eMedicine Specialties > Radiology > Pediatrics

Chiari II Malformation

Lutfi Incesu, MD, Professor, Department of Radiology, Ondokuz Mayis University School of Medicine; Chief, Neuroradiology and MR Unit, Department of Radiology, Ondokuz Mayis University Hospital, Turkey
Anil Khosla, MBBS, Assistant Professor, Department of Radiology, Section of Neuroradiology, Mallinckrodt Institute of Radiology, Washington University School of Medicine, Veterans Affairs Medical Center of St Louis; Michael R Aiello, MD, Radiologist, St Elizabeth Medical Center, Utica, NY

Updated: Oct 15, 2009

Introduction

Background

Between 1891 and 1896, German pathologist Hans Chiari described a series of anomalies of the caudal cerebellum and brainstem on the basis of autopsy observations. In 1891, he described an anomaly consisting of elongated peglike cerebellar tonsils that are displaced into the upper cervical canal through the foramen magnum. This is now designated as the Chiari type I malformation.

Five years later, Han Chiari published a further report on a hindbrain anomaly, now known as the Chiari type II malformation.^{[1 ]}He also reported a single case of cervical spina bifida that was associated with herniation of the cerebellum through the foramen magnum, which has since been called Chiari III malformation. Some authors have added a form of severe cerebellar hypoplasia without displacement of brain through the foramen magnum, the so-called Chiari IV malformation.

The Chiari II malformation is a complex congenital malformation of the brain, nearly always associated with myelomeningocele. This condition includes downward displacement of the medulla, fourth ventricle, and cerebellum into the cervical spinal canal, as well as elongation of the pons and fourth ventricle, probably due to a relatively small posterior fossa.^{[2,3,4,5,6 ]}

For excellent patient education resources, visit eMedicine's Brain and Nervous System Center. Also, see eMedicine's patient education articles Spina Bifida and Normal Pressure Hydrocephalus.

Sagittal T1-weighted magnetic resonance image of posterior fossa abnormalities in Chiari II malformation: (1) colpocephaly; (2) beaked tectum; (3) cascade of an inferiorly displaced vermis behind the medulla; (4) elongated, tubelike fourth ventricle; (5) low-lying torcular herophili; (6) cerebellar hemispheres wrapping around the brainstem anteriorly; (7) concave clivus; (8) medullary spur; and (9) medullary kink.

Sagittal midline sonogram (same patient in Images 3-4 in Multimedia) shows a large massa intermedia (long arrow) and a beaked tectum (short arrow). The image also shows obliteration of the cisterna magna and the fourth ventricle, as well as compression of the pons and brainstem.

Antenatal magnetic resonance image shows a Chiari II malformation in a fetus. Courtesy of Umit Aksoy, MD, Uludag University, Bursa, Turkey.

Pathophysiology

The Chiari II malformation is a complex anomaly with skull, dural, brain, spinal, and spinal cord manifestations. This disorder is almost invariably associated with myelomeningocele. The hindbrain findings of Chiari II malformation are best explained with the theory of McLone and Knepper, which allows the hindbrain disorder to be conceptualized as resulting from a normal-sized cerebellum developing in an abnormally small posterior fossa with a low tentorial attachment.^{[10 ]}

For the last hundred years, numerous theories have been proposed to explain the etiology of the diffuse findings involved in the Chiari II malformation and myelomeningocele. To date, no single theory has been proven completely satisfactory. However, 2 schools of thought have emerged. One attributes the malformation to be primarily a result of mechanical forces, including the traction and hydrodynamic theories, and the other postulates the cause as abnormal embryologic development, including the developmental arrest and/or primary dysgenesis, small posterior fossa and/or overgrowth, neuroschisis, and abnormal neurulation hypotheses.

Chiari and Gardner advocated the hydrodynamic theories.^{[1,2 ]}Chiari thought that posterior fossa herniation was related to supratentorial hydrocephalus. Gardner believed that hydrocephalus and hydromyelia were normal physiologic events in early embryologic development; however, if the pathways for the normal progress of the cerebrospinal fluid did not develop, the neural tube distends and ruptures, resulting in myeloschisis. The hydrodynamic theory, however, does not explain the small size of the posterior fossa, the upward herniation of the posterior fossa contents, the slitlike fourth ventricle, and the multiple supratentorial anomalies.

The traction theory was proposed by Penfield, Cobum and Luchenstein, who suggested that a tethered spinal cord near the myelomeningocele may pull the cerebellum and medulla into the cervical canal. In the Chiari II malformation, however, the spinal cord is not always tethered. Traction from the caudal end of the cord is rapidly dissipated within 4 segments. The traction theory also does not explain the medullary kink.

Cleland proposed a developmental arrest theory with dysgenesis of the hindbrain.^{[11 ]}He thought that primary dysgenesis of the brainstem was the cause of the malformation. Daniel and Stritch^{[12 ]}and Peach^{[13 ]}thought that failure of development of the pontine flexure due to a developmental arrest causes both upward and downward herniation of the elongated brainstem. However, this theory does not explain the associated cerebral malformations.

