Connectivity of an effective hypothalamic surgical target for cluster headache

The purpose of this study was to look at the connectivity of the posterior inferior hypothalamus in a patient implanted with a deep brain stimulating electrode using probabilistic tractography in conjunction with postoperative MRI scans. In a patient with chronic cluster headache we implanted a deep brain stimulating electrode into the ipsilateral postero-medial hypothalamus to successfully control his pain. To explore the connectivity, we used the surgical target from the postoperative MRI scan as a seed for probabilistic tractography, which was then linked to diffusion weighted imaging data acquired in a group of healthy control subjects. We found highly consistent connections with the reticular nucleus and cerebellum. In some subjects, connections were also seen with the parietal cortices, and the inferior medial frontal gyrus. Our results illustrate important anatomical connections that may explain the functional changes associated with cluster headaches and elucidate possible mechanisms responsible for triggering attacks.

Design and fabrication of multichannel cochlear implants for animal research

The effectiveness of multichannel cochlear implants depends on the activation of perceptually distinct regions of the auditory nerve. Increased information transfer is possible as the number of channels and dynamic range are increased and electrical and neural interaction among channels is reduced. Human and animal studies have demonstrated that specific design features of the intracochlear electrode directly affect these performance factors. These features include the geometry, size, and orientation of the stimulating sites, proximity of the device to spiral ganglion neurons, shape and position of the insulating carrier, and the stimulation mode (monopolar, bipolar, etc.). Animal studies to directly measure the effects of changes in electrode design are currently constrained by the lack of available electrodes that model contemporary clinical devices. This report presents methods to design and fabricate species-specific customizable electrode arrays. We have successfully implanted these arrays in guinea pigs and cats for periods of up to 14 months and have conducted acute electrophysiological experiments in these animals. Modifications enabling long-term intracochlear drug infusion are also described. Studies using these scale model arrays will improve our understanding of how these devices function in human subjects and how we can best optimize future cochlear implants.

Deep brain stimulation in craniofacial pain: seven years' experience

Cluster headache (CH) is a primary headache with excruciatingly painful attacks that are strictly unilateral. About 10% of cases experience no significant remission, and about 15% of these do not respond to medication, so surgery is considered. Neuroimaging studies show that the posterior inferior hypothalamus is activated during CH attacks and is plausibly the CH generator. We report on 16 chronic CH patients, with headaches refractory to all medication, who received long-term hypothalamic stimulation following electrode implant to the posterior inferior hypothalamus. After a mean follow-up of 23 months, a persistent pain-free to almost pain-free state was achieved in 13/16 patients (15/18 implants; 83.3%) a mean of 42 days (range 1-86 days) after monopolar stimulation initiation. Ten patients (11 implants) are completely pain-free. A common side effect was transient diplopia, which limited stimulation amplitude. In one patient, a small non-symptomatic haemorrhage into the 3rd ventricle occurred following implant, but regressed 24 h later. Persistent side effects are absent except in one patient with bilateral stimulation, in whom stimulation was stopped to resolve vertigo and worsened bradycardia, but was resumed later without further problems. Hypothalamic stimulation is an effective, safe and well tolerated treatment for chronic drug-refractory CH. It appears as a valid alternative to destructive surgical modalities, and has the additional advantage of being reversible.

Update on neurosurgical treatment of chronic trigeminal autonomic cephalalgias and atypical facial pain with deep brain stimulation of posterior hypothalamus: results and comments

The objective of this study is to describe the therapeutic effect and the technical and surgical problems of deep brain stimulation (DBS) of the posterior hypothalamus over seven years, for treatment of chronic trigeminal autonomic cephalalgias and atypical facial pain. We report a surgical series of 20 patients that underwent DBS of the posterior hypothalamus. This series includes 16 patients with chronic cluster headache (CH), one patient with short-lasting unilateral neuralgiform headache attacks with conjunctival injection and tearing (SUNCT) and three patients with atypical facial pain. All patients of this series were resistant to any pharmacological and conservative treatment. The stimulated target was the same in the whole series even though stereotactic coordinates of the target referred to the midcommissural point differ slight in the Y anteroposterior value due to individual anatomical variability. The commissures based reference system was adapted to individual anatomical landmarks of the brainstem adding to the registration a third reference point below the commissural plane. The stimulation parameters of unipolar stimulation were similar in the whole series: 180 Hz, 60 mus, 1-3 V. In the CH series, at five years follow-up the percentage of total number of days free from pain attacks improved from 1%-2% to 71%. Ten patients of this series had a complete and persistent pain-free state at 18 months follow-up and the patient with SUNCT has complete pain relief. In the three patients with atypical facial pain, the neurostimulation procedure was absolutely unsuccessful. DBS of the posterior hypothalamus produced a significant and marked reduction of pain bouts in CH patients and in the SUNCT patient. The attempts to treat atypical facial pain in three patients failed.

Is vagus nerve stimulation a treatment option for patients with drug-resistant idiopathic generalized epilepsy?

BACKGROUND

The value of vagus nerve stimulation (VNS) for treating patients with drug-resistant idiopathic generalized epilepsy (IGE) is not well documented.

PATIENTS AND METHODS

Twelve patients (2 males, 10 females) with a mean age of 31 years (11-48 years) and with drug-resistant IGE had VNS implanted in the period 1995-2006. All had generalized seizures documented by video-electroencephalogram. Mean follow-up period was 23 months (9-54 months).

RESULTS

There was a total seizure reduction of 61% (P = 0.0002). There was 62% reduction of generalized tonic-clonic seizures (P = 0.0020), 58% of absences (P = 0.0003) and 40% of myoclonic seizures (P = 0.0156). Eight patients were considered responders (>50% seizure reduction); two of these patients became seizure-free. Five out of seven patients with juvenile myoclonic epilepsy were responders. At the last follow-up visit, the patients had reduced the anti-epileptic drug (AED) usage from an average of 2.3 to 1.7 AED per patient (P = 0.0625). Two patients are currently being treated with VNS therapy only. Nine patients reported side effects, which were mostly mild and tended to diminish over time.

CONCLUSION

Our results indicate that adjunctive VNS therapy is a favourable treatment option for patients with drug-resistant IGE. Rapid cycling seems worth trying in some of the non-responders.

Spontaneous neuronal activity of the posterior hypothalamus in trigeminal autonomic cephalalgias

Microrecordings of three neurons were obtained at the target site in three patients with trigeminal autonomic cephalalgias who were implanted with deep brain stimulators in the posterior hypothalamus. Two patients had chronic cluster headache, one short unilateral neuralgiform headache with conjunctival injection and tearing. Average firing rate was around 24 spikes/s. All neurons were firing randomly, and for most of the recordings in tonic fashion. In one patient, tactile stimulation of the ophthalmic branch, contralateral to the recording site, decreased the firing rate. Neuronal activity in these patients was similar to that reported in animal studies of the posterior hypothalamus. Positioning deep brain stimulators in the posterior hypothalamus may offer a tool to better characterise the activity of this part of the brain in humans.

Occipital nerve stimulation for drug-resistant chronic cluster headache: a prospective pilot study

BACKGROUND

Drug-resistant chronic cluster headache (drCCH) is a devastating disorder for which various destructive procedures have been tried unsuccessfully. Occipital nerve stimulation (ONS) is a new, safe strategy for intractable headaches. We undertook a prospective pilot trial of ONS in drCCH to assess clinical efficacy and pain perception.

METHODS

Eight patients with drCCH had a suboccipital neurostimulator implanted on the side of the headache and were asked to record details of frequency, intensity, and symptomatic treatment for their attacks in a diary before and after continuous ONS. To detect changes in cephalic and extracephalic pain processing we measured electrical and pressure pain thresholds and the nociceptive blink reflex.

