Foramen Ovale and Peg Electrodes

Heinz Gregor Wieser

Introduction

The foramen ovale (FO) electrode recording technique was developed in Zürich in 1983.²⁶ Its goal was to make less invasive and simplify the neurophysiologic part of the presurgical evaluation protocol for candidates for selective amygdalohippocampectomy (AHE). Before 1983, stereoelectroencephalography (SEEG) had been used for the majority of patients being evaluated there for epilepsy surgery.²¹^,²³ The accumulated SEEG experience revealed that the mesial temporal lobe (TL) structures, in particular the hippocampus, the parahippocampal gyrus, and the amygdala, play important roles as seizure-generating structures in most patients with temporal lobe epilepsy (TLE). As a consequence of these findings, in 1975, the so-called transsylvian AHE³³ was developed and has since become the operative approach of first choice for surgical treatment of mesial TLE (MTLE) in Zürich as well as in many other centers.

Peg electrodes were developed at the Cleveland Clinic in 1988.²^,³ They are usually used as sentinel electrodes, often in combination with other invasive techniques, for obtaining epidural ictal recordings.

Data obtained from Engel’s survey carried out before the 1992 Palm Desert conference showed that, at that time, 15 centers (15%) used FO electrodes and four centers used epidural pegs. FO electrodes had been used only in 5% of all reported patients operated on from 1986 to 1990 (n = 7,664), but in 21% of 662 reported AHE patients.⁷ Since then, FO electrodes seem to be used increasingly in more centers,¹^,⁶^,¹²^,¹⁵^,¹⁸^,³⁴ whereas peg electrodes are presently used by only two epilepsy surgery centers.

FO and peg electrode recording techniques have been labeled an intermediately invasive or semiinvasive approach.

Indications

Peg electrodes allow artifact-free recording of the electrocorticogram in relatively small but widely separated regions of the cortical convexity. They were designed to determine surgical suitability in patients in whom the clinical, electrographic, and neuroimaging information did not localize the epileptogenic zone well; that is, they were used in circumstances in which exact placement of a subdural grid array was uncertain from the clinical, neuroimaging, and scalp electroencephalogram (EEG) data. Often they were inserted as sentinel electrodes contralateral to subdural grid placement, to confirm that ictal discharges were not beginning contralaterally. Depending on the particular clinical circumstances, peg electrodes may be used with other electrodes including FO electrodes, depth electrodes, and subdural grid arrays.

FO electrodes record from the mesial aspects of the TL. Compared with intracerebral depth electrodes, subdural grid electrodes, and most probably also subdural strip electrodes, FO electrodes are less invasive, but nevertheless the possibility of complications is inherent in the FO electrode technique. Therefore, its use should be restricted to presurgical evaluation of possible candidates for epilepsy surgery. Moreover, it is obvious that its main indication is for patients with TLE, in particular for patients suffering from the syndrome of MTLE.²⁵ Today FO electrodes are mainly used in patients with suspected MTLE with ambiguous seizure onset lateralization or with other noncongruent findings. Bilateral hippocampal sclerosis (HS) and/or bilateral independent interictal spiking are the second most often encountered reasons to insert FO electrodes. HS is the neuropathologic substrate most often found in MTLE. Vossler et al.²² found that marked hippocampal atrophy (HA) and high-grade HS are associated with initial ictal discharges (IID) restricted to the hippocampal formation, whereas low-grade HS and absence of HA are associated with slower (2–4 per second) scalp IID frequencies and with lobar or regional seizure onsets not restricted to medial structures. Therefore, FO electrodes are particularly useful in patients without marked HS and when AHE is intended. However, in such constellations, the combination with subdural strip or grid electrodes is often necessary.

Table 1 Types of FO electrodes and combinations with other invasive recording technique (n = 264)

		Bilateral	Unilateral			Number of electrode contacts
	Total	R+L	R	L	1	3	4	8	10
Patients	264	253	6	5	34⁺	1	138	7	84
FO with thermoelement	14	6	5	3
		FO Electrodes combined with other intracranial electrodes
		+SEEG	+Strip	+Strip + Grid	+Grid
Patients	49	14	32	2	1
%	18.6	5.3	12.1	0.7	0.4
R= right hand side; L= left hand side. ⁺ First FO EEG was recorded from a patient with trigeminal neuralgia. 44 DIXI Electrodes (DIXI microtechniques, France), the remaining were designed in our own laboratory. Thermoelement for research purpose (Landolt et al., 1995).

FIGURE 1. Left: Insertion technique of foramen ovale electrodes according to the technique of Kirschner (1932) using the landmarks of Härtel (1914). 1 is a point 5 cm anterior to external auditory meatus. 2 is the medial pupillary point. 3 is the electrode entry side, 3 cm lateral to oral commissure. The index finger is in the pterygoid fossa. The needle is inserted as described in the text, and then the special splittable cannula is withdrawn and broken.

