title: Forward shifting of posterior dural sac during flexion cervical magnetic resonance imaging in Hirayama disease: an initial study on normal subjects compared to patients with Hirayama disease.
abstract: OBJECTIVE
  Forward shifting of the posterior cervical dural sac is the most important sign in diagnosing Hirayama disease but can also be seen in normal subjects, causing potential diagnostic dilemma. We aim to explore the degree of forward displacement of posterior dural sac in normal subjects compared to that with Hirayama disease.
  
  
  METHODS
  50 healthy male teenagers and 3 patients with Hirayama disease were recruited into the control group and patient group, respectively. MR imaging of the cervical spine was performed in both neutral and flexion positions for all subjects, with the following parameters measured: maximal distance of forward shifting of posterior dural sac, dimension of dural sac and spinal cord.
  
  
  RESULTS
  Forward shifting of the posterior cervical dural sac was depicted in 46% of normal subjects upon flexion position but without associated cord compression due to intrinsic expansion of the spinal canal volume. This intrinsic compensatory mechanism was inadequate in diseased patients leading to cord compression with significant increment in ratio of anteroposterior diameter of forward displacement of posterior dural wall/anteroposterior diameter of spinal canal ("x/y"), and decrement in ratio of anteroposterior diameter of spinal cord/perpendicular transverse diameter of spinal cord ("a/b").
  
  
  CONCLUSIONS
  Depicting of forward shifting of posterior dural sac alone on flexion position cannot reliably diagnose Hirayama disease, which should be established only if there is forward shifting of posterior dural sac, plus increased ratio of x/y and decreased ratio of a/b on flexion position from associated mass effect on the spinal cord.
authors: Department of Radiology, Tuen Mun Hospital, Tuen Mun, Hong Kong. vincentlai@hkcr.org
authors: Vincent
authors: V
authors: Lai
authors: Yiu Chung
authors: YC
authors: Wong
authors: Wai Lun
authors: WL
authors: Poon
authors: Ming Keung
authors: MK
authors: Yuen
authors: Yat Pang
authors: YP
authors: Fu
authors: Oi Wah
authors: OW
authors: Wong
mesh_terms: D000293:Adolescent; D000328:Adult; D002574:Cervical Vertebrae / Q000473:pathology*; D004388:Dura Mater / Q000473:pathology*; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D010865:Pilot Projects; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014897:Spinal Muscular Atrophies of Childhood / Q000473:pathology*; D055815:Young Adult