title: Approximate entropy of self-reported mood prior to episodes in bipolar disorder.
abstract: BACKGROUND
  Approximate entropy (ApEn) measures regularity in time series data, while traditional linear statistics measure variability. Using self-reported mood data from patients with bipolar disorder, this preliminary study addressed whether ApEn could distinguish (i) the 60 days prior to the start of a manic or depressed episode from the 60 days prior to a month of euthymia, and (ii) the 60 days prior to a manic episode from the 60 days prior to a depressed episode.
  
  
  METHODS
  Self-reported mood data from 49 outpatients with bipolar disorder receiving standard treatment were analysed. The data contained 27 episodes (12 manic and 15 depressed), and 43 periods of 1 month of euthymia. For the 60 days prior to episode or euthymia, the ApEn, linear statistics and the correlation between linear and non-linear measures were calculated.
  
  
  RESULTS
  ApEn was significantly greater in the 60 days prior to a manic or depressive episode than the 60 days prior to a month of euthymia. The onset of an episode was associated with greater irregularity in mood. Variability was also significantly larger and correlated with ApEn. ApEn was significantly greater in the 60 days prior to a manic episode than in the 60 days prior to a depressed episode, whereas measures of variability were not significantly different. Mood in the 60 days prior to mania was more irregular than prior to depression.
  
  
  CONCLUSIONS
  Non-linear measures may complement traditional linear measures in the analysis of longitudinal data in bipolar disorder. A larger study is indicated.
authors.affiliation: ChronoRecord Association, Inc., Fullerton, CA 92834, USA. tglenn@chronorecord.org
authors.forename: Tasha
authors.initials: T
authors.lastname: Glenn
authors.forename: Peter C
authors.initials: PC
authors.lastname: Whybrow
authors.forename: Natalie
authors.initials: N
authors.lastname: Rasgon
authors.forename: Paul
authors.initials: P
authors.lastname: Grof
authors.forename: Martin
authors.initials: M
authors.lastname: Alda
authors.forename: Christopher
authors.initials: C
authors.lastname: Baethge
authors.forename: Michael
authors.initials: M
authors.lastname: Bauer
mesh_terms: D000328:Adult; D000339:Affect; D018692:Antimanic Agents / Q000627:therapeutic use; D001714:Bipolar Disorder / Q000175:diagnosis / Q000188:drug therapy / Q000523:psychology*; D003625:Data Collection / Q000706:statistics & numerical data; D042241:Early Diagnosis; D019277:Entropy; D005260:Female; D006801:Humans; D016014:Linear Models; D008137:Longitudinal Studies; D008297:Male; D008432:Mathematical Computing; D008499:Medical Records / Q000706:statistics & numerical data; D008875:Middle Aged; D017711:Nonlinear Dynamics; D011594:Psychometrics / Q000706:statistics & numerical data; D012984:Software; D013223:Statistics as Topic
chemical_list: D018692:Antimanic Agents