Jounral October - posterior fossa abnormalities

Congenital abnormalities of the posterior fossa

1. CONGENITAL ABNORMALITIES OF THE POSTERIOR FOSSA RADIOGRAPHICS 2015 Moderators Dr Suresh B Masimade sir Dr Vikram Patil sir Presenter: Sujay Jajee

2. ANATOMY OF THE NORMAL POSTERIOR FOSSA

3. The cerebellum is divided into three parts by the primary and pre pyramidal fissures. The posterior margin of the brainstem extending from the caudal aqueduct to the obex should be a straight line. The fastigium of the fourth ventricle must lie just below the midpoint of the ventral pons.

4. DANDY WALKER MALFORMATION Most common posterior fossa malformation. Presents before 1 year of age with macrocephaly and symptoms of raised ICT. May present as a complex disruption of the interaction between the developing cerebellum and the developing fossa mesenchyme and its derivatives. Key features on neuro imaging: Hypoplasia {rarely agenesis} of the cerebellar vermis {Typically inferior portion} Dilatation of the cystic appearing fourth ventricle which fills up the entire posterior fossa.

5. The cerebellar hemispheres are typically displaced anterolaterally but their size and morphology are unaffected. Tentorium, torcular and transverse sinus are elevated. They are caused by cystic dilation of the fourth ventricle and result in global enlargement of posterior fossa. Hydrocephalus is associated with DWM in 90% patients. BV angle is increased {> 45}

6. BLAKE POUCH CYST Caused by lack of fenestrations of the Blake pouch, resulting in absence of communication between fourth ventricle and the subarachnoid space - resulting in hydrocephalus. Presents with hydrocephalus and macrocephaly in the neonatal period. Neuro imaging findings include: Cyst in retrocerebellar location which is essentially a diverticulum of the consequently enlarged fourth ventricle. Displaces the choroid plexus inferior to vermis along the anterosuperior aspect of the cyst. Increased BV angle {18-30}

7. MEGA CISTERNA MAGNA Enlarged cisterna magna measuring >10mm with intact vermis, normal fourth ventricle. Represents a true focal enlargement of subarachnoid space in the inferior and posterior portions of the posterior fossa. Communicated freely with fourth ventricle and subarachnoid space resulting in consistence absence of hydrocephalus. It has been suggested that delayed fenestration of Blake pouch results in mega cisterna magna.

8. Key neuro imaging finds: {Must be differentiated from isolated vermian hypoplasia and Blake pouch cyst} Enlarged Cisterna magna >10 mm Presence of normal vermis. Absence of hydrocephalus

9. POSTERIOR FOSSA ARACHNOID CYSTS These are duplications of the arachnoid membrane which produce fluid filled cysts known as arachnoid cysts. They may be located inferior or posterior to vermis, Cranial to vermis in tentorial hiatus (Supravermian), anterior or lateral to cerebellar hemispheres, or anterior to brainstem. They do not communicate with fourth ventricle or the subarachnoid space.

10. On neuro imaging: Well circumscribed, extra axial fluid collection or cyst showing CSF intensity. Arachnoid cysts may enlarge during infancy and produce mass effect on the cerebellum and vermis causing secondary obstruction of the ventricular system. May present with : Macrocephaly. symptoms of increased intracranial pressure. Developmental delay.

11. ISOLATED INFERIOR VERMIAN HYPOPLASIA It is a pathological condition characterised by partial absence of the inferior portion of the cerebellar vermis. Patients present with mild functional deficits in fine motor activity and receptive language may be present.

12. Key neuro imaging: Inferior vermian hypoplasia, best appreciated in mid saggital images. Can be prenatally diagnosed after 18-20 weeks, as incomplete caudal growth of inferior vermis over the fourth ventricle is physiological before 18 weeks. The remainder of the vermis, cerebellar hemispheres, fourth ventricle and the posterior fossa have a normal size and architecture. Increased BV angle {<45}

13. MALFORMATION VERMIS FOURTH VENTRICLE POSTERIOR FOSSA HYDROCEPHALUS BONE SCALLOPING DANDY WALKER MALFORMATION HYPOPLASTIC ENLARGED ENLARGED PRESENT NO BLAKE POUCH CYST NORMAL ENLARGED NORMAL PRESENT NO INFERIOR VERMIA HYPOPLASIA HYPOPLASTIC ENLARGED NORMAL NO NO MEGA CISTERNA MAGNA NORMAL NORMAL ENLARGED NO POSSIBLE ARACHNOID CYST NORMAL NORMAL OR REDUCED NORMAL POSSIBLE YES KEY NEURO IMAGING FINDINGS

