Search PubMed⌕ Search

Biomedical subjects

B J Roth

Publications and source records attributed to B J Roth.

At least 127 records · Page 7Linked to original sources
PubMed1991

Magnetic stimulation of the human cerebral cortex, an indicator of reorganization in motor pathways in certain pathological conditions.

Basic principles of magnetic stimulation of biological tissues are reviewed. Noninvasive magnetic stimulation of the brain delivered over sensorimotor areas evokes movements and less commonly paresthesias in contralateral limbs. We have evaluated the maps of motor outputs in patients with (1) congenital mirror movements, which resulted in marked derangement of the map of outputs of distal hand muscles with enlarged and ipsilateral representations; (2) amputations, which resulted in plastic reorganization of motor outputs targeting muscles immediately proximal to the stump; (3) spinal cord injury, which also resulted in enlargement of the map of outputs targeting muscles proximal to the lesion level; and (4) hemispherectomy performed at an early age for intractable seizures, which resulted in the remaining hemisphere controlling ipsilateral arm muscles. These results demonstrate the potential for reorganization in motor systems following lesions in the peripheral as well as in the central nervous system.

Brain Mapping↗
PubMed1991

Action potential propagation in a thick strand of cardiac muscle.

A theoretical model of action potential propagation in a thick strand of cardiac muscle is presented. The calculation takes into account the anisotropic and syncytial properties of the tissue, the presence of the interstitial space, the effect of the surrounding tissue bath, and the variation of the potential both along the strand length and across the strand cross section. The bidomain model is used to represent the electrical properties of the tissue, and the Ebihara-Johnson model is used to represent the properties of the active sodium channels. The calculated wave front is curved, with the action potential at the surface of the strand leading that at the center. The rate of rise of the action potential and the time constant of the action potential foot vary with depth into the tissue. The velocity of the wave front is nearly independent of strand radius for radii greater than 0.5 mm. The conduction velocity decreases as the volume fraction of the interstitial space decreases. In the limit of tightly packed cells, an action potential propagates quickly over the surface of the strand; the bulk of the tissue is then excited by a slow inward wave front initiated on the surface. This model does not predict an increase in conduction velocity when cells are tightly packed, a hypothesis that has been proposed previously to explain the fast conduction velocity in Purkinje fibers of some species.

Action Potentials↗
PubMed1991

The electric field induced during magnetic stimulation.

The electric field induced in tissue during magnetic stimulation is calculated. There are two sources of the electric field: charge and a time-dependent magnetic field: both sources are important in magnetic stimulation. Charge accumulation on the tissue surface tends to shield the nerve from the stimulus. The induced electric field is generally parallel to the tissue surface. Simulations of both peripheral and central nervous system stimulation are presented.

Electric Conductivity↗
PubMed1991

Topographic maps of human motor cortex in normal and pathological conditions: mirror movements, amputations and spinal cord injuries.

We studied motor evoked potentials to transcranial magnetic stimulation in patients with unilateral upper limb amputations, complete T10-T12 spinal cord transection, and congenital mirror movements and in controls. Different muscles in the trunk and upper and lower extremities were evaluated at rest. In controls, muscles could be activated with stimulation of regions several centimeters wide. These areas overlapped extensively when muscles studied were from the same limb and shifted positions abruptly when muscles were from different limbs. Distal muscles were easier to activate than proximal muscles and normally evidenced exclusively a contralateral representation. Congenital defects in motor control in patients with mirror movements resulted in marked derangement of the map of outputs of distal hand muscles with enlarged and ipsilateral representations. Peripheral lesions, either acquired (amputations) or congenital (congenital absence of a limb), resulted in plastic reorganization of motor outputs targeting muscles immediately proximal to the stump. Central nervous system lesions (i.e., spinal cord injury producing paraplegia) also resulted in enlargement of the map of outputs targeting muscles proximal to the lesion. These results indicate that magnetic stimulation is a useful non-invasive tool for exploring plastic changes in human motor pathways following different types of injury.

Adult↗
PubMed1990

Hematologic neoplasia associated with primary mediastinal germ-cell tumors.

Between September 1983 and December 1988, we observed 16 cases of hematologic neoplasia associated with mediastinal germ-cell tumors. Twenty-eight similar cases have been reported in the literature. A review of the clinical and cytogenetic details in these patients suggests that the hematologic neoplasia is not the result of cisplatin-based chemotherapy of the mediastinal germ-cell cancer. This syndrome was found only in patients with nonseminomatous mediastinal germ-cell tumors, particularly those with serologic or histologic evidence of yolk-sac elements. The two most common hematologic neoplasms seen in this syndrome were acute megakaryoblastic leukemia and malignant histiocytosis. Consistent cytogenetic abnormalities have not yet been identified, but the finding of the marker chromosome isochromosome (12p) in the mediastinal germ-cell tumor and associated leukemic blasts in one patient suggests that these tumors may arise from a common progenitor cell.

