Chiari malformations !

Malformation

In Chiari, part of the cerebellum (and sometimes the brainstem) slips down through the opening at the base of the skull and can block CSF.

Updated 2026-11-01

See it in 3D: Cerebellar tonsils · Vermis · Medulla oblongata · Foramen magnum · Fourth ventricle · Spinal cord

What it is

Chiari malformations are a group of anomalies in which the cerebellum, and sometimes the brainstem, descends through the foramen magnum. Type I, the most common, usually shows up in older children or adults with headache; type II almost always accompanies myelomeningocele and causes hydrocephalus. They link the study of neural tube defects with the posterior fossa.

Key facts

Type I: cerebellar tonsils descend below the foramen magnum; often asymptomatic.
Type II (Arnold–Chiari): vermis, medulla and fourth ventricle descend; associated with myelomeningocele.
Type III: cerebellum and brainstem herniate into an occipitocervical encephalocele; rare and severe.
Type IV: hypoplastic or absent cerebellum, very rare.
Syringomyelia (a fluid cavity in the cord) is associated with Chiari.
Hydrocephalus is common in type II.

Embryological cause

Type II: with myelomeningocele, CSF leaks through the open defect and the posterior fossa fails to expand; the hindbrain stays small and is pulled down.
The cord fixed (tethered) at the lumbosacral defect pulls on the brainstem as the spine grows.
Type I: a posterior fossa too small for its contents (occipital development), with displaced tonsils.
Herniated tonsils or medulla plug the foramen magnum and disturb CSF flow → hydrocephalus, syringomyelia.

How it presents

Type I: occipital headache worse with coughing, sneezing or straining; neck pain, dizziness, gait or swallowing problems.
Syringomyelia: cape-like loss of pain and temperature over shoulders and arms, hand weakness, scoliosis.
Type II in the newborn: stridor, apnea, swallowing difficulty and nystagmus from brainstem involvement.
Type II nearly always with myelomeningocele and hydrocephalus.

Diagnosis

MRI of brain and spine: the test of choice.
Prenatal (type II): 'lemon' and 'banana' signs on ultrasound with spina bifida.
Look for syringomyelia with MRI of the whole cord.

Test yourself

Think of the answer, then tap the question to see it.

Which structure descends in Chiari type I malformation?

The cerebellar tonsils, below the foramen magnum.

Which neural tube defect is Chiari II almost always associated with?

Lumbosacral myelomeningocele.

What sensory pattern does the associated syringomyelia produce?

Cape-like loss of pain and temperature over shoulders and arms, with touch preserved.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

← All of embryology

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Chiari Malformation (National Institute of Neurological Disorders and Stroke, 2024); About Spina Bifida (Centers for Disease Control and Prevention, 2024); Langman's Medical Embryology, 13th ed., ch. 18: Central Nervous System (Wolters Kluwer, 2015).

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Representative model, for educational purposes only.