Posterior Fossa Tumors:
Although raised intracranial pressure is common in children at presentation with posterior fossa tumors, routine placement of a ventriculo-peritoneal shunt (VP shunt) or external ventricular drain (EVD) prior to surgery is no longer commonplace.
This is largely due to the use of pre-operative corticosteroids.
In the setting of posterior fossa tumors and raised intracranial pressure, these drains can be complicated by:
- upward transtentorial herniation
- infection
- tumor seeding via VP shunts
Corticosteroid therapy (dexamethasone) may be necessary pre-operatively in some patients with large tumors to reduce associated cerebral edema. If possible, steroids should not be started until after the initial MRI scan, since corticosteroids may affect tumor contrast enhancement.
The usual corticosteroid dosage is 0.25 to 1 mg/kg/day of Decadron, in divided doses, every 4-6 hours.
Up front suboccipital craniotomy to resect the tumor as completely as possible is usually performed, aiming to:
- Re-establish CSF flow through the fourth ventricle
- Confirm the pathological diagnosis
- Improve local control
Immediately after surgery, an external ventricular drain is placed and only removed when the post-operative swelling has subsided and the normal CSF pathways have been re-established.
As complete a resection as possible is attempted. This is critical to achieve local control. In children there is good evidence that a complete resection helps to reduce the risk of local recurrence. This is true for all of these tumors:
- Medulloblastoma
- Ependymoma
- Posterior fossa astrocytoma.
For medulloblastoma:
- Gross total resection of tumor helps to reduce the risk of local recurrence.
- The smaller the amount of tumor left after resection, the more likely it is that chemotherapy and radiation therapy will be effective.
- Often a thin film of tumor is left on the floor of the fourth ventricle. This is because tumor may be invading into the brainstem and further resection in this area is likely to cause severe neurological damage. The chance of local control does not seem to be significantly altered in these circumstances and it is critical that the plane of the floor of the fourth ventricle is not violated.
Modern techniques have reduced significant operative morbidity to below 5%.
After removal of the tumor and the external shunt, about 30% of patients may not re-establish the normal CSF pathways and will need a permanent internal (VP) shunt because of permanent hydrocephalus.
Post-operative complications are often self-limiting and with time gradually resolve. These include:
- CSF leakage
- Post-operative mutism
- Cranial nerve/Pseudobulbar palsy
- Pseudomeningocele
- Communicating hydrocephalus
Cerebellar Mutism:
- Occurs after 8% of posterior fossa resections (risk is higher in medulloblastoma - perhaps as many as 20% of patients are affected).
- Usually develops during the first post-operative week.
- Transient mutism and not accompanied by long tract signs or cranial nerve palsies.
- Possibly due to damage to the dentate nucleus and/or its outflow tract, the superior cerebellar peduncle.
- Recovery of speech generally starts in 2 weeks to 3 months after surgery.
- May persist for several months and occasionally there are longer term neurological sequelae.
