- Email: [email protected]
Deep magnetic brain stimulation — The end of psychiatric electroshock therapy?
1Medical Hypotheses _I
I
Mrdicnl Hy~~lltwc (1994) 43.69-74 b Longman Group Ltd 1994
Deep Magnetic Brain Stimulation The End of Psychiatric Electroshock Therapy? T. ZYSS Department and Clinic of Psychiatry 501 Cracow. Poland.
CoNegium Medicurn, Jagiellonian
University, Kopernika Street Zla, 3?-
Abstract - The biophysical processes during electroconvulsive therapy (ECT) are discussed. The multilayer head structure causes a shunting and spreading of the major portions of the current all over the extracranial tissues. The final effect is that only a small percentage of the injected current passes into the brain. The second ‘side effect’ of the ECT is the electrical overstimulation of the cortex layer, with self-sustained after-discharge causing convulsive seizure. Therefore, the stimulus transmission into the neurochemically and physiologically disturbed meso-diencephalic region is the most important problem. The stimulation of the brain with rapid-rate time-varying magnetic field pulses makes this problem soluble. The magnetic field passes through all head structures with no attenuation and can stimulate the brain without discomfort. Our calculations of stimulus parameters have assumed that this inductive method may be able to evoke the same therapeutic effects as ECT, but in a safer way (painless, noninvasive and without motor seizure).
Introduction
Electroconvulsive therapy (ECT) is one of the most powerful methods in psychiatric treatment. A variety of biochemical mechanisms have been suggested to explain the therapeutic action of electroshock therapy (1). ECT causes complex functional and structural changes in the meso-diencephalic brain structures (2.3). The principal indication for convulsive therapy is depressive psychosis. The primary depressive syndrome is characterised in a neurophysiological model Date received 29 April 1991 Date :accepted 17 August 1993
by centrencephalic dysfunction. Therefore, the exogenously applied electrical current modifies the regulation of this central dysfunction (4). The most important problem of ECT (in other words: electrical stimulation) is therefore the stimulus transmission into the deep brain regions. In 1985 Barker (5) introduced a new neurological technique of transcranial magnetic stimulation. Since now, most reports have dealt with the effects of stimulation on the superficial layers of brain. There is not much research giving information about the influence of magnetic stimulation on deeper layers of brain. 69
70
MEDICAL HYPOTHESES
Resistivity
e
;1300~ WOO(fl.cm) jl~0:
$5:250; j i
-15000
750
-
scblp s&,
Fig.
1 Conceptual
representation
Pathway of stimulating
of a current
pathway
-
;orlex I cerebra-spinal fluid during
subktantia alba
ECT and resistivity
current
The examination of the path of current from the scalp electrode into the deep brain structures is very useful. This may explain the seizure producing mechanism and the efficacy of ECT (Fig. I) (6). The head structures have different current- transmitting characteristics. The tissues provide a rather complex network of resistance which impedes the electrical stimulus. The resistivity is the physical parameter for describing the current flow possibility in the intervening tissues (p = R-m). This parameter describes the ability of a tissue to oppose the flow of the current: the higher the resistivity, the more the opposition to current flow. Beneath the stimulating electrode is the scalp (skin, skeletal muscles, galea). Its resistivity is higher for current flowing transversly (T = 1000 R*cm) than longitudinally (L = 300 R*cm) (7). This causes a shunting and spreading of the major portion of current all over the extracranial tissues. Below the scalp is the skull with very high resistivity values (5000-15000 Dcm). The bone layer offers considerable hindrance for current flow. Below the skull are the meninges
disturbed centrencephaltc structures
. . . . . . ,, . ..
values (p =
1fi*m)
for tissues between
scalp and brain.
