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FROM: Chiropractic Journal of Australia 2004 (Jun); 34 (2): 58–62
Brian S. Budgell, DC, MSc
School of Health Sciences, Faculty of Medicine, Kyoto University, Kyoto, Japan
Background: As standards for randomized, controlled, clinical trials in medicine evolve, there is debate about whether the RCT model of investigation is appropriate for chiropractic and other forms of so-called "complementary and alternative medicine." There may be some question as to whether the use of placebo interventions can be justified ethically and scientifically given that experimental treatments must eventually compete in a marketplace where there is often already a clinical alternative which is more effective than placebo. Beyond these concerns, design of an appropriate placebo for chiropractic trials is particularly problematic since the therapeutic component of overall chiropractic treatment may be difficult to isolate.
Objective: To compare placebo interventions in current use in chiropractic clinic research with simple somatic stimuli that produce significant physiological effects in a selected group of patients (those suffering from dystonia).
Methods: A literature search was made using medline, with the key words dystonia, sensory trick and geste antagoniste. Articles were reviewed for descriptions of these stimuli. The stimuli were compared, in terms of site and modality, with placebo interventions used in recent chiropractic clinical trials.
Results: Stimuli used as placebo procedures in recent chiropractic clinical trials are quite similar, in terms of site and modality, to the "sensory tricks" that either cause substantial temporary relief, or, alternatively, provocation of symptoms in dystonic patients.
Conclusions: Caution should be used in assuming that control (placebo) procedures used in chiropractic clinical trials - procedures that involve physical contact or positioning of patients - lack specific effects on neuromusculoskeletal symptomatology.
INDEX TERMS: (MeSH) PLACEBO, SENSORY TRICK, GESTE ANTAGONISTE, CHIROPRACTIC
In common parlance, the placebo may be thought of as a sham treatment given to placate the gullible or troublesome patient. In medical practice, it is more often thought of as medication, most often a pill, which has no specific action against the complaint for which it is prescribed. More recently, standards for design of clinical research have demanded more rigorous definition of what has been called “the imaginary term in medicine’s algebraic formula.” 
For purposes of pharmacological research, it is possible to select placebo substances that appear, with a very high level of probability, to be physiologically inert in humans, or at least to have no specific action against a disorder that is the target of investigation. Nonetheless, various studies have indicated that such supposedly inert substances may be associated with impressive levels of therapeutic effects, sometimes rivalling the medications, which are known to have specific pharmacological effects. 
It is common practice in clinical research to compare the effects of an experimental intervention with those of a placebo intervention (one which is believed to have no specific effect on experimental outcome measures). The non-specific effect of a placebo may then be subtracted from the total effect of the experimental intervention in order to derive a “true” effect of the intervention under study (Figure 1). These apparently irrational mathematics are legitimised by practice and have some virtue in reducing the probability that chance will intervene and produce what appears to be a true clinical effect where in fact none exists.
|EFFECT OF EXPERIMENTAL INTERVENTION||less||PLACEBO EFFECT||equals||TRUE EFFECT OF EXPERIMENTAL INTERVENTION|
Figure 1: The non-specific effect of the placebo under study may be subtracted from the total effect of the experimental intervention in order to derive the “true” effect of the intervention under study.
Various mechanisms have been sought to explain the supposed effects of putative placebos. If there is a particular placebo-sensitive personality, this has proven elusive, nonetheless psychology has been generous to excess in the provision of theories regarding the placebo.[3-5] Psychological explanations, however, skirt the issue of how physiological effects are achieved in a physical world, and the demonstration of placebo effects, even in laboratory animals, demands hypotheses with more reference to biochemistry and physiology than to the id and ego.  On the assumption that physiological mechanisms of health have some modicum of specificity, i.e. are less than a panacea, unravelling the placebo means defining it out of existence. In practical terms, placebo has come to refer to effects whose mechanisms appear unfathomable, and so it may be that as we dig deeper into physiology, placebo phenomena will, one by one, come to be assigned to particular physiological mechanisms.
A modern paradigm of this is the discovery of endorphins. Endorphins are endogenously produced substances which bind to opiate receptors in the central nervous system, and which may exert an analgesic effect. Placebo analgesic effects are in some instances at least partly attributable to endorphin activity.  Hence, what was once an apparently non-specific effect whose mechanism was unfathomable becomes a physiological phenomenon subject to study and clinical manipulation, thereby ceasing to be a placebo. Not all placebo analgesia is achieved exclusively via endorphins, nor are all endorphins’ effects limited to analgesia,  i.e. the two are not synonymous. The investigation of endorphins, however, is now directed towards understanding and maximising their clinical effects, rather than controlling and eliminating such effects (see, for example references 9-11).
