Intracranial Monitoring: Depth, Subdural, and Foramen Ovale Electrodes

Margitta Seeck

Donald L. Schomer

Ernst Niedermeyer

INTRODUCTION

In most Western societies, epilepsy affects around 0.7% of the population, that is, 2.1 million in the United States, 3.5 million in Europe, and 47 million worldwide are affected. In about 20% to 30%, drug treatment cannot control seizure occurrence and the possibility of surgical treatment is then considered. Although the optimal profiles of surgical candidates continue to be defined, the precise localization of a focal onset remains a cornerstone of successful surgical treatment. In many patients, localization requires EEG recording directly from the cortex. This is done either by peroperative corticography, that is, directly from the brain in the operating room (see Chapter 34), or by chronically implanted electrodes for a variable duration lasting from a few days to several weeks. The major goals of intracranial EEG (IEEG) recordings are to determine the seizure onset and distinguish the focus from vital cortex (Table 33.1). There are no true evidence-based studies on the yield of chronic IEEG monitoring, that is, studies assigning subjects randomly to “intracranial monitoring” or “no intracranial monitoring.” However, the yield of precisely mapping the cortex in terms of focus localization and vital cortex is evident even without formal studies.

Table 33.1 Requirements and Goals of Intracranial Monitoring

•	Noninvasive exams are inconclusive concerning the site and extent of a surgical intervention
•	The clinical status of the patient is compatible with prolonged intracranial recordings, for example, no major behavioral disturbances, absence of psychosis, and bleeding disorder
•	The patient accepts the procedure and the risk of inconclusive results after intracranial monitoring
•	The precise focus identification requires the following information:
	•	Lateralization of onset (e.g., left or right temporal lobe epilepsy)
	•	Identification of lobe of onset (e.g., temporal vs. frontal lobe epilepsy)
	•	Precise region of onset (e.g., left anterior vs. posterior temporal onset)
•	Localization of vital or eloquent cortex in adjacent cortex is mandatory (e.g., determination of language cortex in a patient with left posterior temporal lobe seizure onset)

Most patients who are candidates for IEEG fall into an intermediate category of operability: a focal onset is suspected or at least not completely ruled out on the basis of the results of non-invasive EEG exams, but immediate resective surgery is considered too risky in terms of neurologic complications. Statistically speaking, the need for IEEG is an unfavorable predictor for postoperative success (1).

The definition of candidacy for IEEG has changed over the past decades, due in large part to more sophisticated brain imaging. Today, in most centers, a patient with a glioma in the anterior mesial temporal lobe and consistent EEG findings would not be considered an IEEG candidate, even if he/she had contralateral interictal spikes. In contrast, a patient with multiple lesions and discordant EEG findings, or with no MRI abnormality at all, would be considered an appropriate IEEG candidate. However, to the best of our knowledge, there are no published consensus guidelines regarding the utility of IEEG. The need for invasive recordings has been discussed extensively for different clinical scenarios for patients with temporal lobe epilepsy (TLE). It has been proposed that the concordance of unilateral hippocampal atrophy and interictal EEG is sufficient to proceed to surgery, that is, even without (scalp) ictal recordings, which may be sometimes misleadingly showing bilateral or even contralateral epileptic discharges (2). However, some centers would envision not only ictal recordings but even IEEG in order to determine unambiguously EEG onset (3). In any case, careful evaluation of each individual patient is mandatory, including functional imaging and neuropsychological data. Ictal scalp recordings help to better identify the patient’s seizure semiology, verify the presumed ictal onset by analysis of the clinical picture, and estimate the burden of epilepsy as well as following up on postoperative persistent seizures. Apart from persistent epileptic seizures, postoperative nonepileptic (psychogenic) seizures have been described in 2% to 8%. These can be difficult to diagnose if the preoperative semiology is not known (4).

Despite improvements to our understanding about who benefits most from this otherwise costly and labor-intensive procedure, each center has its own strategies, which depend on the experience of the epilepsy surgery team, the availability and quality of the noninvasive exams, and overall national health care systems that facilitate (or not) the access to the highest level epilepsy surgery centers and IEEG. Requirements for the equipment and staffing of such centers have been published (5). The diversity in
presurgical epilepsy evaluation across nations or continents became evident in the recent survey. The Pediatric Epilepsy Surgery Subcommittee of the International League Against Epilepsy conducted a survey in Europe, the United States, and Australia (6). It appeared that in the United States children are more often evaluated with intracranial electrodes, even in the presence of an MRI lesion. There is also more access to functional imaging in the United States compared to their European and Australian counterparts. However, more patients are treated with a vagal nerve stimulator, a palliative procedure, in the United States compared to Europe and Australia. Although other characteristics of the patient populations were not different (underlying pathology, affected lobe, etc.), the reasons for these differences are not entirely clear but are probably of multiple origins, which include different national “schools” with different algorithms of epilepsy care or different regulations from health care providers.

