BLOG: How to use OCTA in glaucoma diagnosis, management
Click Here to Manage Email Alerts
Since its birth, OCT revolutionized our ability to diagnosis and manage glaucoma. Fast forward to an extension of OCT, OCT angiography.
Many clinicians are comfortable with OCT and are aware of its contributions; however, OCTA’s place in glaucoma management is still being discovered. Here we will explore the literature to determine whether there is strong evidence to support the use for OCTA in glaucoma. Specifically, I ask: Can OCTA serve as an analogue to OCT? What advantages does OCTA have over standard OCT? Do changes on OCTA correlate with visual field deficits? Can OCTA provide an additional prognostic value?
I will also share what I have learned from personal experience with the technology, afford concrete ways to use this device in the setting of glaucoma and discuss general limitations.
Glaucoma’s connection to retinal vasculature
Abundant evidence shows there is a vascular component to glaucomatous damage (Chan et al.).
One can appreciate the arcuate reduction in vessel density extending from the optic nerve head in an OCTA obtained from a documented glaucoma patient.
OCTA can blueprint vascular networks by detecting subtle changes in the amplitude of reflectance caused by moving red blood cells. Absence of blood flow (or very low blood flow) indicates a reduction in vessel density. All brands of OCTA segment the layers of the retina and give blood flow information for each layer and for each region (superficial, deep, choroidal, etc.). Glaucomatous vessel density changes are most evident when examining the superficial capillary plexus in my experience. Studies of OCTA in cases of macular changes in the setting of glaucoma support the notion that the superficial vasculature is primarily affected in the disease (Takusagawa et al.). These findings should be of no surprise because they agree with the anatomy and the localization of glaucomatous damage.
Relationship between OCTA, RNFL
Data from multiple studies have demonstrated a strong correlation between vessel density loss and retinal nerve fiber layer (RNFL) loss seen within glaucomatous eyes (Akil et al.). These data suggest that OCTA has an equal ability to detect subtle differences in vessel density between normal, pre-perimetric and patients with mild primary open angle glaucoma.
To highlight advantages afforded by OCTA, we need to discuss the limitations of standard OCT. For one, OCT-RNFL databases have trouble detecting pathology in optic nerves with abnormal anatomy or in eyes with structures that influence RNFL thickness, such as myelinated nerve fiber layers or epiretinal membrane. If one is able to assess the vessel density in the area of concern with OCTA, one can confirm or rule out pathology at that moment, removing the need to wait for RNFL loss to confirm disease. Moreover, due to the lack of need for a normative database, OCTA analysis simply calls for different analytical skills than standard OCT — qualitative vs. quantitative — which can be advantageous in some instances. Simply put, OCTA forces clinicians to make decisions based on the anatomy vs. computational algorithms.
The reduction in vessel density emanating superior temporal from the disc corresponds with subtle RNFL loss, as demonstrated by the vertical green line on the TSNIT curve. This finding could be easily overlooked on the TSNIT curve or if one falls into a habit of relying on sector analysis. The minimum rim width (MRW) scans also fail at detecting this subtle change.
OCTA analysis encourages detecting small nuances that can prompt further investigation with other diagnostic testing such as visual field.
OCTA’s correlation with visual field loss
There is strong evidence that glaucomatous vessel density loss strongly correlates with visual field loss, ranging from mild to severe functional loss. Yarmohammadi and colleagues go even further, showing that the visual field loss in glaucomatous patients correlates better with OCTA than with OCT-RNFL.
One theory that explains this is that dysfunctional or stressed retinal ganglion cells (RGCs) can cause decreased visual field sensitivity (Malik et al.) and may have reduced or absent blood flow, which would be reflected as a reduction in vessel density on OCTA. However, these sickly, pre-apoptotic RCGs would not cause a reduction in RNFL thickness because they have yet to atrophy to cause measurable thinning (Munguba et al.). Ergo, vessel density may serve as a better analogue to RGC function than structural loss. These findings suggest that an OCTA scan is warranted in patients who have repeatable visual field deficits in the absence of thinning on standard OCT.
Which is first, RNFL loss or vasculature loss?
Although there is still debate over whether RNFL loss precedes vascular loss or vice versa, the evidence demonstrating OCTA’s strong correlation with early RGC dysfunction provides insight. In theory, cellular dysfunction precedes cellular death; however, more research is needed to answer this question fully. Nonetheless, if an OCTA scan reveals a substantially larger area of vessel density loss compared with corresponding RNFL loss, closer follow-up is warranted. In these patients, vessel density loss likely serves as a harbinger for future RNFL loss.
Whether it is uncertainty due to anomalous anatomy, repeatable visual field defects that have yet to be correlated with structural loss or when simply faced with conflicting data, OCTA has shown it can help elucidate the underlying clinical picture.
However, OCTA does not come without its limitations and by no means should it replace standard OCT imaging at this point. In some instances, the OCTA device will not segment the various layers of the retina without error. Dynamically scrolling through cross sections may be more useful than the static images in these cases. OCTA also collects far more data than standard OCT and, thus, requires a greater deal of patient cooperation. Acquiring a quality scan on an undilated pupil or on a patient that has issues with fixation is challenging. Lastly, OCTA cannot determine the etiology of vessel density loss. Any confounding factors that have influence of retinal perfusion need to be considered during analysis.
References:
- Akil H, et al. PLoS One. 2017;doi:10.1371/journal.pone.0170476.
- Chan KKW, et al. BMJ Open Ophthalmol. 2017;doi:10.1136/bmjophth-2016-000032.
- Malik R, et al. Clin Exp Ophthalmol. 2012;doi:10.1111/j.1442-9071.2012.02770.x.
- Munguba GC, et al. Invest Ophthalmol Vis Sci. 2014;doi:10.1167/iovs.14-14525.
- Takusagawa HL, et al. Ophthalmol. 2017;doi:10.1016/j.ophtha.2017.06.002.
- Yarmohammadi A, et al. Ophthalmol. 2016;doi:10.1016/j.ophtha.2016.08.041.
For more information:
Ray Farmer is a fourth-year optometry student from SUNY College of Optometry. He saw these patients at VA Boston with Doug Rett, OD, FAAO, and is guest writing the column this month.
Collapse