Marin-Padilla and Marin-Padilla proposed a small posterior fossa theory related to a mesodermal deficiency. Although underdevelopment of the occipital bone has been described in hamsters depleted of vitamin A, neither displacement of posterior fossa structures nor the presence of other manifestations of the Chiari malformation were found.

McLone and Knepper proposed a unified theory.^{[10 ]}According to this theory, a primary neurulation deficit results in the normal closure of the ventral portion of the neurocele. Failure of subsequent normal distention of the primitive rhombencephalic ventricular system deprives the basal cranial mesoderm of the inductive bone that is needed to develop the posterior fossa. The tentorium is left low and deficient, and the pontine flexure cannot form. The cerebellum and brainstem are subsequently extruded upward and downward.

Normal distention of the rhombencephalic ventricle probably influences brainstem development. Failure of ventricular distention could result in disorganization of the cranial nerve nuclei. The third ventricle does not become normally distended. The thalamus remains approximated, forming a large massa intermedia. Lack of support for the developing telencephalon results in heterotopia, dysgenesis of the corpus callosum, and disorganization of the cerebral gyri.^{[7 ]}

Lückenschãdel skull is another manifestation of the lack of inductive bores that are transmitted to the surrounding mesenchyme. Defective myelination and hypoplasia of the lower cranial nerve nuclei are also present, as shown at autopsy. The glossopharyngeal and vagus nerves are caudally displaced by the medulla. They and the spinal accessory nerve must travel rostrally in the compressed subarachnoid space, ventral to the medulla, to exit the cranium via the jugular foramen. These nerves are at risk of pressure necrosis as they traverse the bony ridge of the basiocciput and jugular foramen. This mechanism would explain the respiratory stridor, paralyzed vocal chords, and swallowing abnormalities seen in severe cases of Chiari II malformation.

The fetal neural folds fail to become completely neurulated. Consequently, the developing spinal cord wall does not become properly apposed, and the cerebrospinal fluid abnormally drains through the open neural tube into the amniotic cavity. This drainage allows the primitive ventricular system of the brain to collapse, altering the inductive effect of pressure on the surrounding mesenchyme and adversely affecting enchondral bone formation. As a result, an abnormally small posterior fossa forms. Subsequent development of the cerebellum and brainstem within the abnormally small posterior fossa leads to downward herniation of the cerebellar vermis and brainstem through an enlarged foramen magnum into the upper cervical spine.

The dentate ligaments attach to the lateral aspects of the spinal cord and hold it in place. If the medulla extends dorsal to the fixed cord and stretches down further than the dentate ligaments allow the spinal cord to move, a characteristic cervicomedullary kink is formed (70%). The fourth ventricle is positioned low, oriented vertically, and narrowed in its anteroposterior diameter.

Frequency

United States

The Chiari II malformation is the most common serious malformation of the posterior fossa. The frequency is approximately 1 case per 1000 population in the United States.^{[4 ]} Of 69 necropsies performed in hydrocephalic patients, 31 demonstrated a Chiari II malformation.

Mortality/Morbidity

Neonatal Chiari II malformations continue to result in significant morbidity and mortality. Hindbrain dysfunction is the major cause of the mortality. The mortality rate is 15% in the first years of life among patients with a Chiari II malformation. Most authors report long-term mortality rates as high as 50%, regardless of the treatment strategy. Overall infant and childhood mortality rates are high in the immediate perinatal period. These stabilize at 15% by the time the patient is aged 2 years but then increase to 18-19% by 15 years.

Symptomatic Chiari II malformation is the leading cause of mortality in the myelodysplastic population. One third of patients with myelomeningocele develop brainstem dysfunction by the age 5 years. Of these, one third die in infancy.

About 20-33% of patients with a Chiari II malformation and myelomeningocele become symptomatic as a result of hindbrain herniation. Of these, one third do not survive beyond infancy. Cranial nerve and brainstem dysfunction are the most serious and potentially life-threatening problems.

Respiratory difficulties occur in 29-76% of patients; these are the most common and lethal manifestation of the condition. Apnea may progress from inspiratory stridor, or it may be centrally mediated in the form of prolonged expiratory apnea with cyanosis (PEAC). In one series, the onset of PEAC resulted in the death of 56% of patients. The rapidity of neurologic deterioration and the final neurologic status immediately before decompensation were the most important factors influencing the prognosis.

Overall, approximately 10-15% of patients with the Chiari II malformation and myelodysplasia die within the first 2 years. Some studies, however, are encouraging. Vandertop et al reported a mortality rate of 11.7% in infants undergoing surgery before the age of 1 month.^{[14 ]}

Race

No racial predilection exists for Chiari II malformation.

Sex

The incidence of Chiari II malformation is increased in females.

Age

Two distinct age-dependent syndromes exist for Chiari II malformation: one involves infants and the other involves older children. Each syndrome has different symptoms, chronologic courses, and outcomes. Symptoms may manifest in the first days of life, but the most common period for symptom manifestation in infancy is during the first months of life.

Anatomy

The Chiari II malformation is a complex deformity of the calvarium, dura, and hindbrain, and it is almost always associated with myelomeningocele. The spectrum of abnormalities in Chiari II malformation is broad, with many findings reported.