FINDINGS

Two patients were pain free after a follow-up of 16 and 22 months; one of them still had occasional autonomic attacks. Three patients had around a 90% reduction in attack frequency. Two patients, one of whom had had the implant for only 3 months, had improvement of around 40%. Mean follow-up was 15.1 months (SD 9.5, range 3-22). Intensity of attacks tends to decrease earlier than frequency during ONS and, on average, is improved by 50% in remaining attacks. All but one patient were able to substantially reduce their preventive drug treatment. Interruption of ONS by switching off the stimulator or because of an empty battery was followed within days by recurrence and increase of attacks in all improved patients. ONS did not significantly modify pain thresholds. The amplitude of the nociceptive blink reflex increased with longer durations of ONS. There were no serious adverse events.

INTERPRETATION

ONS could be an efficient treatment for drCCH and could be safer than deep hypothalamic stimulation. The delay of 2 months or more between implantation and significant clinical improvement suggests that the procedure acts via slow neuromodulatory processes at the level of upper brain stem or diencephalic centres.

Anatomical identification of active contacts in subthalamic deep brain stimulation

BACKGROUND

Subthalamic Deep Brain Stimulation is a valid surgical procedure for the treatment of idiopathic PD, although its precise mechanism of action is still unclear; moreover, there are no conclusive data about the functional anatomy of the human subthalamic region. Identifying the location of active contacts for StnDBS can yield interesting insights on the mechanisms of action of DBS and the different role played by the anatomical structures of the subthalamic region.

METHODS

Twenty-five patients operated on for bilateral StnDBS were considered. During the surgical procedure, a complete intraoperative neurophysiological study was obtained by means of semimicrorecordings and stimulations. After surgery, an MRI study confirmed the position of the electrodes; MR images were subsequently superimposed onto a stereotactic atlas by using a dedicated workstation. The coordinates relative to the tip of the electrodes and active contacts were then calculated.

RESULTS

Most of the electrode tips are located inside the subthalamus or immediately ventrally to it. Of the active contacts used for chronic stimulation, 96.5% are located in a well-defined anatomical region, which includes subthalamus, zona incerta, and FF.

CONCLUSIONS

Our findings seem to suggest that other structures beyond the subthalamus itself play a clinical role in symptoms control after DBS for PD.

Safety of nerve conduction studies in patients with implanted cardiac devices

Patients with implanted cardiac devices and their physicians may defer important electrodiagnostic testing because of anxiety about potential negative effects on the device. To determine the safety of routine nerve conduction studies (NCS) in this population, 10 patients with permanent dual-chamber pacemakers of various types and five patients with implanted cardiac defibrillators (ICD) underwent nerve stimulation at sites commonly used during NCS. The implanted cardiac device was interrogated before and after the study and there was continuous monitoring of the surface electrocardiogram (ECG) and atrial and ventricular electrograms. Electrical impulses generated during routine NCS were never detected by the sensing amplifier and did not affect the programmed settings of the implanted cardiac device. We conclude that routine NCS is safe in patients with implanted cardiac pacemakers with bipolar sensing configurations and defibrillators.

Management of Cerebral Spinal Fluid Leak Complicating Spinal Cord Stimulator Implantation

Accidental puncture of the dura mater with resultant leakage of cerebral spinal fluid (CSF) and development of postdural puncture headache (PDPH) is a known potential complication of percutaneous placement of spinal cord stimulator (SCS) leads. However, the implications and management strategies for this complication have not been thoroughly reported. We report two cases of SCS lead placement complicated by CSF leak and PDPH.

Spinal cord stimulation for refractory angina pectoris and peripheral vascular disease

Spinal cord stimulation has been used in clinical practice for more than three decades. The primary use of this therapy has been in spine-related disorders. In recent years, the therapy has been used more extensively in diseases of the vascular system. Increasingly, interest has piqued in using this mode of treatment for refractory angina and ischemic pain secondary to peripheral vascular disease. In this publication, we review the current literature on these two indications and present case examples of both therapies.

