Evaluating retinal blood flow autoregulation by means of Optical Coherence Tomography Angiography (OCT-A)

Glaucoma is an eye disease in which degeneration of retinal ganglion cells
results in the loss of the visual field and - if left untreated - blindness.
The concept of an elevated intraocular pressure (IOP) as the only cause of
glaucomatous damage is nowadays considered outdated or at least incomplete,
since some people develop glaucoma without an increase in IOP (normal-tension
glaucoma [NTG]).
The existence of NTG has resulted in the vascular hypothesis of glaucoma, which
focuses on the perfusion of the optic nerve head (ONH) and retinal nerve fiber
layer (RNFL). Indeed, epidemiological studies found associations between low
systemic blood pressure and longstanding hypertension and glaucoma. However, a
blood pressure measurement alone does not reflect ocular perfusion reliably in
individual patients. Therefore, a different approach is needed in order to
better address ocular perfusion.
Optical Coherence Tomography-Angiography (OCT-A) is a recent method developed
to obtain images of the retinal microvasculature with a scanning protocol that
is no different to a regular OCT scan. The novelty lies within the software*s
incorporated algorithm which uses the decorrelation between consecutive B-scans
to generate angiographic images in a matter of seconds. Contrary to the current
gold-standards (Fluorescein and Indocyanine Angiography), OCT-A is
non-invasive. What*s more, it presents excellent repeatability and
reproducibility. By incorporating OCT-A in our study we aim to get a more
accurate evaluation of perfusion by looking directly to the retina.
Arterioles in the retina are characterized by their intrinsic ability to
autoregulate by changing their diameter in response to external stimuli such as
variations in blood pressure. This mechanism is useful in maintaining a
constant blood flow and, thus, constant oxygen supply in the retina. However,
autoregulation fails if the blood pressure is either too low or too
high.
A common issue that occurs when trying to interpret the results of vascular
deficiency in glaucoma is the *chicken-egg* problem. A reduced blood flow could
be either the cause or the result of thinning of the retinal nerve fiber layer
(RNFL). For this reason, we have decided to first include only non-glaucomatous
subjects coming from the full BP range because, in order to unravel the
association between blood pressure and perfusion, it is necessary to first
evaluate the findings of OCT-A in a healthy (that is, non-glaucomatous)
population.
Three aspects are of particular importance:
1) that the population should cover the extremes of blood pressure values.
2) that a subset of individuals presents symptoms associated with autonomous
and/or autoregulatory dysfunction (migraine, cold intolerance, orthostatic
hypotension).
For this reason a subset of this population (cold intolerance) and an equal
number of healthy controls will be evaluated for vasoconstriction after a
cold-provocation test has taken place (Referentie: B.D. Salmenson, J. Reisman,
S.H. Sinclair, D. Burge. Macular capillary hemodynamic changes associated with
Raynaud's phenomenon. Ophthalmology, 99 (6) (1992), pp. 914-919)
3) that we address both early-stage untreated, and long-standing treated
hypertension (each autoregulatory curve might have different properties).

The objective of this study is to compare OCT-A findings between five groups of
non-glaucomatous subjects characterised respectively by a) low symptomatic, b)
low asymptomatic, c) normal asymptomatic, d) untreated high asymptomatic, and
e) treated high blood pressure. Ocular blood flow, autoregulation of the
retinal vessels and RNFL thickness will be addressed.

