Chiari Malformations

Chiari malformations (CMs) are a group of central nervous system (CNS) conditions characterized by the underdevelopment of the posterior cranial fossa with subsequent protrusion of neural structures through the foramen magnum. There are 4 types of CM, with type I being the most common. Headaches are the most common symptom. Diagnosis is made by clinical findings and confirmed by magnetic resonance imaging (MRI). Treatment is surgical, based on decompression of the posterior fossa and restoration of CNS flow. Prognosis depends on the type of malformation.

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Chiari malformations (CMs) are a group of disorders defined by structural deficits in the brain and spinal cord leading to limited space in the posterior fossa, which forces cerebellar structures to protrude through the foramen magnum.


  • Type I: tonsillar herniation > 5 mm inferior to plane of foramen magnum
    • Abnormally shaped cerebellar tonsils
    • No associated brainstem herniation or supratentorial anomalies
    • Associated hydrocephalus and hydrosyringomyelia common
  • Type II: herniation of cerebellar vermis, brainstem, and 4th ventricle into foramen magnum
    • Associated with myelomeningocele and multiple brain anomalies
    • Associated hydrocephalus and syringomyelia are very common.
  • Type III: herniation of cerebellum and brainstem compressing spinal cord
    • High cervical or occipital encephalocele containing herniated cerebellar and brainstem tissue
    • Rare and typically incompatible with life
  • Type IV: incomplete or underdeveloped cerebellum with exposed skull and spinal cord
    • Hypoplasia or aplasia of cerebellum and tentorium
    • Rare and always incompatible with life
Chiari Malformations

Chiari type I and II
Chiari malformations are characterized by and underdeveloped posterior cranial fossa, with subsequent protrusion of neural structures through the foramen magnum. Type 1 is characterized by herniation of the cerebellar tonsils only, while type 2 involves more structures.

Image by Lecturio.



  • Type I:
    • Most common form
    • 1 in 1,000–5,000 live births 
    • Slight female predominance
  • Type II:
    • Approximately 1 in 2,000 live births 
    • Decreased incidence with prenatal folate supplementation
    • No gender predominance
    • Always associated with myelomeningocele 
  • Type III: 
    • Rarest form
    • Makes up 1%–4.5% of all CMs

Associated conditions:

  • Pierre Robin sequence
  • Noonan syndrome
  • Neurofibromatosis type 1

Etiology and Pathophysiology


  • Mechanism for herniation
    • Formation of small posterior fossa ➝ limited growth of cerebellum and nearby structures 
    • Growing structures herniate through foramen magnum or vermis.
  • Multiple proposed causes:
    • Genetics:
      • Abnormal segmentation of hindbrain 
      • Causes abnormal embryonic development of bony and nervous tissues
    • Restricted growth of posterior fossa causes compression of neural tissues.
    • Progressive hydrocephalus pushes structures downward.


Neurologic symptoms are caused by:

  • Compression of central nervous system (CNS; cerebellum, brainstem) against foramen magnum and spinal canal
  • Formation of cavitations in spinal cord (syrinx or syringomyelia) due to backup of CSF outflow
Pressure on the CNS due to Chiari malformation

Syrinx seen with Chiari malformation
Cavities filled with cerebrospinal fluid (CSF) are commonly seen with CMs. They cause symptomatology commonly associated with the malformation by putting pressure on the surrounding neural tissue.

Image by Lecturio.

Clinical Presentation

Type I


  • Asymptomatic in some cases
  • Headache:
    • Most common symptom (60%–70%) 
    • Usually occipital and upper cervical
    • Occipital headache worse on Valsalva maneuver
  • Ataxia and nystagmus (due to compression of cerebellum) 
  • Compression of cranial nerves can lead to:
    • Hoarseness
    • Vocal cord paralysis
    • Tongue asymmetry
  • Central sleep apnea

Physical signs:

  • Syringomyelia and central cord syndrome:
    • Develops at level of C8–T1
    • “Cape-shaped” area of pain and temperature sensation loss due to spinothalamic tract involvement
    • Flaccid paralysis and muscle atrophy due to lower motor neuron involvement
  • Scoliosis: due to asymmetric development of vertebral columns

Type II


  • Signs of brainstem dysfunction (e.g., ataxia, urinary incontinence)
  • Swallowing and feeding difficulties
  • Stridor/difficulty in breathing/apnea
  • Weak cry
  • Nystagmus
  • Occipital headaches

Physical signs:

  • Myelomeningocele: protrusion of CNS and meninges
    • Commonly lumbosacral or thoracic
    • Usually prenatally diagnosed
  • Hydrocephalus (< 10%):
    • Tense/bulging fontanelles 
    • Increased head circumference > 98th percentile
    • Nuchal rigidity and neck tenderness
    • Cognitive deterioration
    • Imbalance and gait disturbances
    • Urinary incontinence

Type III

There is high infant mortality with this type.


