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G. K. Rai, B. Gilmartin, J. S. Wolffsohn; The Effect of Accommodation on Pulsatile Ocular Blood Flow. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1040.
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The association between sustained accommodative effort and the onset and progression of myopia is well documented, although evidence for causation is equivocal. This study examined the effect of accommodation on pulsatile ocular blood flow (POBF) to evaluate a putative vascular aetiology for myopia.
The Ocular Blood Flow Analyser (OBFA, Paradigm medical Instruments Inc., UK) was used to measure the changes in the dependent variables intraocular pressure (IOP); POBF and pulse amplitude (PA) in the right eye while the left eye fixated a zero (low); 1.50 (intermediate) and 4D (high) accommodative target (presented randomly) for 3 minutes (myopes=22; emmetropes=19). The fixating eye was rendered functionally emmetropic with soft contact lenses and the accommodative responses were measured in free dioptric space with the Shin-Nippon NVision-K autorefractor. Partial coherent interferometry was used to measure the axial length (AL) and the ocular perfusion pressure (OPP) was calculated as 2/3 mean arterial pressure (MAP) minus IOP, where MAP = 1/3 systolic pressure + 2/3 diastolic pressure.
Mean (+/- sd) accommodation response levels for low, intermediate and high stimulus levels were respectively -0.14 ± 0.41 D, 1.56 ± 0.29 D and 4.19 ± 0.24 D. Only IOP measures were significantly influenced by accommodation (p=0.01) with a significant reduction for high accommodation levels by 1.10 ± 2.10 mmHg (p=0.002). Positive correlations were found between the level of myopia and the changes in IOP on intermediate (r=0.61, p=0.003) and high (r=0.53, p=0.01) accommodation levels. No differences in the POBF responses to accommodation between refractive groups were evident. However for high accommodation levels PA decreased by 0.57 ± 0.53 mmHg in emmetropes but increased by 0.34 ± 1.46 mmHg in myopes (p=0.014), with the difference being independent of inter-subject variations in refractive error, AL and OPP.
Of the dependent variables evaluated, the IOP is shown to reduce significantly only for high accommodation response levels. Consistent with a previous report, the degree of myopia influences IOP on both intermediate and high accommodation levels such that as the refractive error increases the IOP progressively reduces. PA data indicate that choroidal pulsatility increases in the myopes whilst it decreases in the emmetropes with H accommodation levels, although there is no single parameter which accounts for the relationship. Differences in the vascular regulatory mechanisms between emmetropes and myopes during accommodation are demonstrated.
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