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Chiari malformations

Chiari malformations
Literature review current through: Jan 2024.
This topic last updated: Aug 10, 2023.

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 [1].

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

ANATOMIC ASPECTS

Description — 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.

Classification — Chiari malformations were first described by John Cleland in 1883 [2,3]. They were later classified by Hans Chiari in 1891, into four groups [1,4].

Chiari type I malformation (CM-I) is characterized by abnormally shaped cerebellar tonsils that are displaced below the level of the foramen magnum (image 1).

Chiari type 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 (image 2 and image 3 and figure 1).

Chiari type 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 (image 4).

Chiari type IV malformation (CM-IV) is now considered to be an obsolete term that describes cerebellar hypoplasia unrelated to the other Chiari malformations.

Other subtypes (not widely used) have since been defined [4]. 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 [5], and the Chiari 1.5 malformation, which is a CM-II like malformation without spina bifida [6]. Both of these subtypes show crowding at the foramen magnum.

Chiari I anatomy — CM-I is characterized by cerebellar tonsils that are abnormally shaped and downwardly displaced below the level of the foramen magnum (image 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 syringomyelia with CM-I varies in the literature; in one series of 415 children with symptomatic CM-I, syringomyelia was present in 35 percent [7]. Some patients have a holocord syrinx that extends the whole length of the spinal cord [8].

Atlas assimilation, often associated with the Klippel-Feil anomaly, is common in CM-I [9]. 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.

Chiari II anatomy — The 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 (image 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 [1].

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

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 [1,10-12]. In addition, CM-II is rarely associated with rhombencephalosynapsis, consisting of agenesis of the cerebellar vermis with fusion of the cerebellar hemispheres [13].

Chiari III anatomy — The rare CM-III combines inferior displacement of the medulla with a high cervical or occipital encephalocele that typically contains much of the cerebellum (image 4) and may also involve supratentorial tissue, including the occipital cortex and part of the occipital horn of the lateral ventricle [1,14]. In addition, CM-III may be associated with any of the features of CM-I and CM-II.

Spine and skull abnormalities — With any of the Chiari types, bone abnormalities of the spine or skull may also be seen, including the following [1]:

Atlas assimilation, also called atlanto-occipital assimilation or occipitalization of the atlas (fusion of the atlas [C1] to the occiput)

Atlantoaxial dislocation (loss of stability between the atlas [C1] and axis [C2])

Klippel-Feil anomaly (congenital anomaly consisting of failure of segmentation of any two of the seven cervical vertebrae)

Platybasia (flattening of the skull base)

Basilar invagination, also called basilar impression (protrusion of the odontoid process [dens] of the axis [C2] through the foramen magnum into the intracranial cavity)

Lückenschädel, also known as lacunar skull (an ossification disorder in which the fetal skull appears fenestrated)

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 [1].

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 [1,15]. Another growth abnormality theory proposes that collision between caudally directed cranial growth and rostrally directed cervical growth is the underlying abnormality in Chiari type I malformation (CM-I) [16,17]. 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 [10,18]. In support of this theory, the posterior fossa is abnormally small, and the torcula is displaced downward in patients with Chiari malformations [1,19].

The hydrodynamic pulsion theory suggests that early progressive fetal hydrocephalus pushes down on the brainstem and cerebellum [20-23].

The oligo-cerebrospinal fluid theory proposes that defective closure of the neural tube in early fetal development results in leakage of cerebrospinal fluid (CSF) and thus insufficient cerebrospinal volume to fully distend the embryonic ventricular system, which leads to a small posterior fossa and cerebral disorganization [24,25].

One early theory suggested that the downward displacement of the cerebellar tissue was due to traction by a tethered cord [20-22,26]. 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 is transmitted all the way to the brainstem and cerebellum [23,27,28]. Furthermore, not all Chiari malformations are associated with tethering [1].

CM-I can be due to either neuroectodermal or mesodermal anomalies. Isolated CM-I is thought to be of mesodermal origin. By contrast, CM-I that is associated with syndromic or nonsyndromic craniosynostosis, or associated with other neurological disorders such as intellectual disability and epilepsy, is thought to be of neuroectodermal origin [4]. Occasionally, CM-I can be associated with occult spinal dysraphism (ie, spinal bifida occulta) [8]. The remaining types of Chiari malformations are due to neuroectodermal anomalies [4].

