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