TENS
UNDERSTANDING TRANSCUTANEOUS ELECTRICAL NEURAL STIMULATION
Introduction
If you have recently attended an LVI program or read the LVI Dental Vision magazine it is obvious things are rapidly changing in our profession. The nomenclature and technology you will be exposed to at LVI may be foreign to even the most recent dental school graduate. You will not be at LVI long before you hear Bill Dickerson comment on the need to TENS the patient. TENS? Knowing the esthetic reputation of LVI you immediately imagine an esthetic rating system, as in Bo Derek. Not this time. Bill is referring to Transcutaneous Neural Electrical Stimulation (TENS), a modality used to relax the masticatory muscles prior to diagnosis and treatment. TENS is the procedure of applying a controlled electrical pulse to the nervous system by passing electricity through the skin via electrodes placed on the skin. This is the first of a three part series design to give LVI Vision readers a better understanding of the scientific basis and the clinical use of TENS.
What is TENS?
The use of electrical stimulation for therapy and physical rehabilitation dates to ancient times. Today, application of electrical stimulation to achieve a desired physical effect is common in neurology, cardiology, physical medicine orthopedics and dentistry. The term TENS is now commonly seen in both professional and lay journals. However, not all TENS is the same. There are two distinct categories of TENS; each with distinct electrical parameters and mode of action. It is important that the clinician understand the difference in technical parameters and physiologic action of these valuable modalities.
High Frequency TENS
High Frequency TENS is commonly called pain blocker TENS or dorsal column stimulator. Recently, implanting these devices for control of chronic lower back and other refractory pain has received much attention. High frequency TENS delivers 80-100 asymmetrical biphasic wave stimuli per second (80-100Hz).
This physiologic action of high frequency TENS is very different than that of the Low Frequency TENS you will be using at LVI. We must go back to our basic neurophysiology from dental school to understand the physiologic principle of high Frequency TENS. Pain is carried by C nerve fibers. These fibers are small and relatively unmyelinated which result in relatively slow neural conduction velocity. The pressure, touch and non pain Alpha fibers are larger, more myelinated and have greater conduction velocity than C pain fibers. The Alpha nerve fibers are stimulated by the high frequency TENS.
The Melzak-Wall Theory proposed in 1965, and still not totally understood 35 years later, postulated that there is a constant and dynamic interaction between the large A fibers and smaller C pain fibers. These alpha and C fibers terminate and interact in the substantia gelatinosa, an extremely dense neural tract, located in the midbrain. The substantia gelatinosa cells acts as inhibitory inter-neurons that exert a presynaptic inhibition on both the alpha and C fibers. The large non pain alpha fibers send collateral branches to these inter-neurons thus inhibiting transmission of information to higher levels of the brain. The TENS stimulus floods the Alpha nerve fibers with stimulus. It is a race to the midbrain between the non-pain alpha signals and the painful C nerve fiber signals. The Alpha wins!
The third element to recall from our neurophysiology lesson ( just kidding of course since I slept through neurology class like most of you while waiting to get to real dentistry ) is that pain is not perceived until the pain signal reaches the cerebral cortex. The cortex is the cognitive part of the brain. Block that pain somewhere before it gets to the cortex and you can hit your thumb all day with a hammer and not feel it.
A simple explanation of high frequency TENS can be understood by visualizing a funnel used to siphon gasoline. Place it in the area of the midbrain with the narrow spout facing upward toward the higher level cognitive centers of the brain. Fill that siphon with non pain Alpha signals using high frequency TENS before the C pain signal can get through the narrow spout. Super simplified, that is the Melzak Wall principle of pain blocking.
High frequency TENS is characterized by rapid onset of relief but short residual effect after cessation of stimulation. There is no observable muscle contraction until an amplitude is reached where tetany can be initiated. At this amplitude, which is not a desired therapeutic protocol, muscle fatigue can be caused by the high frequency TENS. This point will be important later when we discuss low frequency TENS and the history of resistance to the Myo-monitor. High frequency TENS creates little or no endogenous opiate liberation and no reversal of naloxone.
High frequency TENS is occasionally used by dentists treating chronic, intractable pain patients. However, Low or Ultra-low frequency TENS is the choice to help relax masticatory and facial muscles.
Low Frequency TENS = THE MYO-MONITOR
Low frequency TENS has also been called acupuncture like TENS, ultra-low frequency TENS and muscle stimulator TENS. These units deliver 0.5- 4.0 stimuli per second. The two units you will be using at LVI are the Myo-monitor and the home therapy BNS-40. Both devices deliver an asymmetrical biphasic wave stimuli every 1.5 seconds (0.67Hz). There is an observable involuntary muscle contraction of the facial and masticatory muscles when the low frequency TENS is advanced to a threshold amplitude characteristic for each individual.
