Chiari malformations
Author
Chaouki Khoury, MD, MS

Section Editor
Marc C Patterson, MD, FRACP

Deputy Editor
John F Dashe, MD, PhD

Last literature review version 19.1: janvier 2011 | This topic last updated: octobre 1, 2010

INTRODUCTION — Chiari malformations are a heterogeneous group of disorders that are defined by anatomic anomalies of the cerebellum, brainstem, and craniocervical junction, with downward displacement of the cerebellum, either alone or together with the lower medulla, into the spinal canal.

This topic will review anatomic and clinical aspects of the various types of Chiari malformations.

DEFINITION — Chiari malformations were first described by John Cleland in 1883. They were later classified by Hans Chiari in 1891, into four groups.

• Chiari I malformation (CM-I) is characterized by abnormally shaped cerebellar tonsils that are displaced below the level of the foramen magnum (picture 1)
• Chiari II malformation (CM-II), also known as Arnold-Chiari malformation, is characterized by downward displacement of the cerebellar vermis and tonsils, a brainstem malformation with beaked midbrain on neuroimaging, and a spinal myelomeningocele (picture 2 and picture 3 and figure 1)
• Chiari III malformation (CM-III) is rare and combines a small posterior fossa with a high cervical or occipital encephalocele, usually with displacement of cerebellar structures into the encephalocele, and often with inferior displacement of the brainstem into the spinal canal (picture 4)
• Chiari IV malformation (CM-IV) is now considered to be an obsolete term that describes cerebellar hypoplasia unrelated to the other Chiari malformations

In addition to the classic types of Chiari malformations, further subtypes (not widely used) have since been defined. These include the Chiari 0 malformation, characterized by anatomic aberration of the brainstem (posterior pontine tilt, downward displacement of the medulla, low lying obex) but with normally placed cerebellar tonsils, and the Chiari 1.5 malformation, which is a CM-II like malformation without spina bifida. Both of these subtypes show crowding at the foramen magnum.

ANATOMIC ASPECTS — As noted above, Chiari malformations are congenital conditions that are defined by anatomic anomalies of the craniocervical junction with downward displacement of the cerebellar structures. Chiari malformations are often associated with spinal cord cavitations (ie, syringomyelia). In most cases of Chiari malformations, the posterior fossa is small, and neural elements are crowded and impacted at the foramen magnum.

With any of the Chiari types, other bony abnormalities may also be seen, including the following:

• Atlas assimilation
• Atlantoaxial dislocation
• Klippel-Feil anomaly (congenital anomaly consisting of failure of segmentation of any two of the seven cervical vertebrae)
• Platybasia
• Basilar invagination (protrusion of the odontoid process through the foramen magnum into the intracranial cavity)
• Luckenschadel, also known as lacunar skull (an ossification disorder in which the fetal skull appears fenestrated)

Chiari type I — Chiari type I malformation (CM-I) is characterized by cerebellar tonsils that are abnormally shaped and downwardly displaced below the level of the foramen magnum (picture 1). The normal cerebellar tonsils may lie up to 3 mm below the foramen magnum in adults. In general, tonsils lying 5 mm or more below the foramen magnum on neuroimaging are considered to be consistent with a Chiari malformation, though there is no direct correlation between how low the tonsils are lying and clinical severity. With infants, however, tonsils as low as 6 mm below the foramen magnum can still be normal.

The frequency of spinal cavitations (syringomyelia, hydromyelia or syringohydromyelia) with CM-I varies in the literature between 40 and 75 percent. Some patients even have a holocord syrinx that extends the whole length of the spinal cord.

Atlas assimilation, often associated with the Klippel-Feil anomaly, is common in CM-I. The Klippel-Feil anomaly may not be evident in infants and young children, as the fusion site might still be cartilaginous at an early age.

The prevalence of hydrocephalus associated with CM-I is approximately 10 percent.

