title: The effect of myopic axial elongation and posture on the pulsatile ocular blood flow in young normal subjects.
abstract: OBJECTIVE
  This study investigated the effect of myopic axial elongation on pulsatile ocular blood flow (POBF) in young normal subjects. A regression equation was derived to quantify the effect. The effect of posture in POBF for eyes with different axial lengths was studied to determine if axial myopia, with different ocular volumes, would result in a change in POBF with various postures.
  
  
  METHODS
  Seventy-nine normal subjects (38 males and 41 females) with different refractive errors were recruited. The mean age of the subjects was 22 years, and only the right eye was used for analysis. The axial length (AL) was measured, followed by Perkins intraocular pressure (IOP) and POBF with an OBF tonometer (OBF Labs [UK] Ltd) in a sitting posture. An "erected arm" blood pressure (BP) was measured with an automatic sphygmomanometer for the calculation of ocular perfusion pressure (OPP), using the formula: mean BP IOP, where the mean BP was calculated as one third of the systolic blood pressure plus two thirds of the diastolic blood pressure. The measurements of IOP, POBF, and BP were repeated in a supine posture.
  
  
  RESULTS
  The POBF was negatively correlated with AL (r = -0.57, p < 0.01). The regression line was in the form of POBF (microl/min) = -78.5 x AL (mm) + 2655.7. There was a significant reduction (17%) in POBF in a supine posture (p < 0.01). The reduction was not significantly correlated with AL. The pulse amplitude also demonstrated a 10% reduction from postural variation (p < 0.01). The OPP increased significantly (33%) in the supine posture (p < 0.01).
  
  
  CONCLUSIONS
  The POBF was significantly influenced by AL. Therefore, AL should be measured in determining the POBF, especially in young myopic subjects. The amount of postural variation in POBF was similar to previous studies with young subjects and was not affected by AL.
authors.affiliation: Department of Optometry and Radiography, The Hong Kong Polytechnic University, Kowloon, Hong Kong SAR, China. orkclam@polyu.edu.hk
authors.forename: Andrew K C
authors.initials: AK
authors.lastname: Lam
authors.forename: Stephen
authors.initials: S
authors.lastname: Wong
authors.forename: Carly S Y
authors.initials: CS
authors.lastname: Lam
authors.forename: Chi-Ho
authors.initials: CH
authors.lastname: To
mesh_terms: D000328:Adult; D000375:Aging / Q000502:physiology*; D001794:Blood Pressure; D005123:Eye / Q000098:blood supply*; D005260:Female; D006801:Humans; D007429:Intraocular Pressure; D008297:Male; D009216:Myopia / Q000000981:diagnostic imaging* / Q000503:physiopathology*; D011187:Posture / Q000502:physiology*; D011673:Pulsatile Flow; D012016:Reference Values; D012039:Regional Blood Flow; D016683:Supine Position / Q000502:physiology; D014463:Ultrasonography