However, while sophisticated epilepsy centers exist today in all Western countries, it should not be forgotten that there are still countries where epilepsy surgery is not done or is only rarely available, which prevents patients from obtaining a potentially very effective epilepsy treatment (7).

In the past 10 to 20 years, the development of new noninvasive imaging techniques was readily embraced in the presurgical epilepsy evaluation (8, 9, 10, 11, 12 and 13), starting with the entry of high-definition MRI and special imaging protocols for epilepsy patients (14). Indeed, the knowledge obtained from these techniques has given us patient populations and helped to specify patient populations where IEEG is more likely to be diagnostic. For example, >10 years ago, invasive monitoring was considered in patients with hippocampal sclerosis, but bilateral temporal spikes or seizures. However, the presence of an MRI abnormality is a good prognostic indicator for seizure-free outcome postoperatively (15), and patients with bilateral spikes or even seizures may very well benefit from surgery on the sclerotic hippocampus (16,17), even without IEEG. However, truly bitemporal lobe epilepsy, as determined by IEEG and MRI, is associated with a lower success rate and only 50% became seizure-free according to a recent study (18). On the other hand, severe unilateral hippocampal atrophy, the so-called burned-out hippocampus, may present with aberrant, even contralateral, scalp ictal discharges. Those patients nevertheless benefit from surgery (19). In some of these patients, IEEG was used to verify ictal onset in the affected hippocampus.

More recently, it has become evident that the presence of a localized or lateralized temporal hypometabolism equals the presence of an MRI lesion. Both MRI lesional and nonlesional TLE have the same postoperative prognosis, that is, a chance of seizure control of approximately 80%, if the temporal hypometabolism in the PET is obvious and unilateral (20). Thus, if this finding is confirmed in more patients, nonlesional TLE patients may not need IEEG anymore if they underwent good-quality PET-imaging and its results are concordant with the remainder of the evaluation. Over the next few years, other investigative measures such as studies on other pathologies, for example, dysplasia or tuberous sclerosis, identifying reliable noninvasive markers of the epileptogenic zone may further reduce the overall number of patients who need IEEG (21, 22 and 23).

BASIC CONCEPTS

IEEG should be considered if it is felt that surgery can cure or at least significantly ameliorate the epilepsy disorder, but pertinent localizing information is still missing from the noninvasive evaluation. This implies that every patient should first undergo a noninvasive evaluation in order to understand the size and nature of the problem. A clear clinical hypothesis is mandatory before implantation, because implanted electrodes record only from a limited cortical volume. Although the exact volume is not known, there are estimates indicating that a cortical area of 10 to 20 cm² is necessary to generate a scalp spike (24). Detailed analysis of scalp discharges and the simultaneously recorded subdural EEG showed that only the early components of spikes and seizures arise from a limited volume, which is, however, still larger than 10 cm². With further propagation, the active brain areas attain 30 cm² or more (25). In this context, imaging methods that suggest point sources of the underlying spikes represent rather a simplification than the true estimation of the size of epileptogenic cortex (26).

Moreover, the surgical act and the fact that the electrodes remain in place for several days or weeks are associated with a significant risk of morbidity but a relatively small risk for mortality (see below), requiring careful elaboration of the goal of the investigation and the sites of implantation.

Several concepts need to be recognized and defined as much as possible before any epilepsy surgical procedure is proposed (27). The epileptogenic area, that is, the area whose resection is necessary and sufficient to abolish further seizures, needs to be clearly identified. This zone is not to be confounded with

the lesional zone, that is, the area that appears morphologically altered but may be smaller or even remote to the epileptogenic zone
the irritative zone, that is, the area of interictal spikes, which can be more widespread than the epileptogenic zone and include even contralateral structures
the symptomatic zone, that is, the area whose activation produces the clinical symptoms, such as nausea or dystonic posturing
the functional deficit zone, that is, area whose activation/deactivation leads to neurologic or neuropsychological deficits, for example, verbal memory impairment or Todd paresis

Overall, IEEG data look different compared to scalp EEG data and appear to be more “epileptogenic,” due to lack of the filter qualities of the skull, although the same rhythms known from scalp EEG can also be retrieved in the IEEG. The determination of ictal EEG pattern is usually at the center of the investigation; however, careful analysis of interictal epileptogenic discharges is probably more important than previously appreciated.