(theirresistivity values have not been reported) and the space with the cerebrospinal fluid (CSF). The resistivity of CSF is relatively low (compared with values for brain tissue) - about 65 R*cm. The electrical current runs easier in the better conducting CSF and spreads over the cortex ‘crawls’ on the cortex layer). The resistivity of the grey and white matter are about 250 and 750 !&cm respectively. This distribution again causes the ‘crawling-effect’, this time in the cortical layer. Finally the cortex is overstimulated in the form of strong self-sustained after-discharge (8). Another problem is connected with a very small percentage (e.g. 5--150/C) of the injected current passing into the brain. In this way, the deep brain tissues are effectively hidden from electrical stimulation by superficially lying skin, fascia, bone and meninges (9). Even though such a small quantity of current reaches brain tissue, the after-discharge mechanism operates as an amplifier for deep stimulus penetration. In this way, the inefficiency of techniques using subconvulsive treatment doses can be explained. The subliminal stimulus dose is insufficient to elicit the after-discharge primary in the cortical layer and to achieve therapeutic changes in the centrencephalic system.
71
DEEP MAGNETIC: BRAIN STIMULATION
current
flow
in neural
domains
Fig. 2 Changing magnetic field B (generated in stimulating coil L) induces electric field causing current flow in neural domains.
The motor seizure (after-discharge in the motor-cortex) indicates stimulus strength inside the cortex layer. The most important problem is therefore the stimulus transmission into the neurochemically disturbed meso-diencephalic regions.
Magnetic stimulation The use of time-varying magnetic field to induce a sufficient excitation in living tissue has long been known. This kind of stimulation of the human retina, the most sensitive structure in humans, was for the first time reported by d’Arsonva1 in 1896 (elementary visual sensations = phosphenes). Since then, there have been many studies of the techniques of magnetic stimulation. In 1964, Ollendorf (10) discussed the possibility of applying a magnetic stimulation to evoke a magnetic brain shock. In 1985, Barker et al showed that this inductive method is able to stimulate the human brain (strictly: the cortex layer). The presented transcranial magnetic stimulation (TMS) excites neural structures in the motor cortex and causes compound muscle responses (motor evoked potentials - MEP). This technique is useful for detection and quantitative estimation of disturbances in the fast cortico-spinal pathways. The stimulus strength is enough to activate the descending motor tract, but it is insufficient to evoke the cortical after-discharge which causes convulsive seizure (11).
Fundamentals
of magnetic stimulation
The magnetic technique uses electrical fields, induced by a time-varying magnetic field, to stimulate the neural tissue (12). An impulse of the magnetic field passes into the head and creates an electric field in the brain by electromagnetic induction. The induced electric field causes flow of ions which in turn result in neuronal activation. This method does not rely on the passage of electric current from electrodes into the brain structures (the current passes through a magnetic coil close to the head) (13). The difference between the electrical and magnetic stimulation is the form of stimulus application and its transmission, but finally the mechanism of stimulation is the same-the current flow (Fig. 2). Magnetic stimulation offers the advantage of being noninvasive and painless, and does not cause serious discomfort to the patient (14).
Stimulus parameter Depth of penetration
Magnetic stimulation can reach deep brain structures that are not easily accessible using electrical stimulation. The depth of penetration of magnetic stimulation is proportional to the diameter of the coil used (number of turns, internal and external diameters, spatial geometry) and also to the power of the stimulator (l&16).
