the whole are not uncommon


Poe etai.

Summary

Congenita/ centra/ nervous system anoma/ies

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We wish to thank Anthony J. Pileggi. M.D.. and Thomas M. Yarnell. M.D. for their as-sistance in studying the subjects In Figures 30 and 16 respectively.

We also would like to thank the Department of Medical Photography, supervisedby Joseph Mentrikoski. and the Medical Transcriptionists. supervised by Lisa Kanour fortheir assistance in the preparation of this manuscript.

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