• Osseous changes
  • Lacunar skull or lückenschãdel: Almost all patients with Chiari II malformation manifest dysplasia of the membranous bones of the calvarium. This is called lacunar skull (lückenschãdel) and appears as clusters of areas of thinning, pits, and fenestrae that are most prominent near the vertex or torcular herophili (see Image below and Image 8 in Multimedia).

Deep scalloping between the bony septations that characterize the lacunar skull (lückenschädel) (arrows) are best appreciated on an axial computed tomography section, as in this patient with a Chiari II malformation.

• A lacunar skull may be observed in utero in a fetus as early as at 8 months of gestation. The lacunar skull typically persists until the age of 1-3 months and then disappears after approximately 6 months of age, regardless of whether progressive hydrocephalus is present. Subtle calvarial thinning and scalloping may persist into adulthood.
• Concave clivus and petrous ridge: The posterior aspects of the petrous temporal bones are often concave (see Image below and Image 2 in Multimedia). The clivus also develops abnormally and is often short, with a concave configuration similar to that of the petrous ridges (see Image below and Image 1 in Multimedia).

Axial computed tomography scan in a patient with a Chiari II malformation. This image shows a gaping, somewhat heart-shaped tentorial incisura (large arrowheads) that appears to be completely plugged with the upwardly herniating cerebellum. The cerebellar hemispheres extend anteromedially (small arrowheads) and almost completely engulf the brainstem. The petrous ridges are concave (arrows).

Sagittal T1-weighted magnetic resonance image of posterior fossa abnormalities in Chiari II malformation: (1) colpocephaly; (2) beaked tectum; (3) cascade of an inferiorly displaced vermis behind the medulla; (4) elongated, tubelike fourth ventricle; (5) low-lying torcular herophili; (6) cerebellar hemispheres wrapping around the brainstem anteriorly; (7) concave clivus; (8) medullary spur; and (9) medullary kink.

• Small posterior fossa and gaping foramen magnum: The posterior fossa is exceptionally small, and the foramen magnum is larger and rounder than usual.
• Low-lying transverse sinuses: The internal occipital protuberance is situated just superior to the foramen magnum. The transverse sinuses and torcular herophili are low lying (see Image below and Images 1 and 5 in Multimedia).

Coronal T1-weighted magnetic resonance image in a patient with a Chiari II malformation. This image shows low-lying transverse sinuses (arrows), hydrocephalus, and a small posterior fossa. A hypoplastic tentorium cerebelli with gaping incisura (arrowhead) is present with a towering cerebellum (small arrows).

• Changes to the dura
  • Fenestrated falx: The falx almost always shows partial absence, hypoplasia, and/or fenestrations (see Image below and Image 7 in Multimedia).

Axial T2-weighted magnetic resonance image in a patient with a Chiari II malformation. This image shows a hypoplastic fenestrated falx cerebri with striking interdigitation of the gyri (arrows).

• Hypoplastic tentorium: The tentorium is hypoplastic and attaches to the occipital bone far caudally, just above the foramen magnum (see Image 1).
• Heart-shaped incisura: The hypoplastic tentorial leaves arise laterally from the low-lying transverse sinuses (see Image 2).
• Changes to the cerebellum, medulla, and spinal cord
  • Cerebellar peg: Protrusion of vermis and hemispheres through the foramen magnum (90%) results in craniocaudal elongation of cerebellum behind the spinal cord (see Images below and Images 1, 3, and 9 in Multimedia).

Sagittal midline T1-weighted magnetic resonance image in a patient (same patients in Images 3-4 in Multimedia) with a Chiari II malformation. This image shows a large massa intermedia (long arrow) and a beaked tectum (short arrow). Other posterior fossa abnormalities, similar to those shown in Image 1 in Multimedia, are also seen in this patient.

A thoracic-level myelomeningocele (short arrow) is seen in a patient with a Chiari II malformation (long arrow) (same patient in Images 9-11 in Multimedia).

• Medullary kink: The medulla is kinked inferiorly (75%) and lies dorsal to the spinal cord, which is unable to descend because of competent dentate ligaments (see Images 1 and 3).
• Towering cerebellum or vermian pseudotumor: The cerebellar hemispheres and vermis also extend above the incisura of the tentorium (see Image below and Image 5 in Multimedia)
• Corners of the cerebellum are wrapped around the brainstem, pointing anteriorly and laterally (see Images 1 and 3).
• Tubelike elongated fourth ventricle: The fourth ventricle is elongated craniocaudally, narrowed transversely, and decreased in anteroposterior diameter (see Images 1, 3, and 9).
• The cerebellopontine cistern and the cisterna magna are obliterated.
• The combined displacements of the spinal cord, medulla, pons, and cerebellum form a cascade of herniations, each of which compresses all of the tissue in front of it, displacing them anteriorly.
• In older patients, a wide subarachnoid space may be seen behind a vermis that is deeply grooved at the level of cervical nerve C1. The vertebral artery frequently loops on itself within the cervical canal and passes caudally to the level of cervical nerve C3. The possible cause of this groove may be the pulsatile effect of the vertebral artery on the adjacent cerebellum.
• Changes to the midbrain and the third and lateral
  • Beaked tectum: Variable degrees of fusion of the colliculi and tectum result in prominent beaking and inferior displacement of the tectal plate (see Images below and Images 1, 3-4, and 6 in Multimedia).

