Posterior fossa malformations

posterior fossa

1. POSTERIOR FOSSA MALFORMATIONS Dr.Archana Koshy

2. Posterior fossa anatomy Chiari malformations Dandy walker malformation Joubert syndrome Rhomencephalosynapsis 4/4/2016 Posterior Fossa Malformations 2

3. 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. 4/4/2016 Posterior Fossa Malformations 3

4. 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 - E mbryonic hindbrain ( rhombencephalon ) M esencephalon - Midbrain structures. (Mesodermal elements give rise to the meninges and bone. 4/4/2016 Posterior Fossa Malformations 4

5. 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 Part of the cerebral aqueduct Fourth ventricle CSF cisterns that surround the brainstem and cerebellum . 4/4/2016 Posterior Fossa Malformations 5

6. BONE Anterior wall – Dorsum sellae of the sphenoid body Clivus of the basioccipital bone Lateral wall – 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 . 4/4/2016 Posterior Fossa Malformations 6

7. 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 4/4/2016 Posterior Fossa Malformations 7

8. 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. 4/4/2016 Posterior Fossa Malformations 8

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13. ARNOLD CHIARI MALFORMATIONS 4/4/2016 Posterior Fossa Malformations 13

14. 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 . 4/4/2016 Posterior Fossa Malformations 14

15. 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

16. 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

17. 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.

18. Radiographic features D istance 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

19. Axial Sections : the medulla is embraced by the tonsils and little if any CSF is present - crowded foramen magnum .

20. Sagittal : tonsils are pointed, rather than rounded and referred to as peg-like sulci are vertically oriented, forming so-called  sergeant stripes

21. ASSOCIATIONS Cervical cord syrinx is present in ~35% (range 20-56%): more common in symptomatic patients H ydrocephalus 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

22. 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.

23. 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.

24. 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

25. Radiographic features MRI MRI 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 f indings may include ◦ Posterior angulation of the odontoid process ◦ Hydrocephalus ◦ Crowded small posterior fossa ◦ Syringohydromyelia ◦ Scoliosis

26. CHIARI II MALFORMATION MYELOMENINGOCOELE   SMALL POSTERIOR FOSSA DESCENT OF THE BRAINSTEM

27. CHIARI II MALFORMATION Relatively common congenital malformation of the spine and posterior fossa ( ~1:1000 live births) Numerous associated abnormalities are also frequently encountered

28. 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).

29. RADIOGRAPHIC FEATURES Antenatal 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 callosal dysgenesis ) may be identified. 

30. 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 )

31. 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.

32. POSTERIOR FOSSA • S mall posterior fossa with a low attachment of the tentorium and low torcula • T he brainstem appears 'pulled' down with an elongated and low lying fourth ventricle • T he tectal plate appears beaked: inferior colliculus is elongated and points posteriorly, with resulting angulation of the aqueduct which results in  aqueductal stenosis and hydrocephalus • C erebellar tonsils and vermis are displaced inferiorly through foramen magnum which appears crowded SPINE • S pina bifida aperta /  myelomeningocele Tethered Cord

33. CHIARI III MALFORMATION E xtremely rare anomaly L ow occipital and high cervical encephalocele H erniation of posterior fossa contents, that is, the cerebellum and/or the brainstem, occipital lobe, and fourth ventricle

34. 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 

35. 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: Dandy-Walker malformation Dandy-Walker variant Mega Cistern Magna Posterior Fossa Arachnoid Cyst • Occurs in1:30,000births • Seen in 4-12% of all babies with hydrocephalus

36. DANDY WALKER MALFORMATION TRIAD OF MALFORMATIONS Cystic dilation of fourth ventricle Complete or partial agenesis of the cerebellar vermis 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

37. 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.

38. 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)

39. 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

40. Joubert syndrome G roup 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 Hypotonia Ataxia O cular motor apraxia, N eonatal breathing dysregulation Intellectual disability of variable severity

41. 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.

42. 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 Macrocephaly Signs of increased intracranial pressure D evelopmental delay. However , these cysts may be asymptomatic and discovered incidentally .Arachnoid cysts do not communicate with the fourth ventricle or the subarachnoid space

43. 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.

44. 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. M ega cisterna magna freely communicates with the fourth ventricle and the cervical subarachnoid space R esults in consistent absence of hydrocephalus.

45. 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 M ay be seen in patients with associated VACTERL T runcal and/ or limb ataxia A bnormal eye movements H ead stereotypies Delyed motor development

46. THE KEY NEUROIMAGING FINDINGS- A genesis or hypogenesis of the vermis continuity (often called fusion) of the cerebellar hemispheres, superior cerebellar peduncles, and dentate nuclei C reates a horseshoe-shaped arch across the midline, resulting in a keyhole-shaped fourth ventricle