Method and apparatus for altering neural tissue function

A method and apparatus for altering a function of neural tissue in a patient. An electromagnetic signal is applied to the neural tissue through an electrode. The electromagnetic signal has a frequency component above the physiological stimulation frequency range and an intensity sufficient to produce an alteration of the neural tissue, the alteration causing the patient to experience a reduction in pain, and a waveform that prevents lethal temperature elevation of the neural tissue during application of the electromagnetic signal to the neural tissue.

BACKGROUND OF THE INVENTION 
The use of radiofrequency (rf) generators and electrodes to be applied near 
or in neural tissue for pain relief or functional modification is well 
known. For instance, the RFG-3C RF Lesion Generator of Radionics, Inc., 
Burlington, Mass., and its associated electrodes enable placement of the 
electrode near neural tissue and heating of that tissue by rf resistive 
power dissipation of the generator power in the tissue. Thermal monitoring 
by thermo sensor in the electrode has been used to control the process. 
Heat lesions with tissue temperatures of 60 to 95 degrees Celsius 
(.degree.C.) are common. Tissue dies by heating at about 45 to 50.degree. 
C., so this process is a heat lesion generation and is designed to elevate 
the neural tissue above this lethal temperature threshold. Often, the 
procedure of heating above 45 to 50.degree. C. causes severe pain to the 
patient which is so unpleasant and frequently unbearable that local or 
general anesthetic is required during the heat procedure. Use of such 
anesthetics has a degree of undesired risk to the patient, and the 
destructive nature of and unpleasant side effects of the rf heat lesion 
are limitations of this technique, which is well known. Heat lesion 
generators typically use continuous wave rf generators with 
radiofrequencies of between 100 KiloHertz to several MegaHertz (viz. the 
rf generators of Radionics, Fischer, OWL, Elekta, Medtronic, Osypka, EPT 
companies). The theory and use of rf lesion generators and electrodes for 
pain and functional disorders is described in various papers; specifically 
see: (1) Cosman, et al. "Theoretical Aspects of Radiofrequency Lesions and 
the Dorsal Root Entry Zone." Neurosurg 15:945-950, 1984; and (2) Cosman E 
R and Cosman B J. "Methods of Making Nervous System Lesions," in Wilkins R 
H, Rengachary S S (eds): Neurosurgery. New York, McGraw-Hill, Vol. III, 
2490-2498, 1984. 
Neural stimulation is also now a common method of pain therapy. Stimulus 
generators with outputs of 0 to 10 volts (or zero to several milliamperes 
of current criteria are used) are typical. A variety of waveforms and 
pulse trains in the "physiologic" frequency ranges of 0 to about 300 Hertz 
are also typical. This output is delivered to electrodes placed near or in 
neural tissue on a temporary basis (acute electrode placement) or 
permanent basis (chronic electrode implants). Such stimulation can relieve 
pain, modify neural function, and treat movement disorders. Typically, the 
stimulation is sustained to have a long-term effect, i.e. usually when the 
stimulus is turned off, the pain will return or the therapeutic neural 
modification will cease after a short time (hours or days). Thus permanent 
implant electrodes and stimulators (battery or induction driven) is 
standard practice (viz. see the commercial systems by Medtronic, Inc., 
Minneapolis, Minn.), and the stimulus is usually sustained or repeated on 
an essentially continuous basis for years to suppress pain or to treat 
movement disorders (viz. Parkinsonism, bladder control, spasticity, etc.). 
Stimulators deliver regular pulse trains or repetitive bursts of pulses in 
the range of 0 to 200 Hertz (i.e., a physiologic range similar to the 
body's neural frequency pulse rates), so this method simulates or inhibits 
neural function at relatively low frequency. It does not seek to heat the 
neural tissue for destructive purposes as in high frequency technique. 
Chronically or permanently implanted stimulators often require battery 
changes or long-term maintenance and patient follow-up, which is expensive 
and inconvenient, often requiring repeated surgery. 
