title: A "throttle" mechanism in the central retinal vein in the region of the lamina cribrosa.
abstract: OBJECTIVE
  To demonstrate a constriction in the central retinal vein in the region of the lamina cribrosa.
  
  
  METHODS
  A prospective comparative interventional study of 13 controls and 19 patients with central retinal vein occlusion (CRVO) using colour Doppler imaging of the central retinal artery and vein in the region of the lamina cribrosa and optic nerve posterior to the globe.
  
  
  RESULTS
  In controls peak velocities in the vein were higher in the region of the lamina cribrosa than the optic nerve, mean 175 mm/second (mm/s) and 49 mm/s respectively, p<0.0001. The velocities in the artery were also higher in the region of the lamina cribrosa, mean 122 mm/s and 92 mm/s, p = 0.007. The variability of the velocities in the region of the lamina cribrosa was 7.4% in the artery and 15.2% in the vein. The mean ratio of the velocities in the vein (4.2 (SD 2.1)) was significantly higher than the mean ratio in the artery (1.4 (SD 0.4), p<0.0001). In CRVO, the mean ratio in the vein was lower in the affected eyes (2.2 (SD 1.9), p<0.01) and fellow unaffected eyes (2.2 (SD 1.0), p = 0.003) than controls. The values were stable in eight after radial optic neurotomy.
  
  
  CONCLUSIONS
  The presence of a constriction of the vein in the region of the lamina cribrosa can be inferred from the presence of higher blood velocities at this site than further back in the optic nerve. In CRVO there may be a more uniform narrowing of the vein along its course in the nerve. Neurotomy did not affect the measurements.
authors.affiliation: Department of Ophthalmology, St Thomas' Hospital, Lambeth Palace Road, London SE1 7NH, UK. tom@retinasurgery.co.uk
authors.forename: Tom H
authors.initials: TH
authors.lastname: Williamson
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001783:Blood Flow Velocity; D019299:Decompression, Surgical / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009900:Optic Nerve / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery; D011446:Prospective Studies; D015203:Reproducibility of Results; D012161:Retinal Artery / Q000000981:diagnostic imaging; D012169:Retinal Vein / Q000000981:diagnostic imaging / Q000503:physiopathology*; D012170:Retinal Vein Occlusion / Q000000981:diagnostic imaging / Q000503:physiopathology* / Q000601:surgery; D012590:Sclera / Q000098:blood supply; D018615:Ultrasonography, Doppler, Color