Case misclassification in studies of spinal manipulation and arterial dissection.

Background: Spinal manipulation has been associated with cervical arterial dissection and stroke but a causal relationship has been questioned by population-based studies. Earlier studies identified cases using International Classification of Diseases Ninth Revision (ICD-9) codes specific to anatomic stroke location rather than stroke etiology. We hypothesize that case misclassification occurred in these previous studies and an underestimation of the strength of the association. We also predicted that case misclassification would differ by patient age.

Methods: We identified cases in the Veterans Health Administration database using the same strategy as the prior studies. The electronic medical record was then screened for the word "dissection." The presence of atraumatic dissection was determined by medical record review by a neurologist.

Results: Of 3690 patients found by ICD-9 codes over a 30-month period, 414 (11.2%) had confirmed cervical artery dissection with a positive predictive value of 10.5% (95% confidence interval [CI] 9.6%-11.5%). The positive predictive value was higher in patients less than 45 years of age vs 45 years of age or older (41% vs 9%, P < .001). We reanalyzed a previous study, which reported no association between spinal manipulation and cervical artery dissection (odds ratio [OR] = 1.12, 95% CI .77-1.63) and recalculated an odds ratio of 2.15 (95% CI .98-4.69). For patients less than 45 years of age, the OR was 6.91 (95% CI 2.59-13.74).

Conclusions: Prior studies grossly misclassified cases of cervical dissection and mistakenly dismissed a causal association with manipulation. Our study indicates that the OR for spinal manipulation exposure in cervical artery dissection is higher than previously reported.