Posterior fossa malformations

POSTERIOR FOSSA MALFORMATIONS

Dr.Archana Koshy

1. Posterior fossa anatomy

2. Chiari malformations

3. Dandy walker malformation

4. Joubert syndrome

5. Rhomencephalosynapsis

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• Congenital posterior fossa anomalies may result from inherited (genetic) or acquired (disruptive) causes.

• A malformation is defined as a congenital morphologic anomaly of a single organ or body part due to an alteration of the primary developmental program caused by a genetic defect .

• Gene mutations causing malformations may be de novo (ie, new in the affected child, rather than present in or transmitted by the parents) or inherited from the parents.


• The cerebellum is one of the earliest cerebral structures to develop.

• Its development is also unusually protracted as cellular proliferation, migration, and maturation extend into the first few postnatal months.

• Neural structures in the posterior fossa - Embryonic hindbrain (rhombencephalon)

• Mesencephalon - Midbrain structures. (Mesodermal elements give rise to the meninges and bone.


• The Posterior fossa is the largest and deepest of all the cranial

fossae .

• Bowl shaped, relatively protected space that lies below the tentorium.

• Contains the HINDBRAIN – Brainstem , the vermis anteriorly and the

cerebellar hemispheres posterolaterally .

• Posterior fossa CSF containing spaces include

1. Part of the cerebral aqueduct

2. Fourth ventricle

3. CSF cisterns that surround the brainstem and cerebellum .


BONE

• Anterior wall –

1. Dorsum sellae of the sphenoid body

2. Clivus of the basioccipital bone

• Lateral wall –

1. Petrous temporal bone

• Floor – Occipital squamae

• Superiorly – with the supratentorial compartment through the

U shaped tentorial incisura

• Inferiorly – Cervical subarachnoid space through the ovoid

foramen magnum .6/8/2017 Posterior Fossa Malformations 6

• Conventional magnetic resonance (MR) imaging allows

detailed evaluation of the anatomy of the posterior fossa

and its contents.

• A midline sagittal T1- or T2-weighted sequence is ideal for

showing the size of the posterior fossa, the shape and size

of the vermis, and the size and morphology of the fourth

ventricle and brainstem


• BRAINSTEM

• The brainstem has three anatomic divisions: The midbrain, pons, and

medulla.

• The midbrain (mesencephalon) lies partly above and partly below the

tentorium.

• The bulb-shaped pons nestles into the gentle curve of the clivus.

• The medulla is the most caudal brainstem segment and represents the

transition from the brain to the spinal cord.

• An important imaging landmark is the prominent “bump” along the

dorsal medulla created by the nucleus gracilis.

• This demarcates the junction between the fourth ventricle (obex) and

central canal of the spinal cord. The nucleus gracilis normally lies above

the foramen magnum.






•ARNOLD CHIARI MALFORMATIONS


• Chiari malformations were first described in the late nineteenth

century by the Austrian pathologist Hans Chiari.

• He described what seemed to be a related group of hindbrain

malformations associated with hydrocephalus and divided them into

three types: Chiari 1-3.

• Chiari 1 and 2 are pathogenetically distinct disorders.


Chiari 1 Malformation

• Most common variant of the chiari malformations

• Characterised by caudal descent of the cerebellar tonsil through the foramen

magnum.

• Symptoms are proportional to the degree of descent

ETIOLOGY

• The pathogenesis of CM1 is incompletely understood and remains

controversial.

• Primary paraxial mesodermal insufficiency with underdeveloped occipital

somites has also been invoked to explain the development of CM1.

• Other theories suggest that disorders of neural crest-derived elements

could lead to hyper- or hypoossification of the basi-chondro-cranium,

resulting in morphometric changes in the posterior fossa.

• A combination of altered bony anatomy and abnormal CSF

hydrodynamics is the most widely accepted concept

PRESENTATION

• Between one-third and one-half of all patients with imaging findings

consistent with CM1 are asymptomatic at the time of diagnosis.

