Biomedical subjects

J Holsheimer

Publications and source records attributed to J Holsheimer.

At least 37 records · Page 2Linked to original sources

PubMed1997

Effect of anode-cathode configuration on paresthesia coverage in spinal cord stimulation.

OBJECTIVE: To provide a theoretical basis for the selection of the anode-cathode configuration in spinal cord stimulation for the management pain when one percutaneous epidural electrode or two electrodes in parallel are used. METHODS: A computer model of spinal cord stimulation at T8-T9 was used to calculate the dorsal column areas recruited in stimulation by various configurations used in clinical practice. RESULTS: Tripolar (or bipolar) stimulation by a single electrode, symmetrically placed over the dorsal columns, recruits the largest area and will give the widest paresthesia coverage. Stimulation by two symmetrically placed electrodes connected in parallel to a single channel pulse generator may give similar results, because of their generally smaller distance from the spinal cord, but a "summation effect" does not exist. A smaller dorsal column area is activated when two offset electrodes are used. An electrode placed laterally or transverse bipolar stimulation results in unilateral, usually segmentary, paresthesia. CONCLUSIONS: The relative positions of cathodes and anodes and their distance from the spinal cord are the major determinants of dorsal column/dorsal root activation and paresthesia distribution. The large interpatient variability of the intraspinal geometry is the main cause of differences in paresthesia coverage among patients having optimally placed electrode(s). Changes of paresthesia coverage over time are more probable when multiple electrodes are used.

Computer Simulation↗

PubMed1997

Effectiveness of spinal cord stimulation in the management of chronic pain: analysis of technical drawbacks and solutions.

OBJECTIVE: A major drawback of currently available spinal cord stimulation (SCS) systems for the management of chronic intractable pain, especially of widespread pain patterns as in reflex sympathetic dystrophy, is the generally limited paresthesia coverage. The aim of this study is to analyze the origin of this problem and to provide solutions. METHODS: Results from theoretical studies, in which a computer model was used to mimic the effects of SCS on spinal nerve fibers, were used to analyze which factors may limit paresthesia coverage. Model predictions were verified by empirical data from clinical literature. RESULTS: When using common SCS electrodes, both perception threshold and motor/discomfort threshold are generally related to dorsal root stimulation. Because these thresholds have a small ratio (approximately 1:1.4), stimulation of dorsal column fibers and paresthesia coverage is limited by this small range of stimulation. When the distance between the epidural electrode and spinal cord is large (midthoracically), the threshold for dorsal column stimulation exceeds discomfort threshold, resulting only in segmental paresthesia. The range of dorsal column stimulation and paresthesia coverage can be improved when using either an optimally dimensioned rostrocaudal bi-/tripole or a transverse tripole ("guarded cathode"). When applying the latter in combination with a dual channel pulse generator providing simultaneous pulses, paresthesias can simply be changed to optimally cover the painful area. CONCLUSION: Paresthesia coverage and pain management by SCS can be improved when using electrodes as proposed.

Chronic Disease↗

PubMed1997

Recruitment characteristics of nerve fascicles stimulated by a multigroove electrode.

The recruitment characteristics of fascicle-selective nerve stimulation by a multigroove electrode have been investigated both theoretically and in acute experiments. A three-dimensional (3-D) volume conductor model of fascicles in a multigroove device and a model of myelinated nerve fiber stimulation were used to calculate threshold stimuli of nerve fibers in these fascicles. After their exposition, fascicles from rat sciatic nerve were positioned in different grooves of appropriate sizes and stimulated separately. The device appeared to be suitable for fascicle-selective stimulation, because both computer simulations and acute animal experiments showed that crosstalk between neighboring fascicles is not a problem, even when monopolar stimulation was used. The threshold stimulus was lower for a small fascicle than for a large one. When the amount of (conducting) medium between contact and perineurium or its conductivity was reduced, threshold stimuli were lower. Moreover, simulations predict that the slopes of recruitment curves are smaller and inverse recruitment order is less pronounced. Simulations also showed that a small contact is preferable to a large on, because a small contact gives a slightly smaller slope of the recruitment curve. Both experimentally and theoretically a significantly smaller slope of recruitment curves was obtained by stimulation with a cathode and an anode at opposite sides of the fascicle, driven by two current sources giving simultaneous pulses with different, but linearly dependent amplitudes.