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 CSF being forced into the central canal because of impaired subarachnoid circulation at the level of the foramen magnum. However, cine phase contrast magnetic resonance imaging 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 CSF 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 [4].

EPIDEMIOLOGY

Prevalence — The true frequency of Chiari type I malformation (CM-I) is unknown. In the magnetic resonance imaging (MRI) era (that is, since approximately 1985), CM-I is increasingly detected, with a prevalence of 1 to 3.6 percent in MRI studies [29-31].

Although once believed to be a disease of adolescence and adulthood, CM-I is now recognized in younger children [4]. 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 [32]. 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 Chiari type II malformation [1].

Associated conditions — In retrospective studies, Chiari malformations have been associated with Robin sequence [33,34], neurofibromatosis type 1 [35], and Noonan syndrome [36,37]. As an example, in one report of 198 patients with neurofibromatosis type 1 who had neuroimaging, CM-I was found in approximately 9 percent [35]. In addition, among 130 patients who had surgery for CM-I, neurofibromatosis type 1 was diagnosed in 5 percent [35]. (See "Neurofibromatosis type 1 (NF1): Pathogenesis, clinical features, and diagnosis".)

CLINICAL MANIFESTATIONS

Chiari I clinical features — The true natural history of Chiari type I malformation (CM-I) has not been established [38]. In most cases, CM-I does not become symptomatic until adolescence or adulthood [1,19,39]. 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 [40].

Although CM-I is typically asymptomatic in young children, one retrospective series identified 39 patients younger than six years who had early surgical treatment [41]. Children up to age two most often presented with oropharyngeal dysfunction, while those ages three to five typically presented with syringomyelia, scoliosis, or headache.

Pain or headache is the most common presentation in older children and adults with CM-I [42].

The manifestations of CM-I include the following categories [1,4,43-45]:

Asymptomatic – Asymptomatic cases are those where CM-I is discovered on magnetic resonance imaging (MRI), often incidentally, in patients without findings referrable to Chiari malformation by history and examination [30,31,42,45-47]. In one report of 218 children with incidentally discovered CM-I, the initial MRI was obtained for various indications including seizures (15 percent), nonspecific headache (15 percent), trauma (10 percent), or developmental delay (8 percent) [46].

Cerebellar dysfunction – Cerebellar symptoms include nystagmus, scanning speech and ataxia, with truncal ataxia being more common than appendicular ataxia [32,43].

Cranial neuropathies/brainstem compression – Cranial neuropathies or brainstem compression can present with hoarseness, vocal cord paralysis, dysarthria, palatal weakness, pharyngeal achalasia, tongue atrophy, recurrent aspiration, nystagmus (especially down-beating), or sleep-related breathing disorders such as central and obstructive sleep apnea [1,4,43,44,48,49]. Less common symptoms and signs include oscillopsia, sensorineural hearing loss, sinus bradycardia, syncope, and hiccups. Other manifestations of brainstem compression include long-tract signs such as weakness, spasticity, hyperreflexia, and Babinski responses. The latter can result from either brainstem or spinal cord compression.

Headache – Headache or pain due to meningeal irritation is the most common presentation in older children and adults with CM-I [42]. The pain is usually either occipital or nuchal in location. The pain is typically paroxysmal, but it may be dull and persistent [32]. Both the pain and a feeling of dizziness are exacerbated by physical activity or by Valsalva maneuvers such as coughing, laughing, or sneezing [4,32,50].

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 [32]. As defined by the International Classification of Headache Disorders, 3rd edition (ICHD-3), headache attributed to CM-I is precipitated by cough or other Valsalva maneuver, is occipital or suboccipital in location, and lasts less than five minutes [51].

Hydrocephalus – Obstructive hydrocephalus is sometimes associated with CM-I; the prevalence of hydrocephalus associated with CM-I in one small study was approximately 10 percent [40]. Common signs and symptoms of hydrocephalus in children include headache, irritability, behavior changes, developmental delays, vomiting, and lethargy.

