This invention is directed to the providing of a transcutaneous nerve stimulator which is designed to be utilized in T.E.N.S. (Transcutaneous Electrical Nerve Stimulation) therapy. T.E.N.S. therapy is based on a non-invasive, non-narcotic concept of pain management which is non-addictive, is not subject to abuse, and does not interact with drugs. T.E.N.S. therapy has already proven to be an effective modality in treating the organic pain problems associated with the following conditions: chronic lumbar and cervical strains or sprains, degenerating disc disease, degenerative arthritic disease, neuropathies, neuralgias, post-lumbar laminectomy syndrome, post-thoracotomy syndrome, bursitis, postphlebitis syndrome, phantom limb syndrome, and tension and migraine headaches.
Early attempts to suppress organic pain and other neurophysical effects utilizing electrical stimulation occurred as early as about 2,000 years ago when it was discovered that gout apparently cuold be successfully treated by placing the diseased extremities in a tub of water filled with electric eels. Later, headaches were treated using a similar approach. A detailed, scientific investigation was finally conducted by Professor Galvani of the University of Bologna, which investigation is credited with ultimately leading to the development in the 1800's of electrical equipment for suppression of organic pain.
The earliest therapeutic devices utilizing electrical simulation for the most part featured a constant amplitude and rate. Examples of the early art are Benz, U.S. Pat. No. 646,793; Raymond et. al., U.S. Pat. No. 872,148; Tibbals, U.S. Pat. No. 1,059,090; and Call, U.S. Pat. No. 1,908,688. A major problem with electrical stimulation therapy was, and still is, accommodation, whereby the nerve being stimulated in effect accommodates itself over time to the electrical charge, such that the effectiveness of the treatment is diminished.
It took scientists a long time to discover, and attempt to address the problem. Nemec, U.S. Pat. No. 2,622,601; DiPerma, U.S. Pat. No. 2,624,342; and Gratzl, U.S. Pat. No. 2,771,554 all disclose electrotherapeutic devices with at least one including means to vary the rate, amplitude or pulse width of the generated electrical pulse. However, merely being able to change either the rate, amplitude or pulse width still resulted in the problem of accommodation occurring, unless an individual manually adjusted the controls prior to the occurrence of accommodation. The process was both labor intensive and inefficient, with respect to the quality of the therapy, since maximum pain relief was not being provided.
In 1967, a Dr. Sweet at Massachusetts General Hospital developed the first T.E.N.S. unit. The effectiveness of T.E.N.S. therapy is based on its incorporating two major pain control theories. Under the so-called Gate Control Theory, pain can be inhibited and suppressed by "closing the gate" on pain signals as such signals arrive at Central Nervous System centers. This theory postulates that by providing electrical stimulation of a sufficiently high amplitude, the electrical signals race up large myelinated fibers faster than the pain signals travel up smaller myelinated or unmyelinated fibers. The neutral impulses transmitting pain information to the brain thus become interrupted, and since the brain fails to receive the pain signals, no pain is perceived. The other theory incorporated in T.E.N.S. units is the Endorphin Theory, also known as the Endogenous Opiate Theory. This theory postulates that the sustained input of T.E.N.S. signals triggers the release of naturally occurring pain making endorphins and enkephalins (morphine-like substances). These natural substances seemingly block pain signals by a mechanism similar to conventional drug therapy, and inhibit pain information from reaching the brain.
However, T.E.N.S. units, like all electrotherapeutic devices, have suffered problems with accommodation. For example, Geerling, U.S. Pat. No. 4,019,519 issued in 1977, disclosed a unit having only its amplitude adjustable. Miller, U.S. Pat. No. 4,084,595 issued in 1978, disclosed a unit having its rate, amplitude, and pulse width all independently, manually adjustable. However, even this advancement resulted in a less than efficient treatment of the problem of accommodation, since either the therapist or the patient had to, in theory, repeatedly adjust the controls in an attempt to avoid accommodation.
Although variation enables one to deal with the problem of accommodation, pain relief is sacrificed. This is in part due to the interaction between amplitude and pulse width. There is a clinical correlation between amplitude and pulse width with regard to the efficacy of the stimulus. As one shortens the duration of a pulse, its amplitude must be increased to maintain the efficacy of the stimulus. This relationship when plotted graphically is known as a strength-duration curve. Thus not only must the ideal T.E.N.S. units have adjustable amplitude and pulse width, but it must also be able to modulate those values in such a way as to approximate the A-gamma-motor strength-duration curve.
In addition to amplitude and pulse width approximating the strenght-duration curve, the rate of the pulses must vary so as to eliminate any potential for accommodation. This explains the short-comings in Reiner, U.S. Pat. No. 2,808,826 which disclosed a unit which permitted instantaneous changes in pulse width and amplitude to two pre-set points along the strength-duration curve, and Maurer, U.S. Pat. No. 4,340,063 which disclosed a unit having its amplitude modulate in response to modulations in pulse width so as to approximate a portion of the strength-duration curve. The rate in Maurer was adjustable, but only to the extent taught by Miller, such that the problem with accommodation still existed.