RESULTS
Using information collected during 25 years of follow- up, we matched 616 cases of Parkinson’s disease to 3080 controls by age, and 565 cases to 2458 controls by age and the confounder scores. In the age matched group, mean age at study entry was 59.1 years, and mean age at identification of Parkinson’s disease was73.8 years (range 47.5–93.9). The participants with Par- kinson’s disease were less likely than controls to have smoked and substantially less likely to be current smo- kers at the time of matching (2.4% v 4.1%). In the age matched group, cases had higher mean comorbidity scores than controls (1.52 v 1.33). Table 1 shows the characteristics of the cases and controls.
Use of non-aspirin NSAIDs
In the age matched cohort, those who ever regularly used NSAIDs had a slightly increased risk of Parkin- son’s disease (odds ratio 1.28 (95% confidence interval 1.05 to 1.56)) (see table 2). The odds ratio decreased to 1.17 (0.94 to 1.46) in the group matched for age and confounder scores. When we excluded NSAID use within five years of the matching date, the association in the age matched group disappeared (odds ratio 1.18 (0.94 to 1.48)).
There was no clear association between NSAID use in the year before diagnosis of Parkinson’s disease and risk of Parkinson’s disease (table 3). There was an increased risk of Parkinson’s disease in men who had regular NSAID use of 1–2 years’ duration (odds ratio 1.35 (1.07 to 1.70)) and of ≥5 years’ duration (odds ratio 1.48 (1.05 to 2.09)) in the age matched group, but in the group also matched for confounder scores the risk was increased only in those with 1–2 years of use (odds ratio 1.35 (1.05 to 1.75)) (table 4). When we excluded NSAID use within five years of the matching date, the association was borderline in the age matched group but no longer apparent in the group also matched for confounder scores. There was no trend towards increased or decreased risk of Parkinson’s dis- ease across categories of increasing duration of regular use.
Use of aspirin
There was a suggestion of an increased risk of Parkin- son’s disease in men who reported aspirin use for more than four years in the group matched for confounder scores (odds ratio 1.33 (1.00 to 1.78)), but there was no association after excluding aspirin use within five years of the matching date (table 2). There was no associa- tion between Parkinson’s disease risk and aspirin use in the year before the disease was reported (table 3). Men who reported regular aspirin use of 5–9 years had an increased risk of Parkinson’s disease (odds ratio 1.38 (1.02 to1.87)), but the association was not significant for longer durations of use and disappeared after excluding aspirin use within five years of the matching date (table 4).
Tests for effect modification
There was no significant effect modification by either age or smoking status on the relation between Parkin- son’s disease and NSAID or aspirin use. Neither a sen- sitivity analysis excluding those with known vascular disease nor one excluding participants with indicators of chronic pain (headache, migraine, arthritis) changed the results substantially.DISCUSSION
Results from this large nested case-control study do not provide evidence that regular NSAID use decreases the risk of Parkinson’s disease. This is despite careful adjustment for comorbid conditions, which can have a substantial influence on the analysis of drug effects in elderly populations.27 Non-aspirin NSAID use among cases was clustered in the few years before diagnosis, and positive associations between non-aspirin NSAID or aspirin use and Parkinson’s disease tended to disap- pear with increasing lag time. These findings under- score the complex relation between NSAID use and Parkinson’s disease and suggest the presence of con- founding by indication.