Sagittal midline sonogram (same patient in Images 3-4 in Multimedia) shows a large massa intermedia (long arrow) and a beaked tectum (short arrow). The image also shows obliteration of the cisterna magna and the fourth ventricle, as well as compression of the pons and brainstem.

Axial T1-weighted magnetic resonance image in a patient with a Chiari malformation. This image shows a beaked tectum (arrows) and colpocephaly (arrowheads).

• Hydrocephalus: Ventricular dilatation is present in 98% of patients, and 90% require a shunt.
• Colpocephaly: The occipital horns and atria are often mildly enlarged because of maldeveloped occipital lobes, especially in the presence of a malformation involving the corpus callosum (see Images 1, 4, and 6).
• Prominent massa intermedia: A prominent massa intermedia, herniation of the third ventricle into the suprasellar cistern, and an enlarged suprapineal recess are often seen (see Images 3-4). Frequently, a large third ventricle is present as well.
• Approximately one third of infants have a partial block at the level of the ambient cisterns.

• Associated anomalies
  • Myelomeningocele (88-100%)
  • Dysgenesis of corpus callosum (80-90%)
  • Obstructive hydrocephalus following closure of myelomeningocele (50-98%)
  • Syringohydromyelia (50-90%)
  • Aqueductal stenosis (70%)
  • Absence of septum pellucidum (40%)
  • Contracted, narrow gyri (stenogyria; 50%)
  • Heterotopias
  • Diastematomyelia
  • Segmentation anomalies (<10%), incomplete C1 arch
  • Malrotation of the posterior arches of C1 and C2 (see Image below and Image 12 in Multimedia)

Inferior operative-like view on 3-dimensional computed tomography scan. This image shows malrotation of the posterior arches of C1 (long arrow) and C2 (short arrow). Courtesy of Duffau et al.

• Low-lying, often-tethered conus medullaris below lumbar nerve L2
• Rare anomalies
• Holoprosencephaly
• Cervical myelocystocele
• Frontometaphyseal dysplasia
• Juvenile distal spinal muscular dystrophy
• Williams syndrome

Presentation

Two distinct, age-dependent syndromes are identified in Chiari II malformations. One syndrome involves infants, and the other involves older children. Each has different symptoms, chronologic courses, and outcomes.

In Chiari II malformations, infants, particularly neonates, demonstrate rapid progressive neurologic deterioration. Symptoms are rarely present at birth or during the first 2 weeks of life. Hindbrain dysfunction is severe in 4-13% of patients with myelomeningocele. Life-threatening symptoms result from dysfunction of the medullary respiratory center and cranial nerves IX and X.

Pollak et al demonstrated that patients with Chiari II malformation did not have brainstem dysfunction at birth, indicating that compressive or ischemic etiologies may be partly responsible for the symptoms. In support of this theory, infants with the Chiari malformation have evidence of hemorrhagic infarction and necrosis within the medulla. They are prone to the rapid development of symptoms, with clinical deterioration occurring over a period of days.

A common and striking symptom that is initially present is inspiratory stridor when the infant cries. Episodes of stridor and apnea frequently herald impending brainstem compromise and subsequent development of dysphagia or nasal regurgitation, aspiration, quadriparesis, and opisthotonic posturing. Apnea may result from bilateral abductor vocal cord paralysis (obstructive apnea), central neural dysfunction (centrally mediated expiratory apnea with cyanosis), or both.

Older children and adolescents have a more insidious presentation with syncopal episodes; nystagmus; oscillopsia; lower cranial nerve palsies; and motor weakness and spasticity, which usually occur in the presence of hydromyelia.

Clinical symptoms and signs of Chiari II malformation are as follows (in order of decreasing frequency):

• Signs and symptoms in infancy
  • Respiratory distress and impaired swallowing (71%)
  • Inspiratory stridor (59%)
  • Episodic apnea (29%)
  • Weak or absent cry (18%)
  • Aspiration (12%)
  • Nystagmus
  • Pain in the upper and lower extremities
  • Weakness or spasticity of the upper and lower extremities (53%)
  • Depressed or absent gag reflex
  • Fixed retrocollis
  • Palsy of the seventh cranial nerve
  • Scoliosis
  • Worsening of bladder and/or bowel function
• Signs and symptoms in childhood
  • Syncopal episodes
  • Nystagmus (both horizontal and rotatory on lateral gazing)
  • Spastic quadriparesis
  • Upper extremity weakness with increased tone
  • Exaggerated deep tendon reflexes
  • Mirror movements
  • Appendicular and/or truncal ataxia
  • Recurrent pneumonia secondary to aspiration
  • Gastroesophageal reflux
  • Depressed or absent cough reflex
  • Gradual loss of function

Preferred Examination

The Chiari II malformation is a complex anomaly with skull, dural, brain, spinal, and spinal cord manifestations. Traditionally, when signs and symptoms were suggestive of a Chiari II malformation, plain radiography of the head and spine was performed, followed by myelography. Because myelography is an invasive procedure, clinicians were reluctant to perform the test until the severity of the symptoms warranted it.