Electrosurgical generators have been in common use for decades cutting and 
coagulating tissue in surgery. They typically have a high frequency, high 
power generator connected to an electrode that delivers a high power 
output to explode tissue for tissue cutting and to cook, sear, and 
coagulate tissue to stop bleeding. Examples are the generators of Codman, 
Inc., Randolph Mass., Valley Labs, Inc., Boulder, Colo., and EMC 
Industries, Montrouge, France. Such generators have high frequency output 
waveforms which are either continuous waves or interrupted or modulated 
waves with power controls and duty cycles at high levels so that tissue at 
the electrode is shattered and macroscopically separated (in cutting mode) 
or heated to very high temperatures, often above cell boiling (100.degree. 
C.) and charring levels (in coagulation or cauterizing mode). The purpose 
of electrosurgery generators is surgical, not therapeutic, and accordingly 
their output controls, power range, duty cycle, waveforms, and monitoring 
is not designed for gentle, therapeutic, neuro-modulating, sub-lethal 
temperature application. Use of an electrosurgical unit requires local or 
general anesthetic because of its violent and high-temperature effect on 
tissues. 
SUMMARY OF THE INVENTION 
The present invention is directed to a modulated high frequency apparatus 
in conjunction with a signal applicator (for example an electrode or 
conductive plate or structure applied to the body) to modify neural 
function, the associated apparatus and method of use being functionally 
different from and having advantages over the rf heat lesioning systems, 
or the stimulation systems, and electrosurgical systems of the type 
described above. Pain relief or neural modification, for instance, can be 
achieved by the present invention system without average heating of tissue 
above 45 to 50.degree. C., without stimulating at frequencies in the range 
of 0 to about 300 Hertz and without burning or cauterizing tissue. Thus as 
one advantage of the present invention, painful rf lesioning episodes at 
high lesion temperatures can be avoided and the need for chronic 
stimulation can be circumvented. 
For example, by using an rf waveform output connected to an electrode 
inserted into the body near or in neural tissue, and by interrupting the 
rf waveform with bursts of rf power with interposed periods of off-time, a 
pain relieving effect or other neural modulating effect is accomplished, 
but the tissue temperature may not on average exceed approximately 
45.degree. C. This avoids the painful heat lesions associated with the 
typical rf lesions which involve tissue temperatures at a region near the 
electrode of substantially greater than 45.degree. C. The modulated rf 
system can be used painlessly and easily, avoiding usual discomforts of 
standard rf heating procedures, yet relief of the pain or the neural 
disfunction (such as for example motor disfunction, spasticity, 
Parkinsonism, tremors, mood disorders, incontinence, etc.) can be long 
lasting using the novel system of the present invention, giving results in 
many cases that are comparable to those of rf heat lesions done at much 
higher temperatures. Some applications of this invention may include such 
examples as relief of back, head, and facial pain by procedures such as 
dorsal root ganglion or trigeminal ganglion treatments, spinal cord 
application for relief of intractable pain, spasticity, or motor control, 
treatment of the basal ganglia in the brain for relief of Parkinsonism, 
loss of motor control, tremors, or intractible pain. This pain relief or 
control or elimination of motor or other neural disfunction can be 
comparable if not more effective than long-term stimulators with implanted 
electrodes, thus avoiding the need for permanent implants, expensive 
implanted devices and circuits, battery changes, involving repeated 
surgery and expense, and repeated application of stimulation energy over 
long periods (months and years). The pain relief or neural modification 
can be accomplished by the present invention in a non-violent, painless 
way, avoiding average tissue temperature elevations into the lethal range 
and violent macroscope tissue separations, and thus the present invention 
is opposite to the objectives, systems, and methods involved in 
electrosurgical systems. 
Forms of the modulated frequency generator and output waveforms are 
disclosed herein in various embodiments. Specific embodiments with 
temperature monitors and thermal sensing electrodes are disclosed which 
are suited to control the modulated system and its use.

DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, an illustration of the present invention is shown in 
block diagram and schematic elements. An electrode with uninsulated 
conductive surface 1 (for example a conductive tip end) is in proximity to 
a region of neural tissue NT (viz. illustrated schematically by the dashed 
boundary). The electrode has an insulated shaft 2 and connection or hub 
portion 3, inside of which there can be electric connections to surface 1. 
Connection 10 electrically connects to the surface 1 through the electrode 
shaft 2 and to electronic supply units 4 and 5 (which are shown outside 
the body, but which may be miniaturized and implanted inside the body). 
Element 5 is a signal generator of signal output (viz., voltage, current, 
or power), and element 4 is a modulator to modulate (for example the 
amplitude of) the high frequency output from 4. The electromagnetic output 
from 4 and 5 is connected to electrode surface 1, and therefore is 
conductively exposed to tissue NT. As an example, element 5 can take the 
form of an rf power source with a continuous wave output (viz. for 
example, similar to the model RFG-3C generator of Radionics, Inc., 
Burlington, Mass.). Element 4 is a pulse modulation unit which switches on 
and off the rf output from 5 at a designed rate and duty cycle. RF output 
generators or supplies and modulation circuits are known in high frequency 
technique (viz. Radio Engineering by Fredereck E. Terman, McGraw-Hill, New 
York, 1947, 3rd Edition). Further shown is a temperature monitoring 
element or circuit 6 which connects by cable 11 to the electrode and to a 
thermal sensor (viz. thermistor or thermocouple) inside the electrode 
applicator or conductive tip 1 to measure the temperature of the tissue NT 
near the tip. (Such thermal sensing circuits and electrodes are 
illustrated by the Model RFG-3C and associated thermal-sensing rf 
electrodes of Radionics, Inc., Burlington, Mass.). Further, reference 
electrode 8 is shown in electric contact to the patient's body B with 
connection wire 12 to generator 5 so as to provide a circuit for return 
current from electrode applicator 1 through the patient B (such reference 
electrodes are common with rf lesion generators; see Cosman, et al., 
1984). Element 7 is a switch or circuit breaker which illustrates that 
such a return circuit could be opened to limit such direct return current, 
and limit such current to inductive or reactive current characteristic of 
time varying circuits such as rf circuits. 
In operation, the voltage or current output from generator 4 and modulator 
5 are impressed upon tissue NT, which may be neural tissue (viz. spinal 
nerves or roots, spinal cord, brain, etc.) or tissue near neural tissue. 
In accordance with the present invention, such electromagnetic output can 
cause energy deposition, electric field effects, and/or electromagnetic 
field effects on the nerve cells in the tissue NT so as to modify or 
destroy the function of such nerve cells. For example, such modification 
of neural function may include reduction or elimination of pain syndromes 
(such as spinal facet, mechanical back pain, facial pain) in some cases, 
alleviating motor disfunction, spasticity, Parkinsonism, etc., epilepsy or 
mood disorders. Because the rf output from 4 is modulated by element 5, 
its percent on-time is reduced so that sustained heating of tissue NT is 
reduced, yet the neural therapeutic effects of the impressed rf voltages 
and currents on the neural tissue NT are enough to produce the pain 
reducing result. The generator 5 can have a power, voltage, or current 
output control 5A (as on the Radionics Model RFG-3C rf generator) to 
increase or decrease the output power magnitude or modulated duty cycle to 
prevent excessive heating of tissue NT or to grade the level of pain 
interruption as needed clinically. Output control 5A may be a knob which 
can raise or lower the output in a smooth, verniated way, or it can be an 
automatic power control with feedback circuits. In this regard, 
temperature monitor 6 can provide the operator with the average 
temperature of tissue NT near electrode tip 1 to interactively prevent 
temperatures near tip 1 to exceed the range of approximately 45.degree. C. 
(on average thermally lethal to tissue NT), and thus to avoid the higher 
temperature ranges for the usual heat lesioning procedures described 
above. For example, 6 may have feedback circuitry to change the modulation 
duty cycle (by, for example, longer or shorter on-times) to hold the 
temperature near tissue NT to below a set value (viz. 40 to 45.degree. 