• Presentation of symptomatic CM1 differs with age.

• Children who are two years and younger most commonly present with

oropharyngeal dysfunction (nearly 80%).

• Those between three and five years present with headache (57%) or

symptoms related to syringomyelia (86%) and scoliosis (38%).

• Uncommon presentations include hypersomnolence and sleep apnea.

• Valsalva-induced suboccipital headache (i.e., with coughing or

sneezing), neck pain, and syncope are common in adults.

Radiographic features

• Distance is measured by drawing a line from

the inner margins foramen magnum

(basion to opisthion)- McRae’s Line, and

measuring the inferior most part of the tonsils

• above foramen magnum: normal

• <5 mm: also normal but the term benign

tonsillar ectopia can be used

• >5 mm: Chiari 1 malformation

Axial Sections :

the medulla is embraced

by the tonsils and little if

any CSF is present -

crowded foramen magnum.

Sagittal :• tonsils are pointed, rather than

rounded and referred to as peg-like• sulci are vertically oriented, forming

so-called sergeant stripes

ASSOCIATIONS

• Cervical cord syrinx is present in ~35%

(range 20-56%): more common in

symptomatic patients

• Hydrocephalus in up to 30% of cases

and both are thought to result from

abnormal CSF flow dynamics through

the central canal of the cord and around

the medulla

• In ~35% (range 23-45%) of cases there

are associated skeletal anomalies :

• ◦ platybasia/basilar invagination

• ◦ atlanto-occipital assimilation

• ◦ Sprengel deformity

• ◦ Syndromic associations

• ▪Klippel-Feil syndrome

• ▪Crouzon syndrome

TREATMENT OPTIONS.

• Asymptomatic tonsillar ectopia in the absence of an associated syrinx or

scoliosis is usually not treated.

• Periodic surveillance of patients with documented hydrosyringomyelia is

generally recommended, as 12% of syringes show increase in size and

may require craniocervical decompression if symptoms worsen.

• Treatment of symptomatic CM1 attempts to restore normal CSF fluid

dynamics at the foramen magnum .

• A suboccipital/posterior C1 decompression with or without partial tonsillar

resection is the most common procedure.

DIFFERENTIAL DIAGNOSIS• Congenital tonsillar descent (CM1) must be distinguished from normal variants

(mild uncomplicated tonsillar ectopia).

• The most important pathological differential diagnosis is acquired tonsillar

herniation caused by INCREASED INTRACRANIAL PRESSURE OR

INTRACRANIAL HYPOTENSION.

• Approximately 20% of patients with idiopathic intracranial hypertension

(“pseudotumor cerebri”) exhibit cerebellar tonsillar ectopia ≥ 5 mm.

• Half of these patients exhibit a peg-like tonsil configuration, and many have a

low-lying obex.

• Other conditions that reduce posterior cranial fossa volume can also displace

the tonsils below the foramen.

• Such causes of cranial constriction include CRANIOSYNOSTOSIS,

ACHONDROPLASIA, ACROMEGALY, AND PAGET DISEASE.

CHIARI 1.5

• Sometimes considered as a Bulbar variant of Chiari I malformation

• Combination of cerebellar tonsillar herniation seen in a case of Chiari I malformation along with caudal herniation of some portion of

brainstem (often medulla oblongata) through the foramen magnum

• Often asymptomatic. Clinical features, if present, may include intermittent neck pain, more on extension of cervical spine

Radiographic features

MRIMRI if the best method for the diagnosis with

sagittal T1 WI to assess tonsillar herniation:

• Descent >6 mm favors chiari I malformation

and >12 mm suggests chiari 1.5

malformation

•Associated findings may include

◦Posterior angulation of the odontoid

process

◦Hydrocephalus

◦Crowded small posterior fossa

◦Syringohydromyelia

◦Scoliosis

CHIARI II MALFORMATION

1. MYELOMENINGOCOELE

2. SMALL POSTERIOR FOSSA

3. DESCENT OF THE BRAINSTEM

CHIARI II MALFORMATION

• Relatively common congenital

malformation of the spine and posterior

fossa ( ~1:1000 live births)

• Numerous associated abnormalities are also

frequently encountered

• CM2 is a disorder of neural tube closure but also involves paraxial

mesodermal abnormalities of the skull and spine.