The introduction of modern imaging techniques, specifically magnetic resonance imaging (MRI), has radically changed the evaluation of symptoms referable to the brain and spinal cord. MRI is usually used for the detailed evaluation of lesions and complications due to Chiari II malformations.^{[3,15,16,17,18,19 ]}

MRI is best used to appreciate the full constellation of findings in Chiari II malformations, and it permits detailed visualization of the cerebellum and spinal cord. MRIs are useful in showing the low position of the cerebellar tissue and in determining whether associated spinal abnormalities, such as diastematomyelia or syringomyelia, are present. In addition, MRI has been used for the diagnosis of fetal craniospinal anomalies.

MRI is widely available, accepted, and easy to perform. It allows imaging in multiple planes, and it has high spatial and contrast resolution, which allows for the optimal evaluation of morphologic features.

Chiari II malformations are also diagnosed with the help of both computed tomography (CT) scanning and ultrasonography (US). CT scanning is especially useful after the neonatal period in following up obstructive hydrocephalus in infants who have undergone a ventriculoperitoneal shunt procedure.^{[20 ]}

CT scanning is useful for appreciating the lückenschãdel skull (see Image 8), and this imaging modality can be used to identify the other bony changes seen in the Chiari II malformation, such as the large foramen magnum, the flat floor of the posterior fossa, and the scalloping of the petrous pyramids. CT scanning is also excellent for assessing and following up ventricular size before and after shunt placement (approximately 80-90% of patients have hydrocephalus).

Many of the typical abnormal Chiari malformation findings depicted on cranial CT scans and MRIs can also be demonstrated on cranial sonograms. US is routinely used during gestation for screening purposes and in the neonatal period for diagnosis and follow-up of hydrocephalus.^{[18,21 ]}

Plain radiographs of the cervical spine, including flexion and extension views, can be used to assess any pathologic spinal movement. Depicted abnormalities include widening of the upper cervical spinal canal and incomplete bony arching of C1 in as many as 70% of cases with replacement by a periosteal band that contributes to neural compression.

Plain radiography is excellent for demonstrating scoliosis, segmentation errors, and lack of fusion of the dorsal laminae in the spine.

Limitations of Techniques

Plain radiographic findings of Chiari II malformation do not have diagnostic importance except for the bone abnormalities associated with scoliosis and diastematomyelia and for ventriculoperitoneal shunt malfunction.

CT scanning is an efficient diagnostic examination in following up infants and children with hydrocephalus, but it exposes the patient to ionizing radiation. The value of CT scanning in diagnosing cerebral gyral malformations and spinal cord pathology is limited.

MRI is relatively expensive, is contraindicated in patients with pacemakers, and is not tolerated by all patients. MRI requires patient cooperation or sedation.

US is limited to the period before the closure of the anterior fontanelle, which serves as an acoustic window. Abnormalities such as gyral, dural, tentorial, and vermian anomalies accompanying Chiari II malformations are difficult to visualize with US.

Differential Diagnoses

Astrocytoma, Spine
Chiari I Malformation
Chordoma
Encephalocele

Other Problems to Be Considered

Ependymoma (fourth ventricle)
Lhermitte-Duclos disease
Rhombencephalosynapsis
Tectocerebellar dysraphia with posterior encephalocele^{[22 ]}

Radiography

Findings

Plain radiography has been used most frequently for examining bone anomalies, scoliosis, and ventriculoperitoneal shunt malfunction. Plain images can show enlargement of the cervical spinal canal and posterior midline fusion defects, as well as anterior bony abnormalities, such as C1 and C2 dislocation.

Routine radiography can demonstrate l ü ckenschãdel in neonates with myelodysplasia. Additional bony changes that can be seen include scalloping of the posterior surface of the petrous pyramids and the clivus.

Degree of Confidence

Plain radiographs have diagnostic importance in the evaluation of cranial and vertebral bony abnormalities and in the evaluation of ventriculoperitoneal shunt disconnection or displacement of the shunt catheter.

Computed Tomography

Findings

CT scans are used most commonly in patients with Chiari II malformation, for the diagnosis of hydrocephalus, and for the evaluation of suspected shunt malfunction.

Lückenschãdel can be demonstrated on CT scans (see Image 8). Abnormalities are composed of areas of thinning or pits within the calvarial bone. The areas of thinning typically occur on the inner surface of the skull, but they have been seen on both the inner and outer aspects. Within each of the depressions on the inner aspect of the skull, brain tissue can be seen bulging into the bony defect.

CT scans may demonstrate an abnormally large foramen magnum, the flat floor of the posterior fossa, and scalloping of the petrous pyramids and clivus (see Image 2). CT scanning also demonstrates tectal beaking, cerebellar tissue wrapping around the brainstem, fenestrations of the falx, manifestations of hydrocephalus, and shunt malfunction.

CT myelography is rarely performed in patients with Chiari II malformation because of the advent of MRI. High-resolution CT scanning of the spine may be essential for surgical planning, particularly in patients with severe scoliosis and associated complex segmentation anomalies.

