Stereotactic Electroencephalography in Epilepsy

Sanjeet Grewal

Karim Refaey

William O. Tatum IV

Epilepsy is a common and serious neurologic disease affecting more than 50-60 million people worldwide.¹^,²^,³^,⁴^,⁵ While many patients’ epilepsy can be controlled with antiseizure drugs, there is a subset of patients who are drug resistant.⁶^,⁷ For this group of patients, epilepsy surgery is the most effective treatment to obtain seizure freedom.⁸^,⁹^,¹⁰ The goal of epilepsy surgery is to completely resect (or disconnect) the cortical areas responsible for the primary organization of the epileptogenic activity and to preserve the areas of functional (eloquent) cortex that overlaps with the seizure onset zone (SOZ).¹¹^,¹²^,¹³

The success of epilepsy surgery is dependent upon accurate preoperative localization of the SOZ.¹¹^,¹²^,¹³^,¹⁴ This localization requires a comprehensive presurgical evaluation including the clinical history and physical examination, anatomic and functional neuroimaging, and interictal/ictal neurophysiological information, to guide a tailored individualized resection for each patient.¹⁵^,¹⁶ The most common noninvasive technique to evaluate patients with drug-resistant focal epilepsy is a standard electroencephalography (EEG) using scalp electrodes.¹⁷^,¹⁸^,¹⁹ However, this technique is limited in both temporal and spatial resolution compared with direct brain recording. When noninvasive data are insufficient to define the SOZ, intracranial electroencephalography is necessary to more clearly delineate the SOZ.

Stereoelectroencephalography (SEEG) has recently gained traction in the United States as a method to define the SOZ anatomically in patients with drugresistant focal epilepsy.²⁰^,²¹^,²²^,²³^,²⁴^,²⁵^,²⁶ This chapter focuses on the clinical aspects of SEEG methodology as a surgical technique to complement other forms of invasive video-EEG monitoring.

INDICATIONS FOR INTRACRANIAL MONITORING

The SEEG method for patients undergoing invasive presurgical evaluation was originally developed by Jean Talairach and Jean Bancaud during the 1950s.²⁷^,²⁸^,²⁹^,³⁰^,³¹ It was a frequently chosen technique in Europe when invasive EEG monitoring was required in patients with disabling drug-resistant focal seizures.²⁷^,²⁸^,³²^,³³^,³⁴ It was not until the late first decade of the 2000s when physicians in the United States, who until then largely favored subdural EEG, began to implement more widespread use of SEEG.²¹^,²⁹^,³⁰^,³¹^,³²^,³⁵ Recent advances such as robotics in the operating room and neuronavigation-assisted electrode implantation led to a resurgence of interest in SEEG.³³^,³⁴^,³⁶^,³⁷^,³⁸^,³⁹^,⁴⁰^,⁴¹^,⁴²^,⁴³ The principles of SEEG remain similar to the principles originally described by Bancaud and Talairach,²⁹^,³⁵^,⁴⁴ based on anatomo-electroclinical (AEC) correlations, with the goal of conceptualizing the three-dimensional (3D) spatiotemporal organization of the SOZ.³⁴^,⁴⁵^,⁴⁶ With this method, the implantation strategy is unique to each patient. Electrode placement is based on preimplantation hypotheses that take into account concordance of all initial noninvasive studies. If the preimplantation hypotheses are incorrect, the placement of the depth electrodes risks inadequate sampling and misinterpretation of the SOZ.⁴⁷^,⁴⁸^,⁴⁹

In most patients undergoing presurgical evaluation, a noninvasive assessment will correctly identify the SOZ. Unfortunately, formulation of a clear and unique hypothesis may not be possible in a significant minority of patients. In such cases, even when focal or focal/regional epilepsy is suspected, the noninvasive (Phase I) evaluation does not allow practitioners to accurately differentiate hemispheric lateralization or lobar localization between one or more potential SOZs.²⁰^,²⁶^,⁴⁸^,⁴⁹^,⁵⁰ Alternatively, there may be a sound hypothesis for localizing the network of the SOZ but not enough information to pinpoint the exact location of the SOZ, its spatial extent, or its overlap with eloquent cortex.⁴⁸ Consequently, these patients may be candidates for an invasive evaluation using intraoperative electrocorticography (ECoG) or extraoperative methods utilizing sensing electrodes placed as subdural strips and grids, depth electrodes including SEEG, or a combination of different types of electrodes.

In our institution, invasive recording is considered when any of the following conditions arise: