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. 6/8/2017 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 - Embryonic hindbrain (rhombencephalon) Mesencephalon - Midbrain structures. (Mesodermal elements give rise to the meninges and bone. 6/8/2017 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 1. Part of the cerebral aqueduct 2. Fourth ventricle 3. CSF cisterns that surround the brainstem and cerebellum . 6/8/2017 Posterior Fossa Malformations 5
6. 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
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 6/8/2017 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. 6/8/2017 Posterior Fossa Malformations 8
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13. ARNOLD CHIARI MALFORMATIONS 6/8/2017 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. 6/8/2017 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 Distance is measured by drawing a line from the inner margins foramen magnum (basion to opisthion)- McRaes Line, and measuring the inferior most part of the tonsils above foramen magnum: normal 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 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
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 findings may include Posterior angulation of the odontoid process Hydrocephalus Crowded small posterior fossa Syringohydromyelia Scoliosis
26. CHIARI II MALFORMATION 1. MYELOMENINGOCOELE 2. SMALL POSTERIOR FOSSA 3. 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.