MEDICAL HYPOTHESES
72
TM- thdamus HT - hjpothamur FR - lormatio WlituloriS CA -corpus
amygdatoideum
Fig. 3
The topographic
profile
of induced electric field strength (E) for various distances below the coil surface
A larger diameter of the coil will deliver an effective magnetic field deeper into the tissues (17,18). The stimulus with the higher power will be more effective at the same depth. The magnetic field decreases in inverse proportion to the distance from the plane of the coil, but the decrease of effectiveness is much less rapid than for percutaneous electrical stimulation (19). It is independent from the resistivity of intervening head layers. The neurological technique of motor evoked potentials limits the magnetic stimulation effects only to the cortical layer. Typically, a magnetic coil affects a large volume of brain and it seems to be possible that it may activate other structures, including deep regions. There are many mathematical models for calculating the electric field induced in the brain during magnetic stimulation (20-22). The values of quasicylindrical magnetic fields can be estimated using the following equations:
Bx=
_[a-j] --L
,I;: a Pcl
By= -n12?r
B=
$2B,+B;
x
a+x (a+x)*fY2
(a - x)* + y*
+
a-y (a-x)*+y’
I
The secondary induced electric field can be calculated from :
E
=
E
-
’-
nI
r.LoIn (a+ Xl2 +Y2 4Kt
(a-
x)'+y*
(B = magnetic field strength, E = electric field strength, n = number of turns in coil, h = permeability constant, I = current through the wire, t = current flow time, a = coil radius, x, y, z = the distances along the main axes in the Cartesian geometry). It is generally known from electrophysiological studies (23), that the electric field strength of about 100 V/m-t is adequate for activating the neuronal cells. Thus during magnetic stimulation the electric field induced 4-6 cm below the surface of the head (the mean depth of disturbed meso-diencephalic structures) should be sufficient to achieve such a value. On the other hand, the electric field strength at the depth of l-2 cm (the cortex layer) must not be too strong due to the motor cortex overstimulation. In neurological transcranial magnetic stimulation (where the motor cortex is stimulated), the induced electric field at the brain surface is 150-300 V/m-t. From the estimation of these values we can calculate a sufficient stimulus intensity in the deep layers for activating them. Figure 3 illustrates the spatial distribution of the electric field strength for various distances below the
73
DEEP MAGNETIC BRAIN STIMULATION
coil surface (coil radius r = 5 cm; number of turns [windings] n = 20 or 25; the current I = 5000 A and its rise time t = 100 ps).
Frequency and duration of stimulation The stimulus repetition rates available from neurological magnetic stimulators are low, typically a maximum of one stimulus every 3-4 s (frequency 0.25-0.33 Hz) with the maximum output. From electrophysiological techniques (24). it is known that this frequency is unable to stimulate the neural tissue. In 1990 a new method of rapid-rate* transcranial magnetic stimulation was demonstrated (25). Prototype stimulators can operate at a rate of a few tens of stimuli per second for This prolonged stimulation time a few seconds. seemed to be very useful in the event of absence of an after-discharge period.
Safety aspect.s The safety of magnetic stimulation in the neurological technique of motor evoked potentials has been discussed in detail elsewhere (26). The calculations of the rapid-rate stimulation show that they are not overstrong, for example: for the charge delivered on the brain by impulse of 11 pC, for frequency of 50 Hz and stimulation time of 1 min, the total charge is 33.3 mC (= 11 pC x 50 Hz x 60 s). For reference, a typical burst of electroconvulsive therapy delivers a total charge of 1OOmCtolC.
Conclusions
References I. Fink M. ed. Convulsive therapy York: Raven Press, 198.5.
- theory and practice.
New
2. Malitz S, Sackeim HA. eds. Electroconwlsive therapy -clinical and basic research issues. New York: Ann N Y Acad Sci, 1986. 3. Small JG, Milstein V, Miller MJ et al. Neurophysiological evidence for action of ECT. Psychopharmacol Bull 1988; 24: 391-395. 4. Abrams R, Taylor M. Diencephalic stimulation and the effects of ECT in endogenous depression. Br J Psychiatr 1976; 129: 482-485. 5. Barker AT, Freeston IL. Jalinous R. et al. Magnetic stimulation of the human brain and peripheral nervous system-an introduction and the results of initial clinical evaluation Neurosurgery 1987; 20: 100-109. 6. Geddes LA. Bourland JD. Tissue stimulation - theoretical consideration and practical applications. Med Biol Eng Comput 1985; 23: 131-137. 7. Geddes LA. Barker LE. The specific resistance of biological material - a compendium of data for the biomedical engineer and physiologist. Med Biol Eng 1967: 5: 271-293. 8. Creutzfeld OD. Epileptische
Entladungen.