The overall yield of IEEG depends on the criteria that determine at which degree of inconsistency intracranial evaluation is offered. There are only very few data on this subject. A recent study, with access to modern imaging tools, described the results in 77 patients with mesial temporal (39%), lateral temporal (12%), and extratemporal (43%) epilepsy (12). In the
remaining five patients (6%), seizures could not be captured and they remained unclassified. Localizing results were obtained in 74%, which corresponds to numbers from other studies (28,29) and to the authors’ experience. Thus, despite the invasive nature of the procedure and the physical and psychological investment by the patient, it is not always certain that IEEG will be followed by a resective procedure. In around 25%, surgery cannot be proposed, which needs to be addressed and reiterated during preparatory talks with the patients.

HISTORY

IEEG focus evaluation has a relatively brief history, starting in Germany with Berger’s discovery of the EEG published in 1929 (30). Otfrid Foerster used preoperative EEG during epilepsy surgery and became a pioneer in using EEG for the determination of the epileptogenic zone. Foerster worked with Wernicke and published a brain atlas, and as an aside he was the treating neurologist for Lenin when he suffered a stroke in 1922. The rising Nationalist government basically destroyed Germany’s scientific culture for the next decades and many researchers left the country. Dr Foerster was the mentor of Wilder Penfield who himself became widely known for the corticography of the human neocortex. Thanks to his research on patients, the cortical representations of the motor cortex (homunculus) became standard knowledge. Dr Penfield was a neurosurgeon and founded the Montreal Neurological Institute. Herbert Jasper joined him as a neurologist and clinical neurophysiologist, and together they trained numerous neurosurgeons and neurologists in the field of epilepsy surgery. They probably represent the first example of a successful relationship between a neurologist and a neurosurgeon. This combination remains a prerequisite for any successful epilepsy surgery team (31).

Wires and a type of “strip” electrode, placed in the epidural space, were the first to be used in epilepsy patients by Jasper and Penfield. Later, subdural electrodes and other electrode types were developed and applied (31). Bancaud and Talairach, another successful team of neurologist and neurosurgeon from Hôpital Sainte-Anne in Paris, introduced multicontact depth electrodes penetrating directly into the cerebral tissue (32,33), allowing a systematic investigation of deep structures. This was a major step forward in epilepsy surgery investigations, since at that time MRI was not available and the evaluation of surgical candidates relied more on neurophysiologic methods. Ten years later, Wyler developed and introduced another electrode type into the field: the subdural electrode, which was being placed onto the brain’s surface without the risks of an intracerebral insertion (34). Initially, strips with four electrodes were described, evolving later to more complex and larger arrangements (see below).

Although the number of inserted electrodes steadily increased, or at least became technically possible, simultaneous recording of >100 electrodes was difficult for EEG systems, requiring the selection of recorded electrodes and even changes during the evaluation. Upgrading of established systems and innovative setups has finally overcome this obstacle (35), and at the time of the writing of this text, EEG systems with up to 256 channels are on the market.

Today, depth and subdural electrodes are the most established electrodes types. Most centers have a preference for one or the other electrode type, which depends on personal experience or technical equipment in the neurosurgical clinic. Around the same time of the arrival of subdural electrodes, foramen ovale electrodes (FOE) (36) and epidural peg electrodes were described by the Zürich group of Wieser et al. and the Cleveland Clinic group of Lüders et al. (37), respectively. The use of the latter two types remains relatively limited and is often used as an intermediate step in determining if, and which, larger set of intracranial electrode setups will be more adequate or as complementary electrode type, if depth or subdurals alone do not appear sufficient.

