Transcranial magnetic stimulation and brain atrophy: a computer-based human brain model study

Abstract

This paper is aimed at exploring the effect of cortical brain atrophy on the currents induced by transcranial magnetic stimulation (TMS). We compared the currents induced by various TMS conditions on several different MRI derived finite element head models of brain atrophy, incorporating both decreasing cortical volume and widened sulci. The current densities induced in the cortex were dependent upon the degree and type of cortical atrophy and were altered in magnitude, location, and orientation when compared to healthy head models. Predictive models of the degree of current density attenuation as a function of the scalp-to-cortex distance were analyzed, concluding that those which ignore the electromagnetic field-tissue interactions lead to inaccurate conclusions. Ultimately, the precise site and population of neural elements stimulated by TMS in an atrophic brain cannot be predicted based on healthy head models which ignore the effects of the altered cortex on the stimulating currents. Clinical applications of TMS should be carefully considered in light of these findings.

Effects of Repetitive Transcranial Magnetic Stimulation on Spike Pattern and Topography in Patients with Focal Epilepsy

“…Theodore et al have shown that an effect on spikecount can be obtained more reliably in patients with neocortical than in patients with deep foci (Theodore et al 2002). This is well explained by the fact that the magnetic field is reaching only a depth of about 2-4 cm within the brain tissue (Wagner et al 2008). However, patient #4 was the only patient with a rather deep epileptic focus (mesial temporal due to hippocampus sclerosis), but the only one who positively responded to inhibitory rTMS.…”

Increasing Human Brain Excitability by Transcranial High-Frequency Random Noise Stimulation

“…There is, however, sufficient evidence to suggest that the current used here can reach the brain. The bone is the structure with highest resistance, and has to be considered primarily when stimulating the head electrically (Wagner et al, 2008). In fact, high bone resistance was the reason why TMS replaced pulsed electrical stimulation in 1985 (Barker et al, 1985) and thereby could avert painful stimulation.…”

The Promising Role of Neuromodulation in Improving Ischemic Stroke Recovery

“…It is thought that cathodal stimulation of the unaffected hemisphere will cause a suppression of activity locally, allowing for the transcallosal disinhibition of the affected hemisphere, thereby increasing neuroplastic response to repair itself [17]. This same mechanism has been proposed for the improvement of neurocognitive deficits in stroke patients; specifically in task related functions [18], as well as, memory and affective complications [19][20][21]. These results, though minimal, are statistically significant in demonstrating that modulation of ipsilesional activity can improve motor recovery, likely by promoting plastic reorganization of spared cortex.…”

“…A review examining DBS with motor and sensory cortical stimulation examines all recent prospective studies examining long-term outcomes in patients and advocate that DBS is superior to motor cortical stimulation for refractory pain syndromes and may be more appropriate than sensory cortical stimulation for certain pain etiologies, including phantom pain after amputation [45]. In regards to pain after stroke, DBS reveals greatest efficacy for stroke patients complaining of burning hyperaesthesia [18].…”