Posterior cranial fossa contains several structures which are pertinent for maintaining normal level of consciousness. Structural lesion involving the posterior cranial fossa may cause impairment of consciousness commonly by virtue of mass effect leading to raised intracranial pressure and brainstem compression. An important differential diagnosis for sudden deterioration in sensorium of patients having posterior fossa lesions is seizure; which may be convulsive or non-convulsive. Seizures may cause altered consciousness or post-ictal weakness which may mimic clinical deterioration from other causes. Unlike supratentorial lesions which are commonly associated with seizures, the incidence of seizures in posterior fossa lesions is rare ranging from 1.8–5% in various series [6, 7, 12].
The mechanism of seizures in posterior fossa lesions, especially the role played by cerebellum in controlling epilepsy is a matter of dispute. Dysfunctional cerebellum has been found to be linked with disinhibition of epileptic activity in the cerebral cortex [14–16]. Pathologic cerebellar alterations lead to decrease in inhibitory purkinje cell output to the dentate nucleus and can result in seizures via the excitatory dentatothalamocortical tracts.
The incidence of seizures either pre or post-operatively is largely dependant upon the type of lesion [6–13] and use of ventricular shunt or ventriculostomy [6, 7]. The highest incidence for seizures was reported with medulloblastoma, cerebellar haemorrhage and during microvascular decompression for cases of neurovascular conflict. The high risk for seizures associated with this entities is plausibly due breach of the blood-brain barrier and intracranial deposition of hemosiderin, which is considered highly epileptogenic.
Majority of patients with seizures in posterior fossa lesions tend to have poor outcome [7, 11, 12]. Early detection and prompt administration of AEDs is necessary as seizures can cause cerebral hypoxia, metabolic acidosis and a further increase in intracranial pressure. However, clinical detection of seizures in posterior fossa lesions is difficult, as convulsive seizures need to be differentiated from decerebrate or decorticate posturing secondary to raised intracranial pressure. Similarly altered sensorium due to non-convulsive status epilepticus (NCSE) needs to be differentiated from coma due to structural lesion. EEG is a vital tool to facilitate seizure diagnosis and initiate prompt treatment. It is mandatory to order continuous EEG monitoring in all patients with a posterior fossa lesions who develop altered mental status before or after surgery.
There is no literature evidence regarding use prophylactic AEDs or choice of particular AEDs for seizure management [6, 7, 9–13]. However, in certain cases associated with high risk of seizures as discussed earlier, it is worth considering the possible use of prophylactic AEDs. Prevention of seizures could result in an improved prognosis for such patients. In addition, the use of prophylactic AEDs may also be cost-effective given the increased morbidity and mortality in those patients who do develop seizures and NCSE. Further studies and possibly large scale multi-centric trials are warranted to investigate whether the use of prophylactic AEDs can be recommended in high-risk posterior fossa lesions.