Diagnosis and Treatment of Equine Musculo-Skeletal Pain. The Role of the Complementary Modalities: Acupuncture and Chiropractic

1. Introduction

The International Association for the Study of Pain has defined pain as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage" [1]. Pain has a physical component that is determined by an individual's nociceptive system to a given stimulus. It also has an emotional component that is determined by the responsiveness of an individual's limbic system to any particular noxious stimulus. One can also consider a rational component of pain that derives from an objective interpretation of pain by the cerebral cortex [2].

Excellent research work has been done in the field of "chronic pain"; however, few physicians and even fewer veterinarians are adequately familiar with the concepts, manifestations, etiology, and appropriate treatment of chronic pain. The many-faceted syndromes of chronic pain have only been studied extensively in human medicine for the past 30 yr. Similar types of syndromes in equine and companion animals are only a glimmer in the world of veterinary medicine. However, an understanding of the concepts of etiology and treatment can be of immense value to the clinical practitioner and the patient.

2. Pain Sources

Chronic pain, as distinct from acute pain, distinguishes itself by its persistence in the absence of any inflammatory disease or any evidence of a detectable injury. It must be emphasized that this type of pain is a very real phenomenon. It is a markedly debilitating condition that is frustrating for both the physician and the patient. Until brought into light by Travell and Simons [3] in their definitive work on the subject, patients with chronic pain were generally considered by the medical profession to be hypochondriacs and psychosomatic cases. Patients were frequently (and, in many instances, still are) referred for psychiatric evaluation, counseling, and therapy. In the equine, there is a strong tendency to dismiss pain as a behavioral or training issue. Because it is a prey animal, the equine reacts to pain by avoidance or by aggressive tactics if flight or avoidance is not an option. Much of the "misbehavior" exhibited by horses is a response to pain.

There are at least two other distinct types of pain that need to be defined and differentiated from chronic pain. These include nociceptive pain of the type that is incurred with a wound, sprain, strain, or a burn (i.e., an immediate pain). In and of itself, the lesions generally heal, and pain manifestations disappear and do not lead to recurrent pain. The other type of pain is associated with inflammatory conditions. Inflammatory pain is generally self-limiting, unless it is associated with conditions such as rheumatoid arthritis. Chronic pain, in contrast to these other types, is distinguished by a lack of evidence of current trauma or inflammation.

However, if one delves (often years back) into the patient's history, it is frequently discovered that there has been previous injury to the now painful area. History may also indicate that, at some period before the onset of chronic pain, significant "overuse" of the painful area was involved. Relating this to equine medicine, overuse situations are a prevalent fact of life in the world of equine competition. Other major factors in the production of chronic pain in all species include the development of spondylosis, and a resultant nerve root inflammation (i.e., radiculopathy). Radiculopathy also is a factor in acute pain of spinal origin.

The field of psycho-neuro-immunology, addressing relationships between body and mind, is an exploding medical field. Although this connection can be somewhat more difficult to define in animals, information obtained from a good history of a patient can help to elicit more information from an observant owner such that a connection between soma and psyche can be drawn in animals as well. Psychological aspects (e.g., fear) create stress, and the body manifests ongoing physiological responses as response to that stress, especially when stress is applied over long periods of time. In the equine, those stresses eventually lead to pain, decreased performance, and eventual subclinical or overt lameness.

In the horse, the history and role of stress cannot be over-emphasized when evaluating the possibility of a diagnosis of chronic pain and acute pain from such stressors as speed (as in racing), strength (as in pulling), isometric contraction (as in dressage), and fatigue (as in endurance). Horses under work and mental stress often have compromised immunity to some degree. The reasons for such stress are many:consider the stress that is implied in heavy showing schedules, transportation, long-distance shipping, isolation, and thwarting of herd behavior.

Immunization can also produce stress and result in an up-regulated adrenal response. Animals under stress from any source are poor candidates for vaccines and worming. The immune response to the vaccines is compromised, and the worst case scenario is that the procedures may add to the level of immune incompetence that the horse is already experiencing. Stress and immune compromise can lead to recurrent manifestations of chronic pain and lameness.

Proven syndromes in the human field that are linked to stress include myofascial pain in the "fright and flight" muscles (e.g., temporal mandibular joint syndrome, irritable bowel syndrome, leaky gut syndrome, and headaches in the human). There is reason to suspect that these same or similar syndromes in our equine patients are related to the soma-psyche stress cycle.

