J Manipulative Physiol Ther 1999 (Nov); 22 (9): 559–564 ~ FULL TEXT
Charlotte Leboeuf-Yde, DC, PhD, Iben Axén, DC, Gregers Ahlefeldt, DC,
Per Lidefelt, DC, Annika Rosenbaum, BAppSc (Chiro)d, Thomas Thurnherr, DC
Medical Research Unit,
Amtsrådhuset, Torvet, Denmark.
A retrospective study conducted in Sweden  has determined that about one in four chiropractic patients experiences some form of "positive nonmusculoskeletal side effect" after spinal manipulative therapy (SMT).
In addition, the percentage of patients who experience positive side effects increases with the number of spinal regions adjusted.
The Swedish researchers asked all members of the Swedish Chiropractors Association (SCA) to participate in the study. Eighty-one percent of the SCA membership complied. Each doctor of chiropractic gathered data from 20 patients over a three-week period for a total of 1,504 valid patient questionnaires. Patients were included if they had been previously adjusted within the last two weeks for musculoskeletal complaints. The patients were asked if after their previous visit they ìexperienced any positive changes that do not seem to have anything to do with your back problem?
At least one positive side effect or reaction was reported by 23 percent of the respondents. The more spinal areas that were adjusted, the better their chances of experiencing at least one positive reaction:
The positive, nonmusculoskeletal reactions appear to cluster into a number of system/organ-related classifications. Of those patients who experienced them, here is the breakdown by percentage:
- Respiratory System: 26%
- Digestive System: 25%
- Circulatory System/Heart: 14%
- Eyes/Vision: 14%
The benefits experienced can also be broken down into subcategories:
- Easier to Breathe: 21%
- Improved Digestive Function: 20%
- Clearer/Better/Sharper Vision: 11%
- Better Circulation: 7%
- Changes in Heart Rhythm/Blood Pressure: 5%
- Less Ringing in the Ears/Improved Hearing: 4%
The authors, as is typical in research papers, are careful to point out the limitations of extrapolating the findings. They note that the study does not demonstrate whether the statistical link between treatment and reaction is causal. They assert that ìthe absence of an untreated control group makes it impossible to say whether these reactions are treatment-specific, or if they simply represent normal fluctuations of common symptoms of physiologic function. While these results are very exciting, it is clear that additional research is needed.
The Types of Improved Nonmusculoskeletal Symptoms Reported After Chiropractic Spinal Manipulative Therapy
J Manipulative Physiol Ther 1999 (Nov); 22 (9): 559–564
OBJECTIVE: To investigate the frequency and types of improved nonmusculoskeletal symptoms reported after chiropractic spinal manipulative therapy.
DESIGN: Retrospective information obtained by chiropractors through standardized interview of patients on return visit within 2 weeks of previous treatment.
SETTING: The private practice of 87 Swedish chiropractors (response rate 81%).
SUBJECTS: Twenty consecutive (presumably naive) patients per chiropractor (1504 valid questionnaires returned, 86% of optimal number of replies).
INTERVENTION: Spinal manipulation with or without additional therapy provided by chiropractors.
MAIN OUTCOME MEASURES: Self-reported improved nonmusculoskeletal symptoms (reactions).
RESULTS: At least I reaction was reported after the previous treatment in 21% to 25% of cases. Of these responses, 26% were related to the airway passages (usually reported as "easier to breathe"), 25% were related to the digestive system (mostly reported as "improved function"), 14% were classified under eyes/vision (usually reported as "improved vision"), and 14% under heart/ circulation (about half of these reported as "improved circulation"). The number of spinal areas treated was positively associated with the number of reactions.
CONCLUSION: A minority of chiropractic patients report having positive nonmusculoskeletal reactions after spinal manipulative therapy but such reports cluster predominantly around specific symptoms. It would be interesting to find out if these can be verified objectively and, if so, to investigate if they are caused by the treatment or if they are signs of natural variations in human physiology.
From the Full-Text Article:
This study showed that approximately 25% of patients, when asked, report improvements of various symptoms that appear to be nonmuskuloskeletal in origin. It is interesting that many of these positive side effects are sufficiently similar to allow several relatively large subgroup classifications. Improved breathing, digestion, circulation, and vision were the most common examples.