An evaluation of the accuracy and precision of a stand-alone submersible continuous ruminal pH measurement system

The objectives of this study were 1) to develop and evaluate the accuracy and precision of a new stand-alone submersible continuous ruminal pH measurement system called the Lethbridge Research Centre ruminal pH measurement system (LRCpH; Experiment 1); 2) to establish the accuracy and precision of a well-documented, previously used continuous indwelling ruminal pH system (CIpH) to ensure that the new system (LRCpH) was as accurate and precise as the previous system (CIpH; Experiment 2); and 3) to determine the required frequency for pH electrode standardization by comparing baseline millivolt readings of pH electrodes in pH buffers 4 and 7 after 0, 24, 48, and 72 h of ruminal incubation (Experiment 3). In Experiment 1, 6 pregnant Holstein heifers, 3 lactating, primiparous Holstein cows, and 2 Black Angus heifers were used. All experimental animals were fitted with permanent ruminal cannulas. In Experiment 2, the 3 cannulated, lactating, primiparous Holstein cows were used. In both experiments, ruminal pH was determined continuously using indwelling pH electrodes. Subsequently, mean pH values were then compared with ruminal pH values obtained using spot samples of ruminal fluid (MANpH) obtained at the same time. A correlation coefficient accounting for repeated measures was calculated and results were used to calculate the concordance correlation to examine the relationships between the LRCpH-derived values and MANpH, and the CIpH-derived values and MANpH. In Experiment 3, the 6 pregnant Holstein heifers were used along with 6 new submersible pH electrodes. In Experiments 1 and 2, the comparison of the LRCpH output (1- and 5-min averages) to MANpH had higher correlation coefficients after accounting for repeated measures (0.98 and 0.97 for 1- and 5-min averages, respectively) and concordance correlation coefficients (0.96 and 0.97 for 1- and 5-min averages, respectively) than the comparison of CIpH to MANpH (0.88 and 0.87, correlation coefficient and concordance correlation coefficient, respectively). The concordance correlation analysis indicated that the ruminal pH data for LRCpH (1- and 5-min averages) vs. MANpH had location shifts that were smaller than those of the CIpH vs. MANpH. However, the scale shift was similar between the LRCpH and the CIpH. The plotted data from both systems closely resembled the line y = x, indicating that both systems were accurate and precise. In Experiment 3, changes in baseline millivolt readings for pH readings after 24, 48, or 72 h of ruminal incubation were not significantly different than zero, indicating that daily standardization of new electrodes was not essential. Results from this study indicate that the LRCpH system can accurately and precisely measure ruminal pH; thus, it provides increased opportunity for researchers to measure ruminal pH and the occurrence of ruminal acidosis in unrestrained cattle.

Can pacemakers, neurostimulators, leads, or guide wires be MRI safe? Technological concerns and possible resolutions

The substantial benefits of magnetic resonance imaging are often denied to patients known to have implanted medical devices such as pacemakers and neurostimulators. Other patients are put at potential risk when they undergo an MRI procedure, even though specific informed consent is required regarding the possible MRI interactions with the implanted device. The medical community is currently divided over the actual extent of the MRI safety problem. In this report, insight is provided into the wide array of results achieved by many researchers; also, several options for producing medical devices that are inherently safe under worst-case MRI conditions are presented. As the problem is very complex and the variety of implants is large, this paper focuses on the problems of MRI-induced lead heating.

The sustained benefits of long-term neurostimulation in patients with refractory chest pain and normal coronary arteries

OBJECTIVES

To assess the long-term efficacy of neurostimulation for treating refractory angina pectoris-like chest pain, we followed patients, treated with either transcutaneous electrical nerve stimulation (TENS) or spinal cord stimulation (SCS).

METHODS

Neurostimulation was judged successful and subsequently continued when initial pain was reduced at least 50%. All patients started with TENS, but if skin irritation occurred during TENS, a SCS system was implanted. The quality of life was measured at baseline and follow-up with the Seattle Angina Questionnaire. Additional information was gathered concerning anti-anginal medication, complaints, and physical condition.