• A number of steps are required for proper neural tube closure and formation of

the focal expansions that subsequently form the cerebral vesicles and

ventricles.

• Skeletal elements of both the skull and vertebral column become “modeled”

around the neural tube.

• Only if the posterior neuropore closes will the developing ventricles expand

sufficiently for a normal-sized posterior fossa to form around the hindbrain.

• If this does not happen, the cerebellum develops in a small posterior fossa

with abnormally low tentorial attachments.

• The growing cerebellum is squeezed cephalad through the tentorial incisura

and stretched inferiorly through the foramen magnum (FM).

RADIOGRAPHIC FEATURESAntenatal ultrasound

Classical signs described on ultrasound include

•Lemon sign

•Banana cerebellum sign

There may also be evidence of fetal ventriculomegaly due to obstructive effects as a result of downward cerebellar herniation.Additionally many of the associated malformations (e.G. Corpus callosaldysgenesis) may be identified.

SKULL AND DURA

• The calvarial vault forms from membranous bone.

• With failure of neural tube closure and absence of fetal

brain distension, normal induction of the calvarial

membranous plates does not occur.

• Disorganized collections of collagen fibers and

deficient radial growth of the developing calvaria

ensue.

• The results a striking anomaly called lacunar skull

(i.e., Lückenschädel)

• Focal calvarial thinning and a

“scooped-out” appearance are

typical imaging findings of

lacunar skull.

• The calvaria appears thinned

with numerous circular or oval

lucent defec ts and shallow

depressions.

• Changes diminish with age and

are mostly resolved by six

months, although some

scalloping of the inner table

often persists into adulthood.

POSTERIOR FOSSA

•Small posterior fossa with a low attachment of

the tentorium and low torcula

•The brainstem appears 'pulled' down with an

elongated and low lying fourth ventricle

•The tectal plate appears beaked: inferior

colliculus is elongated and points posteriorly,

with resulting angulation of the aqueduct which

results in aqueductal stenosis and

hydrocephalus

•Cerebellar tonsils and vermis are displaced

inferiorly through foramen magnum which

appears crowded

SPINE

•Spina bifida aperta / myelomeningocele

• Tethered Cord

CHIARI III MALFORMATION

• Extremely rare anomaly

• Low occipital and high cervical

encephalocele

• Herniation of posterior fossa contents,

that is, the cerebellum and/or the

brainstem, occipital lobe, and fourth

ventricle

Variants

• Chiari IV malformation -extreme cerebellar hypoplasia-now

considered to be an obsolete term

• Chiari V malformation - absent cerebellum , herniation of the

occipital lobe through the foramen magnum

DANDY WALKER COMPLEX

• Malformation of posterior fossa

– Pathogenesis unknown but thought to be due to arrest of

development of hindbrain around 7-10 week gestation.

• Spectrum of disease that includes:

1. Dandy-Walker malformation

2. Dandy-Walker variant

3. Mega Cistern Magna

4. Posterior Fossa Arachnoid Cyst

• Occurs in1:30,000births

• Seen in 4-12% of all babies with hydrocephalus

DANDY WALKER MALFORMATION

TRIAD OF MALFORMATIONS

1. Cystic dilation of fourth ventricle

2. Complete or partial agenesis of the cerebellar vermis

3. Enlarged posterior fossa with displacement of the tentorium and

torcular and lateral sinus.

• Diagnosed after 18 weeks – closure of cerebellar vermis should happen

by that time

DANDY WALKER MALFORMATION

ULTRASOUND

Antenatal sonographic features that would suggest the diagnosis include the

combination of :

• marked enlargement of the cisterna magna (> or = 10 mm)

• complete aplasia of the vermis

• a trapezoid-shaped gap between the cerebellar hemispheres

Antenatal ultrasound may falsely over diagnose the condition if scanned before 18

weeks due to the vermis not being properly formed before that time.