The discovery of endorphins and their relationship to analgesia bears at least two important lessons for research into chiropractic clinical phenomena
To apply this appropriately to chiropractic clinical research, there is need to be extremely cautious in defining placebo interventions and in assuming that their effects are relatively non-specific. 
Chiropractic therapeutic interventions subjected to clinical examination almost invariably involve physical contact with the patient, particularly manipulation of the spine. Hence, not surprisingly, so-called placebo interventions used in such studies frequently involve what is thought to be non-specific physical stimulation. At one extreme, this may involved elaborate combinations of stimuli which mimic chiropractic treatment; at the other extreme the subject may merely be positioned on a chiropractic table and receive no other stimuli at all. [28-36]
At some point on this spectrum, such placebo techniques must become sufficiently similar to the experimental intervention to lose their value as control procedures. Consensus might produce a broadly accepted threshold, for example audible “cavitation” of a spinal joint, which placebo interventions in chiropractic trials ought not to cross. Any such criteria for definition of the chiropractic placebo, however, will need to take account of the considerable potential of apparently inconsequential stimuli to produce substantial changes in the physiology of the human neuromusculoskeletal system. This is illustrated most dramatically by the sensory trick (geste antagoniste) in the dystonic patient. The primary dystonias have in common dysfunction of the basal ganglia, resultant abnormalities of motor inhibition, and therefore sustained involuntary muscle contraction.  In some patients, what might otherwise be trivial stimuli—light touch, or a small postural change—may bring about dramatic relief or aggravation of symptoms; hence the expression “sensory trick” or, in French, “geste antagoniste”. The quite specific effects of these innocuous stimuli may therefore call into question whether the sham techniques used in some clinical trials of physical therapies actually qualify as placebo treatments.
A MEDLINE literature search was conducted, using the search string “dystonia or sensory trick or geste antagoniste.” Articles were examined for descriptions of the site and modality of the “sensory trick.” An additional search was made for randomised clinical trials using the search string “chiropractic or osteopathy or spinal manipulative therapy.” Of 155 articles identified, 65 were in fact randomised controlled clinical trials of spinal manipulation. Of these, a number used patient positioning alone or no treatment as their control procedure, however, 15 studies were identified which employed active control procedures that were referred to as placebo, sham or simulated treatment. The site and modality of the placebo or sham procedures, when adequately described (Table 1), were compared to documented sensory tricks.
Table 1: Placebo Procedures Used in Chiropractic Clinical Trials
Patient positioning, massage, palpation and assorted “low amplitude, low velocity impulses” 
“Light manual contact to the spine” with use of drop-piece 
Contact with a mechanical adjusting instrument “turned to the 0 setting” 
“Sham adjusting with an Activator at a nontension setting” 
“An adjusting instrument (handheld)…altered to produce sound but not stylus excursion…positioned on the skin over the lateral aspect of the middle of the neck” 
“Brief superficial effleurage” plus “high-velocity, short lever, low-amplitude thrust of at least 200 N and not to exceed 400 N” to mid lumbar spine 
“Range of motion activities, light touch, and simulated OMT” consisting of “manually applied forces of diminished magnitude aimed purposely to avoid treatable areas” 
“Held by the nurse for 10 minutes” 
“Manual static and motion palpation and light touch of specific spinal segments” 
“Application of [non-functional] ultrasound head over the area of pain for a total of 10 minutes” 
Comparison of placebo procedures used in chiropractic clinical research (Table 1) with stimuli that are known to relieve (Table 2) or provoke (Table 3) dystonic symptoms indicates that common “placebo” procedures are not significantly different from procedures which specifically effect musculoskeletal symptomatology in dystonic patients. It is well documented that relatively light touch of the head or neck region may provide relief of cervical dystonia. [14-18] Similarly innocuous mechanical stimuli have been documented to relieve or aggravate symptoms in at least some patients with virtually all of the better-described dystonic syndromes (Tables 2 and 3). As seen in Table 1, the use of light physical contact as a placebo treatment is rather routine, and in some instances quite vigorous procedures were labelled by the authors as placebo, sham or simulated treatment.