Adolescent↗
PubMed1990

A theoretical calculation of the electric field induced by magnetic stimulation of a peripheral nerve.

A mathematical model is presented that predicts the electric field induced in the arm during magnetic stimulation of a peripheral nerve. The arm is represented as a homogeneous, cylindrical volume conductor. The electric field arises from two sources: the time-varying magnetic field and the accumulation of charge on the arm surface. In magnetic stimulation both of these contributions are significant. The magnitude of the electric field is greatest near the surface of the arm, and is well localized. Various coil orientations are examined; the smallest electric fields are induced when the coil is perpendicular to the arm surface, the largest when the coil is parallel. These results are consistent with many experimental observations in the literature, and aid in the basic understanding of magnetic stimulation of the peripheral nervous system.

Arm↗
PubMed1990

The magnetic field of cortical current sources: the application of a spatial filtering model to the forward and inverse problems.

We extend our theoretical model based on spatial filtering to determine the ability of a SQUID magnetometer to resolve 2 localized current sources in the brain. We find that in order to resolve 2 separated but coaxial cortical sources, the source-to-pickup coil distance must be comparable to the distance between the 2 sources. The size of the current sources affects the resolving power of a magnetometer, but an anisotropy of 3 in the cortical tissue does not produce a significant effect. The model also provides a solution of the inverse calculation, either to reconstruct the original current source distribution from the measured magnetic field, or to continue the field at the magnetometer inward towards the sources. Both the inverse and inward calculations are limited by the fact that the inverse filter function serves as a high-pass filter, which leads to instabilities at high spatial frequencies, particularly in the presence of noise. The instabilities can be minimized by choosing an appropriate window to attenuate the noise, but this in turn reduces the spatial resolution.

Cerebral Cortex↗
PubMed1990

Effects of coil design on delivery of focal magnetic stimulation. Technical considerations.

The localization of effects from magnetic coil stimulation is not immediately obvious. We measured the magnetic fields produced by several different coils and compared the results with theoretical calculations. Magnetic stimuli were delivered from a Cadwell MES-10 magnetic stimulator using 3 circular coils (one 9 cm in diameter; two with an angulated extension, 5 and 9 cm in diameter) and twin oval coils arranged in a butterfly shape (each coil approximately 4 cm in diameter) and from a Novametrix Magstim 200 using two circular flat-spiral coils (6.7 and 14 cm in diameter). Peak-induced strength of the magnetic field was recorded with a measuring loop (1 cm in diameter) at different distances from the center of the coil. When the measuring loop was moved in the same plane laterally from the center of the coil, for all coils except the butterfly-shaped coil, the field was highest in the center and fell off near the circumference of the coil. The field dropped progressively when measurements were made more distant from the plane of the coils. The electric field induced from the magnetic coil could be calculated from the coil geometry. For all coils except the butterfly-shaped coil, the largest electric field was at the circumference of the coils. The 6.7 cm flat-spiral coil induced currents similar to those induced by the larger coils but more focally. The butterfly-shaped coil induced the largest currents under its center, where the circumferences of the two component coils come together. The component of the electric field parallel to the wire in the center of this coil was the largest and most localized.

Electric Stimulation↗
PubMed1990

A model of the stimulation of a nerve fiber by electromagnetic induction.

A model is presented to explain the physics of nerve stimulation by electromagnetic induction. Maxwell's equations predict the induced electric field distribution that is produced when a capacitor is discharged through a stimulating coil. A nonlinear Hodgkin-Huxley cable model describes the response of the nerve fiber to this induced electric field. Once the coil's position, orientation, and shape are given and the resistance, capacitance, and initial voltage of the stimulating circuit are specified, this model predicts the resulting transmembrane potential of the fiber as a function of distance and time. It is shown that the nerve fiber is stimulated by the gradient of the component of the induced electric field that is parallel to the fiber, which hyperpolarizes or depolarizes the membrane and may stimulate an action potential. Finally, it predicts complicated dynamics such as action potential annihilation and dispersion.

Action Potentials↗
PubMed1990

The serum-effusion albumin gradient in the evaluation of pleural effusions.