14. CEREBELLAR INVOLVEMENT IN MIGRATIONAL DISORDERS Posterior fossa abnormalities including hypoplasia of the pons and cerebellum have been included in migrational disorders. Associated with mutations of RELN and VLDLR genes. Microcephaly. Seizures and marked cognitive impairment. Congenital lymphedema. May present with:

15. Mutation in number of neuron specific alpha and beta tubules genes are associated with a spectrum of posterior fossa abnormalities ranging from sever PCH to mild cerebellar hypoplasia, pontine cleft and/or asymmetric cerebral peduncles. Asymmetry of cerebral hemispheres and peduncles may be present. Dysmorphic basal ganglia with an abnormal internal capsule are a consistent and characteristic supratentorial finding. Migrational abnormalities ranging from lissencephaly to polymicrogyria and callosal hypoplasia may also be present.

16. A severely hypoplastic cerebellum with markedly reduced foliation pattern and severe hypoplasia of the pons are characteritisic findings. Supratentorial brain shows simplified gyration pattern with dysmorphic basal ganglia and an abnormal internal capsule. Key neuro imaging:

17. RHOMBENECEPHALOSYNAPSIS Characterised by absence of the vermis and continuity of the cerebellar hemispheres, dentate nuclei and superior cerebellar peduncle. Most common clinical manifestation include truncal and limb ataxia, abnormal eye movements, head stereotypes, and delayed motor development. It is a key feature of Gomez-Lopez-Hernandez syndrome. May also be associated with VACTERL

18. Key neuro imaging: Agenesis/Hypogenesis of the vermis and continuity of the cerebellar hemispheres (Fusion of the hemispheres), superior cerebellar peduncles and dentate nuclei Horseshoe shaped arch across the midline, resulting in a keyhole shaped fourth ventricle. May be associated with other CNS anomalies such as hydrocephalus due to aqueduct stenosis, forebrain abnormalities including absent olfactory bulbs, dysgenesis of the corpus callous, and absent septum pellucidum.

19. MACROCEREBELLUM Characterised by abnormally large cerebellum with preservation of its overall shape. May be seen as a part of well defined syndromes like Costello syndrome, Sotos syndrome or Neurometabolic disorders like MPS I and II. May present with Ataxia, Hypotonia, intellectual disability, and ocular movement disorders.

20. Key neuro imaging: Disproportionate cerebellar enlargement with preserved architecture and shape seen on saggital and axial images. Cerebellar hemispheres are affected than the vermis and may expand into adjacent anatomic regions by wrapping around the brainstem or herniating upward or downward. Supratentorial findings such as ventriculomegaly or white matter signal abnormalities may be present depending on the underlying disease.

21. CEREBELLAR DYSPLASIA This included a heterogenous distribution of different causes, both malformation and disruptions. Focal findings suggest a disruptive lesion whereas global findings suggest a malformation. Cerebellar dysplasia in the inferior cerebellar hemispheres is a consistent finding in Chudley-McCullough syndrome. Chudley-McCullough syndrome is caused because of mutations in GPSM 2 gene. Additional features include severe SNHL, Developmental delay, Partial agenesis of corpus callosum, and periventricular heterotopia.

22. Key neuro imaging findings: Abnormal foliation pattern Defective/Enlarged/Vertical fissuration. White matter arborization Obscured grey white matter junction. Supratentorial findings including migrational abnormalities, callosal dysgenesis and clastic lesions.

23. CEREBELLAR AND BRAINSTEM MALFORMATIONS

24. PONTOCEREBELLAR HYPOPLASIA Is a group of autosomal recessive neurodegenraitve disorders with a prenatal onset. Characterised by hypoplasia of the cerebellum and pons with superimposed atrophy. Patients with mutations in CASK gene present with ataxia, nystagmus, postnatal microcephaly and are inherited with an X-linked pattern. Also associated with posterior fossa migration disorder and Congenital muscular dystopihies due to defective Dystroglycan-O-Glycosylation.

