title: [Young onset Parkinson's disease and smoking: meta-analysis].
abstract: BACKGROUND
  Previous systemic reviews have reported a protective effect of smoking against Parkinson's disease (PD). However, the protective effect has not been examined before by any systemic review in patients with young onset PD.
  
  
  OBJECTIVE
  Examine the association between young onset PD and tobacco smoking.
  
  
  METHODS
  We conducted a meta-analysis including all published observational studies that investigated this association before January 2003. All languages were included with no restriction for year of publication. Risk estimate and its 95% confidence interval (95% CI) were extracted or calculated for all localized studies in patients with young onset PD.
  
  
  RESULTS
  Five case-control studies investigated the association between young onset PD risk and ever smoking. The fixed-effect pooled analysis was 0.55 (95% CI: 0.38 0.81), with insignificant homogeneity test.
  
  
  CONCLUSIONS
  This meta-analysis shows a protective effect of tobacco smoking against young onset PD. Our results coincide with the other reviews which did not consider the age of diagnosis of PD. However, it is highly recommended prospective studies for the association between young onset PD risk and ever smoking
authors.affiliation: Cátedra de Medicina Preventiva y Salud Pública, Universidad de Córdoba, Facultad de Medicina, Córdoba, España. fm2faahm@uco.es
authors.forename: M F
authors.initials: MF
authors.lastname: Allam
authors.forename: A S
authors.initials: AS
authors.lastname: Del Castillo
authors.forename: R F-C
authors.initials: RF
authors.lastname: Navajas
mesh_terms: D000367:Age Factors; D006801:Humans; D010300:Parkinson Disease / Q000209:etiology*; D012307:Risk Factors; D012907:Smoking