C.), illustrated by the feedback line 14 in FIG. 1. In addition, the high 
frequency waveform from the generator 5 can be free from substantial 
components in the 0 to about 300 to 400 Hertz range (which is much lower 
than radiofrequencies), and this will avoid the stimulation effects that 
are typical for stimulator system applications as described above. 
As an example of a modulated rf waveform that accommodates the system of 
the present invention, FIG. 2 shows schematically a high frequency output 
of voltage amplitude V and of burst duration T1 between which on-time 
bursts there are illustrated periods of zero voltage of duration T2. 
During the on-time T1, the rf signal output is oscillatory with time 
period T3 between maximum voltages V. The reciprocal of T3 is proportional 
to the value of the radiofrequency (viz., 1 Mega Hertz rf output 
corresponds to T3=1 microsecond). This is an interrupted or bursting type 
of modulated high frequency waveform. During the high frequency on-time 
T1, the voltage can oscillate between plus and minus its maximum value V. 
Accordingly, an electric field is produced around the region of the 
electrode applicator (as for instance the exposed electrode tip 1 in FIG. 
1). The electric field has a modifying, or pain-relieving, or 
neural-altering effect on the tissue near or among the nerve cells and 
fibers. Pain relief and neural modification can accordingly be 
accomplished by this high frequency bursting voltage and accompanying 
electromagnetic field, and also accompanying current among the neural and 
tissue cells. During the off period, there is minimal or no voltage (i.e. 
V=0 at the electrode applicator), and thus no electric field or electric 
currents in and among the neural tissue. During that period, no heat 
deposition is present. Thus, over the entire cycle, from on period T1 
through off period T2, the energy deposition, on average, can be adjusted 
so that there is not excessive heating, on average, around the electrode 
applicator. Thus, the usual mechanism of continuous on-time high frequency 
voltage and current, as in previous heat lesion techniques, is avoided, 
and therefore the achievement of high average temperatures near or around 
the applicator tip may be eliminated by the present invention. The usual 
heat lesion process in which tissue temperatures, on average, exceed 
45.degree. can be avoided. In many instances, this avoidance of high 
temperature domains due to high average heat dissipation of the 
radiofrequency power will prevent acute pain of the process to the 
patient. By having the interrupted waveform, as in FIG. 2, the average 
power is thereby reduced and the average heating around the electrode tip 
or applicator is accordingly reduced. However, substantial voltages V (or 
currents) can still be sustained during the on period with their resulting 
therapeutic effect on the tissue. 
To give a representative example of values for parameters in an interrupted 
high frequency waveform as in FIG. 2, the overall pattern of the waveform 
may have a total period of one second, meaning that the sum of T1+T2=1 
second. The on period T1 can be 20 milliseconds, and the off period T2, 
therefore, can be 980 milliseconds. Voltages V in the range of 10 to 30 
volts or more can be used. It can be used with a pain relieving effect in 
certain tissues. Average tip temperature around an electrode tip such as 
the exposed tip element 1 in FIG. 1 can be maintained at or below 
40.degree. C., well below thermo-lethal levels. Electrodes with diameters 
of 1 or 2 mm shaft (for example the shaft 2 of a cannula in FIG. 1), with 
an exposed tip of 1 to 10 mm (such as the tip element 1 in FIG. 1) can be 
used and the electrode can be inserted in around neural structures in the 
brain or peripheral nerves or peripheral nerve ganglia to accomplish pain 
relief or other neurological alteration. Variation of these parameters can 
be made with similar therapeutic effect, and various geometries of 
conductive electrode or applicator can be effective. Illustrations of a 
wide variety of such electrodes are illustrated by the product line of 
Radionics, Inc., Burlington, Mass. Pointed or sharpened electrodes (such 
as illustrated schematically by electrode tip 1 in FIG. 1) are useful for 
penetration of the electrode through the skin to the target neural tissue 
site, and electric or current fields of higher intensity will be present 
at a sharpened point for a given applied voltage (such as V in FIG. 2), 
which will be effective in altering neural function. 
