Posterior Fossa Anomalies

Slide 1 · Posterior fossa anomalies

Chiari I · Chiari II · Dandy–Walker · Joubert syndrome

Morphology, anatomical reasoning, and clinicopathological associations

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Localize abnormalities of the cerebellar tonsils, vermis, brainstem, and fourth ventricle.

Distinguish Chiari I, Chiari II, Dandy–Walker malformation, and Joubert syndrome.

Connect morphology with hydrocephalus, myelomeningocele, and syringomyelia.

Interpret characteristic anatomical patterns without conflating them.

Four anatomical signatures organize the lecture.

Anatomical schematic — not diagnostic imaging

Chiari I

Low tonsils

Chiari II

Small fossa; Descended vermis

Dandy–Walker

Expanded 4th-ventricular cyst

Joubert

Molar tooth configuration

Slide 3 · Posterior fossa malformations primarily involve two regions

The cerebellum may exhibit marked alterations of size, shape, or position.

Brainstem configuration may be distorted together with cerebellar structures.

Associated abnormalities can extend beyond the posterior fossa.

Anatomical location directs morphologic interpretation.

Anatomical schematic — not diagnostic imaging

Cerebellum

Brainstem

Vermis / midline

Tonsil

Slide 4 · The main anatomical landmarks are not interchangeable

Cerebellar tonsils: Inferior cerebellar structures relevant to Chiari I.

Vermis: Midline cerebellar structure altered in Chiari II, Dandy–Walker, and Joubert.

Superior cerebellar peduncles: Part of the Joubert imaging pattern.

Name the involved structure before naming the malformation.

Anatomical schematic — not diagnostic imaging

Cerebellum

Brainstem

Vermis / midline

Tonsil

Slide 5 · The foramen magnum is a major anatomical boundary

Inferior cerebellar structures normally remain above the craniocervical junction.

Descent through the foramen magnum is integral to both Chiari patterns.

The structure that descends differs between Chiari I and II.

Location relative to the foramen magnum matters.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Medulla / cord

Tonsils above line

Posterior fossa outline

Slide 6 · Tonsils and vermis: Two distinct anatomical signatures

Chiari I is defined by low-lying cerebellar tonsils.

Chiari II includes a misshapen midline cerebellum and downward vermian extension.

Equating the two forms with generic “cerebellar descent” loses specificity.

Tonsillar descent ≠ vermian displacement.

Chiari I

Chiari II

Tonsillar descent

Vermian descent

Two different displaced structures · schematic

Slide 7 · The fourth ventricle is central to Dandy–Walker morphology

Dandy–Walker cystic enlargement represents the expanded, roofless fourth ventricle.

Loss or severe deficiency of the normal vermis allows the large midline cystic space.

This is anatomically distinct from cerebellar tissue passing through the foramen magnum.

The Dandy–Walker cyst is a fourth-ventricular malformation.

Anatomical schematic — not diagnostic imaging

Midline cyst / expanded 4th ventricle

Deficient vermis

Schematic; Not a specimen image

Slide 8 · Posterior fossa size separates two major patterns

Chiari II is associated with a small posterior fossa.

Dandy–Walker malformation is characterized by an enlarged posterior fossa.

Size and direction of distortion should be assessed together.

Small and enlarged posterior fossae imply different patterns.

Anatomical schematic — not diagnostic imaging

Small posterior fossa

Enlarged posterior fossa

Slide 9 · Four disorders: Four leading morphological clues

Chiari I → low-lying cerebellar tonsils.

Chiari II → small fossa with caudally displaced, misshapen vermis.

Dandy–Walker → enlarged fossa and large midline fourth-ventricular cyst.

Joubert → vermian hypoplasia and the molar tooth configuration.

Use an anatomical rather than purely eponymic classification.

Anatomical schematic — not diagnostic imaging

Chiari I

Low tonsils

Chiari II

Small fossa; Descended vermis

Dandy–Walker

Expanded 4th-ventricular cyst

Joubert

Molar tooth configuration

Slide 10 · Chiari I: The defining lesion is tonsillar descent

Cerebellar tonsils lie abnormally low.

