Here the authors actually observe the opposite to their starting hypothesis... as always an association does not mean that appendicectomy causes PD as such... it may be that there are other confounding factors that were not measured (residual confounding), something for example that increases both the risk of inflammation of the appendix and also increases the risk of PD...
Mov Disord. 2016 Sep 13. doi: 10.1002/mds.26761. [Epub ahead of print]
Svensson E, Horváth-Puhó E, Stokholm MG, Sørensen HT, Henderson VW, Borghammer P.
OBJECTIVES:
The appendix may be a key site for the initiation of Parkinson's disease (PD) pathology. We examined the hypothesis that appendectomy is associated with lower PD risk.
METHODS:
We used Danish medical and administrative registries to construct a cohort of all patients in Denmark with an operation code of appendectomy during 1980-2010 (n = 265,758) and a matched general population comparison cohort (n = 1,328,790). Using Cox regression, we computed hazard ratios and corresponding 95% confidence intervals for PD, adjusting for potential confounders and stratifying on age at appendectomy (≤45 years / > 45 years), sex, and follow-up time.
RESULTS:
During follow-up ( > 10 years), PD incidence was 0.19 and 0.15 per 1,000 person-years at risk in the appendectomy cohort and in the general population comparison cohort, respectively, yielding a slightly increased risk of PD (adjusted hazard ratio = 1.14; 95% confidence interval 1.03-1.27). Findings were consistent after more than 20 years of follow-up and when stratified on age of appendectomy and sex.
CONCLUSION:
Appendectomy was associated with a small increase in PD risk 10 or more years after surgery. © 2016 International Parkinson and Movement Disorder Society.
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