Kosloff et al. Chiropractic & Manual Therapies (2015) 23:19
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Background The burden of neck pain and headache or migraine among adults in the United States is significant. Survey data indicate 13% of adults reported neck pain in the past 3 months [1]. In any given year, neck pain affects 30% to 50% of adults in the general population [2]. Prevalence rates were reportedly greater in more eco- nomically advantaged countries, such as the USA, with a higher incidence of neck pain noted in office and com- puter workers [3]. Similar to neck pain, the prevalence of headache is substantial. During any 3-month time- frame, severe headaches or migraines reportedly affect one in eight adults [1]. Neck pain is a very common reason for seeking health care services. “ In 2004, 16.4 million patient visits or 1.5% of all health care visits to hospitals and physician offices, were for neck pain ” [4]. Eighty percent (80%) of visits occurred as outpatient care in a physician ’ s office [4]. The utilization of health care resources for the treatment of headache is also significant. “ In 2006, adults made nearly 11 million physician visits with a headache diagno- sis, over 1 million outpatient hospital visits, 3.3 million emergency department visits, and 445 thousand inpatient hospitalizations ” [1]. In the United States, chiropractic care is frequently utilized by individuals with neck and/or headache com- plaints. A national survey of chiropractors in 2003 re- ported that neck conditions and headache/facial pain accounted respectively for 18.7% and 12% of the patient chief complaints [5]. Chiropractors routinely employ spinal manipulative treatment (SMT) in the manage- ment of patients presenting with neck and/or headache [6], either alone or combined with other treatment ap- proaches [7-10]. While evidence syntheses suggest the benefits of SMT for neck pain [7-9,11-13] and various types of headaches [10,12,14-16], the potential for rare but serious adverse events (AE) following cervical SMT is a concern for re- searchers [17,18], practitioners [19,20], professional organi- zations [21-23], policymakers [24,25] and the public [26,27]. In particular, the occurrence of stroke affecting the vertebrobasilar artery system (VBA stroke) has been associ- ated with cervical manipulation. A recent publication [28] assessing the safety of chiropractic care reported, “… the fre- quency of serious adverse events varied between 5 strokes/ 100,000 manipulations to 1.46 serious adverse events/ 10,000,000 manipulations and 2.68 deaths/10,000,000 ma- nipulations ” . These estimates were, however, derived from retrospective anecdotal reports and liability claims data, and do not permit confident conclusions about the actual frequency of neurological complications following spinal manipulation. Several systematic reviews investigating the association between stroke and chiropractic cervical manipulation
have reported the data are insufficient to produce definitive conclusions about its safety [28-31]. Two case – control studies [32,33] used visits to a chiropractor as a proxy for SMT in their analyses of standardized health system databases for the population of Ontario (Canada). The more recent of these studies [32] also in- cluded a case-crossover methodology, which reduced the risk of bias from confounding variables. Both case – control studies reported an increased risk of VBA stroke in as- sociation with chiropractic visits for the population under age 45 years old. Cassidy, et al. [32] found, how- ever, the association was similar to visits to a primary care physician (PCP). Consequently, the results of this study suggested the association between chiropractic care and stroke was noncausal. In contrast to these studies, which found a significant association between chiropractic visits and VBA stroke in younger patients (<45 yrs.), the analysis of a population-based case-series suggested that VBA stroke patients who consulted a chiropractor the year before their stroke were older (mean age 57.6 yrs.) than previously documented [34]. The work by Cassidy, et al. [32] has been qualitatively ap- praised as one of the most robustly designed investigations of the association between chiropractic manipulative treat- ment and VBA stroke [31]. To the best of our knowledge, this work has not been reproduced in the U.S. population. Thus, the main purpose of this study is to replicate the case – control epidemiological design published by Cassidy, et al. [32] to investigate the association between chiroprac- tic care and VBA stroke; and compare it to the association between recent PCP care and VBA stroke in samples of the U.S. commercial and Medicare Advantage (MA) popula- tions. A secondary aim of this study is to assess the utility of employing chiropractic visits as a proxy measure for ex- posure to spinal manipulation. We developed a case – control study based on the experi- ence of commercially insured and MA health plan mem- bers between January 1, 2011 and December 31, 2013. General criteria for membership in a commercial or MA health plan included either residing or working in a re- gion where health care coverage was offered by the in- surer. Individuals must have Medicare Part A and Part B to join a MA plan. The data set included health plan members located in 49 of 50 states. North Dakota was the only State not represented. Both case and control data were extracted from the same source population, which encompassed national health plan data for 35,726,224 unique commercial and 3,188,825 unique MA members. Since members might be enrolled for more than one year, the average Methods Study design and population
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