Chiari type II — The Chiari II malformation (CM-II) is characterized by downward displacement of inferior cerebellar vermis (involving the nodulus, pyramis and uvula), and cerebellar tonsils and medulla through the foramen magnum into the upper cervical canal, in association with a myelomeningocele at the lumbosacral or occasionally a higher level of the spinal cord (picture 2). The malformation obstructs the outflow of cerebrospinal fluid through the posterior fossa, causing hydrocephalus. Almost all patients with a myelomeningocele have CM-II, and most have associated hydrocephalus. A reduced volume of the posterior fossa with an enlarged foramen magnum and low torcula with ventral displacement of the tentorium cerebelli are constant features.

Additional findings that may be associated with CM-II include the following (figure 1 and picture 3):

• Inferior displacement of the fourth ventricle into the upper cervical canal
• Elongation and thinning of the lower pons and the medulla
• Beaking of the quadrigeminal plate
• Kinking of medullary spinal cord junction in the cervical canal
• Stenosis or atresia of the cerebral aqueduct
• Upward displacement of the upper cerebellum into the middle fossa
• Cerebellar dysplasia
• Colpocephaly (abnormal enlargement) of the posterior lateral ventricles

Infrequent supratentorial anomalies found with CM-I and CM-II include dysgenesis or absence of the corpus callosum, agenesis of the septum pellucidum, polymicrogyria of the cerebral hemispheres, and heterotopia of cerebral gray matter. In addition, CM-II is rarely associated with rhombencephalosynapsis, consisting of agenesis of the cerebellar vermis with fusion of the cerebellar hemispheres.

Chiari type III — The rare Chiari III malformation (CM-III) combines inferior displacement of the medulla with a high cervical or occipital encephalocele that typically contains much of the cerebellum (picture 4) and may also involve supratentorial tissue, including the occipital cortex and part of the occipital horn of the lateral ventricle. In addition, CM-III may be associated with any of the features of CM-I and CM-II.

ETIOLOGY AND PATHOPHYSIOLOGY — The pathogenesis of congenital Chiari malformations remains the subject of debate. Multiple theories have been proposed, though none explains all the features.

• The molecular genetic theory postulates that Chiari malformations result from primary defects in the genetic programming of hindbrain segmentation and of growth of associated bone and cranial structures. Another growth abnormality theory proposes that collision between caudally-directed cranial growth and rostrally-directed cervical growth is the underlying abnormality in CM-I. However, most Chiari malformations are sporadic and not inherited. Thus, the cause might be either a spontaneous mutation or deletion, or a mutation induced by an exogenous teratogen.
• The crowding theory postulates that restricted growth of the posterior fossa causes compression of neural tissue, which is then squeezed through the foramen magnum like toothpaste through a tube. In support of this theory, the posterior fossa is abnormally small and the torcula is displaced downward in patients with Chiari malformations.
• The hydrodynamic pulsion theory suggests that early progressive fetal hydrocephalus pushes down on the brainstem and cerebellum.
• The oligo-cerebrospinal fluid theory proposes that defective closure of the neural tube in early fetal development results in leakage of cerebrospinal fluid, and thus insufficient cerebrospinal volume to fully distend the embryonic ventricular system, which leads to a small posterior fossa and cerebral disorganization.
• One early theory suggested that the downward displacement of the cerebellar tissue was due to traction by a tethered cord. However, several studies have demonstrated that the caudal traction on a tethered cord is only transmitted rostrally as far as the caudal-most pair of dentate ligaments, thus disputing the notion that traction transmitted all the way to the brainstem and cerebellum. Furthermore, not all Chiari malformations are associated with tethering.

Chiari type I malformation can be due to either neuroectodermal or mesodermal anomalies. Isolated CM-I is thought to be of mesodermal origin. In contrast, CM-I that is associated with syndromic or nonsyndromic craniosynostosis, or associated with other neurological disorders such as mental retardation and epilepsy, is thought to be of neuroectodermal origin. Occasionally, CM-I can be associated with occult spinal dysraphism (ie, spinal bifida occulta). The remaining types of Chiari malformations are due to neuroectodermal anomalies.