RESULTS

Of 36 patients treated successfully with neurostimulation, we identified 24 patients after a mean (SD) follow-up period of 5.08 (3.86) years; 12 patients dropped out of the study. In 13 of the remaining 24 patients, TENS induced skin irritation. Eight of these 13 patients received successful SCS, while five refused implantation. In the 24 patients, a mean pain reduction of 57% was achieved in conjunction with an increased exercise capacity of 30% and walking distance increased from 0.73 (0.83) to 1.62 (1.62) (p=0.018). Within the Seattle Angina Questionnaire the domain 'disease perception' improved from 38.89 (16.61) to 49.31 (21.83) (p=0.004), the domain 'physical limitation' improved from 29.89 (15.10) to 40.97 (22.63) (p=0.001) and 'anginal frequency' improved from 41.67 (24.08) to 55.00 (23.03) (p=0.005). In addition, nitroglycerin consumption was reduced from 7.85 (8.49) to 1.98 (2.19) (p=0.001).

CONCLUSION

Neurostimulation techniques should thus be of widespread value for treating angina pectoris-like chest pain in patients who are refractory to medication.

Does a Combination of Intensive Cognitive-Behavioral Pain Management and a Spinal Implantable Device Confer any Advantage? A Preliminary Examination

Research suggests that a combination of a somatic and a psychosocial intervention for chronic noncancer pain should be associated with a better outcome than either alone. This study presents data on a series of 31 patients who underwent sequential treatment with an implantable device targeting pain relief and a cognitive-behavioral pain management program that targeted improved function. A combination of treatments was used as there was a suboptimal response to the initial treatment. There were improvements in a range of outcomes at a long-term follow-up. Significant improvements were found in disability, affective distress, self-efficacy, and catastrophizing, but not in average pain severity. Further analyses failed to demonstrate an order effect. These results support the view that combined somatic and psychosocial interventions can achieve better outcomes than either alone in selected chronic pain patients. This approach requires that psychological assessment is essential before the use of an implantable device. This may not only improve patient selection, but also identify psychosocial factors that may be modified to enhance the effectiveness of invasive interventions. In addition, consideration for an implantable device following a suboptimal response to treatment in a cognitive-behavioral pain management program should include a re-evaluation of the patients' beliefs and use of self-management (coping) strategies before deciding on further treatment options.

Deep brain stimulation of the substantia nigra pars reticulata exerts long lasting suppression of amygdala-kindled seizures

Deep brain stimulation (DBS) has been used to treat a variety of neurological disorders including epilepsy. However, we have limited knowledge about effective target areas, optimal stimulation parameters, and long-term effect of DBS on epileptic seizures. Here we examined the effects of DBS of the substantia nigra pars reticulata (SNr) on amygdala-kindled seizures. Microwire electrodes were implanted into the SNr and amygdala of adult male rats. When stage 5-kindled seizures were achieved by daily amygdala kindling, high frequency stimulation was delivered to the SNr bilaterally 1 s after cessation of kindling. Our DBS protocol completely blocked kindled seizures in 10 out of 23 (43.5%) rats studied. Furthermore, when the same amygdala kindling procedure was performed 24 h later without DBS, the kindling failed to elicit any seizure signs in 6 of these 10 rats. Some of the post-DBS period of seizure suppression lasted for up to 4 days. In other 3 rats, only mild stage 1 to 2 seizures appeared following amygdala kindling. Only 1 of the 10 rats for which DBS had blocked kindled seizures exhibited full-scale 5 stage-kindled seizures 24 h after DBS. These results suggest that highly plastic neural networks are involved in amygdala-kindled seizures and that DBS, if well timed with the onset of amygdala kindling, may exert long lasting effects on the networks that may prevent the recurrence of kindled seizures.

Cadaver evaluation of EMG needle insertion techniques used to target muscles of the thorax

STUDY DESIGN

Measure the accuracy of needle insertion in thoracic muscles of human cadavers.

