Missing vermis. Ballooning 4th ventricle. A head that keeps growing because the CSF has nowhere to go.
Here is how to spot it on the prenatal scan and never confuse it with Arnold-Chiari again.
A 3-day-old newborn is brought in for evaluation after a prenatal ultrasound at 32 weeks
flagged a cystic posterior fossa. On exam, the head circumference is above the 97th percentile,
the anterior fontanelle is tense and bulging, and the eyes show the
sunset sign (downward deviation, sclera visible above the iris).
Postnatal MRI confirms agenesis of the cerebellar vermis with a large midline cyst that communicates with the 4th ventricle.
What is the diagnosis?
Arnold-Chiari II malformation
Dandy-Walker malformation
Mega cisterna magna
Blake pouch cyst
Missing vermis + cystic 4th ventricle + bulging head in an infant = Dandy-Walker until proven otherwise.
Arnold-Chiari II is the opposite problem: the vermis and tonsils descend through the foramen magnum, almost always paired with a myelomeningocele on the back. There is no vermis agenesis.
Mega cisterna magna looks like a large fluid space behind the cerebellum, but the vermis is intact. It is an incidental finding, not a disease.
Blake pouch cyst is a milder cousin of Dandy-Walker (delayed opening of foramen of Magendie) where the vermis is preserved.
The sunset sign in a newborn means raised intracranial pressure pushing the eyes down. With a tense fontanelle plus macrocephaly, you are watching obstructive hydrocephalus unfold in front of you.
The four fingerprints
Clues that lock the diagnosis
Every clinical vignette of Dandy-Walker is built from these four pieces. Spot two of them and you are home.
Clue 1 · Cerebellum
Vermis agenesis or hypoplasia
The midline strip of cerebellum (the vermis) never fuses around the developing 4th ventricle.
On MRI the two cerebellar hemispheres look like two separate cauliflowers with empty space between them.
Clue 2 · Ventricle
Cystic 4th ventricle
The 4th ventricle balloons into a large midline posterior fossa cyst that fills the gap where the vermis should sit.
It directly communicates with the ventricle, which is the giveaway versus an isolated arachnoid cyst.
Clue 3 · Head
Hydrocephalus + macrocephaly
CSF cannot exit the 4th ventricle, so pressure builds upstream. Infant shows up with a
head crossing the 97th percentile, a tense fontanelle, sunset sign, and developmental delay.
Clue 4 · Skull base
Torcular-lambdoid inversion
The posterior fossa is so enlarged that the tentorium and the torcular Herophili sit high, above the lambdoid suture.
On lateral imaging the tentorium points up and outward, not its usual downward slope.
Toggle the anatomy
Normal posterior fossa vs Dandy-Walker
Sagittal midline view. Flip between a normal posterior fossa and Dandy-Walker. Watch the vermis disappear, the 4th ventricle balloon into a cyst, and the tentorium ride up.
Normal. Vermis fuses, 4th ventricle is a thin slit, foramen of Magendie drains CSF into the subarachnoid space, tentorium slopes gently downward.
Watch the plumbing
Where the CSF gets stuck
CSF starts in the lateral ventricles, threads through the 3rd, squeezes through the aqueduct of Sylvius, lands in the 4th, and must exit through the foramen of Magendie (midline) and the foramen of Luschka (lateral) to reach the subarachnoid space.
Block the outflow and the 4th ventricle balloons into a cyst.
Normal. CSF flows lateral › 3rd › aqueduct › 4th › out via Magendie and Luschka › subarachnoid space › arachnoid granulations.
Look-alikes
Dandy-Walker vs the impostors
Four entities can land a cystic posterior fossa on the radiology read. Only one is Dandy-Walker. The discriminators are the vermis, the descent of structures, and what is on the back of the baby.
Entity
Vermis
4th ventricle
Posterior fossa
Other clue
Dandy-Walker
Agenesis or hypoplasia
Cystic dilation, communicates with cyst
Enlarged, high tentorium, torcular-lambdoid inversion
Hydrocephalus + macrocephaly in infant
Arnold-Chiari II
Present, but herniates downward through foramen magnum
Elongated, displaced inferiorly
Small, crowded
Myelomeningocele on the back. Tonsils and medulla descend.
Mega cisterna magna
Normal and intact
Normal
Enlarged cisterna magna only
Incidental, no clinical disease. Pure CSF space.
Blake pouch cyst
Normal (preserved)
Mildly enlarged, persistent posterior pouch
Normal or slightly enlarged
Delayed opening of foramen of Magendie. Milder than Dandy-Walker.
Challenge before reveal: the scan shows a posterior fossa cyst. Which single check separates Dandy-Walker from Blake pouch and mega cisterna magna?
The vermis is the hinge. Absent or severely hypoplastic vermis with a cystic 4th ventricle points to Dandy-Walker. Intact vermis with extra posterior CSF points away from it.
From the scanner
What it looks like on MRI
Sagittal T2-weighted MRI showing the Dandy-Walker continuum. Look for the bright CSF-filled space behind the cerebellum where the vermis should sit, and the dilated 4th ventricle communicating with it. Tap the image to enlarge.
Sagittal T2 MRI · Dandy-Walker continuum.
The bright fluid pocket behind the cerebellum represents the cystic 4th ventricle expanding into a posterior fossa cyst. The cerebellar vermis is small or absent in the space above the cyst. Compare the high-riding tentorium to a normal study.
Medically reviewed by Kaitlyn Cocuzzo, MD and Fatima Ali, DO · Last updated June 30, 2026 at 10:52 AM ET
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