Risk of VBA Stroke and Chiropractic Care • Cassidy et al
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Table 2. No. (n) and Percentage (%) of Chiropractic (DC) and Primary Care Physician (PCP) Visits Before the Index Date
Entire Cohort
Age ! 45 yr
Age ! 45 yr
Exposures
Cases (n " 818) Controls (n " 3164) Cases (n " 102) Controls (n " 408) Cases (n " 716) Controls (n " 2756)
Most recent DC visit 0–1 day: n (%) 0–3 days: n (%) 0–7 days: n (%) 0–14 days: n (%) 0–30 days: n (%) Most recent PCP visit 1–1 day: n (%)
6 (0.7) 9 (1.1) 14 (1.7) 27 (3.3) 36 (4.4)
22 (0.7) 40 (1.3) 56 (1.8) 88 (2.8) 125 (4.0) 41 (1.3) 130 (4.1) 290 (9.2) 517 (16.3) 945 (29.9)
* *
*
* *
21 (0.8) 34 (1.2) 42 (1.5) 71 (2.6) 107 (3.9) 35 (1.3) 120 (4.4) 263 (9.5) 471 (17.1) 862 (31.3)
6 (1.5) 14 (3.4) 17 (4.2) 18 (4.4) 6 (1.5) 10 (2.5) 27 (6.6) 46 (11.3) 83 (20.3) 390 (95.6) 13 (3.2)
8 (7.8) 12 (11.8) 13 (12.7) 12 (11.8) 18 (17.6) 25 (24.5) 38 (37.3) 46 (45.1)
6 (0.8) 15 (2.1) 23 (3.2)
63 (7.7) 111 (13.6) 205 (25.1) 311 (38.0) 437 (53.4) 782 (95.6) 21 (2.6) 15 (1.8) 0.13 (0.82) 381 (46.6) 384 (46.9) 53 (6.5) 0.85 (1.09)
51 (7.1) 93 (13.0) 180 (25.1) 273 (38.1) 391 (54.6) 693 (96.8) 16 (2.2)
1–3 days: n (%) 1–7 days: n (%) 1–14 days: n (%) 1–30 days: n (%)
No. of DC visits
None in past month 1 or 2 in past month 3 or more in past month Mean (SD†) in past month None in past month 1 or 2 in past month 3 or more in past month Mean (SD) in past month No. of PCP visits
3039 (96.0)
89 (87.3)
2649 (96.1)
96 (3.0) 29 (0.9)
*
83 (3.0) 24 (0.9)
8 (7.8)
*
7 (1.0)
0.08 (0.52)
0.50 (1.65)
0.09 (0.49)
0.08 (0.60)
0.08 (0.53)
2219 (70.1) 875 (27.7)
56 (54.9) 40 (39.2) 6 (5.9) 0.74 (1.01)
325 (79.7) 79 (19.4)
325 (45.4) 344 (48.0) 47 (6.6) 0.87 (1.10)
1894 (68.7) 796 (28.9)
70 (2.2)
*
66 (2.4)
0.41 (0.73)
0.27 (0.61)
0.43 (0.74)
*Cell size ! 6 and cannot be reported. †SD is standard deviation.
ter control of confounding variables. The case-crossover design controls for time independent confounding fac- tors, both known and unknown, which could affect the risk of VBA stroke. This is important since smoking, obesity, undiagnosed hypertension, some connective tis- sue disorders and other important risk factors for dissec- tion and VBA stroke are unlikely to be recorded in ad- ministrative databases. We also found strong associations between PCP visits and subsequent VBA stroke. A plausible explanation for this is that patients with head and neck pain due to ver- tebral artery dissection seek care for these symptoms, which precede more than 80% of VBA strokes. 23 Since it
these associations were more pronounced in the PCP analyses. However, the data are sparse, and we were unable to compute bootstrap confidence intervals in many cases. Discussion Our study advances knowledge about the association between chiropractic care and VBA stroke in two re- spects. First, our case control results agree with past case control studies that found an association between chiro- practic care and vertebral artery dissection and VBA stroke. 24,25 Second, our case crossover results confirm these findings using a stronger research design with bet-
Table 3. Odds Ratios and 95% Confidence Intervals (CI) and Accelerated and Bias Corrected Bootstrap 95% CI for Case-Control Estimates of the Association Between Chiropractic (DC) Visits and Vertebrobasilar Stroke
Case-Control
Age ! 45 yr
Age ! 45 yr
Exposures
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
1.06 (0.43–2.62) 0.87 (0.42–1.81) 0.98 (0.54–1.77) 1.22 (0.78–1.90) 1.14 (0.78–1.67) 1.59 (0.56–4.54) 1.22 (0.51–2.88) 1.42 (0.71–2.86) 1.36 (0.79–2.33) 0.98 (0.60–1.61)
0.36–2.61 0.40–1.78 0.51–1.79 0.77–1.92 0.75–1.62 0.44–4.67 0.48–3.01 0.66–3.00 0.78–2.34 0.56–1.57
12.00 (1.25–115.36) 3.33 (1.02–10.92) 2.41 (0.98–5.95) 3.07 (1.41–6.70) 3.13 (1.48–6.63)
*
0.55 (0.16–1.85) 0.44 (0.16–1.26) 0.55 (0.23–1.30) 0.82 (0.47–1.45) 0.83 (0.52–1.32) 0.63 (0.14–2.81) 0.41 (0.09–1.77) 0.71 (0.24–2.10) 0.84 (0.41–1.75) 0.63 (0.33–1.19)
0.14–1.93 0.12–1.25 0.21–1.23 0.45–1.47 0.51–1.30 0.00–3.37 0.00–1.72 0.17–2.18 0.35–1.68 0.30–1.13
0.80–14.00 0.80–6.29 1.25–7.31 1.34–7.21
Headache or cervical DC visit 0–1 day
*
* *
0–3 days 0–7 days 0–14 days 0–30 days
5.00 (1.34–18.62) 3.11 (1.16–8.35) 3.27 (1.36–7.90) 3.00 (1.26–7.12)
1.07–9.60 1.23–8.67 1.18–8.00
*Unable to compute due to small numbers.
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