Editorial

Optical Coherence Tomography Angiography:
Considerations Regarding Diagnostic Parameters

Touka Banaee, MD

Department of Ophthalmology and Visual Sciences, University of Texas Medical Branch, Galveston, Texas, USA

ORCID:
Touka Banaee: https://orcid.org/0000-0003-0312-487X

J Ophthalmic Vis Res 2022; 17 (3): 313–316

In this issue of Journal of Ophthalmic and Vision
Research (JOVR), two articles are published
addressing changes in optical coherence
tomography angiography (OCTA) parameters
after acute changes in ocular perfusion pressure
(OPP). The first article by Ashraf Khorasani et al[1]
is a study on changes in vascular density (VD) of
the macula and optic nerve after an acute rise in
intraocular pressure (IOP), and the second paper is
a case report by Mirshahi et al[2] describing OCTA
changes in acute systemic hypertension.

Studies on hemodynamics of the retina and optic
nerve microvasculature is an old but still open area
of research. With technologies evolving over
time, a variety of diagnostic modalities have been
used for detection and documentation of blood
flow and vascular diameter, including fluorescein
angiography, color Doppler ultrasonography,
Doppler velocimetry, laser speckle flowgraphy,
and Doppler OCT.[3–7]

OCTA is a relatively new technology used for
extraction of blood vessels on OCT images by
detecting movement of blood. This technology has
been extensively used to evaluate circulation in
different retinal vascular layers (superficial vascular
plexus, SVP and deep vascular plexus, DVP) and

Correspondence to:

Touka Banaee , MD. Department of Ophthalmology and
Visual Sciences, University of Texas Medical Branch,
300 University Blvd., Galveston, Texas 77555-1106, USA.
E-mail: tkbanaee@gmail.com
Received: 27-05-2022 Accepted: 31-05-2022

Access this article online

Website: https://knepublishing.com/index.php/JOVR

DOI: 10.18502/jovr.v17i3.11567

choriocapillaris. Evaluation of the peripapillary
vascular network and optic disc vessels with OCTA
has also become an active area of research in
glaucoma.

While the retina and optic nerve provide
a unique target for in vivo evaluation of
microvasculature, results of studies on this
intricate system may not always corroborate
with each other. This is due to the complex
interplay between blood pressure, intraocular
pressure, and retinal/optic nerve autoregulation,
where subtle differences in study parameters
can affect the response of blood vessels and
impact the study results. The difference between
ocular arterial pressure and intraocular pressure
(IOP) determines the ocular perfusion pressure
(OPP). As the ocular arterial pressure cannot
be easily measured, brachial artery pressure
has been used as its surrogate in most studies,
although some corrections may be needed in
the upright position.[8] Autoregulation of blood
flow is another factor that affects hemodynamics
of ocular tissues, especially the retina and optic
nerve, and needs to be considered in these types
of studies. As autoregulation may need some time
to fully take place, timing of measurements after a
stimulus/event also seems to be important.[9]

This is an open access article distributed under the Creative Commons
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reproduction in any medium, provided the original work is properly cited.

How to cite this article: Banaee T. Optical Coherence Tomography
Angiography: Considerations Regarding Diagnostic Parameters. J
Ophthalmic Vis Res 2022;17:313–316.

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Editorial; Banaee

Concerning the wide availability of OCTA
machines, this modality has been extensively
used to evaluate changes in retinal/optic nerve
circulation under different conditions. There are
two quantitative measures in OCTA that are mostly
used to describe changes in retinal and optic
nerve vasculature: flux and VD. Flux is the intensity
of flow signals while VD is the proportion of flow
signals on the en-face OCTA image.[10] Although
not as accurate as methods that directly measure
blood flow and vessel diameter such as laser
speckle flowgraphy and Doppler OCT, VD in OCTA
can be considered a useful clinical parameter
to reflect changes in blood flow and vascular
diameter.[11] When using this parameter to study
ocular hemodynamics, some important points
need to be kept in mind:

OCTA detects blood flow above a certain
threshold; above that threshold, the OCTA signal
cannot reflect flow rate. Hence, subtle changes in
blood flow may not be detected by VD. At the same
time, it is very sensitive to changes in small vessels
diameter, even below its resolution.[11]

VD also depends on the total length and
diameter of vessels and will be affected by loss
of small vessels in diseases such as diabetic
retinopathy and retinal vein occlusions.

VD is the sum of flow signals in an image
and includes flow from all vessel diameters in the
specified area, therefore the study area will define
which vessel sizes are included. For example, VD in
the parafoveal area will be a better representative
of small vessels as compared to VD of a whole 3×3
or 6×6 OCTA picture.