The pathogenesis of spinal cord cavitations (syringomyelia) associated with Chiari malformations has been the subject of debate. Initially it was thought to be due to cerebrospinal fluid being forced into the central canal because of impaired subarachnoid circulation at the level of the foramen magnum. However, cine phase contrast MRI studies have shown that the syrinx of Chiari malformations is noncommunicating. Therefore, it is more likely that the syrinx results from craniospinal pressure dissociation due to the blockage of cerebrospinal fluid flow in the subarachnoid space at the level of the foramen magnum. This leads to pressure backup into the venous system, with initial engorgement of the Virchow-Robin spaces. The excess fluid then dissipates into the substance of the spinal cord leading to spinal cord edema. As fluid accumulates beyond the resorptive power of the parenchyma, it dissipates into the central canal and dilates it, leading to syrinx formation.

EPIDEMIOLOGY — The true frequency of CM-I is unknown. In the MRI era (that is, since about 1985), CM-I is increasingly detected, with a prevalence in some studies of 0.1 to 0.5 percent.

Although once believed to be a disease of adolescence and adulthood, CM-I is now recognized in younger children. Prior to the advent of neuroimaging with MRI, CM-I was only diagnosed when patients presented with symptoms that warranted investigation. Therefore, in most early series, there were no children younger than 12 years, and it was mistakenly assumed that CM-I occurred only in adolescents and adults. However, with the widespread use of MRI, asymptomatic or minimally symptomatic patients are being diagnosed at earlier ages. Thus, CM-I is the most common type of Chiari malformation. The most frequent type of Chiari malformation presenting in childhood is CM-II.

Associated conditions — In retrospective studies, Chiari malformations have been associated with Robin sequence, neurofibromatosis type 1, and Noonan syndrome. As an example, in one report of 198 patients with neurofibromatosis type 1 who had neuroimaging, CM-I was found in approximately 9 percent. In addition, among 130 patients who had surgery for CM-I, neurofibromatosis type 1 was diagnosed in 5 percent. (See "Neurofibromatosis type 1 (von Recklinghausen's disease)".)

CLINICAL MANIFESTATIONS — There is a wide spectrum of clinical symptoms associated with Chiari malformations. The major categories of symptoms are those related to obstructive hydrocephalus, abnormal eye movements, cerebellar deficits, and spinal myelomeningocele. Patients with CM-I are spared the latter, as they do not have neural tube defects such as a myelomeningocele.

Chiari type I — The true natural history of CM-I has not been established. In most cases, CM-I does not become symptomatic until adolescence or adulthood. In addition, symptom onset is often insidious. In one study of 43 patients with CM-I, the mean age at presentation was approximately 18 years.

The manifestations of CM-I derive mainly from the following categories:

• Elevated intracranial pressure
• Cranial neuropathies
• Brainstem compression
• Myelopathy
• Cerebellar dysfunction
• Pain (mainly neck pain and occipital headache)
• Syringomyelia (picture 5), often accompanied by scoliosis, and presyrinx

In addition, there are asymptomatic cases where the radiologic diagnosis of CM-I is discovered incidentally.

Cranial neuropathies or brainstem compression can present with hoarseness, vocal cord paralysis, dysarthria, palatal weakness, pharyngeal achalasia, tongue atrophy, recurrent aspiration, and nystagmus (especially down-beating). Less common symptoms and signs include oscillopsia, sensorineural hearing loss, sinus bradycardia, syncope and hiccups. Other manifestations of brainstem compression include central sleep apnea and long-tract signs such as weakness, spasticity, hyperreflexia and Babinski responses. The latter can result from either brainstem or spinal cord compression. Cerebellar symptoms include nystagmus, scanning speech and ataxia, with truncal ataxia being more common than appendicular ataxia.

Scoliosis is usually due to an asymmetric spinal syringomyelia leading to differential growth of the hemicords and the vertebral column. Since the advent of MRI, syringomyelia is often diagnosed prior to reaching the stage of the classic cape-like suspended sensory loss. Thus, scoliosis has become the most common presenting symptom of a spinal syrinx.

