Surgery – Diagnostic testing – Sensitivity to electric stimulus
Reexamination Certificate
2002-06-25
2004-12-14
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Sensitivity to electric stimulus
Reexamination Certificate
active
06830550
ABSTRACT:
STATEMENT OF RELATED APPLICATION(S)
The present application is related to European Patent Office Patent Applications EP-01300493.2 entitled “Stair Step Current CPT Measurement Method and Apparatus” filed on Jan. 19, 2001 in the European Patent Office and European Patent Office Patent Application EP-01305831.0 entitled “Stair Step Voltage Actuated Measurement Method and Apparatus” filed on Jun. 22, 2001 in the European Patent Office. Both of these patent applications are in the name of inventor James Lee Hedgecock and are commonly owned herewith.
FIELD OF THE INVENTION
The present invention relates to the field of medical science, and particularly although not exclusively to a method of and apparatus for utilizing bio-electric stimulation.
BACKGROUND OF THE INVENTION
It is known to perform experimental examination for identifying abnormalities in nerve fibers, by applying an electrical stimulation transcutaneously to a patient.
It is well documented that specific current signal frequencies will selectively stimulate distinct types of nerve fibers, for example 5 Hz selectively stimulates type C nerve fibers, 250 Hz stimulates type A-Delta nerve fibers, and 2000 Hz stimulates type A-Beta nerve fibers. This neuroselectivity of frequencies is exploited by a method employed to measure the lowest level of current intensity a subject can recognize with a transcutaneous electrical stimulus. This method is termed current perception threshold (CPT) diagnosis.
A major problem often encountered during CPT testing is that the cutaneous electrical resistance threshold (CERT), the current signal level required before conduction through skin can occur, may be greater than the current perception threshold, the current at which a patient recognizes that a nerve has been stimulated. In subjects with a greater CERT than CPT the intensity of the diagnosis signal is turned up past the actual CPT without the subject recognizing the stimulus, since the current is not flowing through the skin to the nerve fiber. Once the intensity reaches the CERT and the current begins flowing, then the subject may report a false “high” CPT, which is actually the CERT being reached by the applied signal.
Previously, a “constant current” mechanism of bio-electric stimulation was developed in the 1950s and refined in the early 1980s. This later refinement is disclosed in U.S. Pat. No. 4,305,402 (Katim). Katim's constant current mechanism monitored a sine wave current and regulated it so that once the CERT had been reached the current was maintained automatically so as to sustain the signal intensity at sufficient level to allow a continuous flow of current, even though a manually operated intensity control may be turned to zero. Thereby, on the next measurement in a serial test at a same skin site on a patient, the current is not required again to breach the CERT, and the actual CPT can thereby be more accurately measured.
Katim's constant current mechanism works best with a sinusoidal wave form current. However, a sinusoidal current is quite difficult for a patient to recognize within a very narrow range of intensities. Due to the wide fluctuation in measurements obtained using a sinusoidal voltage, measurements must be averaged before meaningful analysis is possible.
A more recognizable stimulus is that of a modulated square wave signal, and in particular, a modulated square wave current. A square wave form current is used in the prior art Medi-DX 7000 CPT diagnostic device of Neuro-DX Associates Incorporated, 445 Dartmoor Street, Laguna Beach, Calif., 92651-1430. This device enables location and quantification of nerve pathology caused by injury, metabolic, and toxic exposures, and provides a screening method for patients prior to invasive examinations and procedures are undertaken. Results of up to 95% accuracy in the detection and quantification of nerve pathology are achievable.
The U.S. Pat. No. 6,029,090 (Herbst) discloses a multi-functional electrical stimulation system having a variety of wave forms including a sine, saw-toothed or square wave form. As with similar prior art stimulation devices Herbst's device provides for a wave form that may be customized in terms of pulse widths and pulse repetition rates. Similarly, GB 2123698 (Biostim) discloses a biological electrical stimulator capable of generating a variety of electrical stimulation wave forms being adjustable with regard to amplitude, pulse rate and burst.
Further teachings of the use of a square wave form being an electrical stimulation signal can be found in U.S. Pat. No. 5,797,854 (Hedgecock), U.S. Pat. No. 4,646,744 (Zion), U.S. Pat. No. 4,690,145 (Minnesota Mining) and U.S. Pat. No. 5,020,542 (Roosmann) however, none of the aforementioned references address the issue of maintaining a flow of current through the skin to the nerve fiber when attempting to measure and determine the CPT in subjects with a greater CERT.
Referring to
FIG. 1
herein, there is illustrated schematically in perspective view, the known Medi-DX 7000 current perception threshold diagnostic device. The device comprises a casing
100
containing drive electronics for performing current perception threshold measurements on a patient, the casing having a front panel
101
having a first electrical connector port
102
for connection of a probe device
103
; a second electrical connector port
104
for a defuse area electrical contact
105
; a set of frequency selector switches
106
-
108
respectively, for selecting test signals having fundamental frequencies corresponding to 5 Hz, 250 Hz and 2 kHz, for testing type C nerves, type A delta nerves, and A-beta nerves respectively; a current intensity control
109
in the form of a rotary dial, having a graduated scale around a circumference of the dial, the rotary dial capable of varying an output current signal in the range 0 to 10 mA between the probe
103
and second electrical contact
105
; a liquid crystal display device
110
used to calibrate the current amplitude during manufacture and during after sales service; and an on/off power switch
111
.
The usage of the device is known in the art, and is as follows:
A patient is placed into a relaxed position by a medical personnel. The second electrode contact
105
is placed upon a region of the patient's skin to make electrical contact. The second contact
105
is immersed in saline solution, to improve conductivity between the skin and a wide area contact region of the second contact
105
. The probe
103
, comprises a gold plated tubular contact
200
capable of receiving a cotton bud
203
, which is dipped in saline solution to improve conductivity between the gold contact
200
and a patient's skin as illustrated in
FIG. 2
herein. The probe
103
and second electrical contact
105
are placed at various positions around the patient's body, and a square wave electrical signal is passed between the probes through the patient's skin, in order to test various nerves around the patient's body as is known in the art.
To test a particular nerve, the medical operator places the probe and contact at specified positions on the persons body and starting from a zero reading, on the rotary current dial
109
corresponding to zero mean current and gradually increases manually the current by rotating the current control
109
, until the patient indicates that a sensation is felt. Due to variations in connection resistance between the cotton bud on the end of the probe
103
, and the patient's skin, the medical operator repeats this process 3 or more times for every measurement position, in order to reject spurious readings, and to take a set of readings which are consistent with each other, and which can be used to derive an average reading. Since the medical operator relies upon the patient's perception of sensation due to current, the patient may, either voluntarily or involuntarily, give a misleading indication of when a sensation is felt. For example a patient may, by the intonation of the human medical operator's voice, anti
Ritchie David B.
Thelen Reid & Priest LLP
Winakur Eric F.
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