They extend below the foramen magnum into the vertebral canal.

The description does not require the complex Chiari II-associated constellation.

Structural definition: Low tonsils extending into the spinal canal.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Medulla / cord

Descending tonsils

Posterior fossa outline

Slide 11 · Chiari I: Sagittal pattern

Identify the foramen magnum as the reference line.

Follow inferior cerebellar tissue across this boundary.

The tonsils, not the vermis, are the key named structure.

Schematic: Descent of the cerebellar tonsils.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Medulla / cord

Descending tonsils

Posterior fossa outline

Slide 12 · Tonsillar position creates craniocervical crowding

Inferior tonsillar extension places cerebellar tissue at the narrow outlet of the skull.

This positional abnormality is the morphological substrate for local complications.

The extent of symptoms is not specified by morphology alone.

Anatomic crowding helps explain variable manifestations.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Medulla / cord

Descending tonsils

Posterior fossa outline

Slide 13 · Chiari I can impair cerebrospinal fluid flow

The source identifies impaired CSF flow as one route to symptoms.

The abnormality is situated at the skull–vertebral junction.

Flow disturbance provides a link to spinal cord fluid cavities.

Inferior displacement may disrupt CSF movement.

Anatomical schematic — not diagnostic imaging

Tonsils extend low

Craniocervical CSF flow affected

Possible symptoms / syrinx

Slide 14 · Chiari I can produce medullary compression

The medulla passes through the craniocervical region.

Low cerebellar structures may compress adjacent medullary tissue.

Medullary compression is a distinct proposed mechanism of symptoms.

The clinical effect is not limited to a cerebellar abnormality.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Medulla / cord

Descending tonsils

Posterior fossa outline

Slide 15 · Chiari I may coexist with a spinal cord cavity

Hydromyelia involves an expanded central canal.

Syringomyelia involves a fluid-filled cavity within spinal cord substance.

Either as an associated finding in Chiari I.

The spinal cord must be considered in a posterior fossa lesion.

Anatomical schematic — not diagnostic imaging

Spinal cord

Dilated central canal

Cord cross-section — schematic

Slide 16 · Chiari I has a variable clinical expression

Some individuals have a silent anatomical abnormality.

Symptoms may arise from CSF-flow impairment, medullary compression, or a syrinx.

Anatomical recognition and clinical manifestations are related but not identical.

Silent malformation and symptomatic disease are both possible.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Medulla / cord

Descending tonsils

Posterior fossa outline

Slide 17 · Chiari I: The stated treatment implication

Symptoms, when present, can usually be corrected by neurosurgical intervention.

This statement concerns potentially symptomatic Chiari I, not a blanket indication for surgery.

Detailed operative selection criteria are outside this supplied source.

Do not infer treatment for an asymptomatic patient from morphology alone.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Medulla / cord

Descending tonsils

Posterior fossa outline

Slide 18 · Case 1: Low cerebellar tonsils

A patient has low-lying cerebellar tonsils extending into the cervical canal.

A related spinal cord fluid cavity is described.

Which posterior fossa abnormality best matches this anatomical pattern?

Identify the malformation and explain the spinal association.

Anatomical schematic — not diagnostic imaging

Interpret the anatomy

What feature is most discriminating?

Use a source-supported pattern, not an assumed threshold.

Slide 19 · Case 1 reasoning: Chiari I

Tonsillar extension through the foramen magnum defines Chiari I.

Associated syringomyelia/hydromyelia is explicitly.

The anatomical clues do not establish Chiari II without its additional pattern.

Differentiate by the displaced structure and associated morphology.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Medulla / cord

Descending tonsils

Posterior fossa outline

Slide 20 · Chiari II is a complex posterior fossa malformation

Also referred to as Arnold–Chiari malformation in the source.

Its defining pattern includes a small posterior fossa and malformed midline cerebellum.

The cerebellar vermis extends downward through the foramen magnum.