OBJECTIVES

Evaluate the effectiveness of known EMG techniques for sampling thoracic innervated muscles.

SUMMARY OF BACKGROUND DATA

The evaluation of thoracic radiculopathies requires accurate electrodiagnostic techniques for evaluating the thoracic myotomes.

METHODS

An American Board of Electrodiagnostic Medicine certified physician placed needles into pertinent muscles of the thorax and an anatomist serving as a blinded dissector recorded the path and accuracy of needle.

RESULTS

Needle examination of thoracic muscles was as accurate as limb needle examination. No notable risks are noted in multifidus sampling. However, in the cadaver, some risks were noted in association with placements in the intercostal muscles. The target muscle was reliably sampled, but the rib or vertebral level was difficult to landmark in the cadaver.

CONCLUSIONS

Needle examination of the thoracic multifidus and intercostals is reliable, although further confirmation is needed to accurately verify the appropriate vertebral or rib level being sampled.

Large-scale neural ensemble recording in the brains of freely behaving mice

With the availability of sophisticated genetic techniques, the mouse is a valuable mammalian model to study the molecular and cellular basis of cognitive behaviors. However, the small size of mice makes it difficult for a systematic investigation of activity patterns of neural networks in vivo. Here we report the development and construction of a high-density ensemble recording array with up to 128-recording channels that can be formatted as single electrodes, stereotrodes, or tetrodes. This high-density recording array is capable of recording from hundreds of individual neurons simultaneously in the hippocampus of the freely behaving mice. This large-scale in vivo ensemble recording techniques, once coupled with mouse genetics, should be valuable to the study of complex relationship between the genes, neural network, and cognitive behaviors.

A lightweight telemetry system for recording neuronal activity in freely behaving small animals

A miniature lightweight radio telemetric device is described which is shown to be suitable for recording neuronal activity in freely behaving animals. Its size (12 x 5 x 8 mm) and weight (1.0-1.1 g with batteries, 0.4-0.5 g without) make the device particularly suitable for recording neuronal units in small animals such as mice or zebra finches. The device combines a high impedance preamplifier, RC-filters and an FM-transmitter. Using the device we recorded action potentials in field L of freely behaving zebra finches (12-17 g) through chronically implanted tungsten electrodes. In freely behaving birds we observed frequency dependent responses of field L units to auditory stimuli for periods of up to 7 days. We investigated the effect of the device on singing and locomotor activity of the zebra finches. Singing and locomotion were significantly affected on the first day after surgery. Both anesthesia and the presence of the transmitter contributed to the observed effect. After 1 day of recovery, singing activity returned to 99.6% and perch-hopping activity to 55.3% of the baseline levels. It is concluded that the device is well suited for recording spike trains from small animals while they behave freely and naturalistically.

MRI compatible electrodes for the induction of amygdala kindling in rats

The rat electrical kindling model has been widely utilized in epilepsy research. This study aimed to identify the optimum "MRI compatible" bipolar stimulating and recording electrodes to enable serial MRI acquisition in this model. Two types of custom-made electrodes (gold and carbon) were compared with commercial platinum-iridium alloy electrodes for suitability based on size, effect on image quality and kindling induction. The custom-made gold electrodes, based on these parameters, were found to be most suitable. These electrodes enable the study of epileptogenesis utilizing MRI in this model of temporal lobe epilepsy (TLE).

Low frequency electrical stimulation through subdural electrodes in a case of refractory status epilepticus

OBJECTIVE

We delivered low frequency stimulation through subdural electrodes to suppress seizures in a case of refractory status epilepticus (RSE).

METHODS

A 26-year-old female developed RSE after several days of febrile illness. Seizure control required continuous infusion of two anesthetics plus high doses of 2-4 enteral antiepileptic drugs. After 3 months of RSE, subdural strips were placed to determine surgical candidacy. Five independent ictal onset zones were identified. Because she was a poor candidate for epilepsy surgery and had a poor prognosis, the implanted subdural electrodes were used to administer 0.5 Hz stimulations to the ictal onset zones in 30 min trains daily for 7 consecutive days in an attempt to suppress seizures.

