title: Proposed caudal appendage classification system; spinal cord tethering associated with sacrococcygeal eversion.
abstract: BACKGROUND
  The most commonly used classification system for caudal appendages (aka human tails) dates from the 1980s and classifies appendages (tails) as either true tails or pseudotails. Advances in neuroimaging since the 1980s, however, as well as an ever-increasing number of reported cases, have made this system outdated. Sacrococcygeal eversion is a condition in which the distal sacral and coccygeal vertebrae are curved in a retroverted rather than anteverted direction. It can give rise to one type of caudal appendage. Sacrococcygeal eversion has never been associated with spinal cord tethering in any previously published reports.
  
  
  METHODS
  We reviewed all cases of caudal appendage encountered by pediatric neurosurgeons at Children's Hospital Colorado since 2000 in which the appendage would be classified as a true tail by the most commonly used system mentioned above. We also reviewed cases of sacrococcygeal eversion encountered since 2000 by the same group of pediatric neurosurgeons. We searched the hospital electronic medical record system for additional appendages using the terms "caudal appendage" and "persistent human tail."
  
  
  RESULTS
  We found 9 "true" tails (as classified by the most commonly used system). All 9 were associated with tethering or possible tethering of the spinal cord and 6 were associated with a low-lying conus medullaris. There were 8 cases of sacrococcygeal eversion, including 2 associated with Apert or Pfeiffer syndrome and fibroblast growth factor receptor 2 (FGFR2) mutations; these have previously been reported. There was a single case of sacrococcygeal eversion associated with Goldenhar or Turner syndrome; the former was associated with a potentially tethering lesion. Four cases of sacrococcygeal eversion not associated with any known syndrome were also found; two of these were associated with tethering or potentially tethering lesions.
  
  
  CONCLUSIONS
  Most so-called true tails are likely cutaneous markers for spinal dysraphism and spinal cord tethering and are not remnants of the embryonic human tail. Sacrococcygeal eversion can be associated with spinal cord tethering. Based on our cases, and on review of the literature, we devised a five-category classification system for caudal appendages: (1) soft-tissue caudal appendages, (2) bony caudal appendages, (3) bony caudal prominences, (4) true tails, and (5) "other" caudal appendages.
authors: Department of Neurosurgery, Children's Hospital Colorado, University of Colorado School of Medicine, Aurora, CO, USA. charles.wilkinson@childrenscolorado.org.
authors: C Corbett
authors: CC
authors: Wilkinson
authors: Department of Neurosurgery, Wayne State University School of Medicine, Detroit, MI, USA.
authors: Arianne J
authors: AJ
authors: Boylan
mesh_terms: D005260:Female; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D009436:Neural Tube Defects / Q000473:pathology*; D012445:Sacrococcygeal Region / Q000002:abnormalities* / Q000473:pathology*
keywords: Caudal appendage; Human tail; Sacrococcygeal eversion; Spinal cord tethering