This is a cross-sectional, observational study. Expected duration is 6-8 months.
Subjects who responded to our advertisement receive the information letter and
the informed consent form. They also fill out a short questionnaire concerning
their ophthalmic medical history. After informed consent is obtained, subjects
will visit the Laboratory of Experimental Ophthalmology (LEO) where initially
the screening procedure will take place. This will include:
•Blood Pressure measurement in sitting position using a standard electronic
device (Omron K6 comfort)
•Visual Acuity and Refraction: using a standardized letter chart
•Non-contact Tonometry: A pulse of air will be used to assess the subject*s eye
pressure, without any physical contact. A raised intraocular pressure is an
indicator of glaucoma.
•Frequency Doubling Perimetry: Visual fields will be assessed for scotomata
using the C20-1 Frequency Doubling Perimetry test. Subjects will place their
head on a chin-rest and will look into a hemispheric bowl. Weak flickering
sinusoidal gratings will be shown at 17 different locations within the bowl
(corresponding to different locations in the visual field) and subjects will be
asked to press a button if they see the stimulus. This will enable
identification of any visual field defects.
Screening is expected to last approximately 20 minutes. The reasoning behind it
is binary. On the one hand, we wish to exclude any eye disease (other than
simple refractory abnormalities) which will perhaps interfere with OCT-A image
quality (e.g. cataract). On the other hand, this study is designed strictly for
non-glaucomatous subjects for the reasons that have already been covered in
previous sections. The aforementioned tests are used in routinely glaucoma
screening. The eye is not touched during the screening.
All eligible subjects will then be asked to remove their contact lenses (if
applicable) and 1% tropicamide (mydriatic) drops will be applied to both eyes.
Pupil dilation is necessary because optimal image quality is crucial to the
project. During the 20-minute waiting time for pupil dilation, subjects will be
asked to fill one short medical history questionnaire including questions
designed to detect a cluster of symptoms related to vascular or autonomic
abnormalities. In addition, we will record each subject*s Body Mass Index (BMI)
by simply measuring their weight and height.
Documentation of the retinal and vessel health will be recorded via fundus
photography with a Topcon fundus camera: the subjects are instructed to put
their chin on the chin-rest, and then they see a flash of light that might
dazzle them for only a few seconds. Two photos of the retina will be taken from
each eye.
Next, the subject will be asked to place their head on a chin-rest in front of
the OCT system (Canon HS-100) and to focus on a fixation cross. Two regular
pictures of each eye will be taken, one of the fovea and one of the optic nerve
head. These images will show the thickness of the subjects retina. Thinning of
the retina is indicative of glaucoma. Subsequently, two more pictures of each
eye will be taken using the angiographic module. Importantly, OCT-A (as well as
any OCT module) is an entirely noninvasive technique: it determines blood flow
from differences in light reflectance in consecutive images. An image is a
matter of <3 seconds for OCT and <10 seconds for OCT-A. The patient will be
kindly instructed not to blink during each scan. Since image artifacts
considerably hamper the study, it is possible that some of the OCT and OCT-A
measurements will be repeated. For this reason we expect this part of the
protocol to last up to 15 minutes (5 minutes for fundus photography and 10
minutes for OCT scans).
For the second part of the protocol, the subject will rest in supine position
for 5 minutes before being asked to stand for another 3 minutes in order to
assess orthostatic hypotension. Blood pressure as well as heart rate will be
recorded using the same device in both positions. OCT-A images will be
immediately obtained from both eyes in the same way they were obtained before.
If the subject belongs to group #1c (cold intolerance, see section 4.1) then
this part of the protocol will not be performed. Instead, a cold pressor test
will take place: subjects will immerse their hands in cold water (0-4°C) for 2
minutes or less if they feel uncomfortable. Blood pressure and heart rate will
also be recorded at the end of the procedure and OCT-A images will again be
immediately obtained. The cold pressor test will also be performed on an equal
number of randomly selected controls from group 3 (see section 4.1). This part
of the protocol is expected to last approximately 20 minutes. Hence, the whole
procedure, taking the arrival of the patient in the LEO as starting point, is
expected to last less than 90 minutes including instructions and a short break.

OCT-A imaging uses only light and does not pose a risk to the subjects.
Subjects are required to place their chin on a chinrest and focus on a cross,
trying not to blink during each scan. Scanning duration is less than 10
seconds. 1% Tropicamide is commonly used in a clinical setting before an OCT
scan to improve the quality of the images. Ophthalmological examinations that
will be performed during screening do not differ from standard tests performed
on every new patient who enters the ophthalmology clinic. During the
ophthalmologic examination no contact will be made with the eye. Lastly, even
though detection of signs of an eye condition that the subject was unaware of
may affect the individual negatively, an early diagnosis is overall beneficial.