Other Techniques

24 CHAPTER

Other Techniques

Saurabh R. Sinha

Beyond the commonly used diagnostic modalities (eg, scalp and intracranial EEG, MRI and other imaging techniques, neuropsychological testing), there are several new/emerging techniques that may have a role in the evaluation of epilepsy. The role of these techniques remains to be fully defined. Some are routinely used at some epilepsy centers, others remain almost purely investigational. The brief summaries given here are meant purely as an introduction to the techniques and their current status with respect to clinical use.

MAGNETOENCEPHALOGRAPHY

Magnetoencephalography (MEG) is a technique for recording the magnetic fields generated by brain activity. Whenever there is a flow of electrical current, there is an associated magnetic field that is generated. This field is perpendicular to the direction of current flow. The magnetic field is detected when it induces a small current in very sensitive, low-impedance sensors (SQUIDs or superconducting quantum interference devices). A typical MEG has 100 to 300 sensors and looks like a giant hair dryer. The patient must lie still with their head in the device for recording. Although MEG data can be reviewed in a raw format, in most instances, the MEG signal corresponding to activity of interest (eg, an epileptiform discharge or an evoked potential) is used to calculate the likely source of the activity using a model of the head.

Like EEG, activity must occur in a fairly large population of neurons with spatial and temporal summation to be detected by MEG. However, MEG is likely more sensitive, requiring only 3 to 4 cm² of activated cortex for detection of an epileptiform discharge, as compared to 6 to 10 cm² for EEG. Also, like EEG, several factors determine the appearance of cerebral activity on MEG—these include distance of source from the recording sensor and the orientation of the source (radial dipoles generate magnetic fields that are not easily detectable from the exterior surface; tangential dipoles generate fields that extend out further from the brain and are more easily detectable) (1). Owing to these factors, MEG may be more sensitive to activity in certain brain regions than EEG and may offer additional/complimentary data to the scalp EEG. However, MEG recording is usually limited to interictal data due to the need for patients to lie fairly still in a relatively large piece of equipment. Clinical availability of such devices is also limited.

Source Localization

For epilepsy, the main utilization of MEG has been to localize the epileptogenic zone based on interictal discharges. Analysis of MEG data provides locations of interictal discharges. These data may be more precise and more sensitive than scalp EEG. It may be particularly helpful in situations where the scalp EEG (ictal and interictal) is poorly localizing or localizes to regions not well studied with EEG, especially in nonlesional patients or patients with large or multiple potential epileptogenic lesions. For most centers, the presence of an interictal EEG discharge is essential for performing MEG. MEG data can suggest potential targets for intracranial electrode implantation and regions that may need to be included in a planned resection to optimize chances of seizure freedom.

Functional Mapping

Another use of MEG is for mapping of eloquent cortex. By using evoked potential and functional imaging stimulation paradigms, the resulting MEG signal can be used to map regions corresponding to primary motor, primary sensory, and language cortices. Like functional MRI (fMRI) data, such data would rarely be used as the sole basis for surgery, but it can provide a starting point for localization of eloquent cortex. With advances in techniques and validation of results, a larger role in the presurgical evaluation of epilepsy and other neurosurgical patients may be possible.

STEREO-EEG

Stereo-EEG refers to the use of multiple-depth electrodes to record intracranial EEG signals. It has long been the favored technique for intracranial monitoring in Europe and is being used more frequently in North America. Unlike grids and strips, which can only record from the cortical surface/fissures, stereo-EEG can access deeper structures. For stereo-EEG, multiple-depth electrodes are placed through small burr holes in the skull. Planning an implantation requires some knowledge of the location of blood vessels to avoid hemorrhages. In most cases, a cerebral angiogram (conventional, CT or MR) is combined with skull and brain imaging to determine the optimal path for stereotactic placement of electrodes. Like other intracranial monitoring, placement must be planned to both allow localization of the epileptogenic zone and exclude other potential targets and eloquent cortex. In some ways, stereo-EEG requires more precise planning—unlike a grid electrode that can provide a sampling of activity over a relatively large region of cortex (tens of square centimeters), a depth electrode provides a much more localized sampling. In addition to recording spontaneous seizures and interictal activity, stereo-EEG electrodes can also be used for brain mapping using electrical stimulation. After seizure localization and brain mapping, the depth electrodes are removed. Surgical resection may be performed at this time or after a period of recovery.

There are several advantages to stereo-EEG. The biggest is easier access to deep structures like the mesial temporal lobe, orbitofrontal cortex, cingulate gyrus, and the insula. In addition, although the risk of hemorrhage during electrode placement is present, once placed, the electrodes are typically better tolerated than strips or grids. They can typically be left in place for longer periods due to the tolerability and lower infection risk. Also, it is easier to sample multiple, even bilateral, regions with stereo-EEG. In cases where surgical resection is found not be feasible (due to multiple foci or proximity to eloquent cortex), a stereo-EEG patient has been spared a craniotomy. The main disadvantages to stereo-EEG are the limited sampling of cortex (especially for brain mapping purposes), the risk for hemorrhage during placement, the time required for placement of the many electrodes, and the relatively lack of familiarity with the procedure for most North American centers.

HIGH-DENSITY EEG

High-density EEG (HD EEG), sometimes also called dense-array EEG, refers to the use of dense arrays of scalp electrodes (up to 256 channels or even more) to record EEGs. In most cases, such arrays are applied as part of electrode caps rather than precise measurement of locations. The electrode locations may be determined by aligning the cap with bony landmarks or by actually measuring the location of electrodes once placed. The large amount of data generated is then combined with source localization techniques to map the dipole corresponding to activity of interest. For source localization, the smaller the distance between adjacent electrodes, the higher the spatial resolution. Thus, HD EEG can localize the source of recorded activity more precisely than conventional scalp EEG. However, noise considerations often still necessitate averaging of data (eg, multiple interictal epileptiform discharges) before source localization. Ictal EEG data may not easily lend itself to source localization techniques; instead, the raw EEG, usually with a limited number of electrodes often approximating a traditional 10-20 electrode placement, is usually reviewed. HD EEG systems can also be used for brain mapping using evoked potential techniques (2).

NEAR-INFRARED SPECTROSCOPY

Near-infrared spectroscopy (NIRS) measures changes in blood flow based on absorption of infrared light. A fiberoptic probe is used to shine infrared light of different wavelengths (eg, 780 nm and 830 nm) onto the brain through the scalp; a light-sensitive probe is also placed on the scalp to measure the amount of reflected light. The amount of hemoglobin (and relative amount of oxygenated and deoxygenated hemoglobin) determines the amount of light absorbed at different wavelengths. Thus, the measurement can be used to detect changes in total blood flow and relative changes in oxygenation levels of hemoglobin. The point of measurement is estimated to be about 2 cm below the scalp. Multiple light sources and sensors are typically incorporated into a single device to provide spatial resolution. Thus, changes in blood flow in the superficial cortex can be measured. NIRS has been used to demonstrate changes in blood flow associated with both interictal discharges and ictal activity. Because NIRS measures changes in blood flow, the temporal resolution is low. However, there is some intriguing data to suggest that these changes may actually precede the epileptic seizure in some situations (3). NIRS also has the potential for functional brain mapping, as similar to fMRI, one can measure changes in blood flow related to certain functional tasks (motor and language).

Although discussed as a potential tool for years, NIRS is not used clinically for several reasons (4). These include limited assessment of only superficial cortical regions, significant artifacts associated with movement, limited spatial resolution, and limited temporal resolution. In addition, there has been a lack of convincing evidence that it can provide significant additional data in the evaluation of epilepsy.