Chapter 32: Use and Placement of Grid and Strip Electrodes

Introduction

For patients with intractable epilepsy, the role of intracranial surgery has been progressively recognized as a well-tolerated and clearly beneficial treatment option.^1–3 Improvements in quality of life are now frequently documented alongside the success rates with surgical approaches, and early surgical consideration for medical failures has gained acceptance as a routine part of the comprehensive approach to refractory seizure management. Although some patients can proceed to an operative intervention based on clinical presentation and noninvasive tests,^3,4 others require more investigation before surgical resection can be considered. This latter group may have insufficient scalp electroencephalography (EEG) documentation of their seizure disorder or an unknown correlation between seizures with a suspected lesion or substrate; they also may lack localization of an ictal onset that is amenable to surgical resection. In other cases, surgical resection for seizure control may pose significant risk if adjacent regions harbor the eloquent cortex. When language, motor, or sensory function may be involved, intracranial electrodes can be used for functional mapping.

In these more challenging cases, subdural strips and grid electrodes have become more important and increasingly useful.^5,6 Intracranial EEG and stimulation can provide the final necessary information that permits consideration of a surgical approach. This chapter will discuss factors influencing the strategies chosen for subdural electrode use when evaluating a patient with partial epilepsy.

Indications for Intracranial Monitoring

Noninvasive Monitoring: Limits and Complementary Roles

EEG provides a noninvasive physiological examination of the brain that allows characterization of neuronal discharge patterns. It provides a graphic representation of frequency and amplitude, and also demonstrates the temporal relationship and spatial distribution of electrical activity from the cerebral cortex.⁷ EEG identifies both normal and pathologic states, with the latter role being essential in the diagnosis and treatment of seizure disorders.^8,9 However, the role of a standard EEG is limited in that the typical events of interest (seizures) do not reliably occur while the test is performed.^10,11 To address this dilemma, long-term, continuous, noninvasive monitoring can be performed in an epilepsy monitoring unit (EMU). The EMU also provides a safe environment where supervision in an inpatient environment minimizes the risk when medications are tapered to help capture a seizure.

Despite the ability of an EMU to greatly improve the likelihood of capturing an event, there are still significant limitations with scalp EEG.¹² The use of scalp contacts requires that the electrical activity of the brain must be detectable through several layers of tissue, each of which causes various impediments to the process of recording an EEG. When using superficial electrodes, the scalp reduces the amplitude of EEG signals. Even trying to abrade the scalp to improve the conductivity does not provide a significant improvement in the EEG.¹³ Studies of bone indicate that it is a poor ...