FIG. 3 shows a variation of modulated high frequency waveform which 
accomplishes high peak voltage swings with reduced average power deposited 
in tissue. The baseline voltage may be put at zero (viz. V=0), shown by 
dashed line 24. The solid line 21 represents the actual waveform, which 
has rapid oscillations at the radiofrequency and has an overall enveloped, 
represented by dashed line 20, that has high points and low points with an 
approximate on time T1 and a time period between envelope of modulation 
maxima T2. T1, again, could be a percentage on time of 2 percent (as 
described above for 20 milliseconds on time out of 1 second total), and 
this on time T1 may vary considerably while still maintaining substantial 
off time so as to prevent overall average high temperature heating (as in 
the usual rf heat lesion systems). Such a modulation envelope (as dashed 
line 20) can be achieved by using a modulated signal generator that varies 
the input or output gain of a high frequency generator (as element 5 in 
FIG. 1) so as to achieve such a waveform as in FIG. 3. In such circuitry, 
which is commonly used in pulse generation techniques, low frequency 
filtering or selection of modulation parameters can avoid stimulation 
voltage or current components at the physiologic range of 0 to 300 Hertz 
so that unpleasant stimulative effects can be avoided during the 
therapeutic intermittent high frequency lesion process. 
FIG. 4 shows yet another embodiment of an interrupted high frequency 
waveform in accordance with the present invention. Here there is a 
non-periodic variation of the voltage represented by the excursions of the 
voltage V represented by excursions on a vertical axis. The maxima point 
25 can occur at random positions in time. The time difference between 
maxima can also vary in an irregular or even random way. This waveform may 
have no repeating or periodic structure but may be analogous to high 
frequency noise with random amplitudes, peaks, zero crossings, and carrier 
high frequencies. Such a waveform can be generated by random noise 
generators, spark gap signals, or other noisy signals that are known in 
the field of signal generation (viz. Radio Engineering, cited above). 
Filtering can be applied in the wave generator and power amplifier so that 
lower frequencies in the physiologic range will not be present to give 
undesirable stimulation effects. 
FIG. 5 shows yet another possible high frequency waveform of interrupted, 
repeated bipolar pulses with frequency repetitive T3 for example the 
physiologic stimulation frequency range (i.e., 0 to about 300 Hertz). The 
pulse on-time may be low enough so that the power deposition can be kept 
low enough to prevent heating, and yet the peak voltage V is enough to 
alter the neural function. 
Variations of such waveforms are possible with the same intermittent high 
frequency effect for pain on neurological modification. For instance, a 
baseline V=0 may not pertain and a slowly varying baseline of non-zero 
value can be used. The time average of the signal need not be zero. The on 
and off switching of a high frequency signal such as in FIG. 2 can be done 
at a non-periodic or non-regular, repeating rate so that, on average, the 
polarization effects in the tissue are still maintained at a low level. 
The average power deposition can still be maintained at a low level with 
non-periodic, interrupted high frequency waveforms. The high frequency 
carrier frequency (i.e. represented by the inverse of time T3 in FIG. 2 
and FIG. 3) may also be non-constant. Varying or combined or superposed 
high frequency waveforms can be used as the carriers, and these combined 
or composite high frequency waveforms can be interrupted or modulated in 
accordance with the present system and invention. Pulse waveforms with 
high frequency carriers can be shaped in a variety of ways, for example 
with fast rising leading edges and slow or falling off or exponential 
trailing edges. The signal generator waveform can have a peak intensity 
which is much higher than the average or RMS intensity to yield a high 
electromagnetic field or current density on the neural tissue while 
maintaining the average power deposition in the tissue at a sufficiently 
low level to prevent heating above lethal tissue temperatures (viz. 40 to 
50.degree. C.). 
FIG. 6 shows a block diagram of a system for generating modulated high 
frequency signals (similar but in more detail to the block element of high 
frequency generator 5 and modulator 4 of FIG. 1). 
Element 50 represents a signal generator which may create a high frequency 
signal of periodic or non-periodic frequency. This has input to element 
31, which is a filter system which selectively filters out frequencies 
that could cause unpleasant, undesired, or damaging physiological signals. 