3. Signs, Symptoms, and the Diagnosis of Pain and Chronic Pain

Unlike acute pain syndrome, the diagnosis of chronic pain syndrome often has to be achieved by ruling out other sources that can cause pain. A prime rule-out would certainly include recent trauma. It would also include rule-outs in the category of degenerative joint disease. However, it must be noted that some degenerative joint disease can be the result of chronic pain itself. Functional disease (i.e., internal medical conditions) must also be ruled out as a source of the observed pain. Conventional diagnostics, including chemistry, and various imaging techniques should be employed.

The most common form of chronic pain is often manifested as myofascial pain syndrome (MPS). A search of the literature provides at least 45 names used for the syndrome throughout the last 100 yr [4]. This multiple nomenclature has lead to much confusion and disagreement within the medical community. A well-documented and prevalent variant of chronic myofascial pain in the human patient is fibromyalgia. The signs of fibromyalgia can provide the basis for a model adaptable to our non-human patients. The main human patient complaint is that "I hurt all over my body". In the horse, one finds general body soreness. Fibromyalgia (FMS) patients describe the pain as intense and crippling. They indicate that the pain is nagging and ongoing. Though there are periods when such patients are relatively free of pain, they experience recurrent acute episodes.

As previously stated, horses typically communicate pain signs by what are mistakenly thought to be behavioral issues including evasive responses, unwillingness to perform, and decreased levels of performance. These are the signs and history at which the clinician must look. Many equine patients manifest low-grade or sub-clinical lameness (such manifestations have frequently been observed by the author in the immune-compromised equine ~2 - 4 wk after immunization).

Lameness associated with chronic pain does not tend to be localized and identified by conventional means. Lameness associated with myofascial pain is not ameliorated or eliminated by nerve blocks. This form of pain does not exhibit radiographic changes or even notable evidence of its presence with the use of nuclear scintigraphy. The clinician must look back to a history of recurrences of such subclinical lameness or declining performance where there was no history or evidence of associated trauma. The clinician must also look for pain symptoms that occurred after a very minor trauma and evaluate any history that would reveal chronic stress factors.

The syndrome most often seen in horses that produces all the typical signs of MPS is a diffuse myofascial pain response that has, in the author's opinion, been erroneously described by some veterinary acupuncturists as a form of "equine herpes". The syndrome manifested, however, is very real and mimics all the signs and diagnostic difficulties of MPS and fibromyalgia in man.

Signs include performance deficits and pain in the gluteal muscles that is often more prevalent on the left side. The loin area is prone to spasm (especially on the left side) when palpated. Pain in the hamstrings is common and shows a marked pain response to palpation along the groove between the semitendinosus muscle and the biceps femoris. Typically, a sharp withdrawal response is noted on the left side of the horse. Pain may also be elicited when palpating the brachiocephalicus and sternocephalicus muscles. Low-grade lameness is a common finding--the horse is typically described as not "coming through behind". A clinical diagnosis of stifle or hock problems is commonly made, but the pain and lameness or subclinical lameness is not diminished by administration of non-steroidal anti-inflammatory drugs (NSAIDS) such as phenylbutazone. The pain is often exacerbated by the use of steroids. Such pain is not abolished with regional nerve blocks, and nuclear scanning renders no definitive pattern. This syndrome in the equine might most appropriately be described as true MPS.

How then do we diagnose MPS if our usual diagnostics fail? Using a clinical diagnosis of FMS in the human patient as an example, the syndrome is considered confirmed if the patient is exquisitely reactive to the palpation of 15 - 21 "trigger points" that are statistically consistent in FMS patients. As in the case of man, this described form of MPS in the equine also exhibits a very consistent group of acupuncture or trigger points that are exquisitely sensitive.

Trigger points have been described as focal areas of hypersensitive loci located in myofascial structures. When palpated, these foci give rise to a larger area of pain in the adjacent tissues or in the referred areas that may be some distance from the trigger point. Trigger points, like acupuncture points, are found in the muscles, belly, skin, scars (areas of sensory deficit), tendons, ligaments, and joint capsules. Trigger points are known to occur in periosteum as well. These trigger points referred to by the medical profession correlate with 70 - 75% of historical acupuncture points. An acupuncture point, in contrast to a trigger point, is considered to be a zone or field of hyperalgesia that arises spontaneously from excitation of either somatic or visceral structures, which are often significantly distant to the reactive point. Acupuncture points are also activated by local irritation in the local area.