DETERMINATION OF PRECISE ELECTRODE PLACEMENT

Independent of the chosen electrode type, knowledge of the exact position of the electrodes with respect to the individual brain anatomy is of utmost importance for successful surgery in terms of complete focus removal while sparing eloquent cortex. Previously, only radiographs of the skull were possible. These provided only a gross idea of the electrode position (38,39). Now 3D reconstruction of CT and MRI with coregistration of X-rays provides superior electrode localization results (39). Coregistration of CT, MRI before and after electrode placement, or digital photos taken intraoperatively with the patient’s MRI (40, 41, 42, 43, 44 and 45) provides excellent results in terms of electrode recording sites localization. Mean mismatch between real and virtual electrode placement was found between 2 and 4 mm (42,44). Additional external fiduciary markers improve this precision (44). Coregistration of different MR sequences, notably Flair and T2, allow the visualization of smaller lesion inside the OR (46). Implantable fiduciary markers were also used, which led to an even smaller localizing error (˜0.5 mm) (47). Other difficulties inherent to all methods of coregistration between a preimplantation and a postimplantation image are the transitory changes of the brain anatomy due to cerebral edema, hematoma, and brain shift. In most algorithms, the electrode artifacts are used to identify their exact localization, on the basis of either a CT or an MRI. In a second step, the extracerebral tissue (skull, scalp) is stripped away, and brain reconstructed in a 3D fashion, showing the surface with the individual gyri and sulci. In Figure 33.1, coregistration algorithm is proposed.

The exact localization of depth electrodes is simpler, given that there is less of brain shift. Otherwise, similar algorithms to reconstruct the position of the electrodes are employed.

DEPTH ELECTRODES

Technical Aspects

Taking all patient studies together, depth electrodes are probably the most frequently used electrode types (Fig. 33.2), given that they were already in use in 1960s and 1970s in the medical field. They are inserted through Burr holes by stereotactic methods, using guidance from previously acquired brain
imaging. Depth electrodes are built as wires containing evenly spaced electrode contacts of conductive material. The insertion is done by a mandarin whose trajectory is determined by the site of the target(s) through CT and/or MRI of the patient’s brain. Sometimes additional arteriography or MR-angiography is required in order to identify crucial vessels and diminish the risk of hemorrhage. In many centers today, a stereotactic setup is utilized using the patient’s MRI. More recently, a frameless robot-guided insertion has been reported (48), and the future will show if the application of such sophisticated methods will decrease the risk of depth electrode placement and/or shorten operation time significantly.

Figure 33.1 Coregistration of intracranial electrodes and the patient’s individual MRI. Proposition for a workflow aiming fusion of the patient’s preimplantation MRI with the postimplantation MRI or postimplantation high-resolution CT. Courtesy of L. Spinelli, PhD. The preimplantation is normalized and skull stripped, then fusioned with the postimplantation MRI, which underwent the same computation. In this example, particular care is taken to compensate for the brain shift due to compression by subdural grids. This is less of an issue in patients with depth electrodes.

Electrode spacing is most often between 5 and 10 mm, but any configuration, distance, and number of electrodes can be chosen, that is, either at regular distances or only at sites that cover the gray substance. Most electrodes contain 6 to 15 electrodes. The electrodes themselves can be rigid or flexible, but most neurosurgeons prefer flexible electrodes, which may lead to less precise electrode placement (due to involuntary deviation from the initial target), but they have had a lower risk of bleeding. Most companies that produce intracranial electrodes allow the choice between platinum-iridium and nickel-chrome alloys. Only the first combination is MR compatible, but they are somewhat more expensive. The electrodes have multiple contacts (usually between 8 and 12) and can be inserted orthogonally, that is, perpendicular to the lateral surface or longitudinal-parasagittally. Oblique insertions into the frontal lobe toward the basal ganglia have also been employed. However, in all cases, there is only very limited sampling of the neocortical-lateral cortex, which may require additional subdural or scalp electrodes if the cortex needs larger coverage.

Figure 33.2 Depth electrodes. Left: Coregistration of the orthogonally inserted electrodes and the patient’s MRI. The insertion sites of depth electrodes in both hemispheres are marked in white. Upper right: Same patient as shown on left, showing his depth electrode inserted in the left amygdala. In this coregistration, each contact is identified separately (light gray balls). Lower right: Longitudinal insertion of depth electrodes, indicated in light gray.

Given that this electrode type is the only one that goes into the brain parenchyma, the major advantage of depth electrode recordings is the opportunity to record from deep, buried cortex, such as hippocampus, amygdala, or mesial frontal structures. Furthermore, the simultaneous sampling of mesial and lateral cerebral structures, associated to an excellent EEG quality, is a favorable aspect of depth electrodes. They also permit bilateral and multilobar sampling, but there is limited spatial sampling in a given region, for example, identification of language cortex and its extent is not possible with depth electrodes alone. Another significant advantage is that they can be easily removed, which can be accomplished without any anesthesia. They can be removed even at bedside.

