02 -Disorders of Neural Tube Closure

C H A P T E R Disorders of Neural Tube Closure Chiari Malformations Chiari I Chiari II Chiari III Chiari IV Cephaloceles Occipital and Parietal Sincipital and Sphenopharyngeal Nasal Cephaloceles, Dermoids, and Gliornas Atretic Cephaloceles Corpus Callosum Anomalies Corpus Callosurn Agenesis Corpus Callosurn Liporna The formation of the brain and spinal cord is referred to as dorsal induction (see Table 1-1) The two general stages of dorsal induction are primary and secondary neurulation Primary neurulation involves the formation of the brain and upper spine; secondary neurulation refers to formation of the distal spine.1 Disorders of primary neurulation are mostly neural tube closure defects and early central nervous system (CNS) anomalies, typically occurring at around or gestational weeks These include Chiari malformations, cephaloceles, and myelomeningoceles During secondary neurulation, interactions between the notochord and mesoderm form the skull, dura, pia, and vertebrae These occur at to gestational weeks Abnormalities of secondary neurulation result in spinal dysraphic disorders that range from simple, isolated anomalies such as spina bifida occulta to more complex malformations such as meningocele and lipomeningocele, neurenteric cysts, dermal sinus, and the caudal regression syndromes Only the skull and brain anomalies will be discussed here; congenital anomalies of the spine and spinal cord are delineated in Chapter 19 CHIARI MALFORMATIONS German pathologist Hans Chiari described congenital hindbrain anomalies in which cerebellar tissue descends into the cervical canal In 1891 he described an anomaly, now designated as Chiari type I, consisting of elongated peglike cerebellar tonsils displaced into the upper cervical canal In 1896 he described a second type of hindbrain anomaly, known as Chiari type II, in which the vermis, pons, medulla, and an elongated fourth ventricle were displaced in feriorly into the cervical canal.2 He also reported a single case of cervical spina bifida combined with multiple cerebellar and brainstem anomalies that has since been called a Chiari type III malformation.3 Some authors have added a fourth type of hindbrain abnormality to the group of Chiari malformations, but the so-called Chiari IV malformation is actually a form of severe cerebellar hypoplasia and occurs during a later stage of development4 (see Chapter 4) 16 PART ONE Brain Development and Congenital Malformations Chiari I Malformation Pathology Chiari I malformation (sometimes termed congenital tonsillar ectopia) is a relatively simple anomaly that is unassociated with other congenital brain malformations In contrast to the Chiari II malformation (see subsequent discussion), in this disorder the vermis, fourth ventricle, and medulla are normal or only minimally deformed Elongated, pointed, "peglike" cerebellar tonsils are displaced in- feriorly through the foramen magnum into the up per cervical spinal canal (Fig 2-1) Tonsillar position and clinical presentation Statistically significant differences in tonsillar position with age normally occur In general, the cerebella tonsils ascend with increasing age In the first decad of life, mm should be used as the criterion for tonsillar ectopia This decreases to mm in the second Fig 2-1 A, Gross pathologic specimen of Chiari I malformation, lateral view Note the peglike low-lying tonsils (arrows) Only minimal deformities of the fourth ventricle and vermis are present B, Anatomic diagram of the Chiari I malformation Pointed, lowlying tonsils are seen (large black arrow) Syringohydromyelia is indicated by the small black arrows C, Sagittal T2-weighted MR scans in a patient with an incidental finding of Chiari I malformation The foramen magnum is indicated by the large black arrows The cerebellar tonsils (small black arrows) lie 10 mm below the foramen magnum (A, Courtesy E C Alvord, Jr.) Chapter and the third decades, to mm between the fourth to the eighth decades, and to mm by the ninth decade.5 Symptomatic patients with Chiari I malformation often present with long-tract signs and other symptoms that mimic demyelinating disease As a group, patients with brainstem or cerebellar signs have the largest mean inferior tonsillar displacement In a recent series, herniations greater than 12 mm were invariably symptomatic However, nearly 30% of patients with tonsillar displacements ranging from to 10 mm below the foramen magnum were asymptomatic.6 Associated abnormalities Chiari I malformation is usually not associated with other brain anomalies However, spinal cord, skull base, and spine lesions are common in this disorder Spinal cord Accumulation of cerebrospinal fluid (CSF) within the spinal cord is a frequent finding in patients with Chiari I (Figs 2-1, B, and 2-2) Simple Disorders of Neural Tube Closure 17 distention of the ependymal-lined central canal is classically termed hydromyelia, Dissection of CSF through the ependyma to form paracentral cavitations within the cord is termed syringomyelia The distinction between these two conditions is not possible on imaging studies and is sometimes difficult to establish even after detailed histologic examination.7 Therefore the term syrinx, or syringohydromyelia, is used subsequently to describe any pathologic CSF containing cord cavity, whether or not it is continuous with the central canal A syrinx is present in 20% to 40% of all patients with Chiari I If only symptomatic patients are considered, the occurrence of associated syringohydromyelia is even higher, ranging from 60% to 90%.6, The cervical spinal cord is the common site, although occasionally patients with Chiari I have a lesion that involves the entire cord An isolated thoracic cord syrinx is uncommon The etiology of the hydrosyringomyelia associated with Chiari malformations is unknown but is likely