Quantifying retinal blood supply during increased intraocular pressure

Dirk De Brouwere

Introduction

This project is aimed to study the impact of increased intraocular pressure (IOP) on the blood saturation level in the human eye. Glaucoma, worldwide considered as a leading cause for blindness, is commonly associated with high IOP. A major symptom, the narrowing of the visual field (commonly expressed as “tunnel vision”), has been associated with a decreased blood supply to the peripheral blood vessels. The aim of this study is to examine the direct relationship between oxygenation in the retina and IOP by artificially increasing the IOP with an external indentation load.

Sensitivity loss of photoreceptors in the retina can be addressed to a lack of oxygen supplied by the retinal blood vessels and capillaries due to obstruction of the blood flow. High IOP leads to increased mechanical stress on the vessels and capillaries that lead to reduced facility of the blood flow in the retina. In the extreme case, there is no blood flow in the retina when very high IOP blocks the retinal blood vessels.

Based on fast manometric measurements of the IOP, it has been concluded that the blood volume pumped in the retinal vessels during a heart pulse decreases for high values of IOP. A decrease of blood flow in the retina will lead to a loss of oxygenation in the retinal capillaries.

Retinal oximetry is an imaging technique based on the different absorption spectra of oxygenized (Hb0₂) and deoxygenized hemoglobin (Hb) (figure 1) and therefore can measure the concentration of HbO₂ and Hb on a retinal image.

Figure 1 Left: Absorption spectra of HbO2 and Hb. Right: Oximetry map of a portion of a healthy retina (From Johnson et al., JBO 2007).

However high IOP is clinically seen as the major risk factor for the development of glaucoma, it has never been brought in relation with the symptoms of the disease. In this project, we try to find a direct relation between high IOP and the blood supply in the retina in order to obtain a physical model of the glaucoma pathology.

Applying an external load on the eye can artificially increase the IOP. In this project, the load is a lens placed in contact with the cornea allowing us high quality retinal images during indentation.

Figure 2 Indentation principle to increase and calculate IOP

The findings of the study, the impact of the IOP on the oxygenation in the retina, will be the fundaments of a new understanding of the pathology of glaucoma. Based on these observations, a new discussion can start towards new and more efficient treatments in glaucoma.

Methods/Techniques:

Theoretical background

· The absorption spectra of Hb and HbO₂ are shown in figure 1. In a retinal image, the intensity in each pixel is defined by the local absorption coefficient µ_a for a specific wavelength λ. Assuming the presence of only two chromophores, the decomposition of µ_a in the oxidized and deoxidized component leads to following equation:


Where ε represents the extinction coefficient of the chromophore. When comparing images for two different wavelengths, the concentration of Hb and HbO₂ can be calculated.

· The method for increasing the IOP is based on the ocular rigidity, which describes the relation between ocular volume and IOP. The principle is illustrated in figure 2. An external load applied on the cornea changes the ocular volume and thereby increasing the IOP. The calculations for the volume displacement ΔV and IOP are calculated based on the geometry of the load and the cornea.

Experimental Setup

· The optical setup of the retinal oximeter is illustrated in figure 3. The device is based on a typical fundus camera with a white light source. Separate images will be acquired using two different band pass filters. After registration of both images, a retinal image is calculated displaying the oxygen saturation at each pixel.

· Implementation of an indentation lens for synchronous monitoring of the IOP. Analogous to classical tonometry, a load will be applied on the cornea through a lens. The applanation surface will be monitored through the indentation tracker. The load-indentation area relation calculates the IOP. When applying a higher load on the cornea, the IOP can artificially be increased in a controlled way. This technique can be see as an automated Schiotz tonometer.

· Inserting the indentation lens in the optical path of the retinal oximeter will combine both setups mentioned.

· When both instruments are synchronized, healthy subjects will be selected and undergo the experiment. In a sequence ca. 20 seconds, retinal images will be captured. A time versus oxygen saturation level diagram for different loads for various areas in the retina will be calculated. This diagram can be used as the basis to model the influence of the IOP on the blood supply in the retina.

Figure 3 Setup of the optical system

References

F. Delori, "Noninvasive technique for oximetry of blood in retinal vessels," Appl. Opt. 27, 1113-1125 (1988).

Pallikaris IG, Ginis HS, De Brouwere D, Tsilimbaris M, “A Novel Instrument for the Non-Invasive Measurement of Intraocular Pressure and Ocular Rigidity”, Proc. ARVO meeting, May 2006

A. Dastiridou, D. De Brouwere, H.S. Ginis, G. Kymionis, M. Tsilimbaris, I. Pallikaris

“Invasive Measurement Of  The Change In Ocular Pulse Amplitude And Pulsatile Ocular Blood Flow In Relation To The Intraocular Pressure” Proc. Annual ARVO meeting, may 2008