High resolution imaging of the oxygen saturation and hemoglobin concentration in retinal blood
vessels

The proper retinal functioning depends on the availability of a sufficient
amount of oxygen. Therefore, measuring the amount of oxygen present in the
retinal vessels is important in order to detect and monitor diseases such as
glaucoma and diabetic retinopathy (DR). The main chromophore of blood is
haemoglobin. The absorption of haemoglobin varies based on the oxygenation for
most wavelengths except for few wavelengths which are called isosbestic
wavelengths. Haemoglobin oxygen saturation is the percentage of oxyhemoglobin
(HbO2) out of total haemoglobin content in blood. Several retinal diseases
result in reduced oxygen circulation in the retina causing hypoxia, which is
one of the key drivers of angiogenesis. As such, measuring the retinal blood
oxygen saturation will enable monitoring of the development of hypoxia and thus
will allow preventing the loss of retinal tissue due to vasoproliferation
through timely therapeutic interventions. The light intensity after interaction
of light with blood is estimated by the brightness value of the vessel. This
(reflected) brightness value is affected by light absorbance by blood in the
vessel. The reference light intensity is estimated by the brightness value of
next to the vessel of interest (in the tissue). This brightness value is not
affected by light absorbance by the blood vessel. By choosing a brightness
value close to the vessel for the reference, the light intensities are affected
by similar factors except for the light absorbance by the vessel. Measuring the
absorption at least two wavelengths yield the saturation of the blood.

Non-invasive monitoring of haemoglobin concentration would be quite helpful for
various clinical purposes. Continuous monitoring is essential for patients with
blood-related disorders, in emergency situations, in field conditions, or with
treatments that interfere with Hb-forming in the body. Continuous monitoring
might also be beneficial to establish a connection between anaemia and other
age-related or systemic diseases. For example, a recent cross-sectional study
suggested that high haemoglobin levels indicate decreased risk of diabetic
retinopathy. Measuring the Hb concentration together with retinal oximetry and
blood flow would indicate total metabolic demand of the retina.

The study is an observational pilot study, to determine the feasibility of non-
invasive determination of the oxygenation values and the haemoglobin
concentration of retinal arteries and veins using an MCSLO. Thus, we select a
population comprising of healthy volunteers between 18 to 68 years of age.

Primary Objective: Non- invasive determination of the oxygenation values and
the haemoglobin concentration of retinal arteries and veins by means of retinal
imaging.

The study is an observational pilot study, proof of concept to determine the
feasibility of non- invasive determination of the oxygenation values and the
haemoglobin concentration of retinal arteries and veins using a MCSLO.
Day 1: Study subjects undergo a baseline ophthalmological investigation
which will take 30 minutes.
Day 1 and an arbitrary days after day one in case of multiple visits:
Subjects receive mydriatic eye drops to dilate the pupil. Subjects are asked to
look in an optical system where the retina is illuminated with different colors
of light. Time: about 15 minutes per visit. If the subject consents, a point of
care analyzer requiring a finger prick to draw 8 µliter will be used to asses
hemoglobin concentration.

Repeat visits might be necessary due to continuing improvements to the
experimental device and to validate repeatability of the measurements

Risks are minimal, the measurement method is non-invasive, there will be no
contact with the eye. The illumination of the retina is in accordance with
European standard IEC 60825-1. To improve the quality of the MCSLO images,
pupil dilatation is necessary with a topical mydriatic (Tropicamide). Pupil
dilatation may cause a modest amount of transient photophobia and blurred
vision in some participants, lasting several hours. This may interfere with car
driving, so patients are advised not to drive themselves. Adverse ocular or
systemic side effects of Tropicamide are very rare. Baseline ophthalmological
investigations include a slit lamp examination and eye pressure measurement to
exclude risk factors such as increased pressure in the eye or a narrow anterior
chamber angle that could put the participant at risk for angle-closure
glaucoma. Overall, the risks involved in this study are very low and are
acceptable for the subjects participating in the study.