Animals↗

PubMed1996

Transverse tripolar spinal cord stimulation: theoretical performance of a dual channel system.

A new approach to spinal cord stimulation is presented, by which several serious problems of conventional methods can be solved. A transverse tripolar electrode with a dual-channel voltage stimulator is evaluated theoretically by means of a volume conductor model, combined with nerve fibre models. The simulations predict that a high degree of freedom in the control of activation of dorsal spinal pathways may be obtained with the described system. This implies an easier control of paraesthesia coverage of skin areas and the possibility to correct undesired paraesthesia patterns, caused by lead migration, tissue growth, or anatomical asymmetries, for example, without surgical intervention. It will also be possible to preferentially activate either dorsal column or dorsal root fibres, which has some important clinical advantages. Compared to conventional stimulation systems, the new system has a relatively high current drain.

Analgesia↗

PubMed1996

Position-selective activation of peripheral nerve fibers with a cuff electrode.

The degree of spatial selectivity which can be obtained with longitudinal dot tripoles in an insulating cuff was quantified in terms of the overlap between fiber populations activated by different tripoles. Previous studies have failed to take into account the relative influences of transverse current and longitudinal current on position-selective activation, and furthermore have not controlled for the differing sensitivities of large and small nerve fibers to electrical stimuli. In this study, these factors were taken into account. Transverse current from an anode positioned opposite the stimulating cathode was found to improve spatial selectivity, and selectivity was enhanced when the ratio of transverse current to longitudinal current was increased. Large fibers were excited before small fibers, irrespective of fiber position, indicating a combination of position and size selectivity.

Animals↗

PubMed1995

Significance of the spinal cord position in spinal cord stimulation.

The effects of the antero-posterior and medio-lateral positions of the spinal cord in the dural sac on the perception threshold and paresthesia coverage in spinal cord stimulation were analyzed. The distributions of the dorsal cerebrospinal fluid (CSF) layer thickness, measured from transverse MR scans of normal subjects at various spinal levels, were used to calculate the distributions of threshold voltages for the stimulation of spinal nerve fibers by a computer model. These theoretical threshold distributions were shown to fit well to the corresponding distributions of perception threshold measured in patients. It is concluded that the thickness of the dorsal csf layer is the main factor determining the perception threshold and paresthesia coverage in spinal cord stimulation: an increasing thickness raises the threshold and reduces the coverage, and vice versa. The effects of an asymmetrical electrode position with respect to the spinal cord midline were also analyzed by computer modeling. It is concluded that a lateral asymmetry of less than 1 mm gives a significant reduction of perception threshold and may result in unilateral paresthesiae.

Adult↗

PubMed1995

Effects of electrode geometry and combination on nerve fibre selectivity in spinal cord stimulation.

The differential effects of the geometry of a rostrocaudal array of electrode contacts on dorsal column fibre and dorsal root fibre activation in spinal cord stimulation are analysed theoretically. 3-D models of the mid-cervical and mid-thoracic vertebral areas are used for the computation of stimulation induced field potentials, whereas a cable model of myelinated nerve fibre is used for the calculation of the excitation thresholds of large dorsal column and dorsal root fibres. The size and spacing of 2-D rectangular electrode contacts are varied while mono-, bi- and tripolar stimulation are applied. The model predicts that the highest preferential stimulation of dorsal root fibres is obtained in monopolar stimulation with a large cathode, whereas dorsal column fibre preference is highest in tripolar stimulation with small contacts and small contact spacings. Fibre type preference is most sensitive to variations of rostrocaudal contact size and least sensitive to variations of lateral contact size. Dorsal root fibre preference is increased and sensitivity to lead geometry is reduced as the distance from contacts to spinal cord is increased.