Chiari II is not simply a more extensive Chiari I.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Medulla / cord

Vermian descent

Smaller posterior fossa

Slide 21 · Chiari II: Characteristic sagittal anatomy

Small posterior fossa confines abnormal hindbrain structures.

Midline cerebellar tissue is misshapen.

The vermis extends caudally beyond the foramen magnum.

Schematic: Downward vermian extension in a small fossa.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Medulla / cord

Vermian descent

Smaller posterior fossa

Slide 22 · A small posterior fossa is a defining feature of Chiari II

Robbins explicitly describes the posterior fossa as small.

The reduced compartment accompanies downward displacement of hindbrain tissues.

It contrasts directly with the expanded posterior fossa of Dandy–Walker.

Fossa size is an important anatomical discriminator.

Anatomical schematic — not diagnostic imaging

Small posterior fossa

Enlarged posterior fossa

Slide 23 · Chiari II: The vermis extends below the foramen magnum

The midline cerebellum is misshapen.

The inferior displacement involves the vermis.

This differs from the named low-lying tonsils of Chiari I.

Be precise about which cerebellar structure is displaced.

Chiari I

Chiari II

Tonsillar descent

Vermian descent

Two different displaced structures · schematic

Slide 24 · Caudal displacement may involve the medulla in Chiari II

Caudal medullary displacement among additional abnormalities.

The medulla is anatomically contiguous with the cervical spinal cord.

The lesion extends beyond cerebellar vermian shape alone.

Look for combined cerebellar and brainstem distortion.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Medulla / cord

Vermian descent

Smaller posterior fossa

Slide 25 · Hydrocephalus is almost invariable in Chiari II

The source describes hydrocephalus as present almost invariably.

Hydrocephalus denotes excessive CSF accumulation within the ventricles.

Its association should be assessed as part of the syndrome, not ignored.

Strong association: Chiari II ↔ hydrocephalus.

Anatomical schematic — not diagnostic imaging

Excess CSF volume

Ventricular expansion

Hydrocephalus

Slide 26 · Lumbar myelomeningocele strongly accompanies Chiari II

An almost invariable lumbar myelomeningocele.

Myelomeningocele includes neural tissue extending through a vertebral defect.

This extracranial association helps recognize the full malformation.

Posterior fossa and spinal dysraphism can form one pattern.

Anatomical schematic — not diagnostic imaging

Vertebral defect

Neural tissue in outpouching

Slide 27 · The tectum may also be malformed in Chiari II

Malformation of the tectum is listed among associated abnormalities.

This emphasizes that hindbrain anomalies may involve several connected regions.

Robbins does not specify an individual tectal morphology in this excerpt.

Do not invent a specific tectal shape absent from the source.

Anatomical schematic — not diagnostic imaging

Cerebellum

Brainstem

Vermis / midline

Tonsil

Slide 28 · Aqueductal stenosis is an associated Chiari II abnormality

Stenosis of the cerebral aqueduct in the Chiari II constellation.

Aqueductal narrowing can be a point of CSF-flow obstruction.

This provides an anatomical connection with hydrocephalus.

Aqueductal stenosis is one possible associated lesion.

Anatomical schematic — not diagnostic imaging

Aqueductal stenosis

Focal CSF outflow obstruction

Obstructive hydrocephalus

Slide 29 · Cerebral heterotopias may accompany Chiari II

Heterotopias are listed among the possible associated cerebral changes.

The neighboring Robbins section defines these as misplaced collections of neurons.

Thus a hindbrain malformation can coexist with an abnormality of forebrain organization.

The lesion may extend beyond the posterior fossa.

Anatomical schematic — not diagnostic imaging

Misplaced neuronal clusters (schematic)

Slide 30 · Hydromyelia or syringomyelia can coexist with Chiari II

Both spinal fluid-cavity patterns are included among associated abnormalities.

Hydromyelia is dilatation of the ependyma-lined central canal.

Syringomyelia is a cleft-like fluid cavity within the cord.

Recognize the spinal association without conflating its two forms.

Anatomical schematic — not diagnostic imaging

Spinal cord

Dilated central canal

Cord cross-section — schematic

Slide 31 · Chiari II: Obligatory-looking versus optional findings

Core morphology: Small fossa, abnormal midline cerebellum, and vermian descent.

