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KEY FACTS

Terminology
- Chiari 1 malformation (CM1); synonyms: Chiari type 1, Chiari 1 deformity, cerebellar tonsillar ectopia
Imaging
- Pointed cerebellar tonsils extending ≥ 5 mm below foramen magnum (basion-opisthion/McRae line) with effacement of CSF spaces
- ± retroflexed odontoid, horizontal shortened clivus, basilar invagination, atlanto-occipital assimilation
- ± caudal descent of brainstem, brainstem compression, medullary kink
- ± syringohydromyelia, scoliosis
Top Differential Diagnoses
- Normal low-lying cerebellar tonsils
- Chiari 2 malformation
- Tonsillar herniation secondary to increased intracranial pressure
- Intracranial hypotension
Pathology
- Most common cause believed to be small/underdeveloped posterior fossa; no association with open spinal dysraphism
- Can be result of premature closure of sutures - Causes include shunted infantile hydrocephalus, bone dysplasias, genetic syndromes
Clinical Issues
- Most common presenting symptom: Occipital headache - Up to 30% of patients asymptomatic
- Goal of surgery in symptomatic patients: Restore normal CSF flow at foramen magnum - Suboccipital decompression, resection of C1 posterior arch ± duraplasty, cerebellar tonsil cautery
Diagnostic Checklist
- Degree of tonsillar descent does not always correlate with symptoms: CM1 frequently picked up incidentally
- Look for presence of syrinx → makes surgical intervention more likely

TERMINOLOGY

Synonyms
- Chiari type 1, Chiari 1 deformity
Definitions
- Chiari 1 malformation (CM1): Compressed & pointed cerebellar tonsils extending below foramen magnum with effacement of CSF spaces

IMAGING

General Features

Best diagnostic clue

- Pointed cerebellar tonsils (unilateral or bilateral) extending ≥ 5 mm below foramen magnum (basion-opisthion line, a.k.a. McRae line)
        - Mild variations in measurement reported in literature; measurement on its own may not be definitive of diagnosis
        - No consensus statement on exact definition

Location
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- Craniocervical junction (CCJ)
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Radiographic Findings
- Shortened horizontal clivus, basilar invagination, CCJ segmentation anomalies, scoliosis
CT Findings
- Crowding of foramen magnum on axial CT images - Sagittal reconstructed images are very helpful
- Partially imaged superior aspect of spinal cord syrinx may be identified
- Associated osseous anomalies may include small posterior fossa, short horizontal clivus, retroverted dens, basilar invagination, platybasia, hypoplastic occipital condyles, segmentation anomalies (such as atlantooccipital assimilation), scoliosis
MR Findings
- T1WI, T2WI, FLAIR - Pointed (not rounded) cerebellar tonsils extending ≥ 5 mm below foramen magnum - Crowded foramen magnum with small/effaced cisterns ± brainstem compression (kinking) - ± small posterior fossa, elongated 4th ventricle - ± syringohydromyelia/syrinx, scoliosis - Syrinx reported in 30-70% of cases - Patients with syrinx more likely to have scoliotic curve > 20⁰ (~ 70%) than those without syrinx (~ 45%) - Other descriptions usually considered subtypes - Chiari 1.5: Brainstem herniation - Obex located below foramen magnum - Complex Chiari: Medullary kink, retroflexed dens, abnormal clival-cervical angle, atlantooccipital assimilation, basilar invagination, platybasia
- MR cine - Restricted CSF flow through foramen magnum ± ↑ brainstem/cerebellar tonsil motion (pistoning) - Clinical utility of this sequence debatable

Imaging Recommendations

Best imaging tool

- Multiplanar MR
        - Axial True FISP/FIESTA MR of CCJ helpful in assessing degree of foramen magnum crowding
- Spine imaging to look for spinal cord syrinx
        - Syrinx makes surgical intervention more likely

DIFFERENTIAL DIAGNOSIS

Normal Variation of Cerebellar Tonsil Position
- Tonsils may normally lie below foramen magnum - May be accentuated by certain head positions
- Tonsils retain normal rounded configuration
Chiari 2 Malformation
- Numerous intracranial findings centered around very small posterior fossa with hindbrain herniation in setting of open spinal dysraphism
- More severe clinical phenotype than CM1
Tonsillar Herniation Secondary to Increased Intracranial Pressure
- Neoplasm, hemorrhage, hydrocephalus, infarct
Intracranial Hypotension
- Look for "slumped" brainstem
- Sagging midbrain, sunken hindbrain with diffuse dural thickening/enhancement, distended veins/dural sinuses, ± subdural hygromas
Chiari 0
- Syringomyelia without cerebellar tonsillar ectopia; syrinx resolves after posterior fossa decompression
- Diagnosis of exclusion (many other causes of spinal cord syrinx)

