Question

A study comparing smokers to non-smokers history as a risk factor for Parkinson’s Disease reports an incidence rate ratio of 0.3 comparing smokers to non-smokers.

1. What is the study design?

2. If the association is real, do these findings indicate an increase or decrease in risk for Parkinson’s disease?

3. A case-control study comparing Alzheimer’s cases to controls reported cases were less likely to have reported past smoking. Then, what kind of bias may explain these results?

Answer 1

1)          Current or past tobacco smokers are at lower risk of developing Parkinson's disease that non-smokers a long term study of doctors reports. How smoking might person against Parkinson's however remains poorly understood. The many known health risks of tobacco smoking also far exceed "any protective effect", its researchers caution.

               The study "Tobacco smoking and the risk of Parkinson's on disease a 65 year follow up of 30,000 male British doctors," was published in the journal Neurology. Smoking is a well-known risk factor for cardiovascular disease, it appears to exert a protective effect. Previous studies have suggested that Parkinson's incident is inversely associated to smoking, with one study reporting that smoking was associated with 60% lower risk for Parkinson's.  

               But the link between smoking and Parkinson's incident is not well understood as data on smoking habits are usually collected after disease and length of follow up is also limited making conclusions difficult. Researchers at the University of Oxford followed male doctors over 65 years to investigate the risk of Parkinson's disease associated with smoking habits the amount of smoked and the duration of effects among ex-smokers.

2)         The analysis included 29,737 doctors all members of the British Medical Association. Data on smoking habits were collected in seven self-completed questions from 1951 to 1998. Participants were asked to classify themselves as current tobacco smokers, ex-smokers, or those who have never smoked, amount smoked daily, and type of tobacco

                  Researchers observed that the percentage of current smokers declined markedly over the study. At its start in 1951, about 67% of doctors age 65 to 69, were smokers. This number dropped to 8% by 1998, the last year of follow-up. The percentage of those classified as cigarette smokers also decreased from 63% in 1951 to 33% in 1998. Specific cause of death was monitored between 1951 and 2016, participants were followed on average for 35 years of 25,379 deaths registered from 1961 to 2016, the first 10 years of follow-up were excluded, 283 were attributed to Parkinson's disease.

                 The crude death rate from Parkinson's was lower for current smokers , 30 per 100,000 person years, compared to non-smokers 46 per 100,000 person years. The crude death rate is the total number of deaths of residents in a specified geographic areas, divided by the total population for the same area. The next researchers modeled how smoking habits were associated to Parkinson's risk at the start-up and then periodically after each survey questionnaire. Results showed current tobacco smoking was inversely linked to Parkinson's risk at baseline and in the updated model.

                  At baseline 1951 doctors classified as current smokers had a 30% lesser risk of PD compared to those who never smoked. In the updated model, current smokers had a 40% lower disease risk compared with never smokers.Parkinson's risk was also found to depend on the daily amount smoked. A statistically significant inverse correlation in both baseline and updated models was seen between the amount of daily tobacco smoked and Parkinson's incidence.

3)           Doctors who quit smoking for 10 or more years had a 14% lower Parkinson's risk than those who had never smoked , those who had quit smoking for nine or few years had a 29% lower risk. The mechanisms underlying the observed inverse relationship are still not fully understood. Some argue that nicotine may have neuro - protective properties by stimulating the release of dopamine, the brain chemical lacking in Parkinson's patients, but other tobacco compounds may also play a role

                The researchers caution that "the adverse effects of smoking on risks of vascular and respiratory diseases, neoplasms and other noncommunicable diseases for which tobacco is the risk factor greatly exceed any protective effects of current tobacco smoking on the risk" of Parkinson's.

                  Doctors who died of causes other than Parkinson's did so at younger ages a mean of 77, than did those who died of Parkinson's the study noted.

                   The present report demonstrates a causally protective effects of current smoking on the risk of PD(Parkinson's disease), the researchers concluded adding that future studies to understand the mechanisms underlying such a protective effect may be worthwhile and may contribute to better etiologic understanding of PD.