Note: Descriptions are shown in the official language in which they were submitted.
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THERMOSTATICALLY-CONTROT T~FD MICROWAVE
CYCLODESTRUCIION
AS A TREATMENT FOR GLAUCOMA
BACKGROUND
As known in the art, several different cyclodestruction
10 procedures (i.e., procedures for destroying the ciliary body) have
been developed or proposed for treating glaucoma. The clinical
standard cyclodestruction procedure employs cryotherapy. Other
known cyclodestruction procedures include therapeutic
ultrasound and Neodymium:Yag cyclophotocoagulation. However,
15 all of these known cyclodestruction procedures have
demonstrated different negative tissue reactions.
Cryotherapy has been characterized by discomfort and
edema, therapeutic sound by induced scleral changes, and
Neodymium:Yag cyclophotocoagulation has been shown to cause
2 0 characteristic spot-like conjunctival lesions. Other less specific
morbidities have included corneal-scleral thinning, hyphema,
cataract, vitritis, retinal detachment, cystoid macula edema, and
hypotony. These potential complications have defined
cyclodestruction procedures as a last treatment for refractory
2 5 cases.
SUMMARY OF THE INVENTION
The present invention is directed to a microwave
3 0 cyclodestruction procedure which avoids negative tissue reactions
and minimi7es potential complications. The microwaves are
applied to the ciliary body by a novel miniature microwave
applicator placed in contact with a spot on the outer surface of the
sclera. The miniature microwave applicator incorporates a
3 5 thermocouple on its anterior radiating surface, so that the
thermocouple also contacts the spot on the outer surface of the
sclera. The thermocouple thermostatically controls the output of
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the microwave generator energizing the applicator to ensure that
the temperature of the scleral tissue never rises to an unsafe
level. Because scleral tissue absorbs less microwave energy, while
ciliary-body tissue absorbs more microwave energy, most of the
5 applied applied microwave energy penetrates through the sclera
to, and is absorbed by, the underlying ciliary body. This raises the
temperature of the ciliary body to the point at which some
cyclodestruction occurs. This process may be repeated at several
separate spots of the sclera to complete the microwave
l 0 cyclodestruction procedure.
BRIEF DESCRIPTION OF THE DRAVVINGS
FIGURE 1 is a functional block diagram showing the
l 5 relationship between a miniature microwave applicator
incorporating a thermocouple (which may take the form shown in
FIGURE 2) and a thermostatically-controlled microwave generator
for energizing the applicator;
FIGURE 2 illustrates the physical form of a preferred
2 0 embodiment of the miniature microwave applicator incorporating
a thermocouple that is used for microwave cyclodestruction;
FIGURE 3 is a first chart useful in explaining the principles
of the present invention; and
FIGURE 4 is a second chart useful in explaining the
2 5 principles of the present invention.
DESCRIPIION OF THE PREFERRED EMBODIMENT
Referring to FIGURE l, the microwave output of
3 0 thermostatically-controlled microwave generator 100 is applied as
an input to miniature microwave applicator incorporating a
thermocouple 102 (which may take the form shown in FIGURE 2)
over a suitable microwave transmission line 104. The
thermocouple of applicator 102 generates a control signal having a
3 5 value which is a function of the temperature at the microwave
radiating aperture of applicator 102. This control signal, which is
fed back to microwave generator 100 over connection 106 to
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thermostatically control microwave generator 100, prevents
microwave energy from being forwarded from the output of
microwave generator 100 over transmission line 104 to the input
of applicator 102 whenever the temperature of the thermocouple
5 rises to a certain preselected temperature.
Referring to FIGURE 2, applicator 102 comprises thin-wall
metal dielectric-filled waveguide.200. In practice, waveguide 200
is fabricated from a block of ceramic material that exhibits a high
dielectric constant (e.g., 85) that is machined to the proper size
1 0 and shape. The longitudinal surface of this properly sized and
shaped ceramic material is first electrolessly plated with metal
and then electroplated with metal to produce the thin metal wall
of waveguide 200. More specifically, the length of waveguide 200
is preferably about one inch; the width of waveguide 200 is
1 5 preferably about 0.2 inch (i.e., 200 mils); and the thickness of
waveguide 200 preferably tapers from about 0.1 inch (i.e., 100
mils) at at its posterior end, to which microwave input connector
202 is attached, to about 0.15 inch (i.e., 150 mils) at its anterior
end, which forms dielectric radiating aperture 204. Thus, the area
2 0 of dielectric radiating aperture 204 is quite small, being only 0.03
square lnch.
As shown in FIGURE 2, the dielectric anterior surface, which
is preferably flat, has a groove 206 machined therein in which
thermocouple 208 is fixedly secured substantially at the center
2 5 thereof. The thickness of the thermocouple is preferably sufficient
to protrude very slightly from the flat dielectric anterior surface.
Thermocouple output wires 210, connected to thermocouple 208,
extend through the length of groove 206 to the outside of
waveguide 200, as shown in FIGURE 2. Thermocouple output wires
30 210 constitute feedback connection 106 of FIGURE 1.
