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Chronic Pain extracts a "penalty" on society now estimated to be well in excess of $100 million per year. The "penalty" that Chronic Pain extracts from its victims is incalculable. Chronic Pain is a major component of Temporomandibular Disorders. The current neurological theory of the mechanism of chronic TMD pain is explored along with the current modes of treatment and pharmacological management of Chronic Pain in a clinical setting.

Dentists are involved in pain management on a daily basis. Dentists treat pain both prophvlacticlv and in response to specific patient symptoms. Most dental treatment involves some type of pain management. We, dentists, have become very adept at managing acute pain. We have much greater difficulty managing chronic pain. The word "pain" derives from the Greek word for penalty, and appeared to them to be a "penalty" inflicted by the gods. In 1984, Bonica estimated that one-third of all Americans suffered from some kind of chronic pain at a "penalty" to society of $65 Billion annually in medical expenses and lost wages and productivity. This figure is certainly much greater now.

Chronic pain can be a very complex problem that can require a rnultidisciplinarv approach to treatment. Chronic pain in the dental setting is most frequently caused by prolonged Temporomandibular Disorders.

These long-suffering TMD patients will routinely exhibit complex and erratic physical symptoms, inevitably accompanied by deeply entrenched emotional and neurological disturbances. Treatment of these advanced chronic pain patients is often complex and difficult, becoming mentally and emotionally taxing on the doctor and the pain management team. An unrecognized, undiagnosed, or misdiagnosed physical disease, injury or impairment (distalized condyles) can also drive chronic pain. In this second case, correction of the ailment is often enough to resolve the chronic pain issues. These patients can be a source of great pride and satisfaction for the doctor. It is exhilarating to watch these patients’ lives transform and blossom when the burden of chronic pain is lifted from their bodies and souls. Unfortunately, unresolved chronic pain is common in our society. The accompanying psychological problems can interfere with clinical treatment or negatively effect a patient s emotional response to treatment. Therefore, it is important for dentists to understand chronic pain and current treatment protocols.

THE PHYSIOLOGY OF PAIN

Acute pain is short term, usually with an obvious source. It is controlled by either suppressing the pain impulses (local anesthetics, peripheral analgesics) or reducing the patient’s conscious awareness of the pain (nitrous oxide, opiate analgesics). Referred pain is pain where the site of the pain is different from the source of the pain. An example of referred pain is trigger points in the temporalis muscle referring pain to the maxillary teeth. The site of pain awareness is the maxilla, but the source of the pain is the temporalis muscle. Referred pain may be present in both acute and chronic situations. Referred pain is controlled by directing treatment toward the source of the pain. Chronic pain presents with different characteristics. Often the cause of the pain is not easily determined and, in some cases, the initiating factor may no longer even be present! Chronic pain patients present with global complaints of pain, with referred pain inevitable. The term "suffering" is most often associated with chronic pain. This emotional component of chronic pain has been the focus of much research. An entirely new field of science dedicated to the study of this mind-body/physical-emotional linkage has developed in this decade. This field of medical science is called psychoneuroimmunology.

Many of the pain response mechanisms and neural pathways, and the physiological interplay between body and mind have been elucidated. This "Cartesian Split", the concept of separateness of mind and body, has been retired after four hundred years of predominance. The new paradigm is based on the concept of informational transmission on the cellular, intracellular and molecular levels in the endocrine, neuralgic and somatic systems. The physical pathways of pain transmission and cortical cognition are shared, to a certain extent, with their emotional counterparts. There are interconnections between the pain control and the emotional control systems in the brain. The emotional control centers of the CNS are integrated with the initiation and propagation of the pain control systems of the brain. Thus, "Chronic Pain" is now considered a psychophysiological disorder with both emotional and physical components. (Ironically, the old Cartesian divisions are used for clarity of communication.) The major weapons that the body has to fight both chronic and acute pain are the peripheral mechanoreceptors and the central Descending Inhibitory System (DIS) (Figure 1).

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Mechanoreceptors are located in vast numbers throughout the entire body and are a major component of the peripheral nervous system. Stimulation of mechanoreceptors is inhibitory to pain impulses. For example, if we burn our Angers on a hot stove, we instinctively shake our hand about, blow on the burn, or run cold water over the injured site. All these actions inhibit the painful impulses coming from the injury and are examples of how we stimulate the rnechanoreceptors. The DIS is a central nervous system pathway that inhibits the transmission of painful sensations by blocking incoming pain impulse transmission at the level of the medulla or the spinal cord. The DIS has several major synaptic junctions in the brain before reaching the lower level of the CNS. These synapses rely on neurotransmitters for the propagation of the descending inhibitory impulses. (Figure 2)

Constant demand on the DIS strains the neurotransmitter supply. The chronic pain patient has put constant and prolonged high volume of noxious input into the CNS. The body, whose pain control mechanism become overtaxed and overwhelmed, can not respond sufficiently to the constant bombardment of this noxious neural input. The neurotransmitters that modulate the inhibitory systems become depleted. The depletion of neurotransmitters "releases the brakes" on the pain input allowing this condition to become ingrained and chronic. Thus, treatment is based on the replenishment of the neurotransmitters.