25. Key neuro imaging: Global cerebellar hypoplasia Pontine hypoplasia Reduced gyros pattern Unmyelinated corpus callous On coronal T2 images, it has a Dragon Fly appearance created by flattened cerebellar hemispheres {the Wings} and a relatively preserved vermis {the Body}

26. JOUBERT SYNDROME Characterised by Hypotonia, Ocular motor apraxia, Neonatal breathing dysregulation and intellectual disability. Genes associated with Joubert syndrome help on encoding of proteins of the nonmotile primary cilia which play a key role in development and functioning of retinal photoreceptors, epithelial linings of the renal tubules, bile ducts and neurone. Inherited with an autosomal recessive pattern, except mutations in OFD1 which is X-Linked pattern.

27. Key neuro imaging: MOLAR TOOTH SIGN - Diagnostic criteria for Joubert syndrome Consists of Elongated, thickened and horizontally oriented superior cerebellar peduncle Deep interpeduncular fossa Vermian hypoplasia Dysmoprhic tectum and midbrain Thickening and elongation of midbrain. Small pons.

28. PREDOMINANTLY BRAINSTEM MALFORMATION

29. PONTINE TEGMENTAL CAP DYSPLASIA Characterised by flat ventral pons, Vaulted pontine tegmentum. Partial absence of middle cerebellar peduncles. Vermian hypoplasia. Molar tooth like aspect of ponto-mesencephalic junction. Absent inferior olivary prominence. Associated with vertebral segmentation abnormalities, rib malformation and congenital heart defects.

30. HORIZONTAL GAZE PALSY WITH PROGRESSIVE SCOLIOSIS Characterised clinically by congenital absence of horizontal eye movements, preservation of vertical gaze and convergence and progressive development of scoliosis in childhood. On neuro imaging: Butterfly shaped medulla due to missing prominence of gracile and cuneate nuclei and prominent inferior olivary nuclei with respect to medullary pyramids.

31. CEREBELLAR DISRUPTIONS Cerebellar maturation and adaptation follow a highly orchestrated series of programmed developmental process of migration, proliferation and arborization starting in the middle of the 1st trimester and ending at about 2 years of age. The complexity and long duration of cerebellar development place the immature developing cerebellum at high risk for acquired injury.

32. CEREBELLAR AGENESIS Characterised by near complete absence of cerebellar tissue. May represent a malformation resulting from a genetically mediated pathomechanism or a disruption {Hemorrhage that occurs during gestation or in the perinatal period} Patients who survive infancy presents with cerebellar dysfunction {truncal and limb ataxia, dysarthria] 44

33. GLOBAL CEREBELLAR HYPOPLASIA Reported in chromosomal abnormalities [Trisomy of 13 and 18], metabolic disorders {Zellweger syndrome} May also result from disruption due to prenatal infection such as CMV or prenatal exposure to anticonvulsant drugs, alchohol or cocaine. On neuro imaging: Near normal shape of the cerebellum with reduction in volume and prominence of the subarachnoid spaces. Supratentorial calcification or white matter signal abnormalities may suggest prenatal infection such as CMV.

34. UNILATERAL CEREBELLAR HYPOPLASIA Spectrum of features ranging from complete aplasia to mild asymmetry in the size of the cerebellar hemispheres. It is of prenatal origin with haemorrhage as a leading cause. Characterised by variable involvement and volume loss in the cerebellar hemisphere and vermis, with a normal sized posterior fossa. Evidence of haemorrhage can be present and visualised with SWI sequences.

35. CEREBELLAR CLEFT It primarily effects the cerebellar cortical grey matter, extends from the surface of the hemisphere into the parenchyma and may reach the fourth ventricle without involvement of the vermis. Represents residual disruptive changes most often secondary to feral cerebellar haemorrhages. Clinical symptoms may include language and speech disorders, truncal ataxia, ocular movements disorder and behavioural disorders.

36. Key neuro imaging: Cleft extending from surface of the cerebellum through the parenchyma towards the fourth ventricle. Irregular grey-white matter junction. Abnormal barbarization of the white matter confined to the cleft. Volume reduction of the affected hemisphere.

37. VANISHING CEREBELLUM IN MYELOMENINGOCELE In non skin covered myelo meningocele, prenatal hindbrain herniation through the foramen magnum is believed to be secondary to a prolonged prenatal CSF leak. This herniation may result in parenchymal ischemia induced by mechanical distortion and marked reduction in the size of the cerebellar hemispheres.

38. DEPARTMENT CASES