The therapeutic purpose of applicator 102 in the treatment
of glaucoma is to apply sufficient microwave energy to the ciliary
body to effect cyclodestruction without creating collateral eye
damage. This is accomplished by first positioning 0.03 square inch
3 5 dielectric radiating aperture 204 in contact with the anterior
surface of applicator 102 in contact with a 0.03 square inch spot
on the outer surface of the sclera which overlies the ciliary body
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(e.g., a spot displaced about 2 millimeters beyond the outer edge
of the iris). This inherently places thermocouple 208 in in contact
with this spot. The applicator is energized with microwave energy
having a frequency (e.g. 5,000 to 6,000 MHz) which readily
5 penetrates the thickness of the scleral tissue with little absorption
and reaches a corresponding spot of the underlying ciliary body,
where it is readily absorbed. The reason for this is shown by the
FIGURE 3 chart, which will be discussed below.
The microwave energy is applied to the spot for a given
l 0 time (e.g., one minute) which is a sufficient time for the irradiated
spot of the ciliary body to be heated to a high enough temperature
to cause cyclodestruction, while the sclera itself is never heated
enough to raise its temperature sufficiently high to result in
damage thereto. (The FIGURE 4 chart, discussed in more detail
l 5 below, indicates the the relationship between temperature and
time of heating duration that results in damage to different types
of m~mm~lian tissue.) In any event, the thermostatic control of
microwave generator lO0 is set so that the radiated microwave
energy is cut off whenever the temperature of thermocouple 208
2 0 rises to a preselected therapeutic temperature which is below the
temperature at which scleral damage occurs. Thus, the continuous
monitoring of sclera-spot surface temperature by thermocouple
208 maintains the temperature substantially constant at the
therapeutic temperature and also ensures that the operation is
2 5 fail-safe.
In order to complete the microwave cyclodestruction
procedure, the above-described process is applied sequentially to
each of several (e.g., five) displaced spots on the outer surface of
the sclera. More specifically, after the above-described process
3 0 with respect to one of the several displaced spots is completed,
the applicator is displaced by about the width of applicator 102
(200 mils) to another similar scleral spot overlying the ciliary
body. Thus, the resulting several displaced spots tend to lie on the
circumference of a circle having a radius about 2 millimeters
3 5 larger than that of the iris.
Referring to the FIGURE 3 chart, there is shown the
penetration depth as a function of frequency at which l /e (where
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e is the base of natural logarithm) of incident microwave energy is
absorbed by low-water-content human tissue and by high-water-
content human tissue, respectively. It is apparent from this chart
that low-water-content human tissue is much more microwave
5 absorbent than high-water-content human tissue. Scleral human
tissue is low-water-content human tissue and ciliary-body tissue
is high-water-content human tissue. Thus, most of the applied
microwave energy merely passes through the thickness of the
scleral tissue to be then highly absorbed by the underlying
1 0 ciliary-body tissue, thereby preferentially heating the underlying
ciliary-body tissue.
Referring to the FIGURE 4 chart, there is shown
temperature- time duration thresholds for damage to occur in
different types of mammalian tissue. As indicated by the wide
1 5 band of the FIGURE 4 chart, for a given heating duration some
types of tissue (e.g., corneal tissue) are damaged substantially less
than others. It has been found that both corneal and scleral tissue
are not damaged by, and tolerate well, being heated to a
temperature up to about 50~ C for at least one minute. Therefore,
2 0 the aforesaid fail-safe thermostatically-controlled therapeutic
temperature for a heating duration of scleral tissue for one minute
certainly may be set at 50~ C, and perhaps even somewhat higher.
Returning to FIGURE 2, the high dielectric constant of the
dielectric filling of waveguide 200 of applicator 102 serves two
2 5 important purposes. First, by reducing the microwave wavelength
traveling therein for a given microwave frequency, the size of
applicator 102 for transporting that given microwave frequency
may be reduced (i.e., miniaturized). Second, the high dielectric
constant of the dielectric filling of waveguide 200 more nearly
3 0 matches the high dielectric constant of the high-water content
ciliary body, and, therefore, enhances microwave power transfer
from dielectric radiating aperture 204 to the ciliary body. Further,
for microwave power transfer purposes, the impedance at the
microwave input to applicator 102 at the posterior end of
3 5 waveguide 200 should closely match that presented by
transmission line 104, and the impedance at the microwave
output from applicator 102 at dielectric radiating aperture 204
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(located at the anterior end of waveguide 200) should closely
match that presented by the scleral tissue with which it is in
contact. The proper impedance matching at both the posterior and
anterior ends of waveguide 200 is achieved by the above-
discussed tapering of the thickness of waveguide 200 from 100
mils at its posterior end to 150 mils at its anterior end.
The above-described controlled microwave cyclodestruction
procedure has been tested experimentally in the treatment of
induced glaucoma in the eyes of rabbits. Microwave induced
cyclodestruction was successful in reducing the intraocular
pressure in all treated glaucomatous eyes for a 4 week duration.
Two additional glaucomatous eyes were left untreated, served as
controls, and were noted to have persistently elevated intraocular
pressures. Then 6 additional eyes were subjected to an equivalent
treatment (50~C x 1 min. x 5 applications) which resulted in
approximately 180~ of heat treatment just posterior to the
corneal-scleral limbus. These specimens were evaluated by light
microscopy at time 0, 24 hours, and at 7 days after treatment.
Clinical and histopathologic evaluations suggested that
2 0 microwave thermotherapy (delivered under thermometry control)
allowed for chorioretinal/ciliary body destruction which resulted
in reductions of intraocular pressure in glaucomatous eyes.
SuBsTlTuTE S~
