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.
Recent studies
Herweh et al compared 6 patients who had Chiari II malformation with 6 healthy patients to identify damage to the corpus callosum (CC) and anterior commissure (AC). They used T1-weighted 3D imaging and diffusion tensor imaging to determine fractional anisotropy (FA) and the cross-sectional area of the CC and AC. Four of the Chiari II patients had hydrocephalus, 2 had callosal dysplasia, and 4 had hypoplastic CC. There was a strong correlation between both size and FA of the CC: callosal FA was significantly reduced, as was the CC area. AC thickness, however, was significantly increased and associated with a higher FA. According to the authors, the study findings pointed to white-matter damage in addition to that caused by hydrocephalus.7
Callen et al retrospectively reviewed fetal intracranial findings of 89 fetuses, between 1999 and 2007, with sonographically detected myelomeningocele to identify abnormalities in tectal morphologic characteristics ("beaking" of the tectum). Of the 89 cases, 59 (66%) had an abnormal tectal shape. Tectal abnormalities were seen equally well in fetuses before and after 24 weeks and were seen more frequently as the severity of posterior fossa findings increased. The authors suggested that such findings may therefore be useful as a supratentorial indicator of both the presence and, potentially, the severity of Chiari II malformations.8
According to Wong et al, interhemispheric cysts (protrusion of the posterior superior third ventricle) are a common supratentorial feature of the Chiari II malformation, but their presence appears to be unrelated to other features of the Chiari II malformation. In their study, 43% of fetuses diagnosed with myelomeningocele had an interhemispheric cyst. Among fetuses without a central nervous system anomaly, no interhemispheric cysts were detected, but a cyst was detected in 1 of 10 fetuses with ventriculomegaly. According to the authors, although interhemispheric cysts are seen in other abnormal fetuses, their striking prevalence in the Chiari II malformation should lead to a thorough examination for myelomeningocele.9
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 Stritch12 and Peach13 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
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
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 encephalocele22
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References
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Further Reading
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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