Techniques

Foramen Ovale Electrode Design, Insertion, Removal, and Recording

In recent years, several types of FO electrodes have become commercially available. Until June 1999, the FO electrodes used in Zürich were site-made.²⁸ In brief, these FO electrodes consisted of Teflon-insulated, helically wound silver or platinum wires (diameter 0.1143 mm [0.0045 inches]) ending in four, eight, or ten poles. They were mounted on a “surgical” (highly corrosion-resistant) stainless steel wire 0.1 mm in diameter. This carrier, isolated with a special varnish, had adequate mechanical properties. It was flexible enough and had a special end to avoid penetration of the arachnoidal-pial layer. Each pole consisted of 90 parallel windings and was 2 or 4 mm long. The distance between two contacts was 2 to 5 mm, depending on the type of electrode (Table 1). The external diameter of the FO electrode types was such that it passed through a special split Table 18-gauge cannula (produced and marketed by Medialimed SA, CH-1604 Puidoux, Switzerland). This splittable cannula is still in use (Fig. 1). It has an external diameter of 1.23 mm and an internal diameter of 0.93 mm and permits the multipin connector to be mounted and soldered, and the stabilizing insulator poured in well before implantation. The site-made electrode impedance ranged from less than 200 ohms to a maximum tolerated 700 ohms, respectively, whereas the DC offset potential was less than 2 mV. These electrodes could be armed with other specific recording devices.
Temperature-sensitive devices have been used in 14 patients¹⁴ (see Table 1), and special ictal and interictal DC recording with monopolar silver FO electrodes were carried out in five patients (Fig. 6).²⁰

Insertion

Insertion of the FO electrode can be done under local anesthesia. Currently, however, the procedure is done in Zürich under general anesthesia. With the stylet inside it, the special cannula is inserted 3 cm lateral to the oral commissure and directed along the intersection of two orthogonal planes: (a) the plane defined by the insertion point, a point on the lower eyelid corresponding to the medial border of the pupil, and the tip of the electrode directed toward the foramen ovale; and (b) a plane defined by the insertion point, a point 5 cm anterior to the external meatus acusticus, and the tip of the electrode directed toward the FO (Fig. 1).

The patient usually responds to the passage of the needle through the FO with a wince and a brief contraction of the masseter muscle. After withdrawal of the stylet, some cerebrospinal fluid usually leaks, and the electrode can then be carefully positioned under radioscopic control. In most instances, the tip of the electrode slips without any resistance into the caudal
end of the ambient cistern (Fig. 2, see Fig. 7). After the splittable cannula has been withdrawn and broken off, the freed electrode is fixed to the skin by a special clamp. Gauze and adhesive tape cover the electrode where it penetrates the skin. Antibiotic protection is given throughout the recording period and is continued for 3 days after the removal of the electrode.

FIGURE 2. Schematic illustration of implanted 10-contact foramen ovale electrodes (A), as depicted by lateral (B) and AP x-ray (C) and CT imaging (D, E). Electrode contacts are numbered 1 to 10. A is used with permission of Dr. Dominik Zumsteg.

Removal

For the removal of the FO electrode, anesthesia is not necessary. During the withdrawal of the electrode a brief painful sensation in the ipsilateral teeth is relatively common. Therefore, the patients should be informed about this possibility before the explantation.

Montage, Recording, and Analysis

Sophisticated DC recording with FO electrodes has been done hard-wired with 32 channels in the laboratory, but seizure monitoring in AC mode is now realized by 64 or more channels with the patient on the ward. For routine monitoring, an uninterrupted bipolar montage connecting the ten contacts of both FO electrodes is recommended, as shown in FIGURE 3. The other channels are used for simultaneous scalp EEG and other intracranial EEG records, as well as polygraphy, if indicated.

FIGURE 3. Example of a 10-contact foramen ovale (FO) electrode recorded seizure onset at contact 5 of the left FO electrode (phase reversal between channels 15 and 16) with repetitive 7/sec sharp waves and evolving rhythmic more wide-spread spike discharges of 19/sec. Note that the scalp-EEG channels do not show the seizure onset. At the left the montage for recording from 10-contact FO electrodes is shown. For routine long-term seizure monitoring, we prefer the depicted closed chain with 20 channels (number in boxes).

Patient Data of the Zürich FO Electrodes Series

With two exceptions (the first FO electrode patient had no epileptic seizures, but trigeminal neuralgia, and one patient had aggressive outbursts thought to reflect limbic seizures), all patients who underwent FO electrode implantation in Zürich had medically refractory complex partial seizures with or without secondary generalization. In most of these patients, before FO electrode implantation, there was rather strong suspicion of mesiobasal limbic seizure onset, as evidenced from the seizure semiology, interictal and ictal scalp EEG, neuropsychological examinations, and structural (computed tomography [CT], magnetic resonance imaging [MRI]) and/or functional (single positron emission computed tomography [SPECT] and positron emission tomography [PET]) imaging. These patients were evaluated with the aim of demonstrating a unilateral mesiobasal limbic seizure onset with a degree of confidence sufficient for surgical intervention. In recent years, FO electrodes were only used in patients with less clear evidence for mesiobasal TL seizure onset; that is, in patients with some contradictory findings pointing to lateral TL or extra-TL seizure onset, and in patients in whom lateralization was a problem (bilateral MTLE). At the beginning of the use of FO electrodes in Zürich, the majority of these patients were evaluated
simultaneously with FO and stereotactic depth electrodes. In recent years, FO electrodes were often combined with strip and sometimes with grid electrodes. In a small proportion of our FO electrode patients, mesiobasal limbic seizure onset was rather unlikely. These patients underwent long-term monitoring with FO electrodes to rule out mesiobasal limbic seizure onset definitively (in which case they would no longer have been candidates for surgical treatment) or to prove a possible secondary pacemaker role of one mesiobasal TL (in which case a so-called palliative AHE would be considered a treatment option).

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