The present invention relates generally to magnetic stimulation and more particularly to high-speed magnetic stimulation of biological tissue.
For some time, electrical or magnetic stimulation devices have been used to stimulate biological tissue.
Electric stimulation of biological tissue is wellknown and generally involves the use of a plurality of electrodes strategically placed on the tissue of a subject (human or animal). Electric pulses are subsequently generated and applied to the electrodes. The tissue in an area separating the electrodes is thus stimulated by the passage of current therethrough.
Magnetic stimulation of biological tissue is a much more recent development. Magnetic stimulation involves the application of a magnetic field to biological tissue. A magnetic transducer such as a wire wound coil is employed to generate the magnetic field. The coil may be contactless and operate a predetermined distance away from the subject. Alternatively, the coil may contact or may be implanted in the subject.
Magnetic stimulation offers many significant advantages over electrical stimulation. Magnetic stimulation eliminates the need for direct physical electrical contact to the subject and thus removes problems associated with high skin electrical resistance, damage to tissue due to high current flow and the like. Additionally, the subject is exposed to less risk since hazards arising from electrical shock due to malfunctioning equipment are reduced, if not eliminated.
Potential applications of magnetic stimulation also include the stimulation of both the central and peripheral nervous systems. Specifically, magnetic stimulation followed by recordal of the corresponding motor evoked potentials (MEPs) is believed to be a promising application of magnetic neural stimulation. MEPs have been recorded over spinal cord, peripheral nerve and muscle following transcranial electrical stimulation. These MEPs have been demonstrated to accurately reflect the functional integrity of efferent spinal motor pathways of rats and cats subjected to mechanical trauma, compression and ischemia, and also have been well correlated with the degree of clinical recovery. These results suggest major clinical applications for MEPs in evaluation of efferent motor pathways as well as in intraoperative monitoring. Conventional neurophysiological techniques do not permit such examination of central motor pathways.
With respect to diagnostic evaluation of efferent motor pathways, MEP abnormalities have been demonstrated in patients with multiple sclerosis, compressive and radiation induced myelopathies, and hereditary spastic paraparesis. In some cases subclinical dysfunction was detected.
Further, intraoperative monitoring of efferent motor pathways is likely to achieve major importance supplementing somatosensory evoked potential (SEP) monitoring during neurosurgery. SEPs, which monitor principally dorsal cord function, do not adequately predict the integrity of the descending motor pathways. Successful use of MEP monitoring has been reported in a large series of neurosurgical procedures, where monitoring results correlated well with clinical outcome. Although at present transpinal cord stimulation is not known to have been reported, this may also be possible.
Magnetic neural stimulation is also likely to be a useful research tool for brain physiology. Using electrical stimulation, MEP to cerebellar stimulation has been demonstrated and used to study interactions of pyramidal and extra-pyramidal motor systems. MEPs have also been used to study the function of callosal pathways. It is likely that many further research applications for the study of brain physiology will emerge. Magnetic stimulation of the brain provides instrumental, noninvasive access to brain function, with the possibility of modification of that function.
Magnetic neural stimulation offers several potential advantages over conventional electrical stimulation. Magnetic stimulation is painless. Transcutaneous electrical stimulation produces the greatest current density in the most superficial skin layers, the skin layers most sensitive to pain. In contrast, lines of flux produced by magnetic stimulation penetrate the skin essentially unaltered, making it possible to stimulate nerves without exciting overlying cutaneous pain fibers, which have a higher threshold for stimulation.
The painless nature of magnetic stimulation will likely prove a major practical advantage in performing peripheral nerve and SEP studies on children. Moreover, the electrophysiological examination of deeper lying and proximal peripheral nerve, which is inaccessible to study by conventional electrical stimulation because of the painful and potentially damaging currents required to excite deep lying tissue, appears to be feasible using magnetic stimulation.
Rapid electrical stimulation of peripheral nerve is sufficiently painful so that this technique cannot be employed on conscious patients. Fortunately, magnetic stimulation at similarly high rates of stimulation is painless.
Magnetic stimulation produces substantially less stimulus artifact than electrical stimulation. This will facilitate SEP recordings in settings where the time interval between stimulus and response is very short, such as recordings in infants and small children, trigeminal nerve SEPs, and blink reflex recordings.
Magnetic stimulation may make transcutaneous clinical evaluation of small, unmyelinated nerve pathways possible. Although potentially useful techniques for small and selective fiber stimulation have been proposed, these techniques have not been successfully used transcutaneously in a clinical setting, most likely due to the high currents that would be required, as well as the effect of skin and nerve upon the pulse shape. Since magnetic fields are not substantially altered by the skin and nerve sheath, it may be possible to use a magnetic stimulator capable of delivering a shaped magnetic pulse to selectively stimulate smaller myelinated and unmyelinated fibers.
Unfortunately, practical use of magnetic neural stimulation has been limited by the inability of currently available magnetic neural stimulators to selectively stimulate a limited area of neural tissue, especially cerebral, spinal or peripheral neural tissue. Presently available stimulators employ large stimulating coils which are incapable of focusing and limiting the lateral spread of the magnetic field. Additionally, the ability to selectively stimulate more discrete areas of tissue such as the brain will be important in gaining an understanding of the physiology of magnetic nerve stimulation and may also enhance its ultimate clinical utility. Furthermore, many peripheral neurophysiological tests involve determination of nerve conduction velocity, and hence require accurate knowledge of precisely where nerve stimulation occurred.
Present devices also cannot stimulate at rates over about 1 Hz. As a result, clinical MEP testing and monitoring is presently restricted for practical purposes to recording of electromyogram (EMG) activity only. Although the sensitivity of clinical studies would likely be enhanced by recording neural responses over spinal cord and peripheral nerve, such studies would require signal averaging, which is impractical at low stimulation rates.
Present magnetic neural stimulators are also unable to operate at the speed necessary for SEP testing since averaging of response to a prohibitively large number of stimuli is required. Thus, magnetic peripheral nerve stimulation, although pain-free, is also not practical for SEP testing using presently available devices. Present devices are also unable to selectively stimulate a limited area of neural tissue.