Comparison with other studies
A small number of studies provide evidence that NSAID use may decrease risk of Parkinson’s disease. In a pooled analysis of data from the Nurses’ Health Study and Health Professional Follow-up Study, regu- lar users of non-aspirin NSAIDs at baseline had a rela- tive risk of 0.55 (95% confidence interval 0.32 to 0.96) for Parkinson’s disease.10 In another large prospective cohort, ibuprofen use was associated with a lower risk of Parkinson’s disease (relative risk 0.65 (0.48 to 0.89)), whereas aspirin, acetaminophen, and other NSAIDs had no association.9 In one case-control study, a longer duration of regular non-aspirin NSAID use (≥2 years) was associated with a substantially decreased risk of Parkinson’s disease (odds ratio 0.45 (0.26 to 0.74)).14 Two case-control studies suggest that regular users of aspirin have a lower risk of Parkinson’s disease,14 15 while two others suggest an increased risk.8 12
In contrast, we found no convincing protective asso- ciation between aspirin or non-aspirin NSAID use and Parkinson’s disease. The Rotterdam Study, a popula- tion based cohort in which Parkinson’s disease was diagnosed by examination in person, also found an increased risk of Parkinson’s disease among regular users of NSAIDs in a prospective analysis, although the finding was not statistically significant (relative risk 1.50 (0.95 to 2.37)).16 At least three large prospec- tive cohorts found no association between baseline reg- ular use of aspirin and Parkinson’s disease risk,910 although in two of the cohorts a suggestion of decreased risk was seen in users of higher doses (≥2 tablets daily).10
In some ways the Physicians’ Health Study cohort is uniquely suited to investigate the effects of NSAIDs. At baseline, our participants were all tolerant to aspirin and had no imbalance in comorbidities or indications for or against aspirin or NSAID use. Using a matched case-control design, we were able to control for the confounding effect of indications for and contraindica- tions to NSAID use as they developed over timeHowever, a number of important distinctions between the Physicians’ Health Study and prior cohorts must be considered. Firstly, our population was older than other cohorts, with an average age of 74 years at diagnosis of Parkinson’s disease. The effect of a specific exposure can be strongly modified by the
age at which the cohort begins. In addition, the health conscious physicians in our study are exceptionally long lived,28 and 25% of our cases of Parkinson’s dis- ease were diagnosed after age 80. The protective asso- ciation of non-aspirin NSAIDs on Parkinson’s disease may be stronger in younger patients or in those with younger onset disease. If NSAIDs do delay the onset of Parkinson’s disease, this might increase its incidence in old age.29 However, we had no evidence of effect mod- ification by age in our data. Alternatively, younger patients may have different indications for NSAID use or a longer duration of use than older patients.
Secondly, it is possible that, because of the highly selected nature of our cohort, we are missing a group in which a protective benefit of NSAIDs might be seen. However, there is no plausible reason to believe that the effect of NSAIDs on Parkinson’s disease differs in the participants of the Physicians’ Health Study as compared with other populations.
Finally, we were unable to separate the exposure to non-aspirin NSAIDs into individual NSAID types, such as ibuprofen (which may be particularly protec- tive against Parkinson’s disease).9 However, at least in early years of the study, ibuprofen was the most com- monly used non-aspirin NSAID on the US market.30Methodological considerations in non-randomised studies of drug effects
Our findings show the difficulties inherent in the eva- luation of drug effects in non-randomised studies. First is the influence of comorbidity on the relation between NSAID use and Parkinson’s disease. This is particu- larly important in an elderly cohort such as ours. The confounding created by imbalance in comorbidities can be seen in either direction. The positive association between ever use of NSAIDs and Parkinson’s disease disappeared when controls were matched on comor- bidity. While it is difficult to match on individual comorbid conditions, we show that matching on a score is a useful method of balancing comorbidities.31
Another important source of potential bias in non- randomised studies of drugs effects is confounding by indication. We found a positive association between ever use of NSAID and Parkinson’s disease in the age matched group and between cumulative aspirin use and Parkinson’s disease in the group also matched for confounders. These associations tended to disappear with increasing lag time, suggest that NSAID and aspirin use in cases was clustered in the few years before the diagnosis of Parkinson’s disease.
Pain is common in Parkinson’s disease and may be a primary or secondary symptom.32 Primary sensory symptoms originate in the peripheral or central ner- vous system, while secondary pain may be caused by conditions such as dystonia and muscle cramps. Sen- sory symptoms may precede the diagnosis of Parkin- son’s disease for several years, even before motor symptoms develop.19 20 As we did not have the date of symptom onset, it is possible that our patients with Par- kinson’s disease experienced increased pain for a few years before the diagnosis, explaining the increase in NSAID use around this time.