Risks

The major risk of using depth electrodes is intracranial or intracerebral hemorrhage, which occurs in 1% to 4% (49). With modern imaging, the rate seems to be lower. In a study of 50 patients with depth and subdural strip electrodes (50), no deaths, no infections, and no new neurologic deficits were reported. Ross et al. provided a review on previously reported series of depth electrode patients, and when considering the whole group of 1656 patients together, the relative risk was highest for intracranial hemorrhage (1.1%), followed by infection (0.8%). In a recent review of 259 patients, a risk of symptomatic intracranial hemorrhage of 1.2% and 0.7% for permanent neurologic deficit is reported (51). The Montreal group reported the absence of major complications in a total of 6415 electrode implantations (491 patients) between 1976 and 2006, that is, a time span before and after introduction of MRI. Hematoma and infection were noted in 0.8% and 1.8%, respectively (52). Overall, in this study, complications were more frequent in the frontal lobe, perhaps due to the higher number of implanted electrodes. Mortality was basically zero in the series after 1990. Among the hemorrhages, only very few were life threatening. In the authors’ experience, it should be less than 0.1%.

Patient counseling on the avoidance of antiplatelet drugs as well as the implementation of preoperative coagulation parameters is recommended. There is no agreement on the use of prophylactic antibiotic treatment in patients during depth electrode investigations. If the contact between the intra- and extracranial milieu is minimized with sterile bandages and the patient is inhibited from scratching under the bandage, the risk of infection is even less than 0.8%. Even years after depth electrode investigations, discrete gliotic changes along the insertion channel are seen in MRI images, but, to date, there is no evidence that these changes become themselves epileptogenic or are related to new neurologic or cognitive deficits.

SUBDURAL ELECTRODES

Technical Aspects

These electrodes were introduced somewhat later into the field of epilepsy surgery than the depth electrodes and are used for epidural recordings. When Penfield recorded epidurally from the left parietotemporal region in a patient with post-traumatic epilepsy in 1939 (53), he can be probably considered the first to have used this electrode type. Subdural or epidural electrodes consist of disk-shaped electrodes, embedded in transparent silastic or Teflon material, arranged in rows or arrays of all shapes (Fig. 33.3). As with depth electrodes, they are made of platinum-iridium and nickel-chrome alloys, allowing MR imaging only if they are made of platinum. The contacts are put freehand on the cortex, subdurally on the pia mater. If necessary, they can be reduced with sterile scissors or along perforated lines to smaller arrays directly in the OR. There are recent reports that make use of neuronavigation and stereotactic methods to improve the precision of placement, in particular for strips, or for merging imaging data sets that help to take all preoperative imaging information into consideration (54,55). Similar to depth electrodes, determination of electrode positions by MRI or CT imaging is an important issue because smaller or larger shifts may occur after the implantation, in particular with small arrays. The contacts are usually 5 to 10 mm apart and can be arranged as one line (strip) or in a larger array, for example, 8 × 8 contacts (grids of 64 electrodes) (Fig. 33.3). Strip electrodes arranged in a single row can be inserted through Burr holes. However, a large grid, for example, array of 4 × 4 or 8 × 8 electrodes, needs a full craniotomy. This is usually a more invasive procedure than the stereotactic procedure employed for the placement of depth electrodes.

The main indication for subdural electrodes is the precise localization of a seizure focus within a suspicious area. When using grid arrays, two to three lobes can be covered with a single grid, for example, frontal posterior, temporal, and parietal. This requires that noninvasive evaluation narrows down the hypothesis of the site of the focus to one or two lobes. Subdural electrodes also allow for preoperative corticography, with the aim to determine vital cortex. This procedure is described below in section Special Considerations in Pediatric Patients and in a study by Lesser et al. (118). By stimulation with short (2 to 5
seconds) intermittent electrical currents, sensations or involuntary movements are produced, which supposedly reflect the characteristics of the underlying cortex. Primary cortex can thus be reliably identified. However, higher order cortical phenomena, like language, are not always easily identified as shown by Seeck et al. (56)

Figure 33.3 Intraoperative subdural grid placement. Each contact (platinum disk, 0.8 mm diameter) is recorded through a wire, which is led out of the crane through a larger white cable (in the lower left of the image; in this grid, bundles of eight wires each).