Myelopathy – Myelopathy, manifesting as motor weakness, sensory disturbances, or autonomic dysfunction, can result from compression of the cervical spinal cord by herniated cerebellar tonsils, or from the effects of a syrinx [42].

Oropharyngeal dysfunction – Oropharyngeal dysfunction associated with CM-I is most prevalent in children younger than three years of age and manifests with sleep apnea or feeding problems [41,42].

Scoliosis – Scoliosis is usually due to an asymmetric spinal syringomyelia leading to differential growth of the hemicords and the vertebral column [8]. 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.

Sleep-related breathing disorders – Emerging data have linked both central and obstructive sleep apnea as manifestations of CM-I in both children and adults [52].

Syringomyelia (syrinx) – Syringomyelia (image 5), often accompanied by scoliosis, occurs in approximately 35 percent of patients with symptomatic CM-I [7]. Among 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, paroxysmal pruritus, 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 [7,53]. 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 [44,54]. 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 (image 6).

Chiari II clinical features — Because it is nearly always associated with a lumbosacral or thoracic myelomeningocele, Chiari type II malformation (CM-II) is usually detected prenatally or at birth (image 2) [1,39].

Manifestations in infancy may include dysphagia, arm weakness, stridor, apneic spells, and aspiration [55]. In late infancy and childhood, progressive hydrocephalus is a common problem in CM-II [1]. 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 I clinical features' above.)

Despite the extensive malformations, some patients with CM-II have normal intelligence and function well independently [56,57].

Chiari III clinical features — 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 [1,14]. Data are limited regarding the natural history and clinical course of Chiari type III malformation (CM-III) [14]. 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 intellectual disability, epilepsy, hypotonia or spasticity, upper and lower motor neuron signs, and lower cranial nerve palsies.

EVALUATION AND DIAGNOSIS — The diagnosis of Chiari malformations is based upon neuroanatomy. There are no biomarkers in blood, cerebrospinal fluid (CSF), or cultured tissue to confirm the diagnosis [1]. Thus, neuroimaging is of prime importance.

Imaging

MRI of brain and spinal cord – Magnetic resonance imaging (MRI) of the brain and the whole spinal cord is the best imaging modality for evaluation of Chiari malformations [8,39,58,59]. 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.

CSF flow imaging – We suggest obtaining a phase contrast cine MRI for patients with CM-I to look for impairment of CSF flow across the foramen magnum [8,60,61]. This information can be used to select patients for surgical decompression of the foramen magnum in order to establish normal CSF flow. The role of phase contrast cine MRI in the assessment of CSF flow dynamics and management of CM-I is reviewed below. (See 'Management of Chiari I' below.)

CT – For patients who cannot have MRI, high-resolution computed tomography (CT) scan with sagittal reconstructions can be used to make the diagnosis of Chiari malformation [62,63]. CT, especially thin-section multiplanar CT with reformatted images, retains importance in the evaluation of the associated bony abnormalities [8]. (See 'Anatomic aspects' above.)

Fetal ultrasound – In some cases of fetal ventriculomegaly, a Chiari malformation can be diagnosed in utero using fetal ultrasound [64,65].

Diagnosis

Diagnosis of CM-I – 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 [8,66-69]. 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 [67]. (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 [70]. 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 [71].

Diagnosis of CM-II – The diagnosis of Chiari type II malformation (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.

Diagnosis of CM-III – Similarly, the diagnosis of Chiari type III malformation (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.

Neurologic assessment and follow-up — Patients with CM-I should be assessed and followed by a neurologist to determine the symptomatic status of CM-I. This is particularly important for characterization of headache, since inaccurate attribution of a primary headache (eg, migraine) to CM-I could lead to inappropriate surgical intervention [59].

Of note, CM-I has not been associated with epilepsy or autism [59].

Sleep evaluation — Polysomnography should be considered in children or adults with CM-I who present with respiratory symptoms suggestive of sleep apnea or severe tonsillar herniation [49,52,59,72].

MANAGEMENT OF CHIARI I — The management of Chiari malformations depends upon the nature of the malformation and the degree of associated neurologic impairments. The management of hydrocephalus is discussed in detail elsewhere. (See "Hydrocephalus in children: Management and prognosis".)