Almost invariable associations: Hydrocephalus and lumbar myelomeningocele.

Other potential findings: Medullary/tectal changes, aqueductal stenosis, heterotopias, cord cavities.

The source distinguishes a characteristic core from variable additions.

Anatomical schematic — not diagnostic imaging

Editable source-grounded anatomical schematic

Slide 32 · Chiari I and II differ in lesion complexity

Feature

Chiari I

Chiari II

Key descent

Cerebellar tonsils

Midline cerebellar vermis

Posterior fossa

Not specified as small in source

Small

Hydrocephalus

Not identified as almost invariable

Almost invariable

Myelomeningocele

Not defining

Lumbar; almost invariable

The distinction is anatomical and syndromic.

Slide 33 · Case 2: Vertebral defect and hydrocephalus

A patient has a lumbar myelomeningocele and hydrocephalus.

Midline imaging shows a small posterior fossa and caudally displaced cerebellar vermis.

Which malformation explains the combined findings?

Consider the syndrome rather than any single abnormality.

Anatomical schematic — not diagnostic imaging

Interpret the anatomy

What feature is most discriminating?

Use a source-supported pattern, not an assumed threshold.

Slide 34 · Case 2 reasoning: Chiari II

Vermian descent within a small posterior fossa identifies the core morphology.

Hydrocephalus and lumbar myelomeningocele are the strongly linked associations.

This combination fits Chiari II rather than isolated Chiari I.

The conjunction of intracranial and spinal findings is decisive.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Medulla / cord

Vermian descent

Smaller posterior fossa

Slide 35 · Dandy–Walker malformation enlarges the posterior fossa

The posterior fossa is enlarged, not small.

The cerebellar vermis is absent or rudimentary.

A large midline cyst occupies the site of the deficient vermis.

The spatial pattern is the opposite of Chiari II crowding.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Expanded fourth ventricle

Enlarged posterior fossa

Slide 36 · An enlarged fossa frames Dandy–Walker morphology

An enlarged posterior fossa is explicitly part of the malformation.

The expanded compartment is associated with the large midline cyst.

Fossa size should be considered together with vermian development.

Do not interpret the cyst in isolation.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Expanded fourth ventricle

Enlarged posterior fossa

Slide 37 · The vermis may be absent or only rudimentary

The source permits complete absence of the vermis.

Alternatively, a small anterior vermian remnant may be present.

Both patterns are compatible with the described Dandy–Walker malformation.

Vermian status is central, but not an all-or-none criterion.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Expanded fourth ventricle

Enlarged posterior fossa

Slide 38 · The large cyst represents an expanded fourth ventricle

The midline cyst is interpreted as the expanded, roofless fourth ventricle.

Normal vermian formation has not occurred.

The lesion is not merely a nonspecific cyst adjacent to the cerebellum.

Identify the anatomical identity of the fluid space.

Anatomical schematic — not diagnostic imaging

Midline cyst / expanded 4th ventricle

Deficient vermis

Schematic; Not a specimen image

Slide 39 · Dandy–Walker cyst has two different boundaries

Its inner surface is lined by ependyma.

Its outer surface is contiguous with leptomeninges.

These relationships define the cyst morphology.

Ependymal lining and leptomeningeal outer continuity are distinct.

Anatomical schematic — not diagnostic imaging

Midline cyst / expanded 4th ventricle

Ependymal lining

Leptomeningeal outer continuity

Schematic; Not a specimen image

Slide 40 · The roofless fourth ventricle links two abnormalities

Marked vermian underdevelopment alters the normal roof region.

The fourth ventricle becomes a large midline cystic space.

This accounts for the conspicuous altered posterior fossa configuration.

Vermian defect and ventricular expansion are connected anatomically.

Anatomical schematic — not diagnostic imaging

Absent / rudimentary vermis

Roofless fourth ventricle

Large midline ventricular cyst

Slide 41 · Brainstem nuclei may also be dysplastic

Dysplasia of brainstem nuclei is commonly found in association.