In: Creutzfeld OD.
ed. Cortex cerebri - Leistung. strukturelle und funktionelle Organisation der Himrinde. Berlin: Springer, 1983: 154-159. 9. Geddes LA. Optimal stimulus duration for extracranial stimulation. Neurosurgery 1987: 20: 94-99.
cortical
IO. Ollendorf F. Beitrrige zur Elektrodynamik des Elektroschoks III. Der induktive &hock. Arch Electrotechnik 1964; 48: 421-444. 1 I. Dhuna A, Gates J. Pascual-Leone A. Transcranial stimulation in patients with epilepsy. Neurology 1067-1071.
magnetic 1991; 41:
12. Jalinous R. Technical and practical aspects of magnetic nerve stimulation. J. Clin Neurophysiol 1991; 8: 10-25. 13. Barker AT. An introduction to the basic principles of magnetic nerve stimulation. J Clin Neurophysiol 1991; 8: 26-37. 14. Bickford RG. Guidi M, Fortescue P et al. Magnetic stimulation
The modern technique of transcranial magnetic stimulation is able to transmit a sufficient electrical stimulus into the deep brain structures for activating them. The method of application and the form of tissue penetration are different in comparison with electrical stimulation. This inductive method is painless and does not cause a motor seizure. Yet, our calculations at this stage relate only to the theoretical stimtdus parameter. We expect that deep magnetic brain stimulation will be able to evoke the same therapeutic effects as electroconvulsive therapy but in a safer way.
*The name ‘rapid-rate’ is incorrectfrom the physical point of view. The electromagneticfrequencies between 0 and 300 Hz are called extreme/y lowfrequencies (ELF).]
of human peripheral nerve and brain - response enhancement by combined magnetoelectrical technique. Neurosurgery 1987; 20: 1IO-I 16. 15. Geddes LA, Bourland JD. Fundamentals of eddy-current (magnetic) stimulation. In: Chokroverty S. ed. Magnetic stimulation in clinical neurophysiology. Stoneham: Butterworths. 1990: 33-43. 16. Cohen LG. Roth BJ, Nilsson J et al. Effects of coil design on delivery of focal magnetic stimulation - technical considerations. EEG Clin Neurophysiol 1990: 75: 350-357 17. Fortescue P, Bickford R. The design of electromagnetic stimulators, In: Chokroverty S. ed. Magnetic stimulation in clinical neurophysiology. Stoneham: Butterworths. 1990: 45-54. 18. Epstein CM, Schwartzberg DG, Davey KR et al. Localizing the site of magnetic brain stimulation in humans. Neurology 1990: 40: 666-670. 19. Meyer BU, Britton TC. Kloten H et al. Coil placement in magnetic brain stimulation related to skull and brain anatomy.
74
MEDICAL HYPOTHESES Clin Neurophysiol 1991; 81: 238-239.
EEG Clin Neurophysiol 1991; 81: 38-46. 20. Grandori F. Intracranial electric fields during transcranial brain stimulation - modelling and stimulation. Marsden CD, eds.
Non-invasive
In: Rossini PM,
stimulation of brain and
spinal cord - fundamentals and clinical applications
New
York: Alan R Liss, 1988: 145-157. 21. Roth BJ, Saypol JM, Hallett M et al. A theoretical calculation of the electric field induced in the cortex during magnetic stimulation. EEG Clin Neurophysiol 1991: 81: 47-56. 22. Tofts PS, Branston NM.
The measurement of electric field and
the influence of surface charge in magnetic stimulation. EEG
23. Eichhom KFr, Amdt B, Claus D et al. Induktive Stimulation. Biomed Technik 1990; 35: 139.144. 24. lmich W.
Das Grundgesetz der Elektrostimulation.
Biomed
Technik 1989; 34: 158-167. 25. Pascual-Leone A, Gates JR, Dhuna AK.
Determination
of
language dominant hemisphere with rapid transcranial magnetic stimulation. Ann Neurol 1990; 28: 223. 26. Agnew WA, McCreery DB. Consideration for safety in the use of extracranial stimulation for motor evoked potentials. Neurosurgery 1987;20:
141-147.