Three-dimensional (3-D) CT scans and CT angiograms can be used for a precise analysis of complex osseous malformations, of the relationships between vascular and bony structures, and of postoperative changes in the bone (see Image below and Images 12-13 in Multimedia).

Postoperative 3-dimensional computed tomography scan. This image shows osseous decompression with a large opening of the foramen magnum and resection of the posterior arch of the atlas (arrows). Courtesy of Duffau et al.

Degree of Confidence

CT scanning is the routine diagnostic method for following up patients with a Chiari II malformation and associated hydrocephalus. An encysted fourth ventricle is one of the causes of shunt malfunction. Because the internal regulation of the shunt valve is usually affected by intracranial pressure changes, the pressure dynamics between infratentorial and supratentorial compartments are important.

CT scanning also is helpful for follow-up to assess the dynamic changes in the infratentorial and supratentorial ventricular systems.

Chiari II malformation associated with subtle gyral malformations and spinal cord abnormalities, such as a syrinx, are often missed on routine CT images. Thin-section sagittal and/or coronal reformatted CT images may be useful for assessing spinal anomalies. The most common cause of a syrinx of the spinal cord in patients with a Chiari II malformation is ventriculoperitoneal shunt malfunction. All patients with a Chiari II malformation and syrinx (as detected with spinal MRI) should undergo cranial CT scanning and a radiographic shunt series to exclude shunt malfunction as a cause for appearance or worsening of the syrinx.

False Positives/Negatives

When posterior fossa and cervical CT scans are evaluated individually, the craniocaudal elongation of cerebellum in a Chiari II malformation can be confused with cervical spinal tumors and cerebellar tumors that ascend through foramen magnum or cause cerebellar herniation.

Magnetic Resonance Imaging

Findings

Antenatal magnetic resonance image shows a Chiari II malformation in a fetus. Courtesy of Umit Aksoy, MD, Uludag University, Bursa, Turkey.

T1-weighted axial magnetic resonance image. This image demonstrates heart-shaped incisura and a petrous ridge in a patient with a Chiari II malformation.

Sagittal T2-weighted magnetic resonance in a patient with a Chiari II malformation (same patient in Images 9-11 in Multimedia). This image shows a thoracic-level myelomeningocele (arrows). The spinal cord, in addition to the thoracic placode, also extends distally and is further tethered at the sacral level.

Hindbrain anomalies, hydrocephalus, and syrinx cavities in the spinal cord are well demonstrated on T1-weighted images. The extent of the S-shaped medullary kink can be easily evaluated by using sagittal MRI in patients with a Chiari II malformation (see Images 1 and 3). MRI of the head is easily performed to assess the size and position of the ventricles, and it provides important additional information (see Images 1 and 6).

Enlargement of the massa intermedia, occlusion of the cerebral aqueduct, and beaking of the tectum are best evaluated by using a sagittal projection on MRI (see Images 1 and 3). Gyral interdigitation, cerebral gyral anomalies, an engulfed brainstem, and the level of the medullary kink can easily be evaluated by using axial MRI.

MRI enables accurate, objective, and detailed identification of spinal cord and vertebral abnormalities, including syringomyelia and diastematomyelia, both preoperatively and postoperatively (see Image below and Images 9-11 in Multimedia). MRI has also been used as a diagnostic tool during the antenatal period, allowing the intrauterine diagnosis, and even treatment, of spina bifida.

Sagittal sonogram in a patient with a Chiari II malformation (same patient as in Images 9-11). This image shows the thoracic spinal cord (arrowheads) and myelomeningocele (arrows). Note the tethering of the placode at the site of the dysraphic defect.

Degree of Confidence

MRI is reliable and the best diagnostic method for distinguishing the differential diagnosis between Chiari II malformation and other craniocervical pathologies (see Differentials and Other Problems to Be Considered).

Although the soft tissue can be evaluated easily by using MRI, bone anomalies that can be depicted easily on plain radiographs and CT scans may be missed on MRI.

Ultrasonography

Findings

Original reports discuss the limited use of US in patients with hydrocephalus and congenital cerebral anomalies, but current applications greatly expand the role of US in the evaluation of the developing central nervous system.^{[8,9 ]}Many of the typical findings described by using cranial CT scans and MRIs also can be demonstrated on the cranial sonograms.

Commercially available equipment is used with a 3- to 7.5-MHz transducer, depending on the size of the patient's head. In newborns, 5- to 7.5-MHz transducers are used, and 3- to 5-MHz transducers are used in older infants. Scans obtained with the transfontanel approach or with the transducer placed directly over the cervical region can demonstrate the downward displacement of the vermis and the medullary kink.

US is suited especially for evaluating the ventricular system in patients with a Chiari II malformation. The lateral ventricles often take on a characteristic pointed appearance that is best demonstrated in the coronal projection. In addition, the lateral ventricles are frequently asymmetric, and they may appear colpocephalic, such that the cerebral mantle appears thinnest over the occipital horns (see Image 4). The choroid plexus of the lateral ventricle can be demonstrated best by using sagittal US sections; it is unusually prominent and has been described as having a drumstick configuration or dangling choroid.