Thus Dandy–Walker is not necessarily restricted to the cerebellar vermis.

The source does not specify a particular microscopic diagnostic criterion.

Remember the possible associated brainstem abnormality.

Anatomical schematic — not diagnostic imaging

Cerebellum

Brainstem

Vermis / midline

Tonsil

Slide 42 · Chiari II and Dandy–Walker have opposite fossa sizes

Pattern

Chiari II

Dandy–Walker

Fossa size

Small

Enlarged

Vermis

Misshapen; displaced caudally

Absent or rudimentary

Dominant space

Crowded hindbrain

Expanded fourth-ventricular cyst

Other clue

Myelomeningocele ± hydrocephalus

Brainstem nuclear dysplasia may occur

Focus on structural direction, not eponyms.

Slide 43 · Case 3: Large posterior fossa and midline cyst

Imaging describes an expanded posterior fossa.

The vermis is markedly deficient; A large cystic structure occupies the midline.

What is the most specific developmental pattern?

Explain where the cyst comes from anatomically.

Anatomical schematic — not diagnostic imaging

Interpret the anatomy

What feature is most discriminating?

Use a source-supported pattern, not an assumed threshold.

Slide 44 · Case 3 reasoning: Dandy–Walker malformation

The combination of enlarged fossa, vermian deficiency, and midline cyst is characteristic.

The cyst corresponds to the expanded, roofless fourth ventricle.

This differs from a small posterior fossa with hindbrain descent.

The cyst is a malformed ventricular compartment.

Anatomical schematic — not diagnostic imaging

Foramen magnum

Expanded fourth ventricle

Enlarged posterior fossa

Slide 45 · Joubert syndrome has a characteristic hindbrain configuration

The cerebellar vermis is hypoplastic.

Superior cerebellar peduncles show rotation and apparent elongation.

The brainstem contour is altered, together producing the molar tooth sign.

Diagnosis rests on a composite imaging configuration.

Anatomical schematic — not diagnostic imaging

Rotated superior cerebellar peduncles

Molar tooth configuration (conceptual; Not an MRI)

Slide 46 · Vermian hypoplasia is part of the Joubert pattern

The cerebellar vermis is underdeveloped.

This abnormality alone does not distinguish Joubert from Dandy–Walker.

Associated peduncular and brainstem changes establish the described signature.

Use combined anatomy rather than a single feature.

Anatomical schematic — not diagnostic imaging

Rotated superior cerebellar peduncles

Molar tooth configuration (conceptual; Not an MRI)

Slide 47 · Superior cerebellar peduncles change shape and orientation

The source describes rotation of the superior cerebellar peduncles.

They appear elongated on imaging.

These changes must be interpreted together with the vermian and brainstem findings.

Peduncular morphology helps establish the pattern.

Anatomical schematic — not diagnostic imaging

Rotated superior cerebellar peduncles

Molar tooth configuration (conceptual; Not an MRI)

Slide 48 · Altered brainstem shape contributes to Joubert imaging

The posterior fossa phenotype also affects the brainstem contour.

The abnormal peduncles and brainstem shape jointly create the imaging sign.

The source does not attribute the sign to isolated vermian hypoplasia.

A combined configuration yields the characteristic appearance.

Anatomical schematic — not diagnostic imaging

Rotated superior cerebellar peduncles

Molar tooth configuration (conceptual; Not an MRI)

Slide 49 · The molar tooth sign is a composite appearance

Vermian hypoplasia provides an important anatomical context.

Rotated, apparently elongated superior cerebellar peduncles form prominent limbs.

An altered brainstem outline completes the characteristic imaging configuration.

A schematic of an imaging sign is not a patient MRI.

Anatomical schematic — not diagnostic imaging

Rotated superior cerebellar peduncles

Molar tooth configuration (conceptual; Not an MRI)

Slide 50 · Joubert syndrome belongs to a ciliopathy spectrum

Joubert and related disorders to diverse genetic mutations.

The affected genes encode components of the primary (nonmotile) cilium.