Asymptomatic patients — A Chiari type I malformation (CM-I) may be an incidental finding on brain or cervical spine magnetic resonance imaging (MRI) obtained for other indications (algorithm 1).

Asymptomatic without syringomyelia – Asymptomatic patients with an incidental diagnosis of CM-I who do not have syringomyelia on MRI or cerebrospinal fluid (CSF) flow obstruction on phase contrast MRI can be managed conservatively with clinical and MRI surveillance as needed. In our practice, we repeat the MRI for these patients only if there are clinical findings that suggest the development of symptoms or signs related to CM-I [59,73-75].

Asymptomatic with syringomyelia – For asymptomatic or oligosymptomatic patients with CM-I who are neurologically intact but have syringomyelia on MRI, management is controversial [38]. As discussed below (see 'Prognosis' below), there are occasional reports of spontaneous resolution of tonsillar displacement or syringomyelia within the context of CM-I. Therefore, some have argued that a period of watchful observation is warranted in asymptomatic children [76,77]. (See 'Prognosis' below.)

In one report of 218 children with incidentally discovered CM-I, the discovery of occult neurologic dysfunction on evaluation or identification of a syrinx on MRI or led to early (within six months of presentation) decompressive surgery for 22 patients (10 percent); subsequent development of de novo symptoms or syrinx led to later (after six months) surgery for another 14 patients (6 percent) [46].

CSF flow obstruction — We suggest obtaining a phase contrast cine MRI for patients with CM-I to look for CSF flow obstruction.

If the phase contrast cine MRI shows complete CSF flow obstruction, we refer patients for surgery. (See 'Role of surgery' below.)

If the study shows partial CSF flow obstruction and a syrinx, we repeat the study six months later. If the imaging and clinical findings remain stable, we repeat imaging one year later, then again in another one to two years, and thereafter as needed.

If the study shows only partial obstruction and no syrinx, we repeat imaging at one year, and as needed thereafter if stable at one year.

We would refer patients for neurosurgical evaluation and posterior fossa decompression at the first sign of clinical deterioration.

In small uncontrolled studies, obstruction of CSF flow correlated with clinical symptoms; in addition, a postsurgical increase in CSF flow was associated with clinical improvement or stabilization [60,78,79]. Phase contrast cine MRI may also be useful for postoperative follow-up of patients with "failed" surgeries, such as those who experience delayed deterioration [60]. Furthermore, a retrospective report of 130 patients who had decompressive surgery for CM-I found that normal CSF flow by phase contrast cine MRI before surgery was predictive of symptom recurrence after surgery (relative risk [RR] 4.8, 95% CI 1.9-12.5) [80]. 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.

Symptomatic patients — Decompressive surgery is indicated for patients with CM-I who are clearly symptomatic with lower cranial nerve palsies, syringomyelia, myelopathy, cerebellar symptoms, or occipital cough headache [59,74]. (See 'Role of surgery' below.)

Role of surgery — As noted above, we refer patients with CM-I for neurosurgical evaluation if they are clearly symptomatic or have imaging evidence of a large syrinx or complete CSF flow obstruction.

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 [62]. The most common procedure is posterior decompression via suboccipital craniectomy with or without duraplasty. Other procedures include 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 [62,81]. The procedure involves a limited suboccipital craniectomy, C1 laminectomy, duraplasty, and arachnoid dissection. In an international consensus document on the diagnosis and treatment of Chiari malformation and syringomyelia published in 2021, all of the 31 multidisciplinary specialists favored opening the dural sac for patients with CM-I and syringomyelia, while 75 percent favored this approach for patients with symptomatic CM-I without syringomyelia [74]. Potential complications include pseudomeningocele formation, CSF leakage, acute postoperative hydrocephalus, and meningitis [62,81].

A more conservative approach consists 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, suggest that posterior fossa decompression with duraplasty is associated with a higher rate of clinical improvement, particularly for patients with syringomyelia, and a lower rate of need for reoperation compared with the more conservative surgery without duraplasty, but an increased rate of CSF-related complications [82-84]. 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.

It is not clear if the studies 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 [85]. 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 [86]. There were no episodes of CSF leak, meningitis, or postoperative hydrocephalus.

