title: Defining disability: development and validation of a mobility-Disability Severity Index (mDSI) in Charcot-Marie-tooth disease.
abstract: OBJECTIVE
  To develop and validate a reliable patient-reported scale that grades the severity of disability in Charcot-Marie-tooth disease (CMT), from an in-depth analysis of patient and healthcare provider perspectives on what mobility changes constitutes mild, moderate and severe disability.
  
  
  METHODS
  In this prospective, cross-sectional study, a 19-item Disability Questionnaire was developed following literature and expert review. Between 2011 and 2012, the Disability Questionnaire was provided to healthcare providers experienced in CMT attending national scientific meetings, and to patients self-registered with the Inherited Neuropathy Consortium--Rare Diseases Clinical Research Consortium on-line contact registry. Provider and patient responses were compared utilising a two-sided unpaired t test with Bonferroni correction. The questionnaire was then assessed for validity, reliability and unidimensionality.
  
  
  RESULTS
  We analysed 259 Disability Questionnaires (167 patients, 92 providers); these showed perfect agreement between patient and provider responses on qualitative descriptions of disability, but significant differences in quantitative responses on items corresponding to minimal or severe disability (p<0.001). Validity and test-retest reliability of the questionnaire was excellent (Cronbach's α=0.96; intraclass correlation coefficients (ICC)=0.977 (0.951 to 0.993). Exploratory factor analysis and the Mokken Scaling Procedure supported the unidimensionality of the mobility-Disability Severity Index.
  
  
  CONCLUSIONS
  The mobility-Disability Severity Index is a unique instrument, categorising disability from the patient's perspective, and will undergo further cross-validation studies in CMT.
authors.affiliation: Department of Neurology, University of Michigan, Ann Arbor, Michigan, USA.
authors.forename: Sindhu
authors.identifier: http://orcid.org/0000-0002-4346-5719
authors.initials: S
authors.lastname: Ramchandren
authors.affiliation: Department of Neurology, University of Iowa, Iowa City, Iowa, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Shy
authors.affiliation: Department of Neurology, University of Michigan, Ann Arbor, Michigan, USA.
authors.forename: Eva
authors.initials: E
authors.lastname: Feldman
authors.affiliation: Department of Radiology, University of Michigan, Ann Arbor, Michigan, USA.
authors.forename: Ruth
authors.initials: R
authors.lastname: Carlos
authors.affiliation: Department of Neurology, Stanford Hospital and Clinics, Stanford, California, USA.
authors.forename: Carly
authors.initials: C
authors.lastname: Siskind
mesh_terms: D000328:Adult; D000368:Aged; D002607:Charcot-Marie-Tooth Disease / Q000175:diagnosis / Q000235:genetics / Q000503:physiopathology*; D003430:Cross-Sectional Studies; D004185:Disability Evaluation; D005260:Female; D005838:Genotype; D006801:Humans; D008297:Male; D008875:Middle Aged; D051346:Mobility Limitation; D012720:Severity of Illness Index; D011795:Surveys and Questionnaires
keywords: Hmsn (Charcot-Marie-Tooth); Neuromuscular; Neuropathy