Eight cases of improved hearing were also identified. Curiously, all these patients had been treated in the thoracic spine (exact level unknown), although always in combination with some other spinal region. Hence, it is not known if they were associated with the fourth thoracic vertebra, as in the case of D.D. Palmer's famous patient. 
Having identified the frequency and types of reactions, two questions now arise. Was the reporting of reactions correct, and was the link between treatment and reactions causally associated?
Was the reporting of reactions correct?
Many of these presumed reactions were familiar to the research team. Hence, the face validity in relation to the types of reactions appeared reasonable. Overreporting as a result of bias among the patients and the chiropractors was unlikely, considering that the patients were naïve (none had sought care primarily for a nonmuskuloskeletal condition). The belief of the research team was that there is no tradition among Swedish chiropractors in general to emphasize this aspect of patient care. This opinion is supported by the previous clinical epidemiologic study of Swedish chiropractors, in which only 1 of 625 patients consulted primarily for a nonmuskuloskeletal symptom. 
Underreporting could of course also have occurred, for example, if the chiropractor's demeanor indicated that these types of reactions do not occur or should not be reported or if the patients did not understand what was meant by the question or could not recall previous reactions. However, the research team, in personal contact with all the chiropractic participants, formed the opinion that Swedish chiropractors, in general, are genuinely interested in getting more objective information on this topic. Patients were uniformly informed, including being shown a schematic drawing based on experiences during extensive pilot testing of the survey instrument. The recall period was limited to a maximum of 2 weeks.
Another problem is that our estimate of reported reactions (23%) refers to the general patient population. Thus it is not known if the proportion of reactions would be the same in specific subpopulations, such as in patients who have specific organic problems. In other words, the appropriate denominators for the various reactions are unknown.
The clinical reality of these reports is also unclear. For example, some of the “scattered” reactions seem to lack credibility, such as “better prostatic function” (sexuality/sex organs), “not so sensitive to sun” (skin), “back of thigh drier” (skin), and “stronger hair” (other). Some patients may be more sensitive or imaginative than others, such as the person who reported 6 rather peculiar reactions.
However, most patients who noticed an improvement report only 1 reaction, and there are some consistently recurring statements, such as “easier to breathe” and “improved hearing,” which support chiropractic folklore. Nonetheless, it would be necessary to confirm their veracity through objective tests. This is an important issue, as shown in a previous uncontrolled study of SMT in asthmatic patients. Although these patients often reported improved lung status after the treatment, this response did not correlate well with the objective tests that measured the true extent of bronchospasm. 
Was the link between treatment and reaction causally associated?
Whether the statistical link between treatment and reaction is causal or not cannot be shown with the present study design. Although a temporal link is a prerequisite for a causal association, the absence of an untreated control group makes it impossible to say whether these reactions are treatment-specific or if they simply represent normal fluctuations of common symptoms of physiologic function.
In the case of SMT and asthma, again, we are aware of 2 high-quality, published, randomized controlled trials that both failed to show that chiropractic SMT is an effective method to treat asthma. [7, 8] These results surely contrast with the subjective reporting of improvement in the study without a control group.  Obviously, unless more controlled randomized trials are conducted in this area, this issue will never be resolved.
Another factor we looked at was the dose-response gradient. Our hypothesis was that if these reactions were indeed the result of a physiologic rather than a psychologic process, the number of reactions per patient would be positively associated with the number of areas treated. This was confirmed. The occurrence of reported reactions increased gradually with the number of areas treated to be about twice as many in those having been treated in 4 areas than those treated in only 1 area.
Some patients did report an improvement of nonmuskuloskeletal symptoms after chiropractic SMT. Whether these reports reflect real physiologic improvements or are based on subjective impressions is not known; if they are real, it is unclear whether they are the result of the treatment or whether they are coincidental events. However, the finding of a positive dose response does warrant further investigation.
We do not wish to purport any specific hypotheses about this issue. We simply wish to point out that this subject is suitable for further research. We hope that our estimates of the frequency with which the various reactions occur can help in the planning of additional studies into this domain. Ideally, such studies would measure validated physiologic responses of symptomatic and asymptomatic subjects in controlled randomized trials.
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