The objective of the study was to compare the serum-effusion albumin gradient (serum albumin level minus pleural effusion albumin level) to Light's traditional criteria (pleural fluid/serum total protein ratio greater than 0.5, pleural fluid/serum LDH ratio greater than 0.6, and pleural fluid LDH greater than 200 U/L) for identifying exudative pleural effusions. The design included prospective measurement of the serum-effusion albumin gradient and Light's criteria in patients with pleural effusions in an inpatient ward in a military teaching hospital. Fifty-nine consecutive patients with pleural effusions who were undergoing diagnostic or therapeutic thoracentesis in whom the etiology of the effusion could be determined were studied. Serum and pleural effusion fluid chemistries were measured in order to determine both the serum-effusion albumin gradient and Light's criteria. Using an albumin gradient of 1.2 g/dl or less to indicate exudates and greater than 1.2 g/dl to indicate transudates, 57 of the 59 patients (41 exudates; 18 transudates) were correctly classified. Two patients with malignant effusions were misclassified as having transudates. Although Light's criteria correctly identified all of the exudates, five patients with congestive heart failure were misclassified as exudates. Four of these patients had had previous diuretic therapy, and all had a clinical response to further diuretic therapy. We conclude that although Light's criteria for exudates are very sensitive, an albumin gradient of 1.2 g/dl or less tends to be more specific, especially in cases of chronic congestive heart failure.

Adult↗
PubMed1989

Interpretation of skeletal muscle four-electrode impedance measurements using spatial and temporal frequency-dependent conductivities.

Spatial and temporal frequency-dependent conductivities are used to interpret four-electrode conductivity measurements on skeletal muscle. The model qualitatively explains the observed dependence of the experimental data on the temporal frequency of the injected current, the angle between the electrode array and the fibre direction, and the distance between the electrodes.

Electric Conductivity↗
PubMed1989

Current injection into a two-dimensional anisotropic bidomain.

A two-dimensional sheet of anisotropic cardiac tissue is represented with the bidomain model, and the finite element method is used to solve the bidomain equations. When the anisotropy ratios of the intracellular and extracellular spaces are not equal, the injection of current into the tissue induces a transmembrane potential that has a complicated spatial dependence, including adjacent regions of depolarized and hyperpolarized tissue. This behavior may have important implications for the electrical stimulation of cardiac tissue and for defibrillation.

Animals↗
PubMed1989

Dose-intensive chemotherapy in refractory germ cell cancer--a phase I/II trial of high-dose carboplatin and etoposide with autologous bone marrow transplantation.

Between September 1986 and March 1988, 33 patients with refractory germ cell cancer were entered on a phase I/II trial of two courses of high-dose carboplatin plus etoposide with autologous bone marrow support. All patients had extensive prior treatment and had either cisplatin-refractory disease (67%) defined as progression within 4 weeks of the last cisplatin dose or failed at least two cisplatin-based regimens (35%) including a cisplatin-ifosfamide salvage regimen. Patients received a fixed total dose of etoposide of 1,200 mg/m2 with each cycle. The carboplatin dose ranged from 900 mg/m2 to 2,000 mg/m2. Twenty of the 33 patients received the second cycle of therapy. Despite extensive prior therapy with cisplatin, neurotoxicity, nephrotoxicity, or hearing impairment with high-dose carboplatin and etoposide was unusual. The most common nonhematologic toxicity was moderate enterocolitis. The hematologic toxicity of this regimen was substantial at each dose level. All 53 courses were accompanied by granulocytopenic fevers. Seven of the 33 patients (21%) died from treatment. All of these deaths occurred during the granulocyte nadir, and five were related to documented sepsis. Overall, 14 of 32 patients (44%) evaluable for response obtained an objective response, including eight complete remissions. Four patients remain in complete remission, with three patients being continuously free of disease in excess of 1 year. Eight responders (including four complete remissions) had progressed while receiving cisplatin. We conclude that carboplatin and etoposide can be administered in combination at high dosages and this regimen may have curative potential for patients with germ cell tumors resistant to conventional-dose cisplatin-based therapies.

Adolescent↗
PubMed1989

Disseminated testicular cancer: current chemotherapy strategies.

Disseminated testis cancer is a landmark adult solid tumor because of its high curability. In the last decade and a half, a number of clinical trials have been completed or are ongoing. In addition, long-term follow-up of some earlier trials is now available. This report summarizes the current treatment approaches, paying particular attention to the role of cisplatin.

Adult↗
PubMed1989

Cisplatin in the management of breast cancer.

Cisplatin's role in metastatic breast cancer was initially confined to heavily pretreated patients. It had relatively little activity in these patients, except when administered with high-dose intensity (greater than 33.3 mg/m2/week). More recent trials in previously untreated patients have suggested that cisplatin is an active single agent, with response rates approaching 50% when administered at a dose of 30 mg/m2/d for four days every 3 weeks. Cisplatin has been safely incorporated into combination chemotherapy regimens, and has been used in high-dose regimens in conjunction with autologous bone marrow transplantation. Future trials should examine the use of cisplatin or platinum analogues as first-line therapy in alternating non-cross-resistant regimens, and should attempt to circumvent the dose-limiting toxicities of cisplatin when used in a dose-intensive fashion.

Antineoplastic Combined Chemotherapy Protocols↗