Strengths and limitations of study
Our analysis has a number of strengths. Valid data on NSAID use is difficult to obtain through retrospective means or pharmacy databases, since many NSAIDs may be obtained over the counter and are often used periodically. Unlike other studies, we present data on aspirin and non-aspirin NSAID use collected yearly over 25 years. The physicians who participated in this study (a trial of aspirin for prevention of cardiovascular disease and cancer) were particularly motivated to report NSAID use accurately. Participants in the Phy- sicians’ Health St
RESULTS
Using information collected during 25 years of follow- up, we matched 616 cases of Parkinson’s disease to 3080 controls by age, and 565 cases to 2458 controls by age and the confounder scores. In the age matched group, mean age at study entry was 59.1 years, and mean age at identification of Parkinson’s disease was73.8 years (range 47.5–93.9). The participants with Par- kinson’s disease were less likely than controls to have smoked and substantially less likely to be current smo- kers at the time of matching (2.4% v 4.1%). In the age matched group, cases had higher mean comorbidity scores than controls (1.52 v 1.33). Table 1 shows the characteristics of the cases and controls.
Use of non-aspirin NSAIDs
In the age matched cohort, those who ever regularly used NSAIDs had a slightly increased risk of Parkin- son’s disease (odds ratio 1.28 (95% confidence interval 1.05 to 1.56)) (see table 2). The odds ratio decreased to 1.17 (0.94 to 1.46) in the group matched for age and confounder scores. When we excluded NSAID use within five years of the matching date, the association in the age matched group disappeared (odds ratio 1.18 (0.94 to 1.48)).
There was no clear association between NSAID use in the year before diagnosis of Parkinson’s disease and risk of Parkinson’s disease (table 3). There was an increased risk of Parkinson’s disease in men who had regular NSAID use of 1–2 years’ duration (odds ratio 1.35 (1.07 to 1.70)) and of ≥5 years’ duration (odds ratio 1.48 (1.05 to 2.09)) in the age matched group, but in the group also matched for confounder scores the risk was increased only in those with 1–2 years of use (odds ratio 1.35 (1.05 to 1.75)) (table 4). When we excluded NSAID use within five years of the matching date, the association was borderline in the age matched group but no longer apparent in the group also matched for confounder scores. There was no trend towards increased or decreased risk of Parkinson’s dis- ease across categories of increasing duration of regular use.
Use of aspirin
There was a suggestion of an increased risk of Parkin- son’s disease in men who reported aspirin use for more than four years in the group matched for confounder scores (odds ratio 1.33 (1.00 to 1.78)), but there was no association after excluding aspirin use within five years of the matching date (table 2). There was no associa- tion between Parkinson’s disease risk and aspirin use in the year before the disease was reported (table 3). Men who reported regular aspirin use of 5–9 years had an increased risk of Parkinson’s disease (odds ratio 1.38 (1.02 to1.87)), but the association was not significant for longer durations of use and disappeared after excluding aspirin use within five years of the matching date (table 4).
Tests for effect modification
There was no significant effect modification by either age or smoking status on the relation between Parkin- son’s disease and NSAID or aspirin use. Neither a sen- sitivity analysis excluding those with known vascular disease nor one excluding participants with indicators of chronic pain (headache, migraine, arthritis) changed the results substantially.DISCUSSION
Results from this large nested case-control study do not provide evidence that regular NSAID use decreases the risk of Parkinson’s disease. This is despite careful adjustment for comorbid conditions, which can have a substantial influence on the analysis of drug effects in elderly populations.27 Non-aspirin NSAID use among cases was clustered in the few years before diagnosis, and positive associations between non-aspirin NSAID or aspirin use and Parkinson’s disease tended to disap- pear with increasing lag time. These findings under- score the complex relation between NSAID use and Parkinson’s disease and suggest the presence of con- founding by indication.