The signal is then fed into element 33, which is a waveform shaping 
circuit, and will shape the waveform input from element 32, which provides 
amplified modulation and/or frequency modulation and/or phase modulation 
control. Circuits of this type can be found, for instance in Radio 
Engineering by Terman (cited above). Additional waveform shaping can be 
done by element 40 and 41, which can control the amplitude of waveform 
and/or the duty cycle of the waveform, respectively. This resultant signal 
is then fed into a power amplifier represented of element 34. This is a 
wide band amplifier used to increase the signal to power levels 
appropriate for clinical use. This energy is then delivered to the patient 
via an electrode depicted as element 35. 
A temperature sensor or plurality of temperature sensors, represented by 
element 36, can also be placed and connected in proximity to this 
electrode so as to insure that the temperature does not exceed desired 
limits. This temperature sensor signal is fed through element 37, which is 
a special filter module used to eliminate high frequency components, and 
thus not to contaminate the low-level temperature signals. 
The temperature signal is fed to element 38, which is a standard 
temperature measuring unit that converts the temperature signal into a 
signal that can be used to display temperature and/or to control, in a 
feedback manner, either the emplitude and/or the duty cycle of the high 
frequency waveform. In this way, power delivery can be regulated to 
maintain a given set temperature. This flow is represented by element 39, 
which is simply a feedback control device. The dotted lines from element 
39 to elements 40 and 41 represent a feedback connection that could either 
be electronic and/or mechanical. It could also simply be a person 
operating these controls manually, based on the visual display of 
temperature, as for example on a meter or graphic display readout 42. 
FIG. 7 illustrates the use of a conductive plate(s) 1A and 1B to apply the 
electromagnetic signal from generator 5 to the patient's body, in this 
case, generally the head B. The conductive plate 1A can be in contact with 
the surface of the head or in contact with neural tissue within the skull. 
Reference electrode 8 operates in the same manner as previously described 
in connection with FIG. 1. 
FIG. 8 illustrates the use of an electrode with an uninsulated conductive 
surface in proximity to or in direct contact with a region of neural 
tissue with the skull B of the patient's body. As in FIG. 1, the electrode 
has an insulated shaft 2. The electrode is connected to signal generator 5 
with a reference electrode 8 to provide a return circuit. Again, as 
described in connection with FIG. 1. 
As was explained with respect to the disclosed embodiments, many variations 
of circuit design, modulated high frequency waveforms, electrode 
applicators, electrode cannulas will be appreciated by those skilled in 
the art. For example, electrodes or electrode applicators are practical, 
including tubular shapes, square shafts, flat electrodes, area electrodes, 
multiple electrodes, arrays of electrodes, electrodes with side outlets or 
side-issued tips, electrodes with broad or expandable or conformal tips, 
electrodes that can be implanted in various portions of the brain, spinal 
cord, interfecal space, interstitial or ventricular spaces, nerve ganglia 
can be considered within the system of the present invention. 
The frequency range for the so-called high frequency waveforms, as shown 
for instance in FIGS. 2, 3, 4, and 5 can be used over a wide range. For 
example, the "high frequency" characteristic of 1/T3, which may be only 
one of many high frequency components, can be above the so-called 
physiologic stimulation frequency range of 0 to about 300 Hertz. This high 
frequency may also range up into the radiofrequency or microwave range 
(viz. 50 Kilo Hertz to many Mega Hertz). 
Mixtures of frequencies can be done as discussed above. These could be 
admixtures of "high frequencies" (above the physiologic stimulation range 
(say 0 to 300 Hertz) and lower frequencies (within that stimulation range 
of say 0 to 300 Hertz). Thus one skilled in the art could have both 
modulated high frequency and stimulation frequencies for various clinical 
effects, such as stimulation blockage of pain while neural modification is 
being applied according to the present invention. 
In view of these considerations, as will be appreciated by persons skilled 
in the art, implementations and systems should be considered broadly and 
with reference to the claims set forth below.