I
Mrdicnl Hy~~lltwc (1994) 43.69-74 b Longman Group Ltd 1994
Deep Magnetic Brain Stimulation The End of Psychiatric Electroshock Therapy? T. ZYSS Department and Clinic of Psychiatry 501 Cracow. Poland.
CoNegium Medicurn, Jagiellonian
University, Kopernika Street Zla, 3?-
Abstract - The biophysical processes during electroconvulsive therapy (ECT) are discussed. The multilayer head structure causes a shunting and spreading of the major portions of the current all over the extracranial tissues. The final effect is that only a small percentage of the injected current passes into the brain. The second ‘side effect’ of the ECT is the electrical overstimulation of the cortex layer, with self-sustained after-discharge causing convulsive seizure. Therefore, the stimulus transmission into the neurochemically and physiologically disturbed meso-diencephalic region is the most important problem. The stimulation of the brain with rapid-rate time-varying magnetic field pulses makes this problem soluble. The magnetic field passes through all head structures with no attenuation and can stimulate the brain without discomfort. Our calculations of stimulus parameters have assumed that this inductive method may be able to evoke the same therapeutic effects as ECT, but in a safer way (painless, noninvasive and without motor seizure).
Introduction
Electroconvulsive therapy (ECT) is one of the most powerful methods in psychiatric treatment. A variety of biochemical mechanisms have been suggested to explain the therapeutic action of electroshock therapy (1). ECT causes complex functional and structural changes in the meso-diencephalic brain structures (2.3). The principal indication for convulsive therapy is depressive psychosis. The primary depressive syndrome is characterised in a neurophysiological model Date received 29 April 1991 Date :accepted 17 August 1993
by centrencephalic dysfunction. Therefore, the exogenously applied electrical current modifies the regulation of this central dysfunction (4). The most important problem of ECT (in other words: electrical stimulation) is therefore the stimulus transmission into the deep brain regions. In 1985 Barker (5) introduced a new neurological technique of transcranial magnetic stimulation. Since now, most reports have dealt with the effects of stimulation on the superficial layers of brain. There is not much research giving information about the influence of magnetic stimulation on deeper layers of brain. 69
70
MEDICAL HYPOTHESES
Resistivity
e
;1300~ WOO(fl.cm) jl~0:
$5:250; j i
-15000
750
-
scblp s&,
Fig.
1 Conceptual
representation
Pathway of stimulating
of a current
pathway
-
;orlex I cerebra-spinal fluid during
subktantia alba
ECT and resistivity
current
The examination of the path of current from the scalp electrode into the deep brain structures is very useful. This may explain the seizure producing mechanism and the efficacy of ECT (Fig. I) (6). The head structures have different current- transmitting characteristics. The tissues provide a rather complex network of resistance which impedes the electrical stimulus. The resistivity is the physical parameter for describing the current flow possibility in the intervening tissues (p = R-m). This parameter describes the ability of a tissue to oppose the flow of the current: the higher the resistivity, the more the opposition to current flow. Beneath the stimulating electrode is the scalp (skin, skeletal muscles, galea). Its resistivity is higher for current flowing transversly (T = 1000 R*cm) than longitudinally (L = 300 R*cm) (7). This causes a shunting and spreading of the major portion of current all over the extracranial tissues. Below the scalp is the skull with very high resistivity values (5000-15000 Dcm). The bone layer offers considerable hindrance for current flow. Below the skull are the meninges
disturbed centrencephaltc structures
. . . . . . ,, . ..
values (p =
1fi*m)
for tissues between
scalp and brain.
