Open-angle glaucoma (OAG) is a chronic progressive optic neuropathy that is increasing in prevalence worldwide. Currently, intraocular pressure is the only known modifiable risk factor. With lowering of intraocular pressure, the proportion of individuals who experience progression of visual field defects is reduced but continues to occur in some individuals. Many other risk factors have been identified, including decreased ocular perfusion pressure and decreased ocular blood flow. Various imaging methodologies have shown an association between OAG and altered blood flow in the various circulations: retrobulbar, retinal, optic nerve head and choroidal. In addition, different morphological alterations have been found to be associated with OAG. This review will cover the evidence that supports the association between altered ocular blood flow and glaucoma. Furthermore, it serves to describe the future methodologies that will assess ocular metabolism, which will strive to move the field closer to definitively understanding the effect of vascular changes on OAG.
The understanding of glaucoma has come a long way from the identification of IOP as a risk factor for glaucoma. Numerous other risk factors have been identified including decreased OPP, decreased OBF, circadian fluctuations in vascular parameters and vascular dysregulation. However, increased IOP continues to be the only modifiable risk factor for the progression of glaucoma. The association between altered OBF and glaucoma has been repeatedly defined; however, the pathophysiologic effect of altered blood flow on glaucomatous damage remains to be understood. Moreover, there are a lack of progression data for parameters such as OPP and OBF. Owing to a lack of large-scale longitudinal clinical trials, more evidence needs to be present before a recommendation can be made about measuring OBF in a clinical setting. Some of the aforementioned studies have small sample sizes and do not possess large statistical power, which could cause confusion due to reduced reproducibility of the studies. OBF measurements remain to be a research methodology to understand more about the pathophysiology of glaucoma. Nevertheless, the use of the methodologies of to assess OBF are continually providing more information that will be used to further understand the pathophysiology of glaucoma.
From the use of CDI for measuring retrobulbar vessels to SLO angiography measuring retinal and choroidal circulations, the blood flow methodologies are each able to assess a subset of the ocular circulation. However, none are comprehensive in their assessment and a combination of the various methodologies must be used to thoroughly analyze OBF. Furthermore, each methodology has its disadvantages, such as CDI’s inability to measure blood flow volume. The relatively new technologies of retinal oximetry and FD-OCT have shown promise to provide continued useful information, with retinal oximetry’s ability to more directly measure tissue metabolism and FD-OCT’s ability to provide accurate measurements of blood flow in absolute units. For further usefulness of OBF data, a comprehensive and standardized approach needs to be implemented.
- Intraocular pressure is the only known treatable risk factor to decrease progression of open-angle glaucoma.
- Sufficient evidence exists from clinical trials to conclude that ocular blood flow deficits are associated with glaucoma.
- Recent evidence has shown that blood flow deficits lead to structural and functional damage.
- In large population trials, decreased ocular perfusion pressure has been associated with the prevalence and progression of glaucoma.
- Greater fluctuations in ocular blood flow and ocular perfusion pressure have been shown to be associated with the development of glaucoma and progression of visual field loss.
- Currently, there is insufficient evidence to conclude that insufficient blood flow directly causes glaucoma progression.
- Future studies will look at glaucoma progression as it relates to ocular blood flow parameters in longitudinal studies involving an increased number of patients, and more standardized methods.
- Assessment of blood flow will need to move away from surrogate measurers of blood flow and more towards measurement of oxygenation and metabolism of ocular tissues.