Computer Simulation↗

PubMed1995

Modelling selective activation of small myelinated nerve fibres using a monopolar point electrode.

The aim of this study is to investigate theoretically the possibility for activation of small myelinated nerve fibres without activating larger ones when stimulating a nerve fibre bundle using a monopolar point electrode. Therefore, the sensitivity of excitation and blocking threshold currents of nerve fibres to fibre diameter, electrode-fibre distance and pulse duration has been simulated by a computer model. A simple infinite, homogeneous volume conductor and a cathodal point source were used in combination with a model representing the electrical properties of a myelinated nerve fibre. The results show that selective activation of small myelinated fibres may be possible in a region at some distance from the electrode.

Computer Simulation↗

PubMed1994

Selective stimulation of sacral nerve roots for bladder control: a study by computer modeling.

The aim of this study was to investigate theoretically the conditions for the activation of the detrusor muscle without activation of the urethral sphincter and afferent fibers, when stimulating the related sacral roots. Therefore, the sensitivity of excitation and blocking thresholds of nerve fibers within a sacral root to geometric and electrical parameters in tripolar stimulation using a cuff electrode, have been stimulated by a computer model. A 3-D rotationally symmetrical model, representing the geometry and electrical conductivity of a nerve root surrounded by cerebrospinal fluid and a cuff was used, in combination with a model representing the electrical properties of a myelinated nerve fiber. The electric behavior of nerve fibers having different diameters and positions in a sacral root was analyzed and the optimal geometric and electrical parameters to be used for sacral root stimulation were determined. The model predicts that an asymmetrical tripolar cuff can generate unidirectional action potentials in small nerve fibers while blocking the large fibers bidirectionally. This result shows that selective activation of the detrusor may be possible without activation of the urethral sphincter and the afferent fibers.

Animals↗

PubMed1994

MR assessment of the normal position of the spinal cord in the spinal canal.

PURPOSE: To investigate intradural geometry, which strongly influences the effects of epidural spinal cord stimulation. METHODS: Axial MR images with turbo spin-echo were made of 26 healthy subjects at C-4 through C-6, T-5 and T-6, and T-11 and T-12, at T-11 and T-12 both in the supine and the prone position. Measurements were made of the dorsomedial and the ventromedial cerebrospinal fluid layer and the anteroposterior and transverse sizes of both the spinal cord and the dural sac. The samples of all variables were analyzed statistically. The distance between spinal and vertebral midline was also determined. RESULTS: The dorsal cerebrospinal fluid layer was 1.5 to 4.0 mm at C-4 through C-6 and 4.0 to 8.5 mm at T-5 and T-6. At T-11 it was 2.0 to 6.0 mm in the supine position and was increased by approximately 2.2 mm in the prone position. At T-12 these values were 1.5 to 4.5 mm and approximately 3.4 mm, respectively. Differences between the spinal and vertebral midline up to 1.5 to 2.0 mm occurred in approximately 40% of the images. CONCLUSIONS: Because there are variations of the dorsal cerebrospinal fluid layer among subjects by more than a factor of 2, and significant variations of the mediolateral position of the spinal cord, information on these parameters in patients will be essential for the optimal application of epidural spinal cord stimulation.

Adult↗

PubMed1993

Excitation of dorsal root fibers in spinal cord stimulation: a theoretical study.

In epidural spinal cord stimulation it is likely that not only dorsal column fibers are activated, but that dorsal root fibers will be involved as well. In this investigation a volume conductor model of the spinal cord was used and dorsal root fibers were modeled by an electrical network including fiber excitation. The effects of varying some geometrical fiber characteristics, as well as the influence of the dorsal cerebrospinal fluid layer and the electrode configuration on the threshold stimulus for their excitation, were assessed. The threshold values were compared with those of dorsal column fibers. The results of this modeling study predict that, besides the well known influence of fiber diameter, the curvature of the dorsal root fibers and the angle between these fibers and the spinal cord axis were of major influence on their threshold values. Because of these effects, threshold stimuli of dorsal root fibers were relatively low as compared to dorsal column fibers. Excitation of the dorsal root fibers occurred near the entry point of the fibers.

