Mild cases of Arnold-Chiari malformation are often asymptomatic and discovered incidentally during CT or MRI imaging performed for unrelated conditions.

🧠 About Arnold–Chiari Malformation

A developmental abnormality of the craniocervical junction where the cerebellar tonsils herniate through the foramen magnum. ⬇️
This can compress the brainstem and cervical spinal cord → leading to neurological symptoms of variable severity.
Part of the spectrum of congenital hindbrain anomalies; sometimes discovered incidentally on MRI.

📌 Aetiology

Exact cause unknown; thought to be related to mesodermal/CSF flow abnormalities during embryogenesis.
Can be acquired after CSF shunting for hydrocephalus (excess drainage lowers pressure → downward herniation).
Genetic predisposition suspected in familial clusters.

🧩 Clinical Presentation

💢 Suboccipital or upper cervical pain (often worsens with Valsalva manoeuvre e.g. coughing/sneezing).
🎢 Dizziness / vertigo, imbalance, and ataxia.
👅 Cranial nerve palsies (IX, X, XII) → dysphagia, dysarthria, hoarseness, impaired gag reflex.
👀 Downbeat nystagmus due to brainstem compression.
🦴 Syringomyelia → cape-like sensory loss, weakness, scoliosis.
In children with Chiari II: may present with spina bifida + hydrocephalus.

🔎 Investigations

MRI Brain & Spine = gold standard. • Demonstrates cerebellar tonsil herniation (measured in mm below foramen magnum). • Shows hydrocephalus, syrinx, and brainstem compression.
CT head may show hydrocephalus or bony changes but is less sensitive.

⚕️ Management

👀 Asymptomatic / mild cases: Observation + MRI surveillance.
🔪 Surgery: Posterior fossa decompression (suboccipital craniectomy ± duraplasty) when there is significant brainstem compression, syrinx, or disabling symptoms.
💧 Hydrocephalus management: CSF shunting or endoscopic third ventriculostomy (ETV) if raised ICP present.
🧑‍⚕️ Multidisciplinary care: Neurosurgery, neurology, physio, OT, speech therapy.

📊 Classification of Chiari Malformations


Type	Description	Associated Features
Chiari I	Herniation of cerebellar tonsils into foramen magnum. Medulla & 4th ventricle remain in place.	Obstructive hydrocephalus Cranial nerve palsies (IX, X, XII) Downbeat nystagmus Syringomyelia
Chiari II	Herniation of cerebellar tonsils, vermis & medulla, with 4th ventricle displaced downwards.	Myelomeningocele (spina bifida) Aqueductal stenosis → hydrocephalus Corpus callosum hypoplasia/agenesis Colpocephaly (dilated occipital horns) Luckenschadel skull (lacunar skull)
Chiari III	Severe herniation of cerebellum & medulla; 4th ventricle descends below foramen magnum.	Encephalocele / myelomeningocele Syrinx formation Very poor prognosis

💡 Exam Pearls

Think Chiari I in a young patient with headache + syrinx + downbeat nystagmus.
Chiari II is almost always associated with spina bifida (neural tube defect).
MRI spine is essential to look for syringomyelia or tethered cord.

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Arnold Chiari malformation