Clinician-friendly outcomes from a randomized controlled trial of spinal manipulation for the treatment of cervicogenic Headache

Background: The number needed to treat (NNT) for one patient to benefit is considered a useful, clinically meaningful way of reporting binary outcomes from randomized trials. Analysis of continuous data from our randomized controlled trial has previously demonstrated a significant and clinically important difference between spinal manipulation and a light massage control favoring manipulation.

Methods: Eighty participants were randomized to receive spinal manipulation or a light massage control (n = 40/group). Improvements in cervicogenic headache pain (primary outcome), disability, and number in prior four weeks were dichotomized into binary outcomes at two thresholds: 30% representing minimal clinically important change and 50% representing clinical success. Groups were compared at 12 and 24-week follow-up using binomial regression (generalized linear models) to compute the adjusted risk difference (RD) between groups and number needed to treat (NNT) after adjusting for baseline differences between groups. Results were compared to logistic regression results.

Results: For headache pain, clinically important improvement (30% or 50%) was more likely for spinal manipulation: adjusted RD = 17%-27% and NNT = 3.8-5.8 (p = 0.004-0.047). Logistic regression only yielded significant results for 50% improvement. For headache disability and number, statistically significant results favoring manipulation were only found in several cases.

Conclusion: Spinal manipulation demonstrated a benefit in terms of a clinically important improvement of cervicogenic headache pain. RD may be easier to interpret than NNT. The study demonstrated that results may depend on threshold for dichotomizing variables into binary outcomes and on the statistical method used in the analysis.