In children with CM-I, an associated syrinx occurs in 30 percent. Of patients with neurologic deficits due to a syrinx, the earliest sign is loss of the superficial abdominal reflexes. Other signs and symptoms include gait disturbance, radicular pain, dysesthesia, upper motor neuron signs in the legs, and lower motor neuron signs maximally in the arms in those with a cervical syrinx, the most common location associated with CM-I. Of note, patients may also have signs and symptoms of brainstem dysfunction if the syrinx extends into the medulla (syringobulbia).

Presyrinx is a potentially reversible condition characterized by spinal cord edema due to obstruction of cerebrospinal fluid flow. It occurs most often in the cervical region and is detected by MRI, appearing similar to a true syrinx on T2-weighted images but lacking discrete cavitation on T1-weighted sequences (picture 6).

Pain or headache due to meningeal irritation is the most common presentation in patients with CM-I. The pain is usually either occipital or nuchal in location. The pain is typically paroxysmal, but it may be dull and persistent. Both the pain and a feeling of dizziness are exacerbated by physical activity or by Valsalva maneuvers such as coughing, laughing, or sneezing. Thus, patients with CM-I may present with cough headache, and CM-I should be considered as a potential etiology in secondary cough headache. It is postulated that Valsalva maneuvers leads to exacerbation of the pain by causing impaction of the cerebellar tonsils at the foramen magnum.

Although CM-I typically is not symptomatic in young children, one retrospective series identified 39 patients younger than six years who had early surgical treatment. Children up to age two most often presented with oropharyngeal dysfunction, while those ages three to five typically presented with syringomyelia, scoliosis, or headache.

Chiari type II — Because it is nearly always associated with a lumbosacral or thoracic myelomeningocele, CM-II is usually detected prenatally or at birth (picture 2). Manifestations in infancy may include dysphagia, arm weakness, stridor, apneic spells, and aspiration. In late infancy and childhood, progressive hydrocephalus is a common problem in CM-II. In addition, CM-II may be associated with one or more of the syndromes associated with CM-I, such as syringomyelia and scoliosis. (See 'Chiari type I' above.)

Despite the extensive malformations, some patients with CM-II have normal intelligence and function well independently.

Chiari type III — This rare malformation is associated with a high cervical or occipital encephalocele that typically contains much of the cerebellum and may also involve supratentorial tissue, including the occipital cortex and part of the occipital horn of the lateral ventricle. Data are limited regarding the natural history and clinical course of CM-III. Patients with CM-III have a high mortality rate, often due to respiratory failure in infancy. Those who survive beyond the neonatal period often have severe neurologic impairments, such as mental retardation, epilepsy, hypotonia or spasticity, upper and lower motor neuron signs, and lower cranial nerve palsies.

DIAGNOSIS — The diagnosis of Chiari malformations is based upon neuroanatomy. There are no biomarkers in blood, cerebrospinal fluid, or cultured tissue to confirm the diagnosis. Thus, neuroimaging is of prime importance, and MRI is the best imaging modality for evaluation. Sagittal, coronal and axial views of the brain along with sagittal and axial images of the entire spinal cord (cervical, thoracic, and lumbar) using T1 and T2-weighted MRI sequences are useful for detecting cerebellar and brainstem displacement, associated craniocervical junction abnormalities, and hydrosyringomyelia. CT, especially thin-section multiplanar CT with reformatted images, retains importance in the evaluation of the associated bony abnormalities. (See 'Anatomic aspects' above.)

For patients who cannot have MRI, high-resolution CT scan with sagittal reconstructions can be used to make the diagnosis of Chiari malformation.

In some cases of fetal ventriculomegaly, a Chiari malformation can be diagnosed in utero using fetal ultrasound.

