Arnold-Chiari Malformation

When the cerebellum hangs into the foramen magnum. Type I sneaks up on the young adult; Type II crashes the newborn nursery.

Opening case
A 24-year-old graduate student comes in for a 2-year history of throbbing occipital headaches that spike when she coughs, laughs, or strains on the toilet. She has started bumping into doorframes. On exam, she cannot feel a pinprick across her shoulders and upper arms, but vibration and proprioception in her feet are intact. What is going on?

Dandy-Walker malformation

Type I Arnold-Chiari malformation with cervical syringomyelia

Multiple sclerosis with cervical cord plaque

Vestibular migraine

Cough-triggered occipital headache + cape-distribution loss of pain and temperature in a young adult is the Type I Chiari + syrinx fingerprint. The cerebellar tonsils have slipped below the foramen magnum like a plug. Every cough briefly spikes intracranial pressure, the plug presses harder on the brainstem, and the headache fires. The syrinx (fluid cyst inside the spinal cord) sits centrally where the crossing spinothalamic fibers live, so pain and temperature drop out in a shoulder-and-arms cape while the dorsal columns at the back of the cord (vibration, proprioception) stay clean. Dandy-Walker is a cystic posterior fossa from a missing vermis, not a herniation, and it presents in infancy. MS plaques rarely produce a perfectly symmetric cape, and vestibular migraine never explains the dissociated sensory loss.

Type I vs Type II

Same posterior fossa, two completely different patients. Tap a tab.

Type I (young adult)

Type II (infant)

Type I · the slow leak

Cerebellar tonsils descend 5 mm or more below the foramen magnum. Nothing else herniates.

T2 sagittal MRI of a 13-week-old patient with Chiari I malformation showing low-lying cerebellar tonsils — Sagittal T2 MRI shows the cerebellar tonsils dipping below the foramen magnum line in Type I Chiari. Tap to expand.

Who

Young adult, often a woman in her 20s or 30s. May have been silent for decades.

Headache

Occipital, worse with cough or Valsalva. Picture: every laugh or strain briefly jams the plug.

Exam

Cerebellar ataxia, gait wobble, vertigo, downbeat nystagmus. Lower cranial nerves can drop out (hoarseness, swallowing trouble).

Plus-one

Syringomyelia in a big chunk of cases. Cape-distribution loss of pain and temperature, hands and arms.

Workup

MRI brain shows tonsillar descent. Get a spine MRI too to look for the syrinx.

Treatment

Asymptomatic: watch. Symptomatic: suboccipital decompression with duraplasty. Carve more room over the foramen so the tonsils stop choking the brainstem.

Type II · the crash

Cerebellar tonsils PLUS vermis PLUS medulla PLUS 4th ventricle all herniate. Almost always paired with an open spina bifida on the back.

T2 sagittal MRI of a newborn with Chiari II malformation — Sagittal T2 MRI of a newborn with Type II Chiari shows the posterior fossa pulled down through the foramen magnum with dilated ventricles. Tap to expand.

Who

Infant. Diagnosed prenatally or in the first weeks of life.

Spine

Myelomeningocele on the lower back at birth. Open sac. Failed neural tube closure.

Head

Obstructive hydrocephalus. Bulging fontanelle, sunset-eye downward gaze, head circumference racing across the chart. Aqueductal stenosis can pile on.

Brainstem

Inspiratory stridor, apnea, poor swallow. The compressed medulla cannot run its breathing and airway reflexes.

Workup

Prenatal ultrasound + elevated maternal AFP flagged it. Postnatal MRI confirms herniation and measures ventricles.

Treatment

VP shunt to drain CSF, neurosurgical closure of the back defect (early postnatal or fetal in utero), posterior fossa decompression if the brainstem keeps failing.

Sagittal Posterior Fossa

Tap a button to switch views. Watch what crosses the orange foramen magnum line.

Normal anatomy
Cerebellar tonsils sit above the foramen magnum. The 4th ventricle drains CSF down through the cisterna magna and back up into the skull. The medulla and the cord meet right at the foramen line. Nothing crosses where it should not.

Sagittal MRI showing a Type 1 Arnold-Chiari malformation with herniated cerebellar tonsils — A second sagittal MRI view showing the cerebellar tonsils pulled past the foramen magnum in Type I Chiari. Tap to expand.

The Type I plus-one: syringomyelia

A fluid cyst that opens up inside the cervical cord. The reason Type I patients lose pain and temperature in a cape pattern across the shoulders.

Plus-one in Type I Syringomyelia

A fluid-filled cavity forms inside the central cord and slowly widens. The crossing fibers of the spinothalamic tract (pink) run right through that center, so they get clipped first. Dorsal columns (green) sit at the back and stay intact until late.

Clue: A young adult with bilateral loss of pain and temperature across the shoulders and upper arms (the cape), but preserved vibration and proprioception. Often a Type I Chiari sitting upstream.

Tonsils plug the foramen → CSF pressure waves bounce wrong → central canal of the cord expands → cavity (the syrinx) carves out the center → crossing pain/temperature fibers cut first → dorsal columns preserved until the cyst gets huge.

Deep dive: spinal cord syndromes →

The Type II plus-one: myelomeningocele

Open spina bifida. Sitting on the back at birth. Almost every Chiari II baby has one.

Plus-one in Type II Myelomeningocele

Clue: A newborn with an open sac on the lower back containing meninges and exposed neural tissue. Prenatal: elevated maternal alpha-fetoprotein on the second-trimester screen, ultrasound shows the defect and the lemon-and-banana sign in the posterior fossa.

Neural tube fails to close in week 4 → bone of the lower spine never forms over the cord → meninges and spinal cord push out as an open sac → AFP leaks into amniotic fluid (high maternal AFP) → the same neural tube defect mechanics drag the hindbrain down through the foramen magnum (that is the Chiari II).

Move: close the back surgically in the first day of life (or in utero), place a VP shunt for hydrocephalus, monitor breathing and swallowing for medullary compression, lifelong folate-and-folate counseling for future pregnancies.

Deep dive: neural tube defects →