Managing glaucoma with OCTA
Optical coherence tomography angiography (OCTA) is a new imaging technique that has the potential to move rapidly into routine clinical practice. OCTA comprises different OCT-based technologies, which enable the non-invasive assessment of retinal perfusion, based on red blood cell movement in the optic nerve head, the peripapillary and the macular retina.
Optical coherence tomography angiography (OCTA) is a new imaging technique that has the potential to move rapidly into routine clinical practice. OCTA comprises different OCT-based technologies, which enable the non-invasive assessment of retinal perfusion, based on red blood cell movement in the optic nerve head, the peripapillary and the macular retina.
In addition to the benefits offered by the non-invasive nature of OCTA over invasive fluorescein angiography in macular diseases, including age-related macular degeneration and diabetic macula oedema, OCTA has been increasingly investigated for use in the detection of glaucoma and understanding the role of vascular alterations in the development of open-angle glaucoma.
OCTA of the disc and peripapillary retina is available with a variety of OCTA instruments, although measurement results are currently not provided by all systems and it is not possible to convert results between the different systems.
Split spectrum
Most commercially-available systems are based on split-spectrum amplitude-decorrelation algorithm (SSADA), which detects red blood cell movement independently from its direction. Of the parameters that can be analysed, peripapillary angioflow density (the perfusion area expressed in the percentage of the total examined peripapillary area) and whole image angioflow density (the perfusion area expressed in the percentage of the total examined image area) provide the most useful information in glaucoma.
In OCTA, perfusion of the different retinal layers are measured separately on the same image. For diseases involving the retinal nerve fibre layer, such as glaucoma, OCTA of the radial peripapillary capillaries layer provides the most useful information.
Recently, it was shown that the diagnostic accuracy of peripapillary and whole image angioflow density in differentiating between glaucoma eyes and normal eyes was similar to that of retinal nerve fibre layer thickness (RNFLT) measurements.1 In the radial peripapillary capillaries layer (i.e., the retinal nerve fibre layer), peripapillary angioflow density showed strong correlation with RNFLT.2
The measurements have favourable reproducibility that is independent of the disease severity, thus measurements made in advanced glaucoma are no less reliable than those made on healthy eyes.2 The sector OCTA parameters show a strong relationship with the sensitivity and defect values of the spatially corresponding visual field areas, so future research may address the focal perfusion-function relationship, similarly to that of the focal structure-function relationship.
It was also recently shown that a topical medical pressure reduction of at least 50% induced a significant increase in the peripapillary angioflow density in untreated high-pressure open-angle glaucoma and ocular hypertension.3 Thus, the relationship between IOP reduction and increase of peripapillary perfusion may be a new area in OCTA research.
Future direction in OCTA research
Since OCTA is relatively new in clinical practice, there are currently no data on its usefulness in the detection and quantification of glaucomatous progression. In addition to the perfusion-function investigations, determination of the role of OCTA in the long-term management of glaucoma is an important future direction. However, due to the relatively slow progression of glaucoma, results cannot be expected before the next 2 or 3 years.
References
1. Yarmohammadi A, et al. Optical coherence tomography angiography vessel density in healthy, glaucoma suspect, and glaucoma eyes. Invest Ophthalmol Vis Sci. 2016;57:OCT451-OCT459.
2. Holló G. Intrasession and between visit variability of sector peripapillary angioflow vessel density values measured with the Angiovue optical coherence tomograph in different retinal layers in ocular hypertension and glaucoma. PLoS ONE 2016; 11(8):e0161631. doi:10.1371/journal. pone.0161631
3. Holló G: Influence of large intraocular pressure reduction on peripapillary OCT vessel density in ocular hypertensive and glaucoma eyes. J Glaucoma. 2017;26:e7-e10.
E-mail: hollo.gabor@med.semmelweis-univ.hu
Dr Holló works at the Head, Glaucoma and Perimetry Unit of the Department of Ophthalmology, Semmelweis University, Budapest, Hungary.