Electric Stimulation↗

PubMed1993

Analysis of parameters for epidural spinal cord stimulation. 3. Topographical distribution of paresthesiae--a preliminary analysis of 266 combinations with contacts implanted in the midcervical and midthoracic vertebral levels.

As a part of the systematic analysis of parameters involved in electrical epidural spinal cord stimulation therapy, distribution of paresthesiae in accordance with the position of contacts, as related to midline and vertebral level of contacts, was investigated. A preliminary analysis performed for 266 combinations having contacts implanted in the midcervical and low-thoracic vertebral levels showed that the topographical spread of the paresthesiae did not always cover the classical dermatomes. Significant paresthesiae spread was found as follows: C4-midline, unipolar combinations; the hand, forearm and upperarm, bipolar combinations; the hand, forearm; C4-lateral, unipolar combinations: the anterior shoulder, forearm, upper arm and hand, bipolar combinations: the hand, forearm and upper arm; T10-midline, unipolar combinations: the anterior and posterior of thigh, leg, knee, ankle and foot, bipolar combinations: the anterior and posterior thigh, leg, knee and foot; T10-lateral, unipolar combinations: the abdomen, anterior leg, knee and anterior thigh, bipolar combinations: the anterior thigh, anterior leg, knee and foot. This preliminary analysis suggests that more detailed studies would be worthwhile in the investigation of sensory responses to electrical stimulation of the spinal cord with epidural electrodes. Findings from such investigations could also be useful to extend our present anatomical knowledge of central and peripheral sensory neural structures.

Cervical Vertebrae↗

PubMed1992

Recruitment of dorsal column fibers in spinal cord stimulation: influence of collateral branching.

An electrical network model of myelinated dorsal column nerve fibers is presented. The effect of electrical stimulation was investigated using both a homogeneous volume conductor and a more realistic model of the spinal cord. An important feature of dorsal column nerve fibers is the presence of myelinated collaterals perpendicular to the rostro-caudal fibers. It was found that transmembrane potentials, due to external monopolar stimulation, at the node at which a collateral is attached, is significantly influenced by the presence of the collateral. It is concluded that both excitation threshold and blocking threshold of dorsal column fibers are decreased up to 50% compared to unbranched fibers.

Action Potentials↗

PubMed1991

Epidural spinal cord stimulation: calculation of field potentials with special reference to dorsal column nerve fibers.

The effect of electrical stimulation with several electrode combinations on nerve fibers with different orientations in the spinal cord was investigated by computing the steady-state field potentials and activating functions. At first an infinite homogeneous model was used while secondly the spinal cord and its surrounding tissues were modeled as an inhomogeneous anisotropic volume conductor. The effect of mediodorsal epidural stimulation was calculated. It was concluded that with cathodal stimulation, mediodorsally in the epidural space, longitudinal fibers are depolarized, but dorsoventral ones are hyperpolarized. With anodal stimulation the opposite will occur. It was found that parameters substantially affecting the potential distribution in the dorsal columns are the conductivity of the white matter and the width and the conductivity of the csf layer.

Action Potentials↗

PubMed1991

Contact combinations in epidural spinal cord stimulation. A comparison by computer modeling.