Fig 2-2 Sagittal T1-weighted (A) and T2-weighted (B) MR scans in a patient with Chiari I malformation The peglike, low-lying tonsils are indicated by the large black arrows A collapsed syrinx of the cervical spinal cord is present (small black arrows) This 40-yearold female had a 3-year history of long tract signs Cranial MR scan had been requested to evaluate for demyelinating disease No evidence for multiple sclerosis was seen on T2WI of the brain 18 PART ONE Brain Development and Congenital Malformations secondary to pathologic cerebrospinal fluid dynamics Postulated causes include posterior compression at the foramen magnum and anterior indentation of the medulla and cord secondary to basilar invagination.9 Some studies show that syrinx is more frequent in patients with moderate degrees of cerebellar tissue hemiation.8, 10 Others report little correlation between increasing tonsillar descent and more extensive or distended synnxes.11 Skull base and spine Osseous anomalies are seen in about one quarter of all patients with Chiari I malformation and include atlantooccipital assimilation, platybasia, basilar invagination, and fused cervical vertebrae (Klippel-Feil)6 (see box, below) Imaging Tonsillar configuration and position, as well as the presence of associated hydrosyringomyelia, are easily detected on sagittal T1- and T2 weighted MR scans (Figs 2-1, C, and 2-2) CT is less satisfactory for assessing tonsillar position and configuration unless intrathecal contrast is used Chiari II Malformation Etiology Chiari II malformation is a complex anomaly with skull, dura, brain, spine, and cord manifestations Although its exact etiology is unknown, recent evidence suggests that the fetal neural folds fail to neurulate completely, leaving a dorsal opening Consequently, the developing spinal cord walls not appose properly and abnormal drainage of CSF through the dehiscent neural tube into the amniotic cavity results The primitive ventricular system then decompresses and collapses Chiari I Malformation Peglike, pointed tonsils displaced into upper cervical canal At least mm in first decade of life 10-30 years: mm 30-80 years: mm Associated anomalies Brain Usually none Ventricles Mild/moderate hydrocephalus (20%-25%) Spinal cord Syringomyelia in 30%-60% of all patients; 60%-90% in symptomatic patients Skeletal anomalies in 25% Basilar invagination (25%-50%) Klippel-Feil (5%-10%) Atlantooccipital assimilation (1%-5%) Because the primitive ventricular system is inadequately distended, this alters the inductive effect of pressur and volume on the surrounding mesenchyme, adversely affecting endochondral bone formation An abnormally small posterior fossa is the result.12 Subsequent development of the cerebellum and brain stem within the abnormally small posterior fossa leads to upward herniation, resulting in an enlarged tentorial indsura and dysplastic tentorium, and to downward herniation of the cerebellar vermis and brain stem through an enlarged foramen magnum into the upper cervical canal.12, 13 Other relate cerebral and skull anomalies observed in the Chia II malformation (such as a large massa intermedia, corpus callosum dysgenesis, and the so-called luckenschadel, or lacunar skull) have also been accounted for by this unified theory Pathology and imaging manifestations The spectrurn of abnormalities in Chiari II malformation is very broad, with many different findings reported (see box, p 19) The Chiari II malformation has abnormalities of the following: Skull and dura Hindbrain, cerebellum, and midbrain Cerebrospinal fluid spaces Cerebral hemispheres Spine and spinal cord It is helpful to consider these abnormalities seprately Skull and dura The pathologic changes of Chiari II malformation are shown schematically in Figure 2-3 Normal membranous bone formation of the calvarial vault requires distention of the underlying brain and ventricular system that is lacking in the Chiari II malformation Radial growth of the developing calvarium is profoundly altered.12 The result is the so-called lacunar skull (luckenschadel) The gross pathology and plain film radiographic appearance of lacunar skull are striking (Figs 2-4 and 2-5) Focal calvarial thinning and a "scooped-out" appearance are typical These skull changes are most striking at birth and tend to diminish with age Bcause the defects are not caused by hydrocephalus, their resolution is unrelated to surgical intervention and ventricular shunting Lacunar skull should distinguished from prominent convolutional markings that can occur normally or with hydrocephalus after sutural closure In the latter instance, other changes of increased intracranial pressure such as sellar erosion should be present CT and MR scans of infants with a Chiari II malformation also demonstrate scalloping and thinning of the inner calvarial table Although the most obvious manifestations of lacunar skull resolve by about months of age, subtle calvarial thinning and scalloping Chapter Disorders of Neural Tube Closure 19 Chiari II Malformation Skull and dura Calvarial defects (lacunar skull, or luckenschadel) Small posterior fossa with low-lying transverse sinuses Fenestrated falk, Heart-shaped incisura with hypoplastic tentorium Gaping foramen magnum Concave clivus, petrous ridges Brain Inferiorly displaced vermis Medullary spur and kink Beaked tecturn Interdigitated gyri Cerebellum “creeps” around brainstern and "towers" through wide tentorial incisura Associated anomalies: callosal dysgenesis, heterotopias, polymicrogyria, stenogyria Ventricles Whole system: hydrocephalus in 90% Fourth: elongated, tubelike, inferiorly displaced Third: large massa intermedia; may be high riding if corpus callosurn absent Lateral: colpocephaly; scalloped, pointed walls Spine and cord MyeIomeningocele in nearly 100% Syringohydromyelia 50%-90% Diastematomyelia Segmentation anomalies in