There is general agreement among experts that the radiologic diagnosis of CM-I in adolescents and adults is made by MRI when one or both cerebellar tonsils are displaced by ≥5 mm below the foramen magnum. Borderline displacement of the cerebellar tonsils (≥3 to <5 mm below the foramen magnum) is considered pathologic if it is associated with additional features of CM-I, such as other craniocervical junction anomalies or syringomyelia. (See 'Anatomic aspects' above.)

In a series that compared 200 normal subjects and 25 patients with CM-I, using a cutoff of 3 mm below the foramen magnum as the lowest normal position of the cerebellar tonsils predicted symptomatic CM-I with a sensitivity and specificity of 96 and 99.5 percent respectively. In the patients with CM-I, the mean position of the cerebellar tonsils was 13 mm below the foramen magnum (range 3 to 29 mm below). In the normal subjects, the mean position of the tonsils was 1 mm above the foramen magnum (range 8 mm above to 5 mm below). In infants, however, tonsils as low as 6 mm below the foramen magnum may be normal, since the cerebellar tonsils have been shown to ascend with age.

As noted earlier, Chiari malformations are typically associated with a small posterior fossa. On neuroimaging, this may result in obliteration of the subarachnoid spaces at the level of the foramen magnum.

The diagnosis of CM-II should be suspected in a fetus or newborn with clinical evidence of a spinal myelomeningocele. MRI can confirm the diagnosis of CM-II by demonstrating downward displacement of inferior cerebellar vermis and medulla through the foramen magnum into the upper cervical canal.

Similarly, the diagnosis of CM-III is made in a fetus or newborn with clinical evidence of a high cervical/occipital encephalocele and confirmatory MRI showing inferior displacement of the medulla and a high cervical or occipital encephalocele with descent of cerebellar structures into the malformation.

MANAGEMENT — The management of Chiari malformations depends upon the nature of the malformation and the degree of associated neurologic impairments. Particularly for CM-II and CM-III, surgical interventions may include closure of open neural tube defects shortly after birth, treatment for hydrocephalus (most often by use of a shunt), and decompression of tight posterior fossa structures. Medical issues involve management of neurogenic bowel and bladder, neonatal feeding difficulties, respiratory failure, and apnea.

The management of myelomeningocele is reviewed separately. (See "Overview of the management of myelomeningocele (spina bifida)" and "Urinary tract complications of myelomeningocele (spina bifida)" and "Orthopedic issues in myelomeningocele (spina bifida)".)

The management of hydrocephalus is discussed in detail elsewhere. (See "Hydrocephalus".)

Asymptomatic patients with an incidental diagnosis of CM-I who do not have syringomyelia can be managed conservatively with clinical and MRI surveillance (eg, six months after the first encounter, and yearly thereafter). However, not all experts agree with this approach, and a minority advocates prophylactic surgery to prevent development of syringomyelia and other associated complications.

Decompressive surgery is indicated for patients with CM-I who are clearly symptomatic with lower cranial nerve palsies, syringomyelia, myelopathy, cerebellar symptoms, severe neck pain or occipital headache. (See 'Surgery' below.)

For asymptomatic or oligosymptomatic patients with CM-I who are neurologically intact but have syringomyelia on MRI, management is controversial. As discussed below, there are occasional reports of spontaneous resolution of tonsillar displacement or syringomyelia within the context of CM-I. (See 'Prognosis' below.) Therefore, some have argued that a period of watchful observation is warranted in asymptomatic children.

Cine phase contrast MRI may be valuable in certain patients with CM-I to look for impairment of cerebrospinal fluid (CSF) flow across the foramen magnum. This information can be used to select patients for surgical decompression of the foramen magnum in order to establish normal CSF flow. Thus, some have recommended obtaining a cine phase contrast MRI in all patients with CM-I; those patients without CSF flow obstruction would undergo a watchful observation strategy, while patients with CSF flow obstruction on cine MRI, whether symptomatic or asymptomatic, would be treated with surgery.