MRI is the modality of choice

for assessment of DWM, although both CT

and ultrasound will demonstrate the

pertinent features.

Classically DWM consists of the triad of:

• HYPOPLASIA OF THE VERMIS and

cephalad rotation of the vermian remnant

• CYSTIC DILATATION OF THE

FOURTH VENTRICLE extending

posteriorly; usually the cerebellar

hemispheres are displaced anterolaterally,

but with a normal size and morphology

• ENLARGED POSTERIOR FOSSA

WITH TORCULAR-LAMBDOID

INVERSION (torcular lying above the

level of the lambdoid due to abnormally

high tentorium)

DANDY-WALKER VARIANT

– Cerebellar dysgenesis without

enlargement of the posterior fossa

– Variable hypoplasia of the

cerebellar vermis

– Better prognosis

DWV is now considered a mild form

of dandy walker Spectrum

JOUBERT SYNDROME• Group of syndromes in which the obligatory hallmark is the "molar tooth" sign,

a complex mid- and hindbrain malformation that resembles a molar tooth on

axial MR scans.

• Consists of vermian hypoplasia/dysplasia .

• Characterized by

1. Hypotonia

2. Ataxia

3. Ocular motor apraxia,

4. Neonatal breathing dysregulation

5. Intellectual disability of variable severity

• Systemic involvement may be present and includes renal

(nephronophthisis), ocular (colobomas, retinal dystrophy),

hepatic (congenital hepatic fibrosis), and skeletal (various

forms of polydactyly) involvement

• Anomalies of the kidneys, eyes, extremities, liver, and bile

ducts are common in the JSRD spectrum.

POSTERIOR FOSSA ARACHNOID CYSTS

• Duplications of the arachnoid membrane produce fluid-filled cysts

known as arachnoid cysts.

• About 10% of arachnoid cysts in children occur in the posterior fossa

1. Macrocephaly

2. Signs of increased intracranial pressure

3. Developmental delay.

• However, these cysts may be asymptomatic and discovered incidentally

.Arachnoid cysts do not communicate with the fourth ventricle or the

subarachnoid space

BLAKE POUCH CYST

• Caused by the lack of fenestration of the blake pouch

• Absence of communication between the fourth ventricle and the

subarachnoid space .

• The cerebellum has a normal size and shape.

• Blake pouch cyst occurs sporadically, and no recurrence risk has

been reported.

• Hydrocephalus and macrocephaly are the most common

presenting features in the neonatal period.

• In the absence of shunt-related complications, the prognosis is

generally favorable.

MEGA CISTERNA MAGNA

• Enlarged cisterna magna (-10 mm on midsagittal images) with

an intact vermis, a normal fourth ventricle, and, in some

patients, an enlarged posterior fossa

• Represents a truly focal enlargement of the subarachnoid

space in the inferior and posterior portions of the posterior

fossa.

• Mega cisterna magna freely communicates with the fourth

ventricle and the cervical subarachnoid space

• Results in consistent absence of hydrocephalus.

RHOMBENCEPHALOSYNAPSIS

• Characterized by absence of the vermis and continuity of the cerebellar hemispheres, dentate nuclei, and superior cerebellar peduncles .

• The majority of patients are nonsyndromic.

• Key feature of Gómez-López-Hernández syndrome

• May be seen in patients with associated VACTERL

1. Truncal and/ or limb ataxia

2. Abnormal eye movements

3. Head stereotypies

4. Delyed motor development

• THE KEY NEUROIMAGING FINDINGS-

1. Agenesis or hypogenesis of the vermis

2. continuity (often called fusion) of the cerebellar hemispheres, superior cerebellar peduncles, and dentate nuclei

3. Creates a horseshoe-shaped arch across the midline, resulting in a keyhole-shaped fourth ventricle

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Posterior fossa malformations