(theirresistivity values have not been reported) and the space with the cerebrospinal fluid (CSF). The resistivity of CSF is relatively low (compared with values for brain tissue) - about 65 R*cm. The electrical current runs easier in the better conducting CSF and spreads over the cortex ‘crawls’ on the cortex layer). The resistivity of the grey and white matter are about 250 and 750 !&cm respectively. This distribution again causes the ‘crawling-effect’, this time in the cortical layer. Finally the cortex is overstimulated in the form of strong self-sustained after-discharge (8). Another problem is connected with a very small percentage (e.g. 5--150/C) of the injected current passing into the brain. In this way, the deep brain tissues are effectively hidden from electrical stimulation by superficially lying skin, fascia, bone and meninges (9). Even though such a small quantity of current reaches brain tissue, the after-discharge mechanism operates as an amplifier for deep stimulus penetration. In this way, the inefficiency of techniques using subconvulsive treatment doses can be explained. The subliminal stimulus dose is insufficient to elicit the after-discharge primary in the cortical layer and to achieve therapeutic changes in the centrencephalic system.
71
DEEP MAGNETIC: BRAIN STIMULATION
current
flow
in neural
domains
Fig. 2 Changing magnetic field B (generated in stimulating coil L) induces electric field causing current flow in neural domains.
The motor seizure (after-discharge in the motor-cortex) indicates stimulus strength inside the cortex layer. The most important problem is therefore the stimulus transmission into the neurochemically disturbed meso-diencephalic regions.
Magnetic stimulation The use of time-varying magnetic field to induce a sufficient excitation in living tissue has long been known. This kind of stimulation of the human retina, the most sensitive structure in humans, was for the first time reported by d’Arsonva1 in 1896 (elementary visual sensations = phosphenes). Since then, there have been many studies of the techniques of magnetic stimulation. In 1964, Ollendorf (10) discussed the possibility of applying a magnetic stimulation to evoke a magnetic brain shock. In 1985, Barker et al showed that this inductive method is able to stimulate the human brain (strictly: the cortex layer). The presented transcranial magnetic stimulation (TMS) excites neural structures in the motor cortex and causes compound muscle responses (motor evoked potentials - MEP). This technique is useful for detection and quantitative estimation of disturbances in the fast cortico-spinal pathways. The stimulus strength is enough to activate the descending motor tract, but it is insufficient to evoke the cortical after-discharge which causes convulsive seizure (11).
Fundamentals
of magnetic stimulation
The magnetic technique uses electrical fields, induced by a time-varying magnetic field, to stimulate the neural tissue (12). An impulse of the magnetic field passes into the head and creates an electric field in the brain by electromagnetic induction. The induced electric field causes flow of ions which in turn result in neuronal activation. This method does not rely on the passage of electric current from electrodes into the brain structures (the current passes through a magnetic coil close to the head) (13). The difference between the electrical and magnetic stimulation is the form of stimulus application and its transmission, but finally the mechanism of stimulation is the same-the current flow (Fig. 2). Magnetic stimulation offers the advantage of being noninvasive and painless, and does not cause serious discomfort to the patient (14).
Stimulus parameter Depth of penetration
Magnetic stimulation can reach deep brain structures that are not easily accessible using electrical stimulation. The depth of penetration of magnetic stimulation is proportional to the diameter of the coil used (number of turns, internal and external diameters, spatial geometry) and also to the power of the stimulator (l&16).