These electrodes do not necessarily require knowledge of stereotactic procedures and/or a neuronavigation device and, as such, can be performed in neurosurgic centers without experience in stereotactic methods. Subdural electrodes record from a larger cortical surface and capture incompletely signals from the sulci as well as from radial sources. It is a well-established finding that only one third of the brain is accessible from the surface, which also means only one third is accessible in terms of recording. Thus, most brain tissue underneath a subdural grid will not be directly recorded, but may be involved only after propagation to these more superficial cortical regions has occurred.

Risks

The morbidity is low for strip electrodes and includes infections, around 1%, or bleeding when bridge veins are damaged (57). Larger grid arrays are more likely to be associated to more significant complications. The infection rate is higher than with depth or strip electrodes alone, and is reported to be around 8%. This risk can be diminished if the cables exit through a tunnel via a second incision a few centimeters away from the original incision. As for depth electrodes, no consensus exists whether or not prophylactic antibiotic therapy should be given (57,63), albeit this issue is more relevant with subdural electrodes. Based on personal experience, prophylactic antibiotic treatment in patients with large arrays of subdural electrodes was related to lower incidence of infectious problems, for example, Cefazolinum 4 × 500 mg IV.

Most patients with grids experience local edema of the underlying cortex, leading to headache and photophonophobia. This complication occurs less often with strips. Rigorous symptomatic treatment with antipain drugs as well as positioning of the head at a 30° to 45° elevation are helpful. Major or near-fatal raised intracranial pressure is extremely rare, but if discovered, may need immediate surgical decompression. Consequently, a neurosurgeon on call nearby is a necessity.

In a recent study, delayed subdural hematoma was noted to be the most frequent complication (8%) (58). They occurred up to 3 days postimplantation. This “fragile” period is probably somewhat longer and, as for most brain surgeries, spans up to 5 days. A recent study in a pediatric group noted postimplantation problems in >20% of their patients, with predominance of hematoma, but this may be due to the fact that 43% of their patients had already undergone a prior epilepsy surgery procedure (59). Another pediatric series reported a lower complication rate, including hematoma (6%) (60). However, implantation of larger arrays of subdural electrodes remains a relatively delicate procedure, requesting unambiguous agreement on the indication, in particular in children. Patients with a history of high-dose radiotherapy and chemotherapy may develop significant persistent neurologic deficits after subdural grid placement, not explained by brain swelling (61). IEEG in this patient group may be related to more severe complications than in patients without this history.

Experience is also an important factor. The Cleveland Clinic group reported 33% of minor and more serious complications at the beginning of their epilepsy program, which dropped to 19% in more recent years. Overall, the use of more than 100 electrodes, longer recording sessions, that is, more than 2 weeks, and more than one cable exit are related to an increase in complication rates (62,63). It is probably safe to say that complications between 10% and 20% of the cases can be expected, requiring close monitoring by specialized personnel and in a center with 24 hours access to a scanner and an ICU. Regular neurologic exams and monitoring for infectious diseases are mandatory (Table 33.2).

If bleeding complications are present, it is safer to remove the electrodes as soon as possible. If there are signs of local infections, and if enough seizures are recorded, removal of electrodes is recommended. The rising of C-reactive protein (CRP), after attaining postoperatively normal or near-normal values, is a valuable marker. If this is not the case, that is, no or only too few seizures are recorded, introduction or reinforcement of the antibiotic treatment with prolongation of the recording period, perhaps with more rigorous drug and sleep withdrawal, should be considered. Since intracranial electrode implantation is a costly and intensive intervention, in terms of manpower and, more importantly, physical and psychological strain on the patients themselves, reimplantation at another moment is not easily done or recommended. General guidelines are difficult to establish since every center has its own resources, and patients differ in their willingness to cope with unforeseen clinical situations.

