Optic nerve head blood flow in glaucoma

Abstract

These days it is commonly accepted that multiple factors are involved in the etiology of glaucoma. Although many studies have demonstrated that the major risk factor for glaucoma is an increase in intraocular pressure (IOP), some studies, including epidemiologic studies, have suggested an association between glaucoma, especially primary open-angle glaucoma (POAG) and normal-tension glaucoma (NTG), and vascular factors. In this chapter, previous studies regarding the implications of optic nerve head (ONH) blood flow in glaucoma will be reviewed, and then our recent studies will be presented. Some population-based prevalence surveys demonstrated that lower perfusion pressure (blood pressure – intraocular pressure), especially diastolic perfusion pressure, was strongly associated with an increased prevalence of POAG or NTG in the US, Europe and Asia (Tielsch et al, 1995; Bonomi et al, 2000; Leske et al, 2002; Hulseman et al, 2007). These reports suggest that POAG including NTG is associated with alterations in factors related to ocular blood flow. There is also sufficient evidence that optic disc hemorrhage is an important risk factor for glaucoma progression (Daugeliene et al, 1999; Leske et al, 2003; Bengtsson et al, 2008; Prata et al, 2010). Additionally, increasing peripapillary atrophy, which might be related with hypoperfusion to the ONH, was reportedly associated with progressive glaucoma (Araie et al, 1994; Uchida et al, 1998; Daugeliene et al, 1999), and it has been reported that non-use of calcium channel blockers was significantly associated with the progression of visual field loss in NTG (Daugeliene et al, 1999). Clinically usable methods for the measurement of ONH blood flow include fluorescein fundus angiography, scanning laser Doppler flowmetry, and laser speckle flowgraphy. Fluorescein fundus angiography has multiple limitations in quantitatively evaluating ONH blood flow (Hayreh, 1997). Above all, once the dense capillary network in the surface nerve fiber layer of the ONH fills completely with fluorescein-stained blood, underlying ciliary vessels are masked so that no information can be obtained about the circulation in the deeper capillaries. Laser Doppler flowmetry is predominantly sensitive to blood flow changes in the superficial layers of the ONH and gives very little information about the prelaminar and deeper regions of the ONH (Petrig et al, 1999). Laser speckle flowgraphy (LSFG) can detect capillary blood flow in the ONH, probably around the laminar region, and is suitable for monitoring the time-course of its change (Sugiyama et al, 2010). LSFG was develped to facilitate the non-contact analysis of ocular blood flow utilizing the laser speckle phenomenon (Tamaki et al, 1995). Originally, normalized blur and square blur rate had been used as indexes of blood velocity, but later they were experimentally shown to be well correlated with blood flow rate. In the recent version of LSFG, a new parameter, mean blur rate (MBR), which is