MEDICAL HYPOTHESES
72
TM- thdamus HT - hjpothamur FR - lormatio WlituloriS CA -corpus
amygdatoideum
Fig. 3
The topographic
profile
of induced electric field strength (E) for various distances below the coil surface
A larger diameter of the coil will deliver an effective magnetic field deeper into the tissues (17,18). The stimulus with the higher power will be more effective at the same depth. The magnetic field decreases in inverse proportion to the distance from the plane of the coil, but the decrease of effectiveness is much less rapid than for percutaneous electrical stimulation (19). It is independent from the resistivity of intervening head layers. The neurological technique of motor evoked potentials limits the magnetic stimulation effects only to the cortical layer. Typically, a magnetic coil affects a large volume of brain and it seems to be possible that it may activate other structures, including deep regions. There are many mathematical models for calculating the electric field induced in the brain during magnetic stimulation (20-22). The values of quasicylindrical magnetic fields can be estimated using the following equations:
Bx=
_[a-j] --L
,I;: a Pcl
By= -n12?r
B=
$2B,+B;
x
a+x (a+x)*fY2
(a - x)* + y*
+
a-y (a-x)*+y’
I
The secondary induced electric field can be calculated from :
E
=
E
-
’-
nI
r.LoIn (a+ Xl2 +Y2 4Kt
(a-
x)'+y*
(B = magnetic field strength, E = electric field strength, n = number of turns in coil, h = permeability constant, I = current through the wire, t = current flow time, a = coil radius, x, y, z = the distances along the main axes in the Cartesian geometry). It is generally known from electrophysiological studies (23), that the electric field strength of about 100 V/m-t is adequate for activating the neuronal cells. Thus during magnetic stimulation the electric field induced 4-6 cm below the surface of the head (the mean depth of disturbed meso-diencephalic structures) should be sufficient to achieve such a value. On the other hand, the electric field strength at the depth of l-2 cm (the cortex layer) must not be too strong due to the motor cortex overstimulation. In neurological transcranial magnetic stimulation (where the motor cortex is stimulated), the induced electric field at the brain surface is 150-300 V/m-t. From the estimation of these values we can calculate a sufficient stimulus intensity in the deep layers for activating them. Figure 3 illustrates the spatial distribution of the electric field strength for various distances below the
73
DEEP MAGNETIC BRAIN STIMULATION
coil surface (coil radius r = 5 cm; number of turns [windings] n = 20 or 25; the current I = 5000 A and its rise time t = 100 ps).
Frequency and duration of stimulation The stimulus repetition rates available from neurological magnetic stimulators are low, typically a maximum of one stimulus every 3-4 s (frequency 0.25-0.33 Hz) with the maximum output. From electrophysiological techniques (24). it is known that this frequency is unable to stimulate the neural tissue. In 1990 a new method of rapid-rate* transcranial magnetic stimulation was demonstrated (25). Prototype stimulators can operate at a rate of a few tens of stimuli per second for This prolonged stimulation time a few seconds. seemed to be very useful in the event of absence of an after-discharge period.
Safety aspect.s The safety of magnetic stimulation in the neurological technique of motor evoked potentials has been discussed in detail elsewhere (26). The calculations of the rapid-rate stimulation show that they are not overstrong, for example: for the charge delivered on the brain by impulse of 11 pC, for frequency of 50 Hz and stimulation time of 1 min, the total charge is 33.3 mC (= 11 pC x 50 Hz x 60 s). For reference, a typical burst of electroconvulsive therapy delivers a total charge of 1OOmCtolC.
Conclusions
References I. Fink M. ed. Convulsive therapy York: Raven Press, 198.5.
- theory and practice.
New
2. Malitz S, Sackeim HA. eds. Electroconwlsive therapy -clinical and basic research issues. New York: Ann N Y Acad Sci, 1986. 3. Small JG, Milstein V, Miller MJ et al. Neurophysiological evidence for action of ECT. Psychopharmacol Bull 1988; 24: 391-395. 4. Abrams R, Taylor M. Diencephalic stimulation and the effects of ECT in endogenous depression. Br J Psychiatr 1976; 129: 482-485. 5. Barker AT, Freeston IL. Jalinous R. et al. Magnetic stimulation of the human brain and peripheral nervous system-an introduction and the results of initial clinical evaluation Neurosurgery 1987; 20: 100-109. 6. Geddes LA. Bourland JD. Tissue stimulation - theoretical consideration and practical applications. Med Biol Eng Comput 1985; 23: 131-137. 7. Geddes LA. Barker LE. The specific resistance of biological material - a compendium of data for the biomedical engineer and physiologist. Med Biol Eng 1967: 5: 271-293. 8. Creutzfeld OD. Epileptische
Entladungen.