We suggest obtaining a cine phase contrast MRI for asymptomatic or oligosymptomatic patients with CM-I who are neurologically intact but have syringomyelia on MRI. We refer such patients for surgery if the study shows complete CSF flow obstruction. Alternatively, we continue to follow them with clinical and neuroimaging surveillance if the study shows partial or no CSF flow obstruction, but refer patients for posterior fossa decompression at the first sign of clinical deterioration.

In small uncontrolled studies, obstruction of CSF flow correlated with clinical symptoms, and a postsurgical increase in CSF flow was associated with clinical improvement or stabilization. Cine phase contrast MRI may also be useful for postoperative follow-up of patients with 'failed' surgeries, such as those who experience delayed deterioration. Furthermore, a retrospective report of 130 patients who had decompressive surgery for CM-I found that normal CSF flow by cine phase contrast MRI before surgery was predictive of symptom recurrence after surgery (relative risk 4.8, 95% CI 1.9-12.5). This result suggests that even symptomatic patients with CM-I may not benefit from decompressive surgery if there is no evidence of CSF flow obstruction.

Polysomnography should be considered in patients who present with sleep apnea, to demonstrate whether their sleep apnea is central or peripheral. Central sleep apnea is more suggestive of brainstem dysfunction and would prompt referral for surgical decompression.

Surgery — The goals of surgery for Chiari malformations are to decompress the craniocervical junction and restore the normal flow of cerebrospinal fluid (CSF) in the region of the foramen magnum. The most common procedure is posterior decompression via suboccipital craniectomy with or without duraplasty. Other procedures included anterior decompression of the foramen magnum by odontoidectomy, and shunting.

Posterior foramen magnum decompression — The two main surgical approaches to posterior fossa decompression are decompression with dural opening and decompression without dural opening.

• Most surgeons perform a posterior fossa decompression with opening of the dural sac for optimal decompression. The procedure involves a limited suboccipital craniectomy, C1 laminectomy, duraplasty and arachnoid dissection. In an international survey conducted between October 2001 and March 2002, approximately three-quarters of the 76 neurosurgeons who responded favored opening the dural sac. Potential complications include pseudomeningocele formation, CSF leakage, acute postoperative hydrocephalus, and meningitis.
• Other surgeons favor a more conservative approach consisting of posterior fossa bony decompression without opening the dural sac. The major potential advantage of this method is avoidance of CSF-related complications such as CSF leak, pseudomeningocele, and aseptic meningitis.

The available data, though largely retrospective and uncontrolled, suggests that the more conservative surgery without duraplasty is associated with a reduced rate of CSF-related complications and an increased rate of the need for reoperation. A meta-analysis published in 2008 identified seven pediatric studies that compared posterior fossa decompression (PFD) surgery with and without duraplasty for children with CM-I. The following observations were reported:

• There was a higher rate of clinical improvement for PFD with duraplasty compared with PFD without duraplasty (79 versus 65 percent, relative risk [RR] 1.23, 95% CI 0.95-1.59) and a higher rate of postoperative reduction in syringomyelia size (87 versus 56 percent, RR 1.43, 95% CI 0.91-2.25). However, these differences were not statistically significant.
• The reoperation rate was significantly lower for PFD with duraplasty (2 versus 13 percent, RR 0.23, 95% CI 0.08-0.69)
• The rate of CSF-related complications was significantly higher for PFD with duraplasty (19 versus 2 percent, RR 7.64, 95% CI 2.53-23.1)

A later study not included in the meta-analysis reported similar results.

It is not clear if the findings of the meta-analysis and the subsequent report comparing decompression with and without duraplasty are generalizable, since the surgical techniques utilized among the included studies were not standardized, and outcome assessment was not blinded. In addition, the complication rates for individual centers and surgeons vary widely. As an example, a retrospective series of 40 patients (mean age 13.3 years, range 3 to 45 years) with CM-I who had decompression with duraplasty reported a low perioperative complication rate (3 percent), attributable to the development of a pseudomeningocele in one patient. There were no episodes of CSF leak, meningitis, or postoperative hydrocephalus.