No single causal gene is specified in the excerpt.

Genetic heterogeneity converges on the primary cilium.

Anatomical schematic — not diagnostic imaging

Primary, nonmotile cilium

Cell

Slide 51 · Ciliary defects are the molecular category, not a single mutation

The source identifies several different cilium-component genes as possible causes.

Mutational diversity is compatible with a shared anatomical hindbrain phenotype.

The supplied text does not establish genotype–phenotype rules or mutation frequencies.

Avoid false gene-level specificity absent from the source.

Anatomical schematic — not diagnostic imaging

Primary, nonmotile cilium

Cell

Multiple affected cilium-component genes

Slide 52 · Joubert and Dandy–Walker share vermian abnormalities

Feature

Dandy–Walker

Joubert

Vermis

Absent or rudimentary

Hypoplastic

Fossa / cyst

Enlarged fossa; large fourth-ventricular cyst

Not defining in provided description

Imaging discriminator

Expanded, roofless fourth ventricle

Molar tooth sign

Mechanistic association

No specific gene class given here

Primary cilium genes

Their additional morphology separates them.

Slide 53 · Case 4: A molar tooth appearance

Imaging demonstrates vermian hypoplasia, unusual superior cerebellar peduncles, and a distorted brainstem contour.

The combined morphology is called the “molar tooth sign.”

Which malformation group and molecular category are suggested?

Use both the imaging configuration and the genetic class.

Anatomical schematic — not diagnostic imaging

Interpret the anatomy

What feature is most discriminating?

Use a source-supported pattern, not an assumed threshold.

Slide 54 · Case 4 reasoning: Joubert-related disorder

The composite hindbrain pattern identifies Joubert syndrome or a related disorder.

The associated genes encode components of the primary, nonmotile cilium.

This does not identify a particular gene or variant without further evidence.

Molar tooth anatomy plus ciliopathy association.

Anatomical schematic — not diagnostic imaging

Rotated superior cerebellar peduncles

Molar tooth configuration (conceptual; Not an MRI)

Slide 55 · Hydromyelia and syringomyelia are not synonymous

Hydromyelia: Enlargement of the normal ependyma-lined central canal.

Syringomyelia: A new fluid-filled cleft-like cavity within the cord.

Both may accompany the Chiari malformations.

The lining and anatomical location distinguish the two entities.

Anatomical schematic — not diagnostic imaging

Spinal cord

Dilated central canal

Cord cross-section — schematic

Slide 56 · A syrinx damages spinal cord parenchyma

The cavity is associated with destruction of adjacent gray and white matter.

Dense reactive gliosis surrounds the involved region.

This cord injury explains why an associated fluid cavity can be symptomatic.

A syrinx is not simply a benign reservoir of fluid.

Anatomical schematic — not diagnostic imaging

Spinal cord

Syrinx cavity

Cord cross-section — schematic

Slide 57 · Syringomyelia can disrupt crossing sensory fibers

The characteristic syndrome is loss of pain and temperature sensation in the upper extremities.

The interrupted fibers cross through the anterior spinal commissure.

This is a clinicopathological link made in the neighboring Robbins section.

Anterior commissural injury produces a selective sensory pattern.

Anatomical schematic — not diagnostic imaging

Spinal cord

Anterior commissural region

Cord cross-section — schematic

Slide 58 · The relationship between Chiari and syrinx is associative

Syringomyelia as potentially associated with a Chiari malformation.

Other causes include intraspinal tumors and prior traumatic injury.

A syrinx therefore does not by itself prove an underlying Chiari anomaly.

Avoid reverse inference from a nonspecific spinal cord cavity.

Anatomical schematic — not diagnostic imaging

Spinal cord

Dilated central canal

Cord cross-section — schematic

Slide 59 · CSF pathway anatomy explains why obstruction matters

CSF is produced by the ventricular choroid plexus.

It circulates through ventricles to basal cisterns, then subarachnoid space.

Impaired flow or resorption can expand the ventricular system.

Follow the CSF pathway when interpreting hydrocephalus.