The geometrical characteristics of longitudinal fiber populations in the dorsal columns (DC), recruited by various contact combinations in epidural spinal cord stimulation (SCS), were compared in a theoretical study. A 3-dimensional computer model was used, representing the geometry and electrical conductivity of the low-thoracic spinal cord and surrounding tissues, in combination with a model representing the electrical properties of a myelinated nerve fiber. It was calculated that, among usual dorsomedial contact combinations, the ratio of mediolateral and dorsoventral extent of the recruited area in DC only varied by 2-5%. The model predicts that in bipolar stimulation the relative lateral extent was smallest at a contact separation of 3-3.5 times the dorsal cerebrospinal-fluid width, which seems to fit the highest ratio of discomfort threshold/paresthesia threshold. It is concluded that the optimal combination varies with distance between cathode and DC and that a unique 'best contact combination' does not exist. When both a decrease of primary afferent diameter in DC and collateral branching near their entrance are considered, the model predicts that recruitment of cutaneous afferents will start in lateral DC and proceed medially at increasing stimulus, which fits clinical observations on the spread of paresthesia. The model predicts that large fibers in the posterior spinocerebellar tract will also be recruited in SCS.

Cerebrospinal Fluid↗

PubMed1991

How do geometric factors influence epidural spinal cord stimulation? A quantitative analysis by computer modeling.

Effects of both anatomic and electrode geometry on the recruitment of rostrocaudal fibers in the spinal cord were investigated by computer simulation of epidural spinal cord stimulation. A three-dimensional model was used, representing the geometry and electrical conductivity of the spinal cord and surrounding tissues, in combination with a model representing the electrical properties of a myelinated nerve fiber. Recruitment contours in the dorsal columns were calculated at various spinal geometries as a function of electrode position, combination and area. Cathodal position appeared to be most significant. Recruitment areas resulting from different contact combinations of a mediodorsal array were almost identical. It was shown that perception threshold largely depends on both dorsal cerebrospinal fluid width and fiber size. The usual bipolar contact separation appeared to approximate the theoretically optimal value, resulting in maximum fiber recruitment at minimum stimulus.

Analgesia, Epidural↗

PubMed1989

Propagation velocity of epileptiform activity in the hippocampus.

The propagation of epileptiform burst activity was investigated in the CA1 area of the in-vitro hippocampal slice preparation of the guinea pig. This activity was provoked by 0.1 mM 4-aminopyridine in the bathing medium and was recorded in the pyramidal layer with an array of eight electrodes. The delay between the first population spike of a burst recorded with different electrodes was calculated using the cross-correlation function. The propagation velocity was estimated from the delays and the electrode intervals. It was found that the velocity of spontaneous and evoked epileptiform bursts varies between 0.15 and 5 m/s and is not confined to the range of conduction velocities of the fibre systems in CA1 (0.3-0.55 and 1.0-1.8 m/s). Different velocities can be present in different parts of the CA1 area and the initiation of spontaneous bursts is not confined to the CA2-3 areas, but can also occur in CA1. Burst activity also propagated in a low calcium-high magnesium medium. Different mechanisms of propagation are discussed and it is argued that the propagation velocity due to ephaptic interaction may vary largely. It is concluded that epileptiform activity can be propagated not only by synaptic connections at or near the pyramidal layer, but also by way of electrical field effects of population spikes.

4-Aminopyridine↗

PubMed1989

A modeling study of nerve fascicle stimulation.

A nerve stimulation model has been developed, incorporating realistic cross-sectional nerve geometries and conductivities. The potential field in the volume conductor was calculated numerically using the variational method. Nerve fiber excitation was described by the model of McNeal. Cross-sectional geometries of small monofascicular rat common peroneal nerve and multifascicular human deep peroneal nerve were taken as sample geometries. Selective stimulation of a fascicle was theoretically analyzed for several electrode positions: outside the nerve, in the connective tissue of the nerve, and inside a fascicle. The model results predict that the use of intraneural or even intrafascicular electrodes is necessary for selective stimulation of fascicles not lying at the surface of the nerve. Model predictions corresponded with experimental results of Veltink et al. on intrafascicular and extraneural stimulation of rat common peroneal nerve and to results of McNeal and Bowman on muscle selective stimulation in multifascicular dog sciatic nerve using an extraneural multielectrode configuration.

Animals↗