title: Evidence of posterior fossa hypoplasia in the familial variant of adult Chiari I malformation: case report.
abstract: OBJECTIVE
  A case of familial Chiari I malformation and a review of the literature are presented. Recent posterior fossa morphological studies suggest that the sporadic variant of adult Chiari I malformation may be caused by occipital dysplasia and overcrowding of posterior fossa contents. This analysis was applied retrospectively for two of the three members of this familial variant.
  
  
  METHODS
  A family is described in which symptomatic Chiari I malformation occurred in two generations: monozygotic twin sisters and the daughter of one sister. The monozygotic twins developed symptoms within 1 year of each other, and both had associated syringomyelia. The daughter of one of the twins presented with symptomatic tonsillar herniation alone at a young age.
  
  
  METHODS
  All three family members underwent surgical decompression of the craniovertebral junction, with or without syringosubarachnoid shunting, with good results.
  
  
  CONCLUSIONS
  Cases of familial Chiari I malformation are rare, although they are probably under-reported. An evaluation of posterior fossa morphology in these patients suggests that occipital dysplasia and overcrowding of posterior fossa contents may be the substrate for both familial and sporadic cases of Chiari I malformation and suggests a unifying concept of origin.
authors.affiliation: Department of Neurological Surgery, Mayo Clinic, Rochester, Minnesota 55905, USA.
authors.forename: J L
authors.initials: JL
authors.lastname: Atkinson
authors.forename: E
authors.initials: E
authors.lastname: Kokmen
authors.forename: G M
authors.initials: GM
authors.lastname: Miller
mesh_terms: D000293:Adolescent; D000328:Adult; D001139:Arnold-Chiari Malformation / Q000175:diagnosis*; D003388:Cranial Fossa, Posterior; D004200:Diseases in Twins / Q000175:diagnosis*; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D012189:Retrospective Studies; D012886:Skull / Q000002:abnormalities* / Q000473:pathology*; D013116:Spinal Cord / Q000473:pathology; D014430:Twins, Monozygotic