Comparison with other studies
A small number of studies provide evidence that NSAID use may decrease risk of Parkinson’s disease. In a pooled analysis of data from the Nurses’ Health Study and Health Professional Follow-up Study, regu- lar users of non-aspirin NSAIDs at baseline had a rela- tive risk of 0.55 (95% confidence interval 0.32 to 0.96) for Parkinson’s disease.10 In another large prospective cohort, ibuprofen use was associated with a lower risk of Parkinson’s disease (relative risk 0.65 (0.48 to 0.89)), whereas aspirin, acetaminophen, and other NSAIDs had no association.9 In one case-control study, a longer duration of regular non-aspirin NSAID use (≥2 years) was associated with a substantially decreased risk of Parkinson’s disease (odds ratio 0.45 (0.26 to 0.74)).14 Two case-control studies suggest that regular users of aspirin have a lower risk of Parkinson’s disease,14 15 while two others suggest an increased risk.8 12
In contrast, we found no convincing protective asso- ciation between aspirin or non-aspirin NSAID use and Parkinson’s disease. The Rotterdam Study, a popula- tion based cohort in which Parkinson’s disease was diagnosed by examination in person, also found an increased risk of Parkinson’s disease among regular users of NSAIDs in a prospective analysis, although the finding was not statistically significant (relative risk 1.50 (0.95 to 2.37)).16 At least three large prospec- tive cohorts found no association between baseline reg- ular use of aspirin and Parkinson’s disease risk,910 although in two of the cohorts a suggestion of decreased risk was seen in users of higher doses (≥2 tablets daily).10
In some ways the Physicians’ Health Study cohort is uniquely suited to investigate the effects of NSAIDs. At baseline, our participants were all tolerant to aspirin and had no imbalance in comorbidities or indications for or against aspirin or NSAID use. Using a matched case-control design, we were able to control for the confounding effect of indications for and contraindica- tions to NSAID use as they developed over timeHowever, a number of important distinctions between the Physicians’ Health Study and prior cohorts must be considered. Firstly, our population was older than other cohorts, with an average age of 74 years at diagnosis of Parkinson’s disease. The effect of a specific exposure can be strongly modified by the
age at which the cohort begins. In addition, the health conscious physicians in our study are exceptionally long lived,28 and 25% of our cases of Parkinson’s dis- ease were diagnosed after age 80. The protective asso- ciation of non-aspirin NSAIDs on Parkinson’s disease may be stronger in younger patients or in those with younger onset disease. If NSAIDs do delay the onset of Parkinson’s disease, this might increase its incidence in old age.29 However, we had no evidence of effect mod- ification by age in our data. Alternatively, younger patients may have different indications for NSAID use or a longer duration of use than older patients.
Secondly, it is possible that, because of the highly selected nature of our cohort, we are missing a group in which a protective benefit of NSAIDs might be seen. However, there is no plausible reason to believe that the effect of NSAIDs on Parkinson’s disease differs in the participants of the Physicians’ Health Study as compared with other populations.
Finally, we were unable to separate the exposure to non-aspirin NSAIDs into individual NSAID types, such as ibuprofen (which may be particularly protec- tive against Parkinson’s disease).9 However, at least in early years of the study, ibuprofen was the most com- monly used non-aspirin NSAID on the US market.30Methodological considerations in non-randomised studies of drug effects
Our findings show the difficulties inherent in the eva- luation of drug effects in non-randomised studies. First is the influence of comorbidity on the relation between NSAID use and Parkinson’s disease. This is particu- larly important in an elderly cohort such as ours. The confounding created by imbalance in comorbidities can be seen in either direction. The positive association between ever use of NSAIDs and Parkinson’s disease disappeared when controls were matched on comor- bidity. While it is difficult to match on individual comorbid conditions, we show that matching on a score is a useful method of balancing comorbidities.31
Another important source of potential bias in non- randomised studies of drugs effects is confounding by indication. We found a positive association between ever use of NSAID and Parkinson’s disease in the age matched group and between cumulative aspirin use and Parkinson’s disease in the group also matched for confounders. These associations tended to disappear with increasing lag time, suggest that NSAID and aspirin use in cases was clustered in the few years before the diagnosis of Parkinson’s disease.
Pain is common in Parkinson’s disease and may be a primary or secondary symptom.32 Primary sensory symptoms originate in the peripheral or central ner- vous system, while secondary pain may be caused by conditions such as dystonia and muscle cramps. Sen- sory symptoms may precede the diagnosis of Parkin- son’s disease for several years, even before motor symptoms develop.19 20 As we did not have the date of symptom onset, it is possible that our patients with Par- kinson’s disease experienced increased pain for a few years before the diagnosis, explaining the increase in NSAID use around this time.
Strengths and limitations of study
Our analysis has a number of strengths. Valid data on NSAID use is difficult to obtain through retrospective means or pharmacy databases, since many NSAIDs may be obtained over the counter and are often used periodically. Unlike other studies, we present data on aspirin and non-aspirin NSAID use collected yearly over 25 years. The physicians who participated in this study (a trial of aspirin for prevention of cardiovascular disease and cancer) were particularly motivated to report NSAID use accurately. Participants in the Phy- sicians’ Health St
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