Jean-Pascal Lefaucheur

Cathodal, anodal or bifocal stimulation of the motor cortex in the management of chronic pain?

The conditions of motor cortex stimulation (MCS) applied with epidural electrodes, in particular monopolar (cathodal or anodal) and bipolar stimulation, are discussed. The results of theoretical studies, animal experiments and clinical studies lead to similar conclusions. Basically, cortical nerve fibres pointing at the epidural electrode and those normal to this direction are activated by anodal and cathodal stimulation, respectively. Because MCS for the relief of chronic pain is generally applied bipolarly with electrodes at a distance of at least 10 mm, stimulation may actually be bifocal. The polarity and magnitude of a stimulus needed to recruit cortical nerve fibres varies with the calibre and shape of the fibres, their distance from the electrode and their position in the folded cortex (gyri and sulci). A detailed analysis of intra-operative stimulation data suggests that in bipolar MCS the anode of the bipole giving the largest motor response in the pain region is generally the best electrode for pain management as well, when connected as a cathode. These electrode positions are most likely confined to area 4.

Clinical application of laser evoked potentials using the Nd:YAG laser

The clinical interest of a new type of laser evoked potentials (LEPs) using Nd:YAG laser was assessed in the diagnosis of peripheral neuropathies affecting the small-diameter nerve fibres, and of spinal cord lesions, affecting the spinothalamic tract. Twelve patients aged from 26 to 79 years with sensory neuropathies (n = 6) or spinal cord lesions (n = 6) underwent neurophysiological examination of the lower limbs comprising quantitative sensory testing, i.e., the determination of vibratory and thermal thresholds (VT and TT), somatosensory evoked potentials (SEPs) to electrical stimulation and Nd:YAG LEPs. VT and SEPs were used to assess large-diameter afferent nerve fibres and the lemniscal pathways while TT and LEPs were used to assess small-diameter afferent nerve fibres and the spinothalamic tract. In addition, patients with peripheral neuropathy underwent also standard nerve conduction studies to explore large fibres and the recording of sympathetic skin responses (SSRs) to explore small fibres, whereas motor evoked potentials were performed in patients with spinal cord lesion. LEPs were absent bilaterally in all patients with polyneuropathy, even when TT remained within the normal limits and SSRs were present. LEPs were absent after stimulation of the affected limb in all patients with a spinal cord lesion, and allowed to detect subclinical contralateral lesion in two cases. LEPs following Nd:YAG laser stimulation are sensitive in the diagnosis of peripheral and/or central nervous system disorders and they give complementary information as compared to routine electrophysiological tests.

A good preoperative response to transcutaneous electrical nerve stimulation predicts a better therapeutic effect of implanted occipital nerve stimulation in pharmacologically intractable headaches.

Occipital nerve stimulation (ONS) is a surgical approach to treat patients with medically intractable chronic headache disorders. However, no preoperative test has been yet validated to allow candidates to be selected for implantation. In this study, the analgesic efficacy of transcutaneous electrical nerve stimulation (TENS) was tested for 1 to 3 months in 41 patients with pharmacologically intractable headache disorders of various origins, using a new technique of electrode placement over the occipital nerve. ONS electrodes were subsequently implanted in 33 patients (occipital neuralgia [n=15], cervicogenic headache [n=7], cluster headache [n=6], chronic migraine [n=5]) who had responded at least moderately to TENS. Assessment was performed up to five years after implantation (three years on average), based on the mean and maximum daily pain intensity scored on a 0-10 visual analogue scale and the number of headache days per month. Both TENS and chronic ONS therapy were found to be efficacious (57-76% improvement compared to baseline on the various clinical variables). The efficacy of ONS was better in cases of good or very good preoperative response to TENS than in cases of moderate response to TENS. Implanted ONS may be a valuable therapeutic option in the long term for patients with pharmacologically intractable chronic headache. Although we cannot conclude in patients with poor or no response to TENS, a good or very good response to TENS can support the indication of ONS therapy. This preoperative test could particularly be useful in patients with chronic migraine, in whom it may be difficult to indicate an invasive technique of cranial neurostimulation.

Treatment of refractory headache secondary to intracranial endovascular procedure by transcutaneous electrical nerve stimulation of the occipital nerve

This pilot study aimed at assessing the effect of transcutaneous electrical nerve stimulation (TENS) of the occipital nerve (ON) to treat chronic refractory headache secondary to intracranial endovascular procedures (iEVP) in 4 patients. The duration of ON-TENS therapy was only 1 month (n=2) or longer than 4 months (n=2). Overall, pain intensity decreased from 7.8 (on a 0-10 scale) at baseline to 0.8 at 6 months after ON-TENS therapy initiation (-90%), while drug treatment was reduced from 34.3 to 0.8 (-98%) on the medication quantification scale. ON-TENS is a simple technique that may benefit patients with post-iEVP refractory headache.