BREAKING THE PAIN CYCLE

The neurotransmitters of CNS pain control are primarily norepinephrine (NE) and serotonin (5-hydroxytryptamine, 5-HT). Norepinephrine is also associated with anxiety and stress. Serotonin is associated with depression. Both are associated with sleep disturbances. As an impulse is transmitted down a nerve fiber, it reaches the synapse or junction with the next cell. Depolarization of the nerve causes the release of neurotransmitters into the synaptic cleft, propagating an impulse in the next nerve fiber. The neurotransmitter dissipates from the synaptic cleft by one of three methods: reuptake, metabolism, or diffusion. (Figure 3) If the neurotransmitters are eliminated faster than they can be replenished, the supply becomes depleted and the synapse stops working. Melzack and Wahl’s "Gate" is now stuck closed, and the inhibition of pain transmission stops. Chronicity has become established.

In a clinical setting, the dentist is not qualified to make a psychological diagnosis of "depression". However, psychometric tests (TMJ Scale, Chronic Pain Battery) can validly assess the patient for depressive symptoms. An assessment of a TMD patient should always include questions about their sleeping habits. Current theory suggests that depletion of the neurotransmitters is responsible for the sleep disturbances that are an indicator of a chronic condition. The sleep disturbances arise from the loss of inhibition of the reticular activating center (RAC). Located in the brain stem, the reticular formation controls the "arousal" of the brain. All major nerve trunks are connected to the reticular formation. The cortex can not be aroused directly; for the RAC acts as the night watchman for the brain. When inhibition is lost or lowered, the nerves of the RAC become easier to depolarize, and the brain is aroused more easily. Since the brain is more easily aroused the frequency and ease of awaking from sleep increases. Humans rely on deep sleep to restore and refresh both the mind and body. However, we must reach the deep stages of sleep to get this benefit. The body manufactures the CNS serotonin supply during deep sleep. The chronic pain patient loses this ability to achieve and maintain the deep stages of sleep. They become locked into a self-feeding downward cycle, both physically and mentally. The chronic pain patient cycles through an emotional gamut of anger, stress, anxiety, frustration, and depression. The Chronic Pain/TMD patient now requires our intervention to break the cycle. Breaking the cycle of chronic pain for a TMD patient requires coordinated efforts to reduce noxious input to the CNS while increasing the inhibitory capabilities of the endogenous systems. TMD patients present with distalized condyles and inflamed TMJoints. The impingement on the auriculotemporal nerve two thousand times daily during occlusal bracing feeds an unrelenting stream of pain impulses into the trigeminal nucleus in the brain stem and upward toward the cortex. Reducing noxious input to the CNS requires an unloading of this nerve and the inflamed retrodiscal tissues associated with it. This requires movement of the condyle, via an orthopedic orthotic, down and forward from the superior, posterior position commonly seen in TMD patients. (Because numerous works describe this therapy in detail, we shall not. However, this does not detract from the demand that this be the primary mode of treatment.) The chronic inflammation of the joint tissues is commonly treated using NSAIDs, such as Clinoril. Then the omnipresent postural distortions can he addressed through proper physical therapy, massage therapy, or chiropractic treatment. In many cases, these three treatments are enough to break not only the physical cycle of pain but, now freed of the burden of constant pain, the emotional state of the patient can improve without psychological or psychiatric counseling.

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When the patient does not have good emotional support systems. i.e., loving family and friends, or when the pain cycle is too engrained, pharmaceutical intervention may be necessary. Concurrently with the physical reduction of nociceptive input, three additional areas can be addressed simultaneously. Sleep disturbances, emotional disturbances from chronic pain, and depletion of neurotransmitters are addressed together because of their theoretically common etiology.

