title: Insights into the classification of myasthenia gravis.
abstract: OBJECTIVE
  Myasthenia gravis (MG) is often categorized into thymoma-associated MG, early-onset MG with onset age <50 years, and late-onset MG with onset age ≥50 years. However, the boundary age of 50 years old between early- and late-onset MG remains controversial, and each category contains further subtypes. We attempted to classify MG from a statistical perspective.
  
  
  METHODS
  We analyzed 640 consecutive MG patients using two-step cluster analysis with clinical variables and discrimination analysis, using onset age as a variable.
  
  
  RESULTS
  Two-step cluster analyses categorized MG patients into the following five subtypes: ocular MG; MG with thymic hyperplasia (THMG); generalized anti-acetylcholine receptor antibody (AChR-Ab)-negative MG; thymoma-associated MG; and generalized AChR-Ab-positive (SP) MG without thymic abnormalities. Among these 5 subtypes, THMG showed a distribution of onset age skewed toward a younger age (p<0.01), whereas ocular MG and SPMG without thymic abnormalities showed onset age skewed toward an older age (p<0.001 and p<0.0001, respectively). The other 2 subtypes showed normal distributions. THMG appeared as the main component of early-onset MG, and ocular MG and SPMG without thymic abnormalities as the main components of late-onset MG. Discrimination analyses between THMG and ocular MG and/or SPMG without thymic abnormalities demonstrated a boundary age of 45 years old.
  
  
  CONCLUSIONS
  From a statistical perspective, the boundary age between early- and late-onset MG is about 45 years old.
authors.affiliation: Department of Neurology, Tohoku University Graduate School of Medicine, Sendai, Japan.
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Akaishi
authors.affiliation: Division of Biostatistics, Tohoku University Graduate School of Medicine, Sendai, Japan.
authors.forename: Takuhiro
authors.initials: T
authors.lastname: Yamaguchi
authors.affiliation: Department of Neurology, Sendai Medical Center, Sendai, Japan.
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Suzuki
authors.affiliation: Department of Neurology, Hanamaki General Hospital, Hanamaki, Japan.
authors.forename: Yuriko
authors.initials: Y
authors.lastname: Nagane
authors.affiliation: Department of Neurology, Keio University School of Medicine, Tokyo, Japan.
authors.forename: Shigeaki
authors.initials: S
authors.lastname: Suzuki
authors.affiliation: Department of Neurology, Neurological Institute, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Murai
authors.affiliation: School of Health Sciences, Sapporo Medical University, Sapporo, Japan.
authors.forename: Tomihiro
authors.initials: T
authors.lastname: Imai
authors.affiliation: Medical Engineering Course, Department of Engineering, The Faculty of Engineering, Nagasaki Institute of Applied Science, Nagasaki, Japan.
authors.forename: Masakatsu
authors.initials: M
authors.lastname: Motomura
authors.affiliation: Department of Neurology, Tohoku University Graduate School of Medicine, Sendai, Japan.
authors.forename: Kazuo
authors.initials: K
authors.lastname: Fujihara
authors.affiliation: Department of Neurology, Tohoku University Graduate School of Medicine, Sendai, Japan.
authors.forename: Masashi
authors.initials: M
authors.lastname: Aoki
authors.affiliation: Department of Neurology, Hanamaki General Hospital, Hanamaki, Japan.
authors.forename: Kimiaki
authors.initials: K
authors.lastname: Utsugisawa
mesh_terms: D000328:Adult; D017668:Age of Onset; D000368:Aged; D016000:Cluster Analysis; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D009157:Myasthenia Gravis / Q000276:immunology* / Q000473:pathology*; D013945:Thymoma / Q000150:complications / Q000276:immunology / Q000473:pathology; D013953:Thymus Neoplasms / Q000150:complications / Q000276:immunology / Q000473:pathology; D055815:Young Adult