Table 2: Sensory Tricks Documented to Relieve Dystonia in Some Patients
Relief by touching face; [14, 15, 18, 21] |
touching cheek, using telephone or thinking about using telephone; 
touching top of head; [14, 15, 17]
full-body rotation of patient; [14, 15]
electrical stimulation of facial skin, moving fingers, vibration of head, neck or shoulders 
Relief by light touch to face, pressure on eyelid;  |
humming, whistling, yawning, coughing, talking, touching specific areas on face, adjusting glasses, rubbing eyelids, covering one eye, chewing gum, picking teeth 
|Jaw and tongue dystonia||
Relief by smiling, talking, singing and thinking about talking;  touching throat. |
|Dystonia musculorum deformans||
Alterations in body position or contact lead to changes in dystonic postures, for example forward flexion of body leads to extension of previously flexed elbow |
Table 3: Stimuli Documented To Provoke or Aggravate Dystonia
Blepharospasm provoked by talking 
Brachial dystonia provoked by violin-playing 
Facial dystonia provoked by eating or thinking about eating 
Oculogyric crisis provoked by movement stimulus in visual field 
Writer’s cramp provoked by drawing  or vibration 
In most individuals, apparently trivial stimuli, such as light touch or modest changes in posture may not produce responses of any clinical interest. This is not to say, however, that the stimuli were not without effect in contributing to alterations of physiological functions. Where reflex centres function normally and global sensory input is not extreme, relatively modest stimuli are unlikely to significantly change the equation linking sensory input and motor output. On the other hand, one may hypothesise that where the sum of sensory input has brought a central nervous system reflex centre almost to the threshold of generating a quantitatively or qualitatively new response, or where the behaviour of a reflex centre has been altered, otherwise trivial additional stimuli might generate responses of some significance. Hypotheses of this sort have previously enjoyed vogue in both chiropractic and osteopathy, unfortunately at a time when clinical investigations of spinal manipulation were poorly developed.  Such hypotheses deserve to be revisited in the light of the recent formalisation of evidence-based care, since they may have much that is useful to say about the design of clinical trials.
The phenomenon of the “sensory trick” or “geste antagoniste” in the dystonic patient alerts us to the reality that otherwise trivial stimuli can produce changes in neuromusculoskeletal function in some people. Dystonia is a term applied to a variety of presentations that share the characteristic of sustained involuntary muscle contractions. Dystonic postures may be accompanied by co-activation of both agonist and antagonist muscles,  or may be attributable to agonist muscle activation alone,  and patients may retain the ability to correct distortions by voluntary relaxation of the involved agonists.
An interesting feature of the dystonias in some patients is that the symptoms may be dramatically improved in the short term by the application of relatively trivial stimuli. For example, simply touching the face with one finger may transiently relieve cervical dystonia. This feature is found in both tardive (drug-induced) and idiopathic cervical dystonia,  and distinguishes these complaints from acute-onset post-traumatic torticollis (muscle-joint syndrome). 
Even in severely debilitating musculorum deformans, profound distortions of the body may be temporarily relieved by the modest vestibular or cutaneous stimulation involved in changing posture, for example having the sitting patient bend forward, or the standing patient turn their whole body 90o to one side.  Because of the dramatic effects achieved by these simple stimuli, the procedures involved have been called sensory tricks, also known as geste antagoniste. As fascinating as the effect of the sensory trick itself is the finding that even mere contemplation of the sensory trick may provide relief in some patients. Conversely, in sensitive dystonic patients, otherwise trivial stimuli, or the contemplation of those stimuli, may provoke dystonic symptoms. 
The dystonias are characterised by sustained involuntary muscle contractions, and it is probably fair to say that the average chiropractic patient manifests sustained involuntary muscle contractions. It would, however, be painting with a very broad brush to call these patients dystonic. On the other hand, it would be naïve to think that there is not some overlap between the chiropractor’s subluxated patient and the medical neurologist’s dystonic patient. Similarities are not limited to subjective clinical symptoms such as pain and stiffness. They include physiological characteristics such as sustained EMG activity in the absence of voluntary activation, [13, 24] and relief of pain and hypertonicity with botulinum toxins. [25-27]
Without proceeding further down that slippery speculative slope, it seems reasonable to propose that the term placebo, with its implication of non-specificity, should be applied sparingly when referring to procedures that involve physical contact with or positioning of subjects in musculoskeletal research. The “control” thought to be possible in clinical trials of physical therapies may be somewhat illusory, and it might be better to think of such studies as “comparative” until there is a better understanding of the pathophysiology of biomechanical disorders.
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