Adaptive Optics and Retinal Blood Flow- Edmund Arthur, OD

During his talk at a Vision Science seminar at Indiana University School of Optometry auditorium in September 2014, Dr. Jesse Schallek of University of Rochester here in the U.S spoke on the “Non-invasive study of blood flow in the living eye: Imaging the transit of single blood cells through the retinal circulation of mice and men”. His talk expatiated on the imaging of transit blood cells in the retina using adaptive optics scanning laser ophthalmoscope (AOSLO), the concept of neurovascular coupling and a possible quantification and translation of the AOSLO method he was describing into the clinical setting. As a first year Vision Science graduate student, I sat among the audience in the auditorium first of all admiring the confidence and the authority with which he spoke on the subject matter but most important of all, my mind was cerebrating on the possible application of the subject matter in his talk to better understanding the pathogenesis as well as the treatment options for retinal vascular diseases such as diabetic retinopathy (DR), age-related macular degeneration (AMD) etc. as well as neurological conditions such as Alzheimer’s disease (AD).

Age-related ocular conditions such as glaucoma, DR, AMD etc. all have retinal vascular changes explaining their pathogenesis [1-4]. For example retinal hypoxia is believed to be responsible for the apoptosis of retinal ganglion cells [1] in glaucoma and loss of pericytes in DR is believed to affect the integrity of the retinal-blood barrier which can lead to poorly regulated blood flow causing retinal vascular occlusion and retinal hypoxia [2, 3]. Abnormalities in retinal perfusion has also been reported in AMD [4]. In the past few years, the use of the Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO) has been proven to provide in vivo retinal images with superior lateral resolution [5, 6]. These systems produce high contrast, high resolution retinal images by measuring the ocular aberrations with a wavefront sensor and correcting them with a deformable mirror providing near diffraction limited imaging of the human eye. Using the AOSLO, it has been shown that blood velocity can be measured in parafoveal capillaries by tracking leukocytes over time [7, 8].

An alternative use of AOSLO imaging that allows direct measurement of blood flow in medium-sized blood vessels by tracking the movement of erythrocytes across an imaging line has been described [9]. Because this technique is based on direct imaging of the light backscattered by erythrocytes, it does not require the use of any contrast dye [9]. Since erythrocytes scatter over a wide range of wavelengths, a near infrared light source (840 nm central wavelength), which makes the imaging process comfortable for the subjects was used [9]. An advantage was taken of the fact that, when focusing on a blood vessel, erythrocytes can directly be visualized as moving bright dots. Separate from the intermittent large intensity variations arising from leukocytes, much smaller objects could be seen continuously moving within the field, and these was attributed to erythrocytes, which constitute 40% to 50% of the blood volume and appeared as bright light-scattering dots [9]. Since erythrocyte velocity is a good indicator of the general blood velocity, measurements of erythrocyte velocity allowed for the calculation of blood flow by taking into account the lumen diameter of the blood vessel [9]. There was however a limitation relating to the size of vessels they can measure. It was difficult to reliably measure flow in the smallest capillaries, due in part to the low contrast. Similarly for the largest retinal vessels, very high blood velocities are not readily measured due to frequency limitations of the horizontal scanner (8 kHz). The solution was however to use a faster horizontal scanner and to control the angle at which the scanning line crosses the blood vessel [9].

Non- proliferative diabetic retinopathy (NPDR) is characterized by pericyte loss, basement membrane thickening, arteriole wall thickening, and microaneurysm formation in its early to moderate stages [10]. Some later stages include more extensive retinal vascular remodeling, with important clinical changes such as intraretinal microvascular abnormalities (IRMA) and capillary nonperfusion [10]. This means that reduced retinal blood supply in an eye with NPDR might be expected which may lead to hypoxia and subsequent apoptosis of retinal neurons. Hence a hypothesis might arise that there is reduced transit blood cells in an eye with NPDR which is not detected by conventional methods. This hypothesis could be tested by the noninvasive AOSLO procedure to actually quantify the amount of transit blood cells flowing through the retina of NPDR patients as against normal individuals.

Age-related macular degeneration (AMD) has been the leading cause of severe impairment of visual function in aging people in industrialized countries. One manifestation of late AMD is geographic atrophy (GA), an area of focal loss of the retinal pigment epithelium (RPE) and outer retina [11]. The second major clinical phenotype; neovascular AMD is manifested by the invasion of choroidal neovascularization (CNV) into the sub-RPE and subretinal space. In neovascular AMD, there are leakage, hemorrhage, and scarring, with progressive loss of the RPE and overlying photoreceptors [12]. This means there is perturbation in the retinal blood supply to retinal neurons in eyes with neovascular AMD. However, the choroidal neovascularization that is seen in neovascular AMD starts beneath the RPE and gradually proliferates the RPE and the outer retina [13]. Earlier detection and management of this might limit the proliferation of the choroidal vessels into the RPE and outer retina and hence prevent the accompanying visual impairment. The modification of the AOSLO method could be done to image and quantify choroidal blood flow so any anomaly could be earlier detected and managed. Hence reduce the blindness that accompanies it.

