S180 Spine • Volume 33 • Number 4S • 2008
Table 4. Odds Ratios and 95% Confidence Intervals (CI) and Accelerated and Bias Corrected Bootstrap 95% CI for Case-Control Estimates of the Association Between Primary Care Physician (PCP) Visits and Vertebrobasilar Stroke
Case Control
Age ! 45 yr
Age ! 45 yr
Exposure
Odds Ratio (95% CI) Bootstrap 95% CI Odds Ratio (95% CI) Bootstrap 95% CI Odds Ratio (95% CI) Bootstrap 95% CI
Any PCP visit 1–1 day
7.22 (4.70–11.08) 3.61 (2.76–4.73) 3.27 (2.67–4.00) 3.11 (2.61–3.69) 2.76 (2.35–3.24) 32.00 (7.36–139.17) 25.19 (8.78–72.24) 16.72 (8.39–33.29 10.89 (6.53–18.16) 6.96 (4.66–10.41)
4.62–11.23 2.73–4.75 2.73–4.00 2.62–3.65 2.35–3.26
11.21 (3.59–35.03) 9.53 (3.96–22.97) 4.81 (2.57–9.02) 4.66 (2.78–7.84) 3.57 (2.17–5.86) 12.00 (1.25–115.36) 25.64 (3.13–209.78) 37.60 (4.80–294.70) 37.60 (4.80–294.70) 11.45 (3.68–35.62)
2.67–52.00 3.52–28.00 2.40–8.72 2.71–7.85 2.08–6.16
6.65 (4.18–10.58) 3.21 (2.41–4.27) 3.12 (2.52–3.87) 2.95 (2.46–3.54) 2.67 (2.25–3.17) 52.00 (6.80–397.50 25.04 (7.41–84.62) 14.39 (6.88–30.08) 9.48 (5.56–16.19) 6.42 (4.17–9.89)
4.16–10.73 2.38–4.32 2.52–3.87 2.47–3.54 2.26–3.17
1–3 days 1–7 days 1–14 days 1–30 days
Headache or cervical PCP visit 1–1 day
*
* * * *
* *
1–3 days 1–7 days 1–14 days 1–30 days
8.69–104.00 8.52–35.63 6.59–18.76 4.68–10.42
6.88–35.31 5.56–16.61) 4.20–10.18
3.50–53.57
*Unable to compute due to small numbers.
is unlikely that PCPs cause stroke while caring for these patients, we can assume that the observed association between recent PCP care and VBA stroke represents the background risk associated with patients seeking care for dissection-related symptoms leading to VBA stroke. Be- cause the association between chiropractic visits and VBA stroke is not greater than the association between PCP visits and VBA stroke, there is no excess risk of VBA stroke from chiropractic care. Our study has several strengths and limitations. The study base includes an entire population over a 9-year period representing 109,020,875 person-years of obser- vation. Despite this, we found only 818 VBA strokes, which limited our ability to compute some estimates and bootstrap confidence intervals. In particular, our age stratified analyses are based on small numbers of ex- posed cases and controls (Table 2). Further stratification by diagnostic codes for headache and neck pain related visits imposed even greater difficulty with these esti- mates. However, there are few databases that can link
incident VBA strokes with chiropractic and PCP visits in a large enough population to undertake a study of such a rare event. A major limitation of using health administrative data are misclassification bias, and the possibility of bias in assignment of VBA-related diagnoses, which has previ- ously been raised in this context. 24 Liu et al have shown that ICD-9 hospital discharge codes for stroke have a poor positive predictive value when compared to chart review. 35 Furthermore, not all VBA strokes are second- ary to vertebral artery dissection and administrative da- tabases do not provide the clinical detail to determine the specific cause. To investigate this bias, we did a sensitiv- ity analysis using different positive predictive values for stroke diagnosis (ranging from 0.2 to 0.8). Assuming nondifferential misclassification of chiropractic and PCP cases, our analysis showed attenuation of the estimates towards the null with lower positive predictive values, but the conclusions did not change ( i.e. , associations re- mained positive and significant—data not shown). The
Table 5. Odds Ratios and 95% Confidence Intervals (CI) and Accelerated and Bias Corrected Bootstrap 95% CI for Case-Crossover Estimates of the Association Between Chiropractic (DC) Visits and Vertebrobasilar Stroke
Case Crossover
Age ! 45 yr
Age ! 45 yr
Exposure
Odds Ratio (95% CI) Bootstrap 95% CI Odds Ratio (95% CI) Bootstrap 95% CI Odds Ratio (95% CI) Bootstrap 95% CI
Any DC visit 0–1 day 0–3 days 0–7 days 0–14 days 0–30 days
1.77 (0.66–4.79) 1.14 (0.52–2.51) 0.80 (0.41–1.56) 1.50 (0.89–2.52) 1.25 (0.76–2.06) 6.67 (1.59–27.90 2.42 (0.88–6.66) 1.77 (0.80–3.94) 3.16 (1.57–6.36) 2.17 (1.09–4.31)
0.49–5.60 0.50–2.76 0.35–1.85 0.84–2.74 0.74–2.13
5.04 (0.82–30.99) 3.44 (0.96–12.28) 12.19 (2.52–58.98) 4.49 (1.60–12.63) 3.60 (1.39–9.35)
* * * *
1.09 (0.30–4.02) 0.61 (0.20–1.84) 0.30 (0.12–0.77) 0.98 (0.51–1.87) 0.86 (0.47–1.56) 2.67 (0.45–15.96) 0.70 (0.14–3.40) 5.18 (0.16–1.66) 1.40 (0.58–3.34) 1.03 (0.45–2.39)
0.00–4.84 0.18–2.11 0.47–2.01 0.45–1.57 *
1.46–10.84
Headache or cervical DC visit 0–1 day
*
*
* * * * *
*
0–3 days 0–7 days 0–14 days 0–30 days
0.70–8.00 0.68–4.68 1.43–7.35 0.97–4.68
17.70 (2.04–153.32)
0.00–6.00
*
*
33.61 (4.24–266.38) 29.47 (3.60–241.54)
0.45–3.41 0.37–2.42
*Unable to compute due to small numbers.
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