Given the absence of high-quality comparative studies, the utility of duraplasty for CM-I is uncertain, and more rigorous studies are needed to resolve this issue.

Anterior foramen magnum decompression — Anterior decompression of the foramen magnum, typically via transoral odontoidectomy, is an alternative surgical approach to treating Chiari malformations. It is most often used for patients who fail posterior decompression. Some experts advocate this approach to reoperation when cine phase contrast MRI imaging shows anterior obstruction of CSF flow. Anterior decompression has also been used alone or in combination with posterior decompression for patients who have pronounced ventral brain stem compression associated with a Chiari malformation.

Shunting procedures — Syrinx shunt placement has been used mainly for patients with CM-I who fail posterior decompression due to progressive symptoms or syrinx enlargement. In addition, some surgeons employ shunting rather than posterior fossa decompression as primary treatment for syringomyelia associated with CM-I, while others routinely combine posterior fossa decompression with shunting for such cases. However, syrinx shunts are controversial, particularly as primary therapy for CM-I related syringomyelia, on the basis of lesser benefit than bony decompression reported in some studies, and the potential for acute deterioration due to shunt malfunction.

PROGNOSIS — The clinical course of CM-I is unpredictable. Some patients remain asymptomatic, and occasional patients have spontaneous resolution of tonsillar displacement or spinal cord syringomyelia. Others have a relentlessly worsening syrinx. Also, long-standing disease may lead to scarring with limitation of surgical benefit, especially if the syrinx has been present for more than three years.

As noted earlier, there are few data regarding the natural history of CM-I. Limited evidence suggests that most patients with minimal or no symptoms remain stable. In one report, a series of 22 children with CM-I who had mild or absent clinical manifestations were followed without surgery for a mean period of 5.9 years (range 3 to 19 years). At last follow-up, symptoms were absent or improved for 17 patients (77 percent), while clinical deterioration was noted for five (23 percent). The deterioration was mild in two patients. The remaining three patients required surgery. Syringomyelia was present at study entry in only one child and did not change over 19 years of follow-up. However, cervical syringomyelia developed de novo in three children during follow-up.

Surgical outcome for CM-I varies. There are no data from randomized controlled trials, but in largely retrospective series, postoperative improvement or stabilization has been reported for the majority of patients who have posterior fossa decompression with or without duraplasty.

• One retrospective series reported 157 patients (mean age 38 years, range 16 to 75 years) with Chiari-related syringomyelia who had posterior fossa decompression with dural opening. At a median follow-up of 88 months, clinical improvement or stabilization was noted in 63 and 31 percent, while deterioration or death occurred in 6 and 1 percent. Factors associated with a poor outcome were older age at surgery and the presence of a long-tract deficit. The extent of the syrinx and level of tonsillar descent before surgery were not associated with outcome. However, syrinx size on postoperative MRI was a predictor of poor outcome.
• Another series retrospectively analyzed 130 children (mean age 11 years, range 2 months to 20 years) with CM-I who had posterior fossa decompression (with duraplasty in 129). At a mean follow-up of 4.2 years, improvement in preoperative pathologies was noted in 83 percent.
• A third retrospective study included 96 children (ages 0.5 to 18 years) with CM-I who had posterior fossa decompression surgery. At a mean follow-up of 2.3 years, improvement was reported for approximately two-thirds of patients. Favorable outcome was significantly higher for children who were younger than 8 years of age at the time of surgery (odds ratio [OR] 3.0, 95% CI 1.2-7.5). Surgical technique (dural scoring versus duraplasty) had no effect on outcome.

In these series, reoperation for persistent or progressive symptoms was performed in 4 to 13 percent of patients. Reoperative procedures have included repeat posterior fossa decompression (or rarely odontoidectomy) after incomplete initial decompression, and ventriculoperitoneal shunt placement for symptom progression and persistent large syringomyelia despite complete initial decompression. (See 'Surgery' above.)