The most common muscles that develop myofascial trigger points and acupuncture points in response to overuse or stress are the muscles of "fright and flight". These would include muscles in the forehand such as the trapezius muscles, the sternocleidomastoid group (brachiocephalicus and sternocephalicus), the masseter muscles, and the infraspinatus muscles. In back pain, the paravertebral extensor muscles very commonly develop points and zones of exquisite sensitivity. Because of the high level of innervation within the paraspinal muscles, trigger point/acupuncture point formation and pain to palpation is a very common finding. In the hindquarters, the gluteal muscles and the "hamstring" group of muscles are the other muscles most typically involved. All the listed muscles undergo shortening when in "fright and flight" posture.

4. Neuropathic Qualities of Pain:Spondylosis and Radiculopathy

To properly understand chronic pain, it is necessary to review the role of afferent and efferent nerves. Neuropathy by definition is a nerve that is not functioning properly and eliciting its normal trophic (controlling) responses. There is a sub-optimal level of signal transmission. A neuropathic nerve exhibits both functional and pathologic changes. According to Gunn [5], spondylosis of the spine is the most common cause of neuropathy. He further states that spondylosis leads to radiculopathy (an inflammatory response in a nerve root). In spondylosis, morphologic alterations occur in the intervertebral disc. As the process continues, it leads to structural disintegration and morphologic change. Other causes of neuropathy that need to be recognized and ruled out include trauma, inflammation, metabolic changes, toxic changes, infection, neoplasia, and degenerative changes.

The spondylosis/radiculopathy complex is a nearly universal condition in both people and animals. It increases with age and in some instances, is familial. The onset of spondylosis is gradual and relapsing. Pain response can remain quiescent until triggered by insults that are minor and commonly occur in ordinary day-to-day living situations. These initiating trauma insults are not usually noticed by the patient.

The recognized features of neuropathic pain are numerous. The most significant and key feature of chronic pain is the recognition of pain in the absence of ongoing tissue damage. Other features include a delay in onset after a precipitating injury, a searing, burning, or deep aching pain, the occurrence of a brief "shooting pain" or stabbing pain, and/or a pain that occurs in a region of sensory deficit such as scars. It is noteworthy that a mild stimulus can cause allodynia. Allodynia is described as an extreme and hyper-reactive response to a minimal pain stimulus. With repetitive stimulation of triggers, such as applying light to moderate pressure over the muscle with the flat side of a needle cap, a stronger pain response is elicited. In the myofascial covering, there is what is termed a marked summation response. Summation response is exhibited as an increasing level of pain each time stimuli are repeated. The increasing pain is accompanied by spasms of the target muscles. In response to the muscle shortening, significant loss of range of motion in joints can occur, because, often, both the associated agonist and antagonist muscles undergo shortening.

Before introducing the clinical manifestations of the spondylosis/radiculopathy/neuropathy complex, it is necessary to introduce two additional concepts. Cannon and Rosenblueth [6] of the University of Washington Medical School defined a process that they referred to as a "denervation" and a "supersensitivity" phenomenon. It is basic neurology to understand that normal nerve or muscle function depends on intact innervation to provide a regulatory (trophic) effect. Their research has shown that any condition that blocks the flow of motor impulses can, over time, cause a condition called "disuse supersensitivity" in the target and in the associated spinal reflex. A lack of adequate efferent response causes the target to be over-responsive to even a very minor stimulus. In the case of muscles, the result is shortening, pain, and possibly, spasm [7].

An amazing number of targets are subject to the partial denervation and its resulting supersensitivity syndrome. Clinically, skeletal muscle would be first on the list. However, many other critical structures such as smooth muscle, spinal neurons, sympathetic nerves, ganglia, adrenal glands, sweat glands, and even brain cells are known targets. Recognition of the importance and wide distribution of the denervation-supersensitivity phenomenon requires that we look at many disease processes in a totally new way. This is an area ripe for veterinary medicine research.

Clinical manifestations are those associated with the exquisitely reactive trigger and acupuncture points. Additionally, there are autonomic/trophic (regulatory) responses, such as patchy sweat areas, and piloerector responses seen in the human patient. In the equine athlete, areas of patchy sweat may occur hours after exercise. An understanding of radiculopathy and the responses to it may point the way to recognizing a cause for the mysteriously delayed onset of sweating in the equine. It merits further research.