VD is affected by multiple parameters in addition
to blood flow, including low signal strength. So,
considering media quality (especially presence
of cataracts) and excluding low signal strength
images is crucial to ensure validity of data.[12]

Previous studies have used various methods to
increase IOP in human eyes, including dark room
prone positioning in eyes prone to angle closure[13]
and application of a suction cup.[6] There are
also studies investigating changes in vasculature
during IOP rise after laser iridoplasty or intravitreal
injections.[9, 14] The response of the vascular
structure may be different with acute rises in
IOP such as when a suction cup is applied, or
after intravitreal injections as compared to slower
increases in IOP as happens after laser iridoplasty
or dark room prone positioning.[13] Therefore,

results of different studies may not be readily
comparable.

The article by Ashraf Khorasani et al is an
interventional study, evaluating the effect of acute
IOP rise by application of a suction cup on VD
in the DVP and SVP, the peripapillary region and
inside the disc in 12 healthy individuals and 12
patients with mild-moderate NPDR. They achieved
a mean IOP elevation of 13.93 ± 3.41 mmHg and
found that VD in both SVP and DCP decreased
in healthy individuals, while in diabetic patients
there was a significant reduction of VD only in
the DVP. In both groups, VD inside the disc was
decreased significantly, while changes in the radial
peripapillary capillaries was not significant.

The two groups had different baseline OCTA
parameters, which can be explained by the
difference in age and the presence of diabetes with
its associated effect on ocular vessels.[15, 16] As
the DVP is considered to be mostly on the venous
side of the retinal circulation,[17] it may be more
affected by changes in intraocular pressure. On the
other hand, older patients with diabetes may have
stiffer arteries that are less affected by changes
in perfusion pressure; hence the SVP, which is
closer to the arteries, may be less affected by a
moderate amount of IOP rise. In this study, changes
in VD in response to IOP rise was not different
between the two groups. These results should be
interpreted reservedly, as the study may have been
underpowered to detect such a difference because
of small sample size.

Another factor to consider is that stress during
placement of the suction cup may cause blood
pressure to rise and may partially mitigate the effect
of IOP rise by increasing ocular prefusion pressure.
Also, the degree of blood pressure rise may differ
based on age and presence of diabetes. Although
blood pressure has been reported to be steady in
healthy subjects during application of a suction cup
in a prior study,[6] it would have been informative
if blood pressure readings at the time of IOP rise
were available.

The different response of the optic disc vessels,
originating from choroidal arteries, and the
peripapillary radial vessels which originate from
the central retinal artery, to acute IOP rise can
be a confirmation of the previously known fact of
different pressures within the retinal and choroidal
systems.[18] Prior studies have not reported a
different response by radial peripapillary capillaries

314 JOURNAL OF OPHTHALMIC AND VISION RESEARCH Volume 17, Issue 3, July-September 2022



Editorial; Banaee

and vessels inside the disc after an acute rise in IOP
by intraocular injections.[9, 19] As stated before,
the complex interplay of BP, IOP, retinal/optic disc
autoregulation and the timing of the measurements
after IOP rise affects the results of studies and
makes standardization and comparisons difficult.

Although a small study with its limitations, this
paper adds to the body of the evidence in this field
and the authors should be congratulated for their
work.

The second article by Mirshahi et al is
a case report describing OCTA findings
in acute hypertensive retinopathy. Acute
hypertensive retinopathy can cause various
tissue reactions alongside the increase in
ocular perfusion pressure: vascular contraction
due to autoregulation, vascular leakage, and
thrombosis.[20] Although an increase in BP
increases OPP, autoregulation counteracts it and
with vascular leakage and thrombosis happening
in acute cases, there may be a decrease in blood
flow and ischemia in OCTA contrary to what is
expected from an increase in OPP. Ischemia is
especially evident in the B scan OCTA images of
this case as parafoveal acute middle maculopathy
(hyper-reflectivity of the outer plexiform layer),
coinciding with locations where capillary loss is
visible in the SVP and DVP. Depth-enhanced
visualization of blood vessels, a characteristic of
OCTA, has enabled better visualization of retinal
capillary loss. Better visualization of ischemia in
the choriocapillaris in comparison to FA can be
attributed to the longer wavelength of infrared light
used in OCTA that penetrates better through the
RPE as compared to the blue light of fluorescein
angiography (FA).

These two articles remind us of the fact that
in biological systems, the response to stimuli
may not follow gross physical rules and need
to be interpreted taking into account variability
of intricate biological responses which may not
always be simple to elucidate.

REFERENCES

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2. Mirshahi A, Roohipour R, Karkhaneh R, Rajabi MB,
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