For some epilepsy surgery candidates, the noninvasive Phase I evaluation cannot accurately localize the epileptogenic region. In these cases, the use of intracranial electrodes is required.
A neurosurgeon places the intracranial electrodes in regions of brain that are suspected to cause seizures. A variety of electrodes are available. The type that is used depends on the region of brain that is being studied. The neurosurgeon places the electrodes directly onto the brain through either a burr hole or larger opening in the skull (craniotomy), depending on the electrode array. When a specific kind of electrode (called an "intraparenchymal electrode") is needed, MRI-guided stereotactic technology is utilized.
In addition to recording seizures, intracranial electrodes may be used to identify regions of brain which support vital functions such as language and movement. The testing is called cortical mapping, and is performed by a trained neurologist. The neurologist passes a small electrical current through the electrodes, and monitors the patient's response. This kind of testing insures a safe surgical resection by defining the margins of vital cortex also called eloquent cortex.