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 with craniovertebral junction malformations (eg, basilar invagination) who fail posterior decompression [74]. 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 [87,88].

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 [89,90].

MANAGEMENT OF CHIARI II AND III — Patients with Chiari type II malformation (CM-II) and Chiari type III malformation (CM-III) should be referred for neurosurgical evaluation. 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 [1]. Medical issues involve management of neurogenic bowel and bladder, neonatal feeding difficulties, respiratory failure, and apnea.

The management of myelomeningocele is reviewed separately. (See "Myelomeningocele (spina bifida): Management and outcome" and "Myelomeningocele (spina bifida): Urinary tract complications" and "Myelomeningocele (spina bifida): Orthopedic issues".)

PROGNOSIS — The clinical course of Chiari type I malformation (CM-I) is unpredictable. Some patients remain asymptomatic, and occasional patients have spontaneous resolution of tonsillar displacement or spinal cord syringomyelia [38,91-93]. Others have a relentlessly worsening syrinx [94]. 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 [95].

As noted earlier, the natural history of CM-I is not well-defined [96]. 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) [38]. 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.

As an example, 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 [97]. 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 magnetic resonance imaging was a predictor of poor outcome.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Congenital malformations of the central nervous system".)

SUMMARY AND RECOMMENDATIONS

Anatomy and classification – There are three main types of Chiari malformations (see '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 (image 1).

Chiari II malformation (CM-II) is characterized by downward displacement of the cerebellar vermis and tonsils, a brainstem malformation with beaked midbrain on neuroimaging, and a spinal myelomeningocele (image 2 and image 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 (image 4).

Epidemiology – CM-I is the most frequent of the Chiari malformations. The true prevalence of CM-I is unknown but may be 1 percent or more. (See 'Epidemiology' above.)

Clinical manifestations – CM-I is often asymptomatic; manifestations may include cough, headache, syringomyelia, cerebellar dysfunction, brainstem symptoms and oropharyngeal dysfunction, cranial neuropathies, hydrocephalus, myelopathy, or scoliosis. Manifestations of CM-II in infancy may include dysphagia, arm weakness, stridor, apneic spells, and aspiration, later development of hydrocephalus, and any of the syndromes associated with CM-I. CM-III has a high mortality rate in infancy due to respiratory failure, and severe neurologic impairments for those who survive the neonatal period. (See 'Clinical manifestations' above.)

Evaluation and diagnosis – Magnetic resonance imaging (MRI) is the best imaging modality for evaluation of Chiari malformations. (see 'Evaluation and diagnosis' above):

CM-I in adolescents and adults is diagnosed by MRI when one or both cerebellar tonsils are displaced by ≥5 mm below the foramen magnum.

CM-II is diagnosed in a fetus or newborn with a spinal myelomeningocele and MRI showing downward displacement of inferior cerebellar vermis and medulla through the foramen magnum.

CM-III is diagnosed in a fetus or newborn with a high cervical/occipital encephalocele and MRI showing 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 (see 'Management of Chiari I' above):

Asymptomatic patients with an incidental diagnosis of CM-I who do not have syringomyelia or CSF flow obstruction can be managed conservatively with clinical and MRI surveillance (algorithm 1).

For asymptomatic or oligosymptomatic patients with CM-I, we obtain MRI of the whole spine with a phase contrast cine MRI; we refer the patient to surgery if the studies show a large syrinx or complete cerebrospinal fluid (CSF) flow obstruction; we follow the patient with clinical and neuroimaging surveillance if there is no syrinx (or a small syrinx) and only partial or no CSF flow obstruction.

Patients with CM-I who are symptomatic with lower cranial nerve palsies, myelopathy, cerebellar symptoms, or occipital headache related to the Chiari malformation or are symptomatic due to syringomyelia should be referred for surgical evaluation and consideration of foramen magnum decompression.

For CM-II and CM-III, surgical interventions may include closure of open neural tube defects shortly after birth, treatment for hydrocephalus, and decompression of tight posterior fossa structures. Medical issues involve management of neurogenic bowel and bladder, neonatal feeding difficulties, respiratory failure, and apnea.

Outcomes – 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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Topic 13507 Version 24.0

References

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