Table 33.2 Proposed Surveillance Algorithm for Patients Admitted for Subdural Intracranial EEG Monitoring

•	24 hours supervision by nurses, technicians, and/or other persons, for example, medical students or family members
•	Postimplantation 24-hour surveillance in the ICU
•	When admitted for EEG monitoring
	•	CRP, blood: every day or every second day
	•	Temperature: two times per day
	•	Brief neurologic exam (pupils, responsiveness, motor functions) every 2-4 hours for the first 2 days, then every 4-6 hours for the next 3-4 days
•	Optional: prophylactic antibiotic treatment (e.g., cefazolin) starting from the day of the implantation through the monitoring period
•	Optional: dexamethasone during the first 3-4 days

Despite higher complication rates with subdural electrodes, the rates of permanent neurologic deficits do not seem to differ from the numbers for depth electrodes and are situated around 1% to 2% (63). The removal can be done at bedside, if strips were used, but will require another craniotomy during which the surgical resection of the focus is carried out as well, if indicated, if grids are used.

FORAMEN OVALE ELECTRODES AND EPIDURAL ELECTRODES

FOE, initially developed in 1985 by the Zürich group of Wieser et al., are used somewhat less frequently in most centers (36). FOE record epidurally and extracerebrally from the mesial temporal lobe structures. They contain four to six contacts, and can be inserted under local anesthesia (Fig. 33.4). They can also be easily removed at bedside, similar to depth electrodes. Major complications are rare, but they are not “semi-invasive” as initially advocated. Subarachnoidal hemorrhage and transient brainstem symptoms, for example, trigeminal neuralgia, are observed. Complication rates of 1% to 9% are reported, mostly transient, that is, similar to rates reported for depth or subdural electrodes. FOE yield good recordings from the mid and posterior portions of the hippocampus, but less well from the anterior aspect and the amygdala and/or small subcompartments of the mesial temporal structures. The classical indication for FOE is the lateralization of temporal lobe onset. They may be combined with subdural or scalp electrodes for more extensive sampling from lateral temporal or extratemporal structures, and consequently the indication may be extended to other indications.

Epidural peg electrodes share configuration aspects with subdural electrodes and are inserted through burr holes or twist drill holes (37). They can be placed bilaterally, similar to scalp electrodes, but cannot be used for cortical mapping. Electrical stimulation through epidural electrodes is painful because the rest of the dura is innervated and irritated by electrical current. Precise sublobar focus localization is not possible with epidural electrodes alone since their spatial sampling is not sufficient. However, they have a low morbidity (<0.5%), consisting mainly of discomfort when the electrodes are inserted through the temporalis muscle and some superficial wound infections. Their main indication is the verification of the absence of epileptogenic discharges and/or recording of selected sites where scalp electrodes would be too contaminated by muscle artifacts.

Figure 33.4 Foramen ovale electrodes. Left: Insertion procedure through the base of the skull (i.e., through the foramen ovale). (Courtesy of CCND Winnipeg.) Right: Foramen ovale electrodes. (Courtesy of Dixi electronics.)

SURVEILLANCE

There is no consensus guidelines concerning the degree of medical and paramedical staff surveillance and concomitant treatment in patients admitted for EEG monitoring. However, a “semi-intensive” care unit with a lower nurse-patient ratio compared to normal care units appears the most appropriate setting. There are still many centers that cannot offer professional surveillance at night or on weekends, leading to seizurerelated complications, for example, autonomous electrode removal during the postictal confusional state.

The choice on whom to rely on during the patient’s monitoring session is influenced by budget and legal considerations. Although we know relatively well the risk of IEEG complications such as bleeding or infection, there is disagreement across surgical epilepsy centers on who watches the patient: parent/close family member; nonmedical staff, for example, out-of-work people recruited on through a placement agency; EEG technicians; or nurses. Patients who are recorded after drug withdrawal carry a risk for more prolonged or more intense seizures (i.e., more often generalized tonic-clonic seizures) resulting in trauma and/or prolonged postictal confusion. Pulling out the electrodes is also encountered. In a recent review, 3 out of 491 patients (0.6%) pulled out their electrodes during a seizure or during the early postimplantation period (52). CSF leakage, bleeding, or infection may result from such an act, particularly if larger subdural electrodes arrays are removed.

Family members or nonmedical people may not be enough, since they are often unable to judge the clinical context or severity of a given situation. There are no studies on the guidelines on the size and composition of the surveillance staff necessary to minimize these complications and to reduce the recording time. For obvious reasons, professional surveillance by trained
nurses and technicians, in collaboration with an experienced epileptologist or neurologist, is the optimal setting. Legal aspects also need to be considered in order to be protected against “under-surveillance,” and these differ markedly between countries. In some countries, like the United States, legal action against the hospital is more readily taken if complications are encountered, and therefore, different solutions and their legal implications need to be more thoroughly studied.

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