In: Creutzfeld OD.
ed. Cortex cerebri - Leistung. strukturelle und funktionelle Organisation der Himrinde. Berlin: Springer, 1983: 154-159. 9. Geddes LA. Optimal stimulus duration for extracranial stimulation. Neurosurgery 1987: 20: 94-99.
cortical
IO. Ollendorf F. Beitrrige zur Elektrodynamik des Elektroschoks III. Der induktive &hock. Arch Electrotechnik 1964; 48: 421-444. 1 I. Dhuna A, Gates J. Pascual-Leone A. Transcranial stimulation in patients with epilepsy. Neurology 1067-1071.
magnetic 1991; 41:
12. Jalinous R. Technical and practical aspects of magnetic nerve stimulation. J. Clin Neurophysiol 1991; 8: 10-25. 13. Barker AT. An introduction to the basic principles of magnetic nerve stimulation. J Clin Neurophysiol 1991; 8: 26-37. 14. Bickford RG. Guidi M, Fortescue P et al. Magnetic stimulation
The modern technique of transcranial magnetic stimulation is able to transmit a sufficient electrical stimulus into the deep brain structures for activating them. The method of application and the form of tissue penetration are different in comparison with electrical stimulation. This inductive method is painless and does not cause a motor seizure. Yet, our calculations at this stage relate only to the theoretical stimtdus parameter. We expect that deep magnetic brain stimulation will be able to evoke the same therapeutic effects as electroconvulsive therapy but in a safer way.
*The name ‘rapid-rate’ is incorrectfrom the physical point of view. The electromagneticfrequencies between 0 and 300 Hz are called extreme/y lowfrequencies (ELF).]
of human peripheral nerve and brain - response enhancement by combined magnetoelectrical technique. Neurosurgery 1987; 20: 1IO-I 16. 15. Geddes LA, Bourland JD. Fundamentals of eddy-current (magnetic) stimulation. In: Chokroverty S. ed. Magnetic stimulation in clinical neurophysiology. Stoneham: Butterworths. 1990: 33-43. 16. Cohen LG. Roth BJ, Nilsson J et al. Effects of coil design on delivery of focal magnetic stimulation - technical considerations. EEG Clin Neurophysiol 1990: 75: 350-357 17. Fortescue P, Bickford R. The design of electromagnetic stimulators, In: Chokroverty S. ed. Magnetic stimulation in clinical neurophysiology. Stoneham: Butterworths. 1990: 45-54. 18. Epstein CM, Schwartzberg DG, Davey KR et al. Localizing the site of magnetic brain stimulation in humans. Neurology 1990: 40: 666-670. 19. Meyer BU, Britton TC. Kloten H et al. Coil placement in magnetic brain stimulation related to skull and brain anatomy.
74
MEDICAL HYPOTHESES Clin Neurophysiol 1991; 81: 238-239.
EEG Clin Neurophysiol 1991; 81: 38-46. 20. Grandori F. Intracranial electric fields during transcranial brain stimulation - modelling and stimulation. Marsden CD, eds.
Non-invasive
In: Rossini PM,
stimulation of brain and
spinal cord - fundamentals and clinical applications
New
York: Alan R Liss, 1988: 145-157. 21. Roth BJ, Saypol JM, Hallett M et al. A theoretical calculation of the electric field induced in the cortex during magnetic stimulation. EEG Clin Neurophysiol 1991: 81: 47-56. 22. Tofts PS, Branston NM.
The measurement of electric field and
the influence of surface charge in magnetic stimulation. EEG
23. Eichhom KFr, Amdt B, Claus D et al. Induktive Stimulation. Biomed Technik 1990; 35: 139.144. 24. lmich W.
Das Grundgesetz der Elektrostimulation.
Biomed
Technik 1989; 34: 158-167. 25. Pascual-Leone A, Gates JR, Dhuna AK.
Determination
of
language dominant hemisphere with rapid transcranial magnetic stimulation. Ann Neurol 1990; 28: 223. 26. Agnew WA, McCreery DB. Consideration for safety in the use of extracranial stimulation for motor evoked potentials. Neurosurgery 1987;20:
141-147.