H.H. Dixon, a world known muscle physiologist at the University of Oregon pioneered the use of TENS electrical parameters that would best relax muscle. He concluded, Experimental work with the myograph and chemical analysis indicates that fatigued muscle restores its energy with light, free motion at a rate below 60 contractions per minute. Fatigue spasm can be reduced by electrical stimulation." Ref.
It is impossible to create muscle fatigue using low frequency TENS because there is time for recovery between stimuli. However, in recent months we have seen certain outspoken opponents of the neuromuscular principles and instrumentation make comments that "The Myo-monitor fatigues muscle" or "You can't relax muscle by shocking muscle". These are ad hominem mis-statements more appropriate for evaluating motives and intellectual limits of the proponent, rather than the scientific merits of their statement. It is impossible to fatigue muscle with a stimulus periodicity of less than 4 Hz. The latency between stimuli assures recovery at both the myo-neural junction and myosin/actin binding sites. Physiology 101.
Muscle in spasm is shortened and unable to readily return to its normal resting length due to depletion of its normal metabolic resources. In addition, a patient with muscle spasm has no pumping mechanism to initiate the pressure gradient required for controlled flow within the network of lymphatic tubules. The local effects of muscle contraction are the pump that maintains lymphatic flow. Lack of this muscle pump in shortened hypertonic muscles reduces lymphatic flow. Buildup of waste metabolic molecules in the interstitial fluid results in pain and edema of the affected muscle.
The pumping effect of the Myo-monitor stimulated muscle contraction is the first step toward normalizing muscle metabolism. Without this involuntary contraction, the toxic waste products of metabolism will create a self-perpetuating cycle of muscle hypertonicity.
Shortened, hypertonic muscle contracts and compresses the blood vessels within the muscle, resulting in diminished blood flow. In cases of chronic or intense contraction, blood flow decreases to a point where the oxygen, glucose and Ca++ concentrations fall to a level inadequate to sustain aerobic muscle metabolism.
Adenosine Triphosphate (ATP) is the source of energy driving the actin/myosin contractile apparatus of muscle. The supply of available ATP must be continually replenished during and after muscle contraction. Muscles that have been in chronic spasm have a small endogenous reserve of ATP in the form of creatinine phosphate that is inadequate to sustain normal muscle function.
Glucose is the source of ATP. Reduced blood flow results in reduced glucose and subsequently reduced ATP which then disrupts the contractile mechanisms. Glucose is obtained either from the blood or from glycogen stored in muscle. The glucose molecule is changed to pyruvic acid through glycolysis to produce ATP. In normal muscle with adequate blood flow the pyruvic acid can enter the citric acid cycle (remember the Krebs cycle from biochemistry) to produce large amounts of ATP molecules, 32 ATP molecules from each glucose molecule.
In hypertonic muscle with reduced blood flow the Krebs cycle is inoperative and pyruvic acid must be stored as lactic acid to allow glycolysis to proceed. Under anaerobic conditions only four ATP molecules are produced from each glucose molecule, inhibiting normal ATP production that is necessary for actin/myosin function.
Low frequency TENS acts on the efferent motor fibers to produce involuntary muscle contraction. This is the contraction which is observed in facial and masticatory muscle when the Myo-monitor is advanced to Clinical Threshold, defined as the first observable upward movement of the mandible under involuntary Myo-monitor stimulus. The periodic involuntary muscle contraction initiated every 1.5 seconds by the Myo-monitor pumps fresh blood into the muscle, increasing oxygen, glucose, and Ca++ necessary for a shift to the Krebs cycle. This allows free muscle movement and return of muscle to it's resting length.
In addition to the local muscle effect, the literature also strongly suggests that low frequency TENS mobilizes endogenous opiates in the midbrain, probably in the substantia gelatinosa. This endorphin effect is often observable after 20-30 minutes of Myo-monitor treatment. The patient will become relaxed, sleepy and feel and sense of euphoria consistent with increased endorphin levels.
Summary
The desirability of diagnosing and establishing occlusion within a relaxed muscle environment is self evident. Disturbed actin/myosin/ATP muscle mechanisms and muscle hyperactivity is present in most pathologies of the teeth, temporomandibular joints and muscles. Restoration of muscle to normal metabolism before occlusal diagnosis and treatment is central to the neuromuscular paradigm. Low frequency TENS, the Myo-monitor, is the clinical workhorse used to achieve relaxation of facial and masticatory muscles.
The next issue of LVI Dental Vision will explain indications, contra-indications and proper clinical use of the low frequency TENS, Myo-monitor.
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