PATHOLOGY

General Features

Etiology

- Primary congenital malformation vs. secondarily acquired morphologic changes
- Primary: Posterior fossa underdevelopment theory most common
        - Underdevelopment of endochondral occipital bone → small posterior fossa vault + downward hindbrain herniation
        - Not all Chiari 1 patients have small posterior fossa
- Secondary: Premature closure of cranial sutures &/or generalized abnormal bone formation
        - Shunted infantile hydrocephalus
        - Calvarial thickening of bone dysplasias or thalassemia
        - Genetic syndromes
- Seen in 2-10% of patients with idiopathic intracranial hypertension (a.k.a. pseudotumor cerebri)

Genetics

- Small posterior fossa in isolation is heritable
        - Multiple potential causative genes on chromosome 1 & 22 identified
- Syndromic/familial associations (up to 60% of cases)
        - Craniosynostosis syndromes (ERF-related, FGFR-related: Apert, Crouzon, Pfeiffer)
        - Osteopathic syndromes (achondroplasia, rickets)
        - Vertebral anomalies (Klippel-Feil, VACTERL)
        - Craniofacial anomalies (Pierre-Robin, Goldenhar)
        - Macrocerebellum (Costello syndrome, Sotos syndrome, macrocephaly-capillary malformation syndrome, Alexander disease)
        - Increased brain volume (NF1/RAS/MAPK mutations/RASopathies, PTEN-PI3K/AKT mutations/PTENopathies)
        - Many others: Ehlers-Danlos syndrome, Marfan syndrome, Williams syndrome, Kabuki syndrome

Gross Pathologic & Surgical Features
- Herniated cerebellar tonsils become atrophic/gliotic/necrotic
- Arachnoid scarring & adhesions at foramen magnum

CLINICAL ISSUES

Presentation

Most common signs/symptoms

- Occipital headache
        - Exacerbated by cough, Valsalva, neck extension, or physical exertion
- Less common: Cerebellar, brainstem, bulbar, cord motor/sensory symptoms
        - Cerebellar symptoms: Ataxia, dysarthria, oscillopsia, nystagmus
        - Brainstem and Bulbar symptoms: Vertigo, diplopia, dysphagia, aspiration, apnea, syncope, bradycardia, sudden death (rare)
        - Spinal cord dysfunction: Motor and sensory losses, hyporeflexia, hyperreflexia, clonus, gait disturbance, neuropathic joint, urinary incontinence, positive Babinski sign, scoliosis
- 15-30% of adults with CM1 are asymptomatic, up to 35% of children with 5-10 mm of tonsillar herniation are asymptomatic
- Not much difference in clinical symptoms between complex Chiari & typical Chiari 1

Demographics
- True prevalence is unknown given how frequently it is picked up incidentally
- Epidemiology: 0.5-3.5% of general population
- Age: Evenly distributed in adult & pediatric patients - 3% of children & 1% of adults have imaging findings of CM1, though age of clinical presentation unclear - One series reports median age of presentation in children ≈ 8 years
- Sex: F > M (as high as 3:1)
Natural History & Prognosis
- Natural history not clearly understood - Many patients asymptomatic for prolonged periods - Increasing ectopia + time → ↑ likelihood of symptoms
- Children respond better to treatment than adults
- Patients selected for nonsurgical management usually have benign course, though spontaneous improvement & worsening have been described
Treatment
- Posterior fossa decompression: Suboccipital craniectomy with C1 laminectomy ± duraplasty, arachnoid opening/dissection, cerebellar tonsil cautery/resection - ↓ of syrinx size in majority of patients after decompression
- Complex Chiari 1 may also require odontoid resection or craniocervical junction fusion
- Scoliosis may improve from decompression alone but often requires bracing or additional surgery
- Postoperative complications in approximately 20% of adults and 37% of children - Most common: CSF leak, pseudomeningocele, infection - Increased risk with duraplasty - 1-11% postoperative mortality
- Conservative management for asymptomatic or minimally symptomatic children without syrinx

DIAGNOSTIC CHECKLIST

Consider
- Degree of tonsillar descent does not always correlate with symptoms: CM1 frequently picked up incidentally
Image Interpretation Pearls
- Description of additional posterior fossa findings helpful for surgical planning - Look for variant occipital venous sinus (at site of future decompression)

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References

Selected References

Differential diagnosis

Cisterna Magna Mass

DDX:047add0c-7e4f-40a0-9933-8d6fa00a24f7

Congenital Cerebellar Malformation

DDX:e0a671d3-a236-4ff9-a232-a9684218d010

Foramen Magnum Mass

DDX:a79c61b3-b26a-48c8-bcfb-bf8afd3ef25e

Images

Selected Images

Sagittal graphic demonstrates pointed cerebellar tonsils extending below the foramen magnum to the inferior aspect of the C1 posterior arch. The obex is inferiorly displaced as well.

Sagittal T1 MR of an 8-year-old with boy incidentally noted Chiari 1 malformation (CM1) demonstrates pointed, low-lying cerebellar tonsils reaching the level of the posterior C1 arch as well as downward displacement of the brainstem with cervicomedullary junction just below C1 posterior arch .

