Research Review by Dr. Michael Haneline©


Dec. 2007

Study Title:

Safety of chiropractic manipulation of the cervical spine: A prospective national survey


Thiel H et al.

Author Affiliations: Department of Research and Professional Development, Anglo-European College of Chiropractic, Bournemouth, United Kingdom; and Department of Pharmacy and Biomedical Sciences, University of Portsmouth, Portsmouth, United Kingdom.

Publication Information:

Spine. 2007; 32(21):2375-8; discussion 2379.


Serious complications following cervical spine manipulation (CSM) are thought to be uncommon, with incidence estimates ranging from 1 in 200,000 to 1 in several million manipulative neck treatments. However, their exact incidence is unknown because estimates have been derived from case reports, case-control studies, and other retrospective data. Accordingly, this study was conducted prospectively, in an effort to provide a more accurate estimate of the risk of serious, as well as minor adverse events following CSM in a sample of chiropractors from the United Kingdom (U.K.).

All 1183 members of the British and Scottish Chiropractic Associations were invited to participate in the study. Using standardized forms, the participating chiropractors recorded details on treatment and outcomes in qualifying patients who received at least 1 CSM procedure (defined as a high-velocity/low-amplitude or mechanically assisted thrust to the cervical spine).

Participating chiropractors recruited patients over a 6-week period on either 100 consecutive treatment consultations or, if the target of 100 was not attained, to the end of the 6-week period. The chiropractors recorded certain details of treatment at the conclusion the CSM treatment session that dealt with any worsening of initial symptoms or the onset of any new symptoms, as well as any adverse event that appeared immediately after CSM.

Any significant (serious) event (defined as “referred to hospital A&E and/or severe onset/worsening of symptoms immediately after treatment and/or resulted in persistent or significant disability/incapacity”) was to be reported on a separate form.

Patients who returned for follow-up visits were queried about the worsening of existing symptoms and onset of any new symptoms in the intervening period. When a patient failed to return for follow-up treatment, the treating chiropractor was contacted to obtain information about any adverse event. If that contact failed, the patient was contacted by telephone and, as a last resort, the patient’s general practitioner was contacted to determine if the patient reported any problems that could possibly be related to chiropractic treatment.

The authors used Hanley's “rule of three” to estimate risk, in case no serious adverse events were reported. This rule states that if none of n patients exhibits the event under investigation, then it can be assumed with 95% confidence that the chance of this event occurring is, at worst, 3 in n. Thus, they would need a sample size comprised of 50000 CSMs to produce a frequency of 1 serious adverse event in approximately 17000 interventions (i.e., 50000/3 = 16666).

Pertinent Results:

  • 377 chiropractors participated in the study, which was 31.9% of the target population
  • 19,722 patients were involved
  • data from 50,276 distinct CSMs and 28,109 treatment consultations were recorded
  • 698 patients failed to return for a scheduled follow-up treatment: it was determined by contacting the chiropractor, the patient or the general practitioner that no serious adverse event had occurred in 285 of these cases (thus, data representing 413 treatment consultations (1.4% of the total) were not recorded)
  • 15,520 patients completed an “intervening period” and reported details at the next treatment session
  • no significant adverse events were reported
  • applying Hanley's “rule of three” to this sample, this means that, with 95% confidence:
  • at worse, the chance of a serious adverse event occurring immediately following a typical chiropractic treatment involving at least 1 CSM would be 1 in 9,600
  • at worse, the chance of a serious adverse event occurring up to 7 days following a typical chiropractic treatment involving at least 1 CSM would be 1 in 5,200
  • at worse, the chance of a serious adverse event following a single CSM would be 1 in 16,800
  • patients reported any worsening of presenting symptoms or onset of any new symptoms following CSM as follows:
  • immediate worsening of presenting symptoms was in most cases musculoskeletal in nature and involved the neck most of the time (1.7% of treatments)
  • immediate onset of new symptoms most commonly involved fainting/dizziness/light-headedness (1.5% of cases)
  • the onset of new or worsening of presenting symptoms in the follow-up period most commonly involved increased neck pain (7.3% of cases)
  • other new or worsening symptoms that occurred in the follow-up period included shoulder/arm pain (4.8% of cases) and reduced neck and upper limb movement (3.9% of cases)

Conclusions & Practical Application:

This study was the first large-scale prospective survey that dealt with CSM, specifically to document details about adverse events that occurred following treatment. Several pertinent findings were presented:
  • The estimates of serious adverse events in this study were categorized as being low to very low.
  • The side effects (worsening or new symptoms) of CSM that were reported by the patients in this study are lower than what has previously been reported.
  • The reported risk rates are similar to those associated with prescription drugs commonly utilized by general practitioners to treat musculoskeletal conditions.
  • The reported risk rates are lower than those reported for acupuncture, which was described as being a “very safe intervention in the hands of a competent practitioner.”
The authors pointed out a number of limitations to the study:
  1. Because serious adverse events following CSM are so rare, a larger sample may have provided a more accurate estimate of the associated risks.
  2. All adverse events associated with the consultations were recorded, even if they were not causally related to CSM.
  3. Reported side effects could actually have been preexisting symptoms or new symptoms that developed apart from the treatment.
  4. The chiropractors could have been dishonest and the researchers had no way of knowing if they were. Consequently, the chiropractors could have recruited patients in a biased way, instead of consecutively, as they were asked to do. Also the chiropractors could have underreported adverse events to avoid recrimination or simply because of poor information gathering skills.
  5. The patients may have been hesitant about reporting negative reactions directly to their chiropractors due to acquiescence bias.
  6. Since only 32% of the profession participated, it is possible that those who volunteered would have had a different risk rate than those who declined to participate.
  7. It is possible that one or more serious adverse events actually did occur among the patients who were lost to follow-up.
In spite of its limitations, however, the authors thought the results of the survey provided evidence that CSM is a relatively safe procedure when administered by registered chiropractors in the U.K.