The characteristic prominence of the massa intermedia and beaked tectum that are often associated with the Chiari II malformation are also visible (see Image 4). The enlarged massa intermedia may appear to fill the third ventricle. A prominent anterior commissure, herniation of the third ventricle into the suprasellar cistern, and an enlarged suprapineal recess are often seen on midline sagittal sections. Dysgenesis of the hindbrain with downward displacement and elongation of the fourth ventricle, medulla, and the cerebellum results in a relatively small-appearing posterior fossa (see Image 4).

In the neonatal period, spinal US can be performed by using areas of dysraphism as an acoustic window. In this way, spinal abnormalities in patients with Chiari II malformation can be detected (see Image 11).

US is routinely used during the antenatal period as a screening method. With careful examination, cranial and spinal pathologic changes related to Chiari II malformations in the fetus can be seen. Reportedly, a lacunar skull can be detected by using US in the fetus.

Intraoperative US is used for surgical purposes, especially lateral ventricular shunt tube placement in infants, and for aiding in the intraoperative placement of catheters into cavities in the syrinx.

Degree of Confidence

US is heavily operator dependent. Findings associated with the Chiari II malformation, such as pachygyria, polymicrogyria, heterotopias, and dural abnormalities, may be missed with the use of US.

Angiography

Findings

Digital subtraction angiography (DSA) can be helpful in excluding abnormal cases of Chiari II malformation that involve the vertebral artery before surgery. This procedure can be performed noninvasively by using MR angiography or CT angiography. MR angiography can be preferred, especially in infants and children, because of the associated radiation exposure with CT angiography.

Intervention

Knowledge of the potential life-threatening symptoms of Chiari II malformation has resulted in the advocacy for early surgical intervention, especially in infants. Conservative treatment may lead to irreversible changes. Early recognition of the Chiari malformation is important because it is the leading cause of death in patients treated for myelomeningocele.

Neonates do more poorly with surgery. In one study, 23% died and 16% had a poor outcome. The reason may have been a delay in surgery with resultant brainstem infarction and hemorrhage. However, if surgery is needed, earlier intervention may result in a better prognosis.^{[23,24 ]}

Medicolegal Pitfalls

• Failure to diagnose a Chiari II malformation and its associated anomalies in a neonate or a child can cause serious brain and spinal complications.
• Failure to identify early cranial and spinal cord lesions may result in delayed intervention and permanent neurologic deficit.
• Failure to make the correct diagnosis (eg, cerebellar tumor instead of Chiari II malformation) can cause morbidity and mortality resulting from surgery.

Special Concerns

• US can be performed in high-risk pregnancies, and with a careful US examination, the Chiari II malformation can be diagnosed antenatally.
  • US is routinely used during the antenatal period as a screening method. With careful examination, cranial and spinal pathologic changes related to Chiari II malformations in the fetus can be seen. Reportedly, a lacunar skull can be detected by using US in the fetus.
  • To confirm the diagnosis and gain detailed information, MRI can be used antenatally.
• MRI has been used as a diagnostic tool during the antenatal period, allowing the intrauterine diagnosis, and even treatment, of spina bifida.

Multimedia

Media file 1: Sagittal T1-weighted magnetic resonance image of posterior fossa abnormalities in Chiari II malformation: (1) colpocephaly; (2) beaked tectum; (3) cascade of an inferiorly displaced vermis behind the medulla; (4) elongated, tubelike fourth ventricle; (5) low-lying torcular herophili; (6) cerebellar hemispheres wrapping around the brainstem anteriorly; (7) concave clivus; (8) medullary spur; and (9) medullary kink.

Media file 2: Axial computed tomography scan in a patient with a Chiari II malformation. This image shows a gaping, somewhat heart-shaped tentorial incisura (large arrowheads) that appears to be completely plugged with the upwardly herniating cerebellum. The cerebellar hemispheres extend anteromedially (small arrowheads) and almost completely engulf the brainstem. The petrous ridges are concave (arrows).

Media file 3: Sagittal midline T1-weighted magnetic resonance image in a patient (same patients in Images 3-4 in Multimedia) with a Chiari II malformation. This image shows a large massa intermedia (long arrow) and a beaked tectum (short arrow). Other posterior fossa abnormalities, similar to those shown in Image 1 in Multimedia, are also seen in this patient.

Media file 4: Sagittal midline sonogram (same patient in Images 3-4 in Multimedia) shows a large massa intermedia (long arrow) and a beaked tectum (short arrow). The image also shows obliteration of the cisterna magna and the fourth ventricle, as well as compression of the pons and brainstem.

Media file 5: Coronal T1-weighted magnetic resonance image in a patient with a Chiari II malformation. This image shows low-lying transverse sinuses (arrows), hydrocephalus, and a small posterior fossa. A hypoplastic tentorium cerebelli with gaping incisura (arrowhead) is present with a towering cerebellum (small arrows).

Media file 6: Axial T1-weighted magnetic resonance image in a patient with a Chiari malformation. This image shows a beaked tectum (arrows) and colpocephaly (arrowheads).

Media file 7: Axial T2-weighted magnetic resonance image in a patient with a Chiari II malformation. This image shows a hypoplastic fenestrated falx cerebri with striking interdigitation of the gyri (arrows).