Pharmacological management of these patients is commonly achieved using anti-depressants. The two major categories are tricyclic anti-depressants (TCAs), and selective serotonin reuptake inhibitors (SSRIs). TCAs work by inhibiting peripheral 5-HT receptor sites and increasing the supply centrally. Amitriptyline (Elevil) has been the gold standard, though trazadone (Desyrel), Nortryptyline (Pamalor), and doxepin (Sinequan) are alternatives. SSRIs slow the reuptake of 5-HT hence the name. Fluoxetine (Prozac) is the most common SSRI and is the most prescribed drug in the world. Sertraline (Zoloft) and paroxetine (Paxil) are common alternatives. SSRIs generally have fewer side effects than the TCAs. Dosing of TCAs and SSRIs is titrated upward on a weekly basis until an effective clinical dosage is achieved. The anti-depressant effects often take 4-6 weeks to achieve: yet, improved sleep can occur in a few days. (These are potent pharmaceuticals with side effects and contraindications! ) Dentists who are not comfortable prescribing or managing these drugs, or who are prohibited by state laws should discuss their findings with the patient’s physician, who will usually agree to manage these medications.) Usually, with the initiation of the above protocols, the patient becomes pain-free. Decreasing or eliminating the pain causes a positive mental change for the patient. Feelings of hopelessness and despair vanish. The improved mood and energy of the patient reinforces the healing process and the patient’s emotional state. While the joint tissues heal, the brain is restoring proper neurotransmitter function and cooling down the hyper-excited neural pathways. As proper physical therapy restores the biomechanics of the body, pain-inhibiting impulses from the somatic mechanoreceptors replaces the noxious nociceptor (pain) input. The self-feeding destructive cycle previously experienced by the patient becomes a positive, self-reinforcing pattern of restored health and a return to a normal lifestyle. This is our "Prime Directive" as doctors, to help our patient restore their physical and emotional health.

Author's Note: The neural mechanisms described in this article are moss simplifications of the extremely complex neural constructs and functions. The use of tricyclic antidepressants and SSRIs is an example of efficacious treatment being rendered without a solid theory for the mechanisms of action of that treatment. There are many problems with the neurotransmitter-depletion theory of chronic pain that have yet to be resolved. It provides a convenient and simple explanation of complex neurological processes. The theories about chronic pain will undoubtedly be modified numerous times and may never become fact. None the less, we must continue to treat these suffering patients with all the care, skill and knowledge that we possess.

Perhaps the only thing in the universe beyond the understanding of the human mind is the human mind itself.

About the Author: Dr. David B. Miller received his O.D.S. degree from the University of Illinois in 1980. He is a Fellow of the Academy of General Dentistry, the International College of Craniomandibular Orthopedics, and the American Back Society. A Life Fellow of the American College of Forensic Examiners, Dr. Miller is a Diplomate of the American Academy of Pain Management, the American Board of Forensic Examiners, the American Board of Forensic Medicine, and the American Board of Forensic Dentistry; concentrating on the forensic analysis and objective documentation of unresolved soft tissue injuries to the craniomandibular/temporomandibular/cervical complex. He is a Founder, Diplomate and Vice President of the College of Forensic Orthopedics. Dr. Miller is Editor of the CFO newsletter. He serves on the Advisory Committee of the American Board of Forensic Dentistry. Dr. Miller has been a featured speaker at the national meetings of the American College of Forensic Examiners, the American Association for Functional Orthodontics, and the BioResearch Corporation. Dr. Miller maintains a general practice focusing on Neuromuscular Dentistry and TMD in Sacramento, California.

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REFERENCES

1. Borne RF: "Serotonin: The Neurotransmitter for the ’90s". Internet, University of Mississippi, 1998

2. Goldstein JA: Betrayal by the Brain: The Neurologic Basis of Chronic Fatigue Syndrome, Fibromyalgia Syndrome, and Related Neural Network Disorders. New York: Haworth Medical Press. 1996

3. Guyton AC and Hall JE: "Textbook of Medical Physiology, 9th Ed. Philadelphia: W.B. Saunders Co., 1996

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5. Miller DB: "Yesterday’s Dentistry. Today’s Chronic Pain", The Functional Orthodontist, Vol. 14, No.3, May/June/July. 1997, pp., 30-33

6. Morris DB: The Culture of Pain, Berkeley: University of California Press, 1991

7. Okeson JP: Bell’s Orofacial Pain. 5th Ed. Carol Stream, IL: Quintessence Publishing, 1995

8. Pert CD: Molecules of Emotion: New York: Scribner, 1997

9. Pilling LF: "Pain: The Psychological Effect on the Patient" in Coy RE, Ed., Anthology of Craniomandibular Orthopedics. Vol I. St. Louis, MO: New Visions, 1991, pp 3-22

10. Spahl TJ and Witzig JW: The Clinical Management of Basic Orthopedic Appliances Vol. III, Temporomandibular Joint. St. Louis, MO: Mosby-Year Book, 1991

11. Thomas RF: The Brain: A Neuroscience Primer. New York: W. H. Freeman and Co., 1996

12. Tollison CD, Ed.: Handbook of Pain Management. Baltimore: The Williams and Wilkins Co., Baltimore, 1994

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