  • Severe neurological, developmental, and cranial nerve defects
  • Seizures
  • Respiratory insufficiency
  • Upper and lower motor neuron paralysis

Physical signs:

  • Encephalocele: protruding defect that contains part of cerebellum and higher structures
  • Spastic or flaccid paralysis

Type IV

  • CNS is undeveloped.
  • Infants die shortly after birth.



  • Rule out other causes of tonsillar herniation (e.g., intracranial mass lesion, hydrocephalus).
  • Antenatal:
    • Obstetric ultrasounds looking for congenital abnormalities in 18th week of gestation (absent vermis)
    • Amniocentesis and karyotyping
  • Postnatal:
    • Ultrasound of head preferred in neonates
    • MRI scan preferred in older children and adults


  • Clinical findings with diagnostic imaging to confirm
  • Can be done prenatally
Chiari I malformation

Magnetic resonace imaging scan showing a type I CM. Notice the tonsillar herniation.

Image: “Sagittal MRI scan of brain of patient with Chiari malformation” by Raymond F Sekula Jr et al. License: CC BY 2.0


  • Favorable prognosis with normal life expected after intervention
  • Chiari III malformation has the poorest prognosis.

Management and Complications

Surgical therapy

  • Decompression of cervicomedullary junction → restoring normal CSF dynamics 
  • Options:
    • Posterior fossa craniectomy 
    • Electrocautery of cerebellum via high-frequency electric currents
    • Spinal laminectomy to remove pathological bony roof of spinal canal

Conservative management

  • Analgesics and muscle relaxants for headaches and neck pains
  • Occasional use of soft neck collar
  • Special education needed in event of delayed developmental milestones


  • Hydrocephalus: accumulation of CSF within cranial cavity; CM is obstructive, or non-communicating, cause of hydrocephalus
  • Pseudomeningocele formation and CSF leakage
  • Meningitis and wound infection post-surgery
  • Neurovascular injury during surgery
  • Cerebellar ptosis in large occipital craniectomy

Differential Diagnosis

  • Hydrocephalus: potentially life-threatening condition caused by excess accumulation of CSF within the ventricular system. Clinical presentation is nonspecific and includes headache, behavioral changes, developmental delays, or nausea and vomiting. Diagnosis is confirmed with neuroimaging (ultrasound, head CT, or MRI) showing ventriculomegaly. Treatment is placement of CSF shunt.
  • Meningitis: inflammation of leptomeninges, usually due to infectious agent. Patients will present with headache, fever, and a stiff neck. Diagnosis is suspected by clinical presentation and confirmed by lumbar puncture. Treatment is aimed at the causative infection. 
  • Neural tube disorders: disorders caused by failure of the neural tube to close properly during embryological development. Symptoms range from asymptomatic to very severe malformations of spine and brain. Etiologies are multifactorial, ranging from maternal nutrition to genetic determinants. Prenatal diagnosis is by ultrasound and maternal α-fetoprotein level. Management is mainly surgical.


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  2. Schijman, E. History, anatomic forms, and pathogenesis of Chiari I malformations. Childs Nerv Syst 20, 323–328 (2004).
  3. Khoury, C. (2020). Chiari malformations. UpToDate. Retrieved December 1, 2020 from
  4. Abd-El-Barr MM, Strong CI, Groff MW. Chiari malformations: diagnosis, treatments and failures. J Neurosurg Sci. 2014 Dec. 58 (4):215–21.
  5. McClugage, S and Oakes, J. The Chiari I malformation. JNSPG 75th Anniversary Invited Review Article. DOI:
  6. Langridge B, Phillips E, Choi D. (2017). Chiari Malformation Type 1: A Systematic Review of Natural History and Conservative Management. World Neurosurg.

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