Media file 8: Deep scalloping between the bony septations that characterize the lacunar skull (lückenschädel) (arrows) are best appreciated on an axial computed tomography section, as in this patient with a Chiari II malformation.

Media file 9: A thoracic-level myelomeningocele (short arrow) is seen in a patient with a Chiari II malformation (long arrow) (same patient in Images 9-11 in Multimedia).

Media file 10: Sagittal T2-weighted magnetic resonance in a patient with a Chiari II malformation (same patient in Images 9-11 in Multimedia). This image shows a thoracic-level myelomeningocele (arrows). The spinal cord, in addition to the thoracic placode, also extends distally and is further tethered at the sacral level.

Media file 11: Sagittal sonogram in a patient with a Chiari II malformation (same patient as in Images 9-11). This image shows the thoracic spinal cord (arrowheads) and myelomeningocele (arrows). Note the tethering of the placode at the site of the dysraphic defect.

Media file 12: Inferior operative-like view on 3-dimensional computed tomography scan. This image shows malrotation of the posterior arches of C1 (long arrow) and C2 (short arrow). Courtesy of Duffau et al.

Media file 13: Postoperative 3-dimensional computed tomography scan. This image shows osseous decompression with a large opening of the foramen magnum and resection of the posterior arch of the atlas (arrows). Courtesy of Duffau et al.

Media file 14: Antenatal magnetic resonance image shows a Chiari II malformation in a fetus. Courtesy of Umit Aksoy, MD, Uludag University, Bursa, Turkey.

Media file 15: T1-weighted axial magnetic resonance image. This image demonstrates heart-shaped incisura and a petrous ridge in a patient with a Chiari II malformation.

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Keywords

Chiari II malformation, neural tube defects, Arnold-Chiari malformation, meningomyelocele, hydrocephalus, Cruveilhier-Cleland-Chiari malformation, Chiari I malformation, Chiari III malformation, Chiari IV malformation, hindbrain anomaly, posterior fossa, myelomeningocele

Contributor Information and Disclosures

Author

Lutfi Incesu, MD, Professor, Department of Radiology, Ondokuz Mayis University School of Medicine; Chief, Neuroradiology and MR Unit, Department of Radiology, Ondokuz Mayis University Hospital, Turkey
Lutfi Incesu, MD is a member of the following medical societies: American Society of Neuroradiology and Radiological Society of North America
Disclosure: Nothing to disclose.

Coauthor(s)

Anil Khosla, MBBS, Assistant Professor, Department of Radiology, Section of Neuroradiology, Mallinckrodt Institute of Radiology, Washington University School of Medicine, Veterans Affairs Medical Center of St Louis
Anil Khosla, MBBS is a member of the following medical societies: American College of Radiology, American Roentgen Ray Society, American Society of Neuroradiology, North American Spine Society, and Radiological Society of North America
Disclosure: Nothing to disclose.

Michael R Aiello, MD, Radiologist, St Elizabeth Medical Center, Utica, NY
Michael R Aiello, MD is a member of the following medical societies: American College of Radiology, American Institute of Ultrasound in Medicine, American Medical Association, Radiological Society of North America, Society of Breast Imaging, and Society of Cardiovascular and Interventional Radiology
Disclosure: Nothing to disclose.

Medical Editor

Charles M Glasier, MD, Professor, Departments of Radiology and Pediatrics, University of Arkansas for Medical Sciences; Chief, Magnetic Resonance Imaging, Vice-Chief, Pediatric Radiology, Arkansas Children's Hospital
Charles M Glasier, MD is a member of the following medical societies: American College of Radiology, American Society of Neuroradiology, Radiological Society of North America, and Society for Pediatric Radiology
Disclosure: Nothing to disclose.

Pharmacy Editor

Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand
Disclosure: Nothing to disclose.

Managing Editor

Marta Hernanz-Schulman, MD, FAAP, Professor, Radiology, Radiological Sciences, and Pediatrics, Director, Department of Pediatric Radiology, Radiologist-in-Chief, Director, Department of Diagnostic Imaging, Vanderbilt University Medical Center, Vanderbilt Children's Hospital
Marta Hernanz-Schulman, MD, FAAP is a member of the following medical societies: American Institute of Ultrasound in Medicine and American Roentgen Ray Society
Disclosure: Nothing to disclose.

CME Editor

Robert M Krasny, MD, Resolution Imaging Medical Corporation
Robert M Krasny, MD is a member of the following medical societies: American Roentgen Ray Society and Radiological Society of North America
Disclosure: Nothing to disclose.

Chief Editor

James G Smirniotopoulos, MD, Professor of Radiology, Neurology, and Biomedical Informatics, Chairman, Department of Radiology and Radiological Sciences, Uniformed Services University of the Health Sciences
James G Smirniotopoulos, MD is a member of the following medical societies: American College of Radiology, American Roentgen Ray Society, American Society of Head and Neck Radiology, American Society of Neuroradiology, American Society of Pediatric Neuroradiology, Association of University Radiologists, and Radiological Society of North America
Disclosure: Nothing to disclose.

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