RESULTS

After 1 day of stimulation, one anesthetic agent was successfully discontinued. Seizures only returned by the 4th day when the second anesthetic had been reduced by 60%. Upon returning, seizures arose from only one of the 5 original ictal onset zones. Unfortunately, RSE persisted, and she eventually died.

CONCLUSIONS

In this case of RSE, low frequency stimulation through subdural electrodes transiently suppressed seizures from all but one ictal onset zone and allowed significant reduction in seizure medication.

SIGNIFICANCE

Low frequency cortical stimulation may be useful in suppressing seizures.

Single-pulse electrical stimulation identifies epileptogenic frontal cortex in the human brain

OBJECTIVE

To assess the value of single-pulse electrical stimulation (SPES) to identify frontal epileptogenic cortex during presurgical assessment.

METHODS

SPES (1-millisecond pulses, 4 to 8 mA, 0.1 Hz) has been used during chronic recordings in 30 patients with intracranial electrodes in the frontal lobes. As a result of presurgical assessment, 17 patients were considered to have frontal epilepsy and 13 extrafrontal epilepsy.

RESULTS

Two types of responses to SPES were seen: 1) early responses: starting immediately after the stimulus and considered as normal responses; 2) late responses: two types of responses seen in some areas after the initial early response: a) delayed responses: spikes or sharp waves occurring between 100 milliseconds and 1 second after stimulation. Frontal delayed responses were seen in 11 frontal patients and 1 extrafrontal patient, whereas extrafrontal delayed responses were seen in 1 frontal and 10 extrafrontal patients. b) Repetitive responses: two or more consecutive sharp-and-slow-wave complexes, each resembling the initial early response. Repetitive responses were seen only when stimulating the frontal lobes of 10 frontal patients. Among the 17 frontal patients, 13 had late responses exclusively in the epileptogenic frontal lobe, whereas only 3 showed them in both frontal lobes. Frontal late responses were associated with neuropathologic abnormalities, and complete resection of abnormal SPES areas was associated with good postsurgical seizure outcome.

CONCLUSIONS

Single-pulse electrical stimulation (SPES) could be an important additional investigation during presurgical assessment to identify frontal epileptogenicity. SPES can be useful in patients who have widespread or multiple epileptogenic areas, normal neuroimaging, or few seizures during telemetry.

Evaluation of extraocular electrodes for a retinal prosthesis using evoked potentials in cat visual cortex

OBJECTIVE

To assess the efficacy of a device using extraocular electrodes as a retinal prosthesis by evaluating the responses evoked in the visual cortex to electrical stimulation.

METHODS

In anaesthetised cats, a lateral orbital dissection and ipsilateral parietal craniotomy was performed. Two extraocular retinal prosthesis (ERP) disc electrodes were sutured to the sclera on the lateral and superior aspects of the globe. Retinal stimulation was performed with charge-balanced constant-current pulses. Potentials evoked in the visual cortex were measured with a ball electrode placed on the lateral gyrus after removal of the dura.

RESULTS

Stable attachment of the ERP electrodes to the globe was achieved with scleral sutures. Visual cortex responses were recorded with the electrodes in bipolar and monopolar configurations. The evoked response consisted of an early component with a peak around 8 ms, and a late component with a peak after 50 ms. Thresholds for evoking a response occurred at current intensities as low as 500 microA. Through extrapolation from evoked response amplitude data, thresholds as low as 300 microA were calculated. Cathodal monopolar stimulation demonstrated lower thresholds than anodal stimulation for evoking cortical responses.

CONCLUSIONS

The ERP electrodes can be easily attached to the globe and are effective in electrically stimulating the retina, evoking responses in the primary visual cortex. Threshold charge-density was within safe limits for neural stimulation.