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Review: Posterior Fossa Anatomy and Tumors
David Halevi
Anatomy: Osseus Borders
Superior: Tentorium cerebelli
Inferior: Foramen magnum
Anterior: apex of the petrous temporal.
Posterior: enclosed by the occipital bone.
Lateral: portions of the squamous temporal and mastoid part of the temporal bone
Anatomy: Internal Structures
Foramen magnumTransmits the medulla, the ascending portions of the spinal accessory nerve (XI), and the vertebral arteries.
Internal acoustic meatusLies in the anterior wall of the posterior cranial fossa. It transmits the facial (VII) and vestibocochlear (VIII) nerves into a canal in the petrous temporal.
Jugular foramenLies between the inferior edge of the petrous temporal bone and the adjacent occipital bone and transmits the internal jugular vein (actually begins here), the glossopharyngeal (IX), the vagus (X) and the spinal accessory (XI) nerves.
Anterior condylar (hypoglossal) canalLies at the anterolateral margins of the f. magnum and transmits the hypoglossal (XII) nerve
Tumors
Cerebellar astrocytoma
• 33% of all posterior fossa tumors in children
• 25% of all pediatric tumors
• Average age at presentation is 9 years
• Frequently a benign, slowly growing cystic tumor
• Most favorable of all intracranial neoplasms
• May arise in the hemisphere or vermis
Medulloblastoma
• Malignant
• Midline from the cerebellar vermis
• Cerebellar hemisphere in older patients
• Large tumours completely fill the 4th ventricle
• Presentation: Hydrocephalus & cerebellar dysfunction
Ependymoma & Ependymoblastoma
• Arises from the floor of the 4th ventricle
• May infiltrate, the underlying brainstem
• 50% present < 3 years
• Anaplastic ependymoma (ependymoblastoma)
• Hydrocephalus, Facial weakness (dorsal brainstem)
Choroid plexus papilloma & carcinoma
• 0.4 - 0.6% of all intracranial tumors
• More frequent in children than in adults
• 60% occur in the lateral ventricle
• 30% in the fourth ventricle
• CSF overproduction may occur
Hemangioblastoma
• 7-12% of all posterior fossa tumors
• Age of presentation is 30-40 years old
• More common in males
Brainstem glioma
15% of all brain tumors
25-30% of all brain tumors in children
Most are low-grade astrocytoma
Predominantly in the Pons
Less frequently in the medulla
May infiltrate extensively throughout the brainstem
Metastatic tumors
3% occur in the brainstem
18% occur in the cerebellum
1°: breast, lung, skin, and kidney
Clinical presentation
Depends on:• Location • Aggressiveness
Due to:• Compression on cerebellum or midbrain• Raised intracranial pressure
Clinical presentation
• ↑ICP• Headache • Vomiting • Strabismus • Blurring of vision due to papilledema• Meningismus• Macrocephaly in children• Hydrocephalus
Clinical presentation
Brain stem compression:
• Ocular palsy
• Diplopia
• Hemiparesis
Clinical presentation
Cerebellar compression:
• Truncal or limb ataxia
•Nystagmus
•Dysmetria
Surgery
• Anaesthesia
• Sitting or prone position
• Vertical midline incision
• Posterior fossa craniotomy
• Excision
Treatment
• Surgery
• Radiotherapy
• Chemotherapy
• Anticonvulsant therapy
• CSF shunt (risk of tumour spread!)
• Steroid medication