Excerpt from Posterior Fossa Tumors

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A brain tumor is one of the most devastating forms of human illness, especially when occurring in the posterior fossa. Brainstem compression, herniation, and death are all risks in tumors which occur in this critical location.

History of the Procedure

Hippocrates, who likely performed it for headache, epilepsy, fractures, and blindness, first described trephination.

Cushing probably was the first to report a large series of posterior fossa tumors. He published information about 61 patients with cerebellar medulloblastoma with mostly fatal outcome.¹

Advances in brain surgery for tumors primarily were due to discovery of anesthesia, asepsis, neurological localization, and the ability to achieve hemostasis.

Problem

Tumors in the posterior fossa are considered critical brain lesions. This is, primarily, because of the limited space within the posterior fossa and the potential involvement of vital brain stem nuclei.

Some patients should undergo an emergency operation, especially if they present with acute symptoms of brain stem involvement or herniation.

Frequency

• Posterior fossa tumors are more common in children than the adults.
• Between 54% and 70% of all childhood brain tumors originate in the posterior fossa.
• About 15-20% of brain tumors in adults occur in the posterior fossa.
• Certain types of posterior fossa tumors, such as medulloblastoma, pineoblastoma, ependymomas, primitive neuroectodermal tumors (PNETs), and astrocytomas of the cerebellum and brain stem, occur more frequently in children.
• Some glial tumors, such as mixed gliomas, are unique to children. They are located more frequently in the cerebellum (67%) and are usually benign.

Etiology

No specific causes for posterior fossa tumors exist. However, genetic factors, such as dysfunction of some tumor suppressor genes (p53 gene) and activation of some oncogenes, may play a role in their development.² Environmental factors such as irradiation and toxins may also play a role.

Pathophysiology

Common types of posterior fossa tumors include the following:

Cerebellar astrocytoma

Cystic cerebellar astrocytoma comprises about 33% of all posterior fossa tumors in children. It represents 25% of all pediatric tumors. Average age at presentation is 9 years. Typically, cerebellar astrocytoma presents as a laterally located cyst with a well-defined solid component. The tumor may be solid or cystic and may be located medially in the vermis or laterally in the cerebellar hemisphere.

Primary neuroectodermal tumors

PNETs include medulloblastomas, medulloepitheliomas, pigmented medulloblastomas, ependymoblastomas, pineoblastomas, and cerebral neuroblastomas. These tumors originate from undifferentiated cells in the subependymal region in the fetal brain. PNETs are second to the cerebellar astrocytoma in frequency, comprising 25% of intracranial tumors in children.

Medulloblastoma

Medulloblastoma initially arises in the inferior medullary velum and grow to fill the fourth ventricle, infiltrating the surrounding structures. Some erroneously thought it arose from medulloblast cells, which do not exist. It is better included in the family of PNETs.

Ependymoma and ependymoblastoma

Ependymomas are derived from ependymal cells. They occur more frequently in females, with 50% presenting in children younger than 3 years. An ependymoma has a much better prognosis than an anaplastic ependymomas (ependymoblastoma). Plastic ependymoma can mold itself to the available spaces inside or outside the ventricle without adhering to the ventricle.

Choroid plexus papilloma and carcinoma

Choroid plexus papilloma and carcinoma represent 0.4-0.6% of all intracranial tumors. They are more frequent in children than in adults (3% of childhood brain tumors). Sixty percent occur in the lateral ventricle and 30% in the fourth ventricle. The third ventricle and cerebellopontine angle are rare locations for this tumor. Cerebrospinal fluid (CSF) overproduction may occur, sometimes reaching more than 4 times normal volumes. In most of cases, CSF analysis demonstrates increased protein, xanthochromia, or both.

Dermoid tumors

Dermoid tumors arise from incomplete separation of epithelial ectoderm from neuroectoderm at the region of the anterior neuropore; this usually occurs during the fourth week of gestation. The cyst wall often includes hair follicles, sweat glands, and sebaceous glands. The cyst grows slowly and gradually becomes filled by desquamated epithelium, sweat, and sebaceous materials. Aseptic meningitis is a sequela of cyst rupture. More commonly, the cyst occurs in the posterior fossa, at or near the midline. It may be extradural, vermian, or intraventricular. A dermal sinus may be connected to the mass. It may be detected clinically or by MRI.

Hemangioblastoma

Hemangioblastoma represents about 7-12% of all posterior fossa tumors. About 70% of hemangioblastomas occurring in the cerebellum are cystic. Age of presentation is 30-40 years old. Hemangioblastomas are more common in males. Hemangioblastoma may be associated with von Hippel-Lindau disease.

Metastatic tumors

Three percent of all cranial metastatic lesions occur in the brainstem and 18% occur in the cerebellum. Originating sites include breast, lung, skin, and kidney. Solitary metastasis is better treated by surgical removal before radiation therapy. Surgery also should be considered in case of radiosensitive original tumors or when the primary source is unknown.

Brainstem gliomas

Brainstem gliomas constitute 15% of all brain tumors. In children, brainstem glioma represents 25-30% of all brain tumors. Most brainstem gliomas are low-grade astrocytoma.

Clinical

The clinical presentation depends on the site of the tumor, biological behavior and aggressiveness of the tumor, and the rate of growth. At the time of presentation, the patient may be very ill from severe headache or frequent vomiting due to associated hydrocephalus.

Symptoms may be caused by focal compression of the cerebellum or brain stem centers and increased intracranial pressure.

Symptoms due to focal brainstem compression include cranial nerve dysfunction. This commonly involves the nuclei or tracts of the third, fourth, or sixth cranial nerves, resulting in ocular palsies and diplopia and long tract signs (hemiparesis).

Symptoms due to focal compression of the cerebellum include characteristic eye findings and vermian syndrome. Truncal ataxia is a common finding in midline tumors, such as medulloblastomas, ependymomas, and vermian astrocytomas. It is manifested by a tendency to fall frequently and a widely based gait.

Hemi-cerebellar syndrome involves limb ataxia, nystagmus, and dysmetria. Tumors that occur in the cerebellar hemisphere, such as metastases, cerebellar astrocytomas, or cystic hemangioblastomas, may present by ataxia of the contralateral limbs. Nystagmus usually occurs late in the disease. Vertical nystagmus suggests a lesion in the anterior vermis, periaqueductal region, or craniocervical junction. Horizontal nystagmus implies involvement of the cerebellar hemisphere.

Intracranial hypertension causes the following 8 symptoms:

• Headache: This is the most common symptom in patients with posterior fossa tumors. Associated neck pain, stiffness, or head tilt suggest tonsillar herniation into the foramen magnum. Headache is insidious and intermittent. It is most severe in the morning or after a nap because of increased intracranial pressure from recumbency and hypoventilation during sleep. Headache manifests in children as irritability and difficulty to be handled.
• Vomiting: It may be due to generalized intracranial hypertension or irritation of the vagal nuclei in the medulla oblongata or area postrema of the fourth ventricle. Vomiting (including projectile vomiting) may occur, usually in the morning. Vomiting sometimes relieves headache.
• Strabismus: It is secondary to sixth nerve palsies from intracranial hypertension. Third nerve palsies may also occur.
• Blurring of vision due to papilledema
• Meningismus
• Dizziness
• Macrocephaly in children
• Hydrocephalus

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