title: Incidence of progressive multifocal leukoencephalopathy in patients with rheumatoid arthritis: a national population-based study.
abstract: BACKGROUND
  Cases of progressive multifocal leukoencephalopathy (PML), a rare but serious disease, have been reported in patients with rheumatoid arthritis (RA) in association with biological therapy, but little is known about the incidence of PML in patients with RA in the absence of treatment exposure.
  
  
  OBJECTIVE
  To estimate the incidence rate of PML in patients with RA compared with the general population, with and without exposure to biological agents.
  
  
  METHODS
  Patients with adult onset RA, exposure to biological agents and a diagnosis of PML from 1999 through 2009 were identified from national registries and linked using each Swedish resident's unique personal identification number. General population comparators matched on age, sex and county were also identified. Crude and age- and sex-standardised incidence rates (cases per 100 000 person-years) were calculated with 95% CI.
  
  
  RESULTS
  66 278 patients with RA and 286 949 general population comparators were included in the study. The incidence rate of PML in the overall RA population was 1.0 (95% CI 0.3 to 2.5) compared with 0.3 (95% CI 0.1 to 0.6) in the general population. The difference in incidence rate was 0.7 (95% CI -0.3 to 17). Among all patients exposed to biological agents, only one patient was diagnosed with PML.
  
  
  CONCLUSIONS
  Data from this national population-based cohort study suggest that patients with RA may have an increased rate of PML compared with the general population.
authors: Clinical Epidemiology Unit (T2), Department of Medicine, Karolinska Institutet, Stockholm SE-171 76, Sweden. earkema@post.harvard.edu
authors: Elizabeth V
authors: EV
authors: Arkema
authors: Ronald F
authors: RF
authors: van Vollenhoven
authors: Johan
authors: J
authors: Askling
mesh_terms: D000911:Antibodies, Monoclonal / Q000627:therapeutic use; D018501:Antirheumatic Agents / Q000627:therapeutic use; D001172:Arthritis, Rheumatoid / Q000175:diagnosis / Q000188:drug therapy / Q000453:epidemiology*; D015897:Comorbidity; D005260:Female; D006801:Humans; D015994:Incidence; D007968:Leukoencephalopathy, Progressive Multifocal / Q000175:diagnosis / Q000453:epidemiology*; D008297:Male; D012042:Registries; D013548:Sweden / Q000453:epidemiology
chemical_list: D000911:Antibodies, Monoclonal; D018501:Antirheumatic Agents