Associated with the presence of reactive points is an increase in muscle tone and muscle shortening. It is this ongoing muscle tension and shortening that results in the manifestation of pain. The key to myofascial pain of neuropathic origin lies in the shortening of the muscles. No muscle shortening means no pain. One of the values of acupuncture lies in the ability to depolarize and relax muscles [8].

A major consequence of muscle shortening that, perhaps, does not receive enough clinical weight and credence is the continuous and unremitting pull on the structures to which the muscle attaches. Pressure from muscle contraction that spans a joint can create pressure within the joint and result in abaxial weight bearing. Non-aligned weight bearing will result in arthralgia, joint malalignment, and eventually, degenerative joint disease. Therefore, a diagnosis of degenerative joint disease may not rule out chronic pain as the root cause. The degenerative joint disease may, in fact, be the result of chronic pain [7].

The presence of muscle shortening has deleterious effects on the tendons and ligaments and subsequently, the joints. Unrelenting muscle tension on these structures may be the precipitating factor in a number of pathologic conditions that we frequently see in horses. Such conditions likely include bicipital bursitis, epicondylitis, hock and stifle degenerative changes, and suspensory pain and tearing in the chondromalacia of the stifle joint.

Continuous muscle tension creates an ongoing, repetitive micro-trauma to the collagen structure of the associated tendons and to the ligamentous structures when use of the joint occurs (as in locomotion). The result is that the replacement collagen in tendons and ligaments is weaker and lacks adequate cross-links. The tendon response is a thickened (enesthesopathic) tendon that remains weak and thus, subject to further trauma. Acupuncture and trigger-point therapy can ablate the pain sites and allow the muscles and associated structures to relax and thus, take pressure off the involved joint. This response will likely prove to be the most significant reason that acupuncture and trigger-point work can provide relief in these conditions. The relief is sometimes profound and can be very long lasting.

A special and common condition (especially in companion animals) associated with the neuropathy incited by spondylosis and nerve root inflammation is a shortening of the paraspinal muscles. Contraction of spinal extensors and flexors compress intervertebral discs and thus, create a self-perpetuating cycle of contraction, pain, and inflammation that increases the radiculopathy even more. In addition, the pressure that is created on the spinal joint surfaces can lead to "facet joint disease" and further degeneration. It may also be a key factor in "kissing spine" conditions in the equine. An understanding of this concept will lead to an understanding of why acupuncture and trigger-point therapy is often very effective in treating discogenic back pain and "kissing spines".

The single most effective treatment known at this point in time for acute and chronic muscle hypertonicity is the placement of needles into the hyperactive neural points, regardless of whether they are acupuncture or trigger points. Active points may include a significant number that are neither traditional nor transpositional acupuncture points. The procedure, as advocated by Gunn [7], is referred to as "intramuscular stimulation" (IMS). In the case of myofascial pain (MFP), latent points and the Chinese "Ahshi Points" may well be more important than one of the anatomically defined transpositional or traditional points. This may also be compared with Japanese acupuncture in which precedence is given to the "presently active point" rather than to the strict anatomically described location of a point. In treatment of MFP, it is important to eliminate all such points, or the patient may actually exhibit worse symptoms after treatment.

Dry needling into the points has been found to be as effective as, and safer than, injecting compounds such as cyanocobalamin or local anesthetics into the points. The placement and stimulation of "dry" needles has proven to be more effective than the use of acupressure or photon therapy ("cold" lasers). This is not to say that these types of therapy have no value in the treatment of chronic pain.

The placement of needles creates an area of micro-damage within the tissues. Two mechanisms involved in the pain-control cycle are the release of inflammatory products and the creation of a "current of injury". Acupuncture needles, placed and repeatedly manipulated in and out, injure the tissue and result in a "current of injury" that provides an ongoing stimulus from 5 to 10 days during the healing phase of the micro-trauma. The nature and history of "current of injury" is well documented and explained by Becker and Selden [9]. To achieve the same level of results, lasers, deep-tissue massage, and acupressure all require more frequent treatments, and the treatments need to be continued over an extended time frame.

