title: The effect of axial length on ocular blood flow assessment in anisometropes.
abstract: BACKGROUND
  Pulsatile ocular blood flow (POBF) assessment measures the choroidal circulation and therefore provides data with diagnostic value in certain ocular diseases, such as glaucoma. The technique assumes a constant pressure-volume relationship. The current study investigated the effect of axial length on POBF from subjects with axial anisometropia. Ocular blood supply in the ophthalmic artery was also determined using colour Doppler ultrasonography.
  
  
  METHODS
  Thirty-one normal, anisometropic subjects were recruited, whose ages ranged from 20 to 34 years. They had axial anisometropia (expressed by spherical equivalent) of at least 2 D. After Goldmann tonometry, the POBF of each eye was measured in a supine posture by one examiner, followed by a measurement of the blood flow velocity in the ophthalmic artery using colour Doppler ultrasonography in the same posture by another examiner. There was a 10 min rest between the two techniques. All the measurements were made at around the same time to eliminate any effect from diurnal variation.
  
  
  RESULTS
  The mean anisometropia (expressed by spherical equivalent) was 3.89 +/- 1.96 D and the mean inter-ocular axial length difference was 1.49 +/- 1.00 mm. The anisometropia and axial length were significantly different between the two eyes (paired t-tests: p < 0.001). However, the intra-ocular pressure was similar between the two eyes (paired t-test: p = 0.41). The POBF was significantly lower in the eye with the longer axial length (459.3 microL min-1) than the fellow eye (590.8 microL min-1), paired t-test: p < 0.001. The pulse amplitude was also significantly lower in the eye with the longer axial length (1.61 mmHg) than the fellow eye (1.89 mmHg), paired t-test: p < 0.001. However, the blood flow velocity from colour Doppler ultrasonography did not demonstrate any significant difference between the two eyes (paired t-test: p > 0.05).
  
  
  CONCLUSIONS
  The POBF and pulse amplitude were found to be reduced in the eye with the longer axial length but colour Doppler ultrasonography did not show any significant difference. This suggests that a new pressure-volume relation should be considered in deriving POBF. Practitioners should measure the axial length in POBF assessment.
authors.affiliation: Department of Optometry and Radiography, The Hong Kong Polytechnic University, Hong Kong SAR, China. orkclam@polyu.edu.hk
authors.forename: Andrew K C
authors.initials: AK
authors.lastname: Lam
authors.forename: Suk-Tak
authors.initials: ST
authors.lastname: Chan
authors.forename: Ben
authors.initials: B
authors.lastname: Chan
authors.forename: Henry
authors.initials: H
authors.lastname: Chan
mesh_terms: D000328:Adult; D015858:Anisometropia / Q000000981:diagnostic imaging / Q000473:pathology / Q000503:physiopathology*; D001783:Blood Flow Velocity; D005123:Eye / Q000098:blood supply* / Q000473:pathology; D005260:Female; D006801:Humans; D007429:Intraocular Pressure; D008297:Male; D009880:Ophthalmic Artery / Q000000981:diagnostic imaging / Q000503:physiopathology; D011673:Pulsatile Flow; D018615:Ultrasonography, Doppler, Color