Glaucoma prevalence is high with the disease prevalence increasing with age after age 40 years, leading to approximately 66 million people affected globally [14,15]. Glaucoma is characterized by the loss of retinal ganglion cells [16]. Open-angle glaucoma (OAG) is the predominant form of glaucoma in Western countries. Although high intraocular pressure (IOP) is the main causal factor, OAG is increasingly considered a neurodegenerative and neuroophthalmologic disorder [17]. The retinal ganglion cell loss in OAG is believed to be caused by hypoxia in the retina [1]. The mechanism underlying the retinal hypoxia and the subsequent retinal ganglion cell loss is not fully understood. Whether or not high IOP is responsible for causing restriction in the flow of blood at optic nerve head where the main ophthalmic artery and vein enters and leaves the retina respectively is not known. This could be easily determined by measuring and comparing the amount of transit blood cells flowing through the retina in eyes with high, low and normal IOPs. The use of the AOSLO method could be used to non-invasively measure retinal blood flow in eyes with varying IOP and hence determine if high IOP causes restriction in retinal blood flow and hence the accompanying hypoxia.

Another school of thought underpinning the reduced blood flow or hypoxia in glaucoma has to do with the concept of neurovascular coupling. Neurovascular coupling states that increased metabolic activity in the retinal neurons calls for increased blood supply [18, 19]. Hence the following question concerning the etiology of glaucoma comes into play; does loss of retinal ganglion cells also cause reduced retinal blood flow since there is reduced metabolic activity due to reduced retinal neurons? This could be investigated by the AOSLO method where the retinal blood flow profile in patients with significant thinning of retinal nerve layer (RNFL) is compared to patients with normal RNFL thickness.

Dementia is an interesting condition that could be explained by the process of neurovascular coupling and further investigated by AOSLO. The retina of the human eye is believed to be an extension of the brain and this is substantiated by retinal conditions such as papilledema which is normally concomitant with hydrocephalus in the brain. Hence the retina serves as open window to study many disorders of the brain in vivo using non-invasive techniques. Dementia and glaucoma are both common neurological diseases, and the prevalence of both increase with age. The estimated prevalence of dementia is approximately 6 to 8% in adults aged ≥ 65 years, and increases from approximately 1.5% at 65 to 69 years of age to ≥ 25% at ≥ 85 years [20]. As a result, it affects more than 25 million people worldwide [20]. The vast majority of dementia cases are related to neurodegenerative diseases, AD in particular. It has been suggested that AD and glaucoma are associated through common risk factors or common mechanisms, such as an abnormal difference of pressure between cerebrospinal fluid pressure and IOP [21], changes in the subarachnoid space of the optic nerve [22] etc. Patients with glaucoma of age ≥ 72 years are believed to be four times at risk of getting AD within a three year period [23]. Additionally, case–control studies evaluating RNFL measured by optical coherence tomography have reported a loss of RNFL in AD patients compared to control groups [24-26]. This is however the point of interest, if neurovascular coupling suggests a relationship between retinal blood flow and neuronal activity [18, 19], then what actually happens to blood flow in people with reduced RNFL layer. Hence this research question could arise: what is the rate of retinal blood flow in patients with AD as compared to a control group? Thus to say retinal blood flow may serve as a potential biomarker in the retina to the detection of AD as a result of the reduced RNFL layer.

So with my research interest in DR and AMD as well as retinal imaging, may be in the course of my PhD or a possible post-doctoral research I would be able to answer some of the research questions I have stated in this piece that I was asking myself as I sat amid the audience in that auditorium. I hope to see a time where AO may help answer some of these questions and hence help better understand the pathogenesis of these retinal vascular and neurological disorders and so yes! AO could yet be a new era of hope for us all involved in the patient eye care as we hope to improve the management of the ocular conditions of our patients.

Bio: Edmund Arthur is a Doctor of Optometry (OD) who graduated from Kwame Nkrumah University of Science and Technology, Kumasi-Ghana in June 2014 and is currently a PhD student in Vision Science with an interest in Diabetes, Diabetic Retinopathy and Retinal Imaging at Indiana University School of Optometry, Bloomington, IN. He works in Dr. Elsner’s lab and is also a student member of the American Academy of Optometry (AAO) with membership number #74683.

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