Cervical magnetic stimulation: the role of the neural foramen

Magnetic stimulation of cervical nerve roots is a promising new technique, limited in part by uncertainty about the site of nerve depolarization. We used a modified "butterfly" stimulus coil with an easily defined excitation field to activate the C-8/T-1 nerve roots, recording over abductor digiti minimi. Locating both the lowest threshold for stimulation and the points of maximum stimulation, we determined the optimum rostral-caudal position and orientation for the stimulus coil over the posterior neck and upper trunk. The most favorable positions corresponded to the C-8/T-1 neural foramina, and the most favorable orientations to the roots within them. Additional measurements of depth and electric field suggested that the stimuli used should have been insufficient to activate nervous tissue in a homogeneous medium. A simple model indicates that the induced current is intensified where it passes through a bony foramen and explains preferential excitation of the nerve root at this site.     

Transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) allows non-invasive study and modulation of cortical excitability in humans. Changes in cortical excitability in physiological and pathological conditions can be tracked by measurements such as motor threshold, motor evoked potentials, recruitment curves, intracortical facilitation and inhibition. The central motor conduction time can estimate neural transmission in central motor pathways. Changes in areas of representation in sensorimotor cortex can be studied with cortical mapping. Modulation of cortical processing can be used to evaluate different brain functions. Therapeutic use in depression, Parkinson's disease and epilepsy has raised great interest over the past decade. Non-invasive cortical mapping may be achieved by combining TMS to other neurophysiological/ neuroimaging techniques. TMS has great potential both as an investigational and as a therapeutical tool in Neurology and Psychiatry.     

Brain stimulation procedures. Transcranial magnetic stimulation, magnetic seizure therapy and deep brain stimulation

Brain stimulation methods are promising treatment options in severe treatment-resistant psychiatric disorders. A safe and noninvasive method is transcranial magnetic stimulation, but the clinical application is not clear. Magnetic seizure therapy is a further development of transcranial magnetic stimulation, by which generalized seizures are induced under anaesthesia. Previous results with regard to the antidepressant effects and the fewer cognitive side effects were significant. Deep brain stimulation is an invasive procedure in which electrodes are stereotactically implanted in special brain areas. The effects in severe therapy-resistant obsessive-compulsive disorders and depressions are promising. However, the evaluation of ethical issues remains an important task.     

Cervical root stimulation in the diagnosis of radiculopathy

Cervical root stimulation (CRS) was compared with conventional EMG, nerve conduction, and late response studies in 34 patients with possible cervical radiculopathy. Cervical roots were stimulated by monopolar needles inserted into paraspinal muscles, recording compound muscle action potentials in biceps, triceps, and abductor digiti minimi muscles. In 18 patients with clinical evidence of radiculopathy, EMG was abnormal in 11 (61%), but CRS was abnormal in all 18. Of 16 patients with symptoms but no signs of radiculopathy, EMG was abnormal in 5 (31%) and CRS was abnormal in 9 (56%).     

Transcranial magnetic stimulation and neuroplasticity

We review past results and present novel data to illustrate different ways in which TMS can be used to study neural plasticity. Procedural learning during the serial reaction time task (SRTT) is used as a model of neural plasticity to illustrate the applications of TMS. These different applications of TMS represent principles of use that we believe are applicable to studies of cognitive neuroscience in general and exemplify the great potential of TMS in the study of brain and behavior. We review the use of TMS for (1) cortical output mapping using focal, single-pulse TMS; (2) identification of the mechanisms underlying neuroplasticity using paired-pulse TMS techniques; (3) enhancement of the information of other neuroimaging techniques by transient disruption of cortical function using repetitive TMS; and finally (4) modulation of cortical function with repetitive TMS to influence behavior and guide plasticity.     

Transcranial magnetic stimulation in depression

Among the new therapeutic techniques in psychiatry, transcranial magnetic stimulation (TMS) seems to bring a profit in the treatment of depressions. It uses the principle of inductance to generate a magnetic current, which in turn activates cortical neurons. Stimulation is highly focused and interests specific regions of the cerebral cortex. This therapeutic technique is generally well tolerated. Side effects are rare, the most hampering one is epileptic seizures. It is favored by high frequencies (above 5 Hz) and arises mainly with patients having a history of personal or family epileptic seizures. The first open trials, quickly confirmed by controlled studies showed the efficiency of TMS in depression. With depression, double blind randomized trials, using high frequencies, stimulation of the left dorsolateral prefrontal cortex give positive results with significant decrease of scores on depressive scales applied to resistant and non resistant depressions. Some studies have stimulated the right dorsolateral prefrontal cortex using low frequencies. The decrease of scores is also significant on depressive scales. The modulating effect of rTMS on cortical excitability of the brain justifies this distinction between high and low frequencies, high frequencies having a facilitating effect whereas low frequencies have an inhibitory effect.     

Transcranial magnetic stimulation in neurology

Transcranial magnetic stimulation (TMS) is a non-invasive tool for the electrical stimulation of neural tissue, including cerebral cortex, spinal roots, and cranial and peripheral nerves. TMS can be applied as single pulses of stimulation, pairs of stimuli separated by variable intervals to the same or different brain areas, or as trains of repetitive stimuli at various frequencies. Single stimuli can depolarise neurons and evoke measurable effects. Trains of stimuli (repetitive TMS) can modify excitability of the cerebral cortex at the stimulated site and also at remote areas along functional anatomical connections. TMS might provide novel insights into the pathophysiology of the neural circuitry underlying neurological and psychiatric disorders, be developed into clinically useful diagnostic and prognostic tests, and have therapeutic uses in various diseases. This potential is supported by the available studies, but more work is needed to establish the role of TMS in clinical neurology.     

Transcranial magnetic stimulation and neuroimaging

Ebmeier KP. Transcranial magnetic stimulation and neuroimaging. Bipolar Disord 2002: 4(Suppl. 1): 96–97. © Blackwell Munksgaard, 2002