5. Acupuncture in Treating Acute and Chronic Pain

Micro-trauma induced by the placement of needles also stimulates a local inflammatory response at the site. Perhaps because of the great number of nerve endings and the increased microcirculation in and around acupuncture points, the inflammatory response is greater than when needles are placed in "sham points". Substances such as bradykinin release histamine from degrading mast cells. Degrading mast cells also release serotonin, which in turn potentiates the action of bradykinin on Group IV afferent nerve fibers. Sensitized C-sensory afferents release substance P, histamine from mast cells, and serotonin from platelets. This process sets up descending pain inhibition from the CNS that blocks further afferent pain response.

Because stress plays an important role in the etiology of chronic pain, it has been the experience of this author that it is helpful to include acupuncture points that stimulate the immune system. Acupuncture-induced responses regarding the immune system include enhanced humoral immunity, increased white blood cell counts, increased phagocytic activity, increased antibody levels, and increased interferon levels [8]. The enhancement of healing is aided by the increased microcirculation that occurs in acupuncture.

6. Role of Chiropractic in Acute Pain Control

Chiropractic has an extensive history in the treatment of pain. An adjoining pair of vertebrae along with their associated structures (spinal nerve root, associated vascular structures, and adnexal tissues) is known as a "motion segment". When spinal joints are restricted in their range of motion, chemoreceptors are activated, and low grade inflammation occurs. The inflammatory response leads to radiculopathy and further fixation of the segment. This leads to spondylosis:one or more of the joints (one or more facet-to-facet or discogenic changes) may not allow normal joint movement within that motion segment. The inflammatory response and radiculopathy result in neuropathy. In neuropathy, there is poor signal transmission of the afferent signal and thus, the nature and appropriateness of the efferent signal is affected. Muscles become shortened (i.e., hypertonic), and shortened muscles manifest pain. Microcirculation is diminished, and this tissue temperature is lowered.

The properly performed chiropractic manipulation moves the involved joint or joints through both active and passive ranges of motion and into the paraphysiologic range of motion. This must be accomplished without disturbing the anatomic barrier of the joint or joints. Facet joint surfaces are richly endowed with nociceptive and other receptor types. This adjustment and manipulation recruits high levels of receptor sites and enhances normal afferent signal transmission into the cord, brainstem, or brain for interpretation. Correct interpretation allows an appropriate efferent response [10].

Loss of range of motion may occur in one or several vectors. Before attempting correction, a proper and in-depth knowledge of anatomy and physiology is of paramount importance in establishing the vectors involved in a motion segment. A properly performed manipulation (typically a high-velocity, low-amplitude thrust of proper force along the determined line of correction) reestablishes full range of motion. The effect of the manipulation decreases the hypertonic/hyperesthetic response of shortened muscle fibers and reduces muscle electroactivity [11,12]. Diminished pain response has been shown by algometry in horses and in humans by visual analog scores [13,14]. When the range of motion is reestablished, the radiculopathy diminishes along with muscle shortening and the associated pain. In a properly executed chiropractic manipulation, like acupuncture and trigger-point therapy, there is evidence of beta-endorphin release [15]. The effects of endorphins on pain are well established [8].

In the chronic pain category of MPS, dysfunction of the motion segment tends to soon recur. Thus, best results are achieved when acupuncture or trigger-point therapy is followed by chiropractic or osteopathic manipulation.

Other follow-up therapies of value include the use of lasers (photon therapy) and microcurrent therapy. The latter is applied through instrumentation, and current is typically in the range of 60- to 800-μA current. Recalling the phrase "no muscle shortening equals no pain", stretching and myofascial release work can also be of significant benefit, because these physical-therapy modalities help maintain muscle length and relaxation. Deep-tissue massage can also stimulate the release of both acupuncture and trigger points. These latter modalities (stretching, myofascial release, and deep-tissue massage) can be readily demonstrated to the client. The client can then do beneficial follow-up therapy and minimize the incidence of recurrences.

7. Conclusions

The physiology of treating pain with pain is a fascinating concept. Though the neurology was unknown to primitive cultures, they effectively used acupuncture needles, chiropractic adjustments, and deep muscle work. Considering the evidence for MFP and the chronic pain concepts of spondylosis/radiculopathy, denervation/supersensitivity phenomena, and unremitting tension on muscles, tendons, ligaments, joint capsules, etc., these very real conditions present new and exciting challenges for the way that veterinary medicine evaluates pain sources and their causes. Much is to be learned about the appropriate treatment of chronic pain. It is the author's hope that veterinary medicine will, with an open mind, rise to the challenge.