Coronal T1 MR in the same patient demonstrates the left cerebellar tonsil is lower than the right. In CM1, either one or both cerebellar tonsils may be involved.

Axial FIESTA MR through the foramen magnum in the same patient demonstrates posterior displacement of the dens with associated deformity of the ventral caudally displaced medulla. Partial effacement of the CSF spaces surrounding the cerebellar tonsils denotes moderate foramen magnum crowding.

Sagittal T2 MR in a 5 year old with occipital headaches demonstrates pointed, low-lying cerebellar tonsils with associated effacement of CSF spaces. Note the short horizontal clivus , retroverted dens , and segmentation anomaly at the C3-4 levels .

Coronal T2 MR in the same patient demonstrates pointed, low-lying cerebellar tonsils extending below the C1 ring , typical of CM1.

Sagittal phase contrast cine MR (with hyperintense signal at sites of active CSF or venous flow) in the same patient shows absence of CSF flow at the craniocervical junction due to the CM1.

Sagittal phase contrast cine MR in a 16-year-old girl with occipital headaches being worked up for CM1 demonstrates normal CSF flow-related signal ventral and dorsal to the brainstem. No CM1 was identified on conventional sequences.

Sagittal bone CT in a patient with hyperreflexia and severe CM1 demonstrates odontoid retroflexion and enlargement of the anterior C1 ring (which abnormally articulates with the remodeled clivus).

Sagittal T2 MR of the cervical spine in the same patient shows the odontoid process retroflexion as well as the cerebellar tonsillar ectopia and associated syringohydromyelia .

A 5-year-old girl with CM1 demonstrates downward displacement of the cerebellar tonsils below the plane of the foramen magnum caudal to the posterior arch of C1 with an abnormal pointed morphology.

Sagittal T1 MR in the same patient status post suboccipital decompression with suboccipital craniectomy, C1 laminectomy, expansive duraplasty, and cerebellar tonsillar shrinkage is shown. There is no residual cerebellar ectopia, and the inferior cerebellum has a normal, rounded morphology.

Axial FIESTA MR in the same patient with CM1 status post posterior fossa decompression demonstrates absence of the posterior arch of C1 from C1 laminectomy and expansive duraplasty . There is patent CSF surrounding the cervical spinal cord with no evidence of crowding.

Sagittal T2 MR depicts marked cerebellar tonsillar ectopia . Clivus is mildly foreshortened. There is central edema in the cervical spinal cord without frank syringohydromyelia, a finding that has been described as presyrinx edema.

Sagittal T1 MR in a patient with osteopetrosis shows cerebellar tonsillar ectopia with extension of the elongated cerebellar tonsils to the C2/C3 level. The hypointense marrow signal reflects diffuse sclerosis.

Axial T2 MR in the same patient reveals characteristic crowding of the foramen magnum with extension of the ectopic cerebellar tonsils into the upper cervical spinal canal.

Additional Images

Sagittal T2 MR in an asymptomatic Chiari 1 patient demonstrates severe cerebellar tonsillar ectopia . The tonsils produce deformation of the upper cervical spinal cord. There is abnormal T2 prolongation in the upper spinal cord reflecting edema and potentially a presyrinx state.

Sagittal T2 MR from a 10-year-old patient demonstrates pointed cerebellar tonsils extending below the foramen magnum to the lower C1 level , typical of CM1. The CSF is largely effaced at the craniocervical junction, and a syrinx is partially seen in the cervical spinal cord.

Axial true FISP/FIESTA MR through the foramen magnum in the same patient shows crowding and effacement of the CSF spaces by the low cerebellar tonsils .

Sagittal T1 MR of the cervical spine in the same patient further demonstrates the cerebellar tonsillar ectopia and large cervicothoracic spinal cord syrinx .

30 KiB Raw Permalink Blame History

KEY FACTS

Terminology

Imaging

Top Differential Diagnoses

Pathology

Clinical Issues

Diagnostic Checklist

TERMINOLOGY

Synonyms

Definitions

IMAGING

General Features

Best diagnostic clue

Location

Radiographic Findings

CT Findings

MR Findings

Imaging Recommendations

Best imaging tool

DIFFERENTIAL DIAGNOSIS

Normal Variation of Cerebellar Tonsil Position

Tonsillar Herniation Secondary to Increased Intracranial Pressure

Chiari 0

PATHOLOGY

General Features

Etiology

Genetics

Gross Pathologic & Surgical Features

CLINICAL ISSUES

Presentation

Most common signs/symptoms

Demographics

Natural History & Prognosis

Treatment

DIAGNOSTIC CHECKLIST

Consider

Image Interpretation Pearls

References

Selected References

Differential diagnosis

Cisterna Magna Mass

Congenital Cerebellar Malformation

Foramen Magnum Mass

Images

Selected Images

Additional Images

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Raw Permalink Blame History