Anatomical schematic — not diagnostic imaging

Ventricular CSF

Basal cisterns

Subarachnoid space

Arachnoid absorption

Slide 60 · Obstructive hydrocephalus: Where continuity is lost

Aqueductal stenosis can create focal ventricular obstruction.

This is termed noncommunicating or obstructive hydrocephalus.

Aqueductal stenosis as an associated Chiari II abnormality.

Keep anatomical obstruction distinct from the general diagnosis.

Anatomical schematic — not diagnostic imaging

Aqueductal stenosis

Focal CSF outflow obstruction

Obstructive hydrocephalus

Slide 61 · Recognize lesions by their dominant location

Main anatomical clue

Best matching anomaly

Low tonsils entering cervical canal

Chiari I

Misshapen descended vermis + small fossa

Chiari II

Midline ventricular cyst + enlarged fossa

Dandy–Walker

Peduncular rotation + abnormal brainstem (“molar tooth”)

Joubert spectrum

Tonsils, vermis, ventricle, and peduncles answer different questions.

Slide 62 · Recognize lesions by their associated abnormalities

Malformation

Major explicitly stated association

Chiari I

CSF-flow impairment, medullary compression, syrinx; may be silent

Chiari II

Hydrocephalus and lumbar myelomeningocele (almost invariable)

Dandy–Walker

Dysplasia of brainstem nuclei can occur

Joubert-related

Diverse mutations affecting primary cilium components

Associations increase confidence without replacing core morphology.

Slide 63 · Case 5: A two-part differential

A posterior fossa study shows vermian abnormality but no description of fossa size.

One report emphasizes a large midline fourth-ventricular cyst; Another emphasizes atypical peduncles and brainstem shape.

Which additional feature differentiates Dandy–Walker from Joubert?

Identify the discriminating relationship, not simply “vermis abnormal.”

Anatomical schematic — not diagnostic imaging

Interpret the anatomy

What feature is most discriminating?

Use a source-supported pattern, not an assumed threshold.

Slide 64 · Case 5 reasoning: Identify the signature structure

An enlarged fossa with expanded, roofless fourth ventricle supports Dandy–Walker.

Peduncular rotation/apparent elongation plus altered brainstem shape supports Joubert.

Vermian deficiency alone is not sufficiently specific.

Pattern combinations are more reliable than isolated terms.

Anatomical schematic — not diagnostic imaging

Editable source-grounded anatomical schematic

Slide 65 · Four malformations: Comparative morphology

Feature

Chiari I

Chiari II

Dandy–Walker

Joubert

Dominant finding

Low tonsils

Descended vermis

4th-ventricular cyst

Molar tooth sign

Posterior fossa

Not defining

Small

Enlarged

Not defining

Vermis

Not defining

Misshapen; descended

Absent / rudimentary

Hypoplastic

Key association

Syrinx ± symptoms

Hydrocephalus + myelomeningocele

Brainstem nuclei dysplasia

Primary cilium genes

Use this summary to interpret a previously unseen anatomical description.

Slide 66 · Take-home anatomical rules

Chiari I: Identify the low-lying cerebellar tonsils.

Chiari II: Identify the small fossa, descended vermis, and major spinal/CSF associations.

Dandy–Walker: Identify the enlarged fossa and roofless fourth-ventricular cyst.

Joubert: Identify the combined molar tooth configuration and ciliopathy link.

Anatomy first; Associated findings second; Inference last.

Anatomical schematic — not diagnostic imaging

Chiari I

Low tonsils

Chiari II

Small fossa; Descended vermis

Dandy–Walker

Expanded 4th-ventricular cyst

Joubert

Molar tooth configuration

Slide 67 · Primary reference and source boundaries

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th edition (2026), Chapter 28, pp. 1132–1136.

Main section: “Posterior Fossa Anomalies” (p. 1135).

Context: hydrocephalus, neural tube malformations, syringomyelia and hydromyelia.

Original editable schematics are explanatory and are not reproduced textbook figures or diagnostic images.

No original chapter PDF or licensed Fig. 28.8 was available for figure verification.