title: Changes in intraocular pressure due to surgical positioning: studying potential risk for postoperative vision loss.
abstract: METHODS
  Parallel group design.
  
  
  OBJECTIVE
  Compare the intraocular pressure responses in the prone flat versus prone Trendelenburg's position.
  
  
  BACKGROUND
  Postoperative vision loss (PVL) complicates approximately 0.05% of spine surgeries. Prone positioning is considered a risk factor because it increases intraocular pressure, which may decrease perfusion pressure to the optic nerve (perfusion pressure = mean arterial pressure - intraocular pressure [IOP]). The prone Trendelenburg's position is often used during spine surgery; however, its effect on optic nerve perfusion is unknown. The purpose of this study is to compare the IOP responses in the prone flat versus prone Trendelenburg's positions to determine if prone Trendelenburg's position also risks PVL.
  
  
  METHODS
  Twenty subjects randomized into 2 groups. Group 1 lay in the prone flat position (0 degrees). Group 2 lay in the prone Trendelenburg's position (-7 degrees). IOPs were measured with a hand-held applanation tonometer while seated, 1 minute after assuming the group's position (Time 0), and at 10-minute intervals for 60 minutes.
  
  
  RESULTS
  The differences in mean IOPs with respect to positions and time were significant (P = 0.0001, P = 0.000). There was a significant difference between sitting and all other times for both groups. In Group 1, there was a significant difference in IOP between Time 0 and all other times prone flat (P < 0.05). In Group 2, there was a significant difference in IOP between Time 0 all other times prone Trendelenburg (P < 0.05).
  
  
  CONCLUSIONS
  IOP increases in the prone Trendelenburg's position, and when combined with other factors, may be a risk factor for PVL. The pathophysiology is discussed and suggestions for clinicians are made.
authors.affiliation: Department of Orthopaedics and Rehabilitation, Brooke Army Medical Center, TX, USA. Kristina.walick@amedd.army.mil
authors.forename: Kristina S
authors.initials: KS
authors.lastname: Walick
authors.forename: John E
authors.initials: JE
authors.lastname: Kragh
authors.forename: John A
authors.initials: JA
authors.lastname: Ward
authors.forename: John J
authors.initials: JJ
authors.lastname: Crawford
mesh_terms: D000328:Adult; D005260:Female; D018475:Head-Down Tilt / Q000009:adverse effects / Q000502:physiology*; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology*; D008297:Male; D009798:Ocular Hypertension / Q000209:etiology* / Q000503:physiopathology; D011183:Postoperative Complications / Q000209:etiology* / Q000517:prevention & control; D016684:Prone Position / Q000502:physiology*; D011897:Random Allocation; D012016:Reference Values; D012306:Risk; D014065:Tonometry, Ocular; D014786:Vision Disorders / Q000209:etiology* / Q000517:prevention & control