title: A systematic review of the incidence and prevalence of comorbidity in multiple sclerosis: overview.
abstract: BACKGROUND
  Comorbidity is an area of increasing interest in multiple sclerosis (MS).
  
  
  OBJECTIVE
  The objective of this review is to estimate the incidence and prevalence of comorbidity in people with MS and assess the quality of included studies.
  
  
  METHODS
  We searched the PubMed, SCOPUS, EMBASE and Web of Knowledge databases, conference proceedings, and reference lists of retrieved articles. Two reviewers independently screened abstracts. One reviewer abstracted data using a standardized form and the abstraction was verified by a second reviewer. We assessed study quality using a standardized approach. We quantitatively assessed population-based studies using the I² statistic, and conducted random-effects meta-analyses.
  
  
  RESULTS
  We included 249 articles. Study designs were variable with respect to source populations, case definitions, methods of ascertainment and approaches to reporting findings. Prevalence was reported more frequently than incidence; estimates for prevalence and incidence varied substantially for all conditions. Heterogeneity was high.
  
  
  CONCLUSIONS
  This review highlights substantial gaps in the epidemiological knowledge of comorbidity in MS worldwide. Little is known about comorbidity in Central or South America, Asia or Africa. Findings in North America and Europe are inconsistent. Future studies should report age-, sex- and ethnicity-specific estimates of incidence and prevalence, and standardize findings to a common population.
authors.affiliation: Department of Internal Medicine, University of Manitoba, Canada/Department of Community Health Sciences, University of Manitoba, Health Sciences Center, Canada rmarrie@hsc.mb.ca.
authors.forename: Ruth Ann
authors.initials: RA
authors.lastname: Marrie
authors.affiliation: Mellen Center for MS Treatment and Research, Cleveland Clinic, USA.
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Cohen
authors.affiliation: Department of Neurology and Neurotherapeutics, University of Texas Southwestern, USA.
authors.forename: Olaf
authors.initials: O
authors.lastname: Stuve
authors.affiliation: Department of Basic Medical Sciences, Neurosciences and Sense Organs, University of Bari, Italy.
authors.forename: Maria
authors.initials: M
authors.lastname: Trojano
authors.affiliation: Department of Neurology, Copenhagen University Hospital Rigshospitalet, Denmark.
authors.forename: Per Soelberg
authors.initials: PS
authors.lastname: Sørensen
authors.affiliation: Scientific and Clinical Review Associates, LLC, USA.
authors.forename: Stephen
authors.initials: S
authors.lastname: Reingold
authors.affiliation: Department of Biostatistics, University of Alabama at Birmingham, USA.
authors.forename: Gary
authors.initials: G
authors.lastname: Cutter
authors.affiliation: Department of Internal Medicine, University of Manitoba, Canada.
authors.forename: Nadia
authors.initials: N
authors.lastname: Reider
mesh_terms: D015897:Comorbidity; D006801:Humans; D015994:Incidence; D009103:Multiple Sclerosis / Q000453:epidemiology*; D015995:Prevalence
keywords: Multiple sclerosis; comorbidity; incidence; prevalence; systematic review