SUMMARY AND RECOMMENDATIONS

• There are three main types of Chiari malformations (see 'Definition' above and 'Anatomic aspects' above):

• Chiari I malformation (CM-I) is characterized by abnormally shaped cerebellar tonsils that are displaced below the level of the foramen magnum (picture 1)
• Chiari II malformation (CM-II), also known as Arnold-Chiari malformation, is characterized by downward displacement of the cerebellar vermis and tonsils, a brainstem malformation with beaked midbrain on neuroimaging, and a spinal myelomeningocele (picture 2 and picture 3 and figure 1)
• Chiari III malformation (CM-III) is rare and combines a small posterior fossa with a high cervical or occipital encephalocele, usually with displacement of cerebellar structures into the encephalocele, and often with inferior displacement of the brainstem into the spinal canal (picture 4)

• CM-I is the most frequent of the Chiari malformations. The true prevalence of CM-I is unknown, but available evidence suggests the prevalence is 0.1 to 0.5 percent. (See 'Epidemiology' above.)
• There is a wide spectrum of clinical symptoms associated with Chiari malformations. The major categories of symptoms are those related to obstructive hydrocephalus, abnormal eye movements, cerebellar deficits, and spinal myelomeningocele. Patients with CM-I are spared the latter, as they do not have neural tube defects such as a myelomeningocele. (See 'Clinical manifestations' above.)
• The diagnosis of Chiari malformations is based upon neuroanatomy. There are no diagnostic biomarkers. MRI is the best imaging modality for evaluation (see 'Diagnosis' above):

• The radiologic diagnosis of CM-I in adolescents and adults is made by MRI when one or both cerebellar tonsils are displaced by ≥5 mm below the foramen magnum.
• The diagnosis of CM-II is made in a fetus or newborn with clinical evidence of a spinal myelomeningocele and confirmatory MRI showing downward displacement of inferior cerebellar vermis and medulla through the foramen magnum into the upper cervical canal.
• The diagnosis of CM-III is made in a fetus or newborn with clinical evidence of a high cervical/occipital encephalocele and confirmatory MRI showing a high cervical or occipital encephalocele with descent of cerebellar structures into the malformation.

• The management of Chiari malformations depends upon the nature of the malformation and the degree of associated neurologic impairments (see 'Management' above):

• For CM-II and CM-III, surgical interventions may include closure of open neural tube defects shortly after birth, treatment for hydrocephalus (most often by use of a shunt), and decompression of tight posterior fossa structures. Medical issues involve management of neurogenic bowel and bladder, neonatal feeding difficulties, respiratory failure, and apnea.
• Asymptomatic patients with an incidental diagnosis of CM-I who do not have syringomyelia can be managed conservatively with clinical and MRI surveillance.
• For patients with CM-I who are clearly symptomatic with lower cranial nerve palsies, syringomyelia, myelopathy, cerebellar symptoms, severe neck pain or occipital headache related to the Chiari malformation, we recommend surgical decompression of the foramen magnum (Grade 1C) 

• For asymptomatic or oligosymptomatic patients with CM-I who are neurologically intact but have syringomyelia (picture 5) on MRI, management is controversial. We suggest obtaining a cine phase contrast MRI for such patients; if the study shows complete cerebrospinal fluid (CSF) flow obstruction, we refer the patient to surgery. If there is only partial or no CSF flow obstruction, we follow the patient with clinical and neuroimaging surveillance, and refer for posterior fossa decompression at the first sign of clinical deterioration.
• The goals of surgery for Chiari malformations are to decompress the craniocervical junction and restore the normal flow of CSF in the region of the foramen magnum. The most common procedure is posterior decompression via suboccipital craniectomy with or without duraplasty. Other procedures included anterior decompression of the foramen magnum by odontoidectomy, and shunting. (See 'Surgery' above.)
• The clinical course of CM-I is unpredictable. Limited evidence suggests that most patients with minimal or no symptoms remain stable. In largely retrospective series, postoperative improvement or stabilization has been reported for the majority of patients who have posterior fossa decompression. (See 'Prognosis' above.)

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