Corrélats fonctionnels cérébraux de la douleur chronique spontanée dans les neuropathies périphériques

Les mecanismes centraux de la douleur neuropathique restent meconnus dans le contexte des neuropathies peripheriques. L’objectif de notre etude etait de comparer la connectivite cerebrale dans deux groupes de 13xa0patients atteints de neuropathie peripherique, l’un etant douloureux, l’autre non, au moyen d’une technique d’IRM fonctionnelle en «xa0resting statexa0». Les patients douloureux avaient des douleurs chroniques spontanees d’intensite superieure a 3/10xa0depuis plus de 3xa0mois avec composante neuropathique (DN4xa0>xa04). Les patients non-douloureux ne devaient jamais avoir presente de douleurs dans le cadre de leur neuropathie. Chaque patient a realise une IRM etudiant le reseau de mode par defaut, les connectivites thalamocorticales, et les «xa0pain matricesxa0». Les deux groupes de patients ne presentaient de differences significatives dans le reseau par defaut, contrairement a la connectivite thalamocorticale. Trois regions thalamiques, respectivement connectees au cortex moteur primaire, premoteur, et temporal ont montre une franche reduction de connectivite fonctionnelle avec differentes regions parietales, temporales, occipitales et cerebelleuses chez les patients douloureux par rapport aux non-douloureux. Concernant l’etude des «xa0pain matricesxa0», les connections «xa0internesxa0» aux differentes matrices, notamment la matrice «xa0nociceptivexa0» (insula posterieur, opercule insulaire, cortex cingulaire median), etaient augmentees chez les patients douloureux, alors que les connectivites «xa0inter-matricesxa0» etaient augmentees chez les non-douloureux, notamment avec la matrice «xa0emotionnellexa0» (cortex orbito-frontal et prefrontal anterolateral). Notre etude a donc mis en evidence chez les patients presentant une neuropathie peripherique douloureuse une reduction de la connectivite fonctionnelle cerebrale concernant deux types de reseauxxa0: –xa0les projections thalamocorticales impliquant notamment les regions thalamiques connectees au cortex moteur primaire et premoteurxa0; –xa0les connections entre les differentes «xa0pain matricesxa0», notamment entre les matrices «xa0nociceptivexa0» et «xa0emotionnellexa0». Ces resultats apportent notamment un eclairage tout a fait nouveau sur les mecanismes d’action de la stimulation corticale pour le traitement des douleurs neuropathiques d’origine peripherique.

MEPs to identify the optimal cathode position in MCS

Objective. In the subthalamic area, the subthalamic nucleus (STN) is the main target for deep brain stimulation (DBS) in idiopathic Parkinson’s disease (PD). We aimed at analysing relationships between the MRI anatomy and spontaneous neuronal activity in order to confirm the potential of microelectrode recording to assist in the determination of the optimal surgical target. nMethods. Thirty trajectories were analysed. Procedures (8 bilateral surgeries) were performed under local anaesthesia after identification on 1.5T MRI (T2 weighted) of anatomical structures, STN, above STN (zona incerta, forel’s fields) and substantia nigra (SN). Spontaneous neuronal activity was recorded (30 seconds, 0.5 mm step), simultaneously along the distal 10 mm on a central (covering optimally STN) and on a 2 mm anterior track. Intra operative X-Ray controls checked that the exploration tracts followed the planed trajectories. Chronic DBS (on central tract 14 times out nof 16) dramatically improved PD. We calculated cell numbers and mean firing rates (MFR, average firing on 30 seconds) on 276 isolated neurons (after threshold and principal component analysis) regrouped according to anatomical structures. MFR were compared with a non parametric statistical test. nResults. 60% of spontaneously active cells were in the dorsal and lateral STN. MFR increased entering STN and SN (p<0.03): 5.88 +/- 1.06 spike.sec-1 above STN; 9.23 +/- 1.22 spike.sec-1 in STN; 14.46 +/- 2.32 spike.sec-1 in SN. nConclusions. Anatomical boundaries and unitary recordings seem to be linked. We found a maximum number of spontaneously active neurons in the dorsal and lateral STN which is also known as a good surgical target for DBS in severe PD.Objectif : plusieurs auteur ont decrit une somatotopie dans le globus pallidus (GP) de l’homme, a partir des enregistrements electrophysiologiques effectues au cours de neurochirurgies, en considerant des mouvements volontaires ou passifs des patients, ou encore sur les effets cliniques des stimulations cerebrales profondes (SCP) effectuees pour traiter les mouvements anormaux. n Le present travail montre une somatotopie des mouvements indesirables (dystonies) observee dans le pallidum d’un patient parkinsonien pendant la stimulation aigue. Ces donnees sont mises en correspondance avec l’anatomie IRM. nMethode : un patient parkinsonien souffrant principalement de dyskinesies des membres et du tronc (16 ans d’evolution de la maladie, 68 ans) a beneficie d’une CSP bilaterale du GP interne (GPi) en condition stereotaxique avec une approche en visee directe. Une IRM T2 permet de discerner les differentes structures pallidales : GP externe (GPe) et les subdivisions du GPi (laterale GPil et mediale GPim). Des mouvements dystoniques controlateraux ont ete observes a l’aide de 2 x 3 electrodes paralleles et separees de 2 mm pendant la stimulation effectuee sous anesthesie locale. Les mouvements dystoniques concernant le visage, les membres superieurs et inferieurs ont ete visualises dans les structures anatomiques etudiees. nResultats : chacune des structures GPe, GPil et GPim semble presenter une organisation rostro-caudale avec le visage represente dans la partie superieure, les membres superieurs dans la region intermediaire et les membres inferieurs dans la portion inferieure. nConclusion : l’analyse des effets moteurs indesirables (dystonie) est ici un outil interessant pour dessiner une somatotopie. Cette somatotopie pallidale, observee chez un patient, doit cependant etre validee.