Researching possible biomarkers for early diagnosis of the Parkinson ‘s disease

Researchers at the University of Iceland are currently investigating whether there is a link between brain changes caused by Parkinson’s disease and alterations in the retinal blood vessels of the eye. If such a link can be confirmed, it could potentially be used for early diagnosis of the disease and improve the quality of life of those who are diagnosed with it.

It is estimated that around 10 million people worldwide suffer from Parkinson‘s disease and around 1,200 people in Iceland. Therefore, the disease is the second most common neurodegenerative disease behind Alzheimer‘s and its prevalence increase with age. In people 60 and over the rate of the disease is at 1% and 4% of those diagnosed are under 50 years of age. The number of cases is expected to increase considerably by 2050 when more than 25 million people are projected to be living with the disease worldwide.

Þórunn Scheving Elíasdóttir, associate professor in the Faculty of Nursing and Midwifery, leads the group of researchers, which is a collaboration between the university, Landspitali – University Hospital, and Reykjalundur Rehabilitation.

No reliable method for early diagnosis

“The Parkinson’s disease is characterised by increasing loss of dopamine-forming cells in the brain’s substantia nigra and abnormal deposits and accumulation of so-called alpha synuclein proteins in the brain. These proteins have also been found in other areas of the central nervous system, e.g. in the inner retina,” Þórunn explains.

Diagnosis of the Parkinson’s disease is based on clinical movement symptoms when 60 to 80% of dopamine forming cells in the brain have stopped working but people can start noticing symptoms related to the disease at least five years prior. Þórunn points out that today there is no reliable method or known biomarkers for early diagnosis before the clinical movement symptoms arise. However, early diagnosis could prove immensely important to increasing the patient’s quality of life.

“The aim of this study is to evaluate whether there are changes in the oxygenation in patient’s retinal blood vessels that could be used as a biomarker for early diagnosis of the disease,” Þórunn explains.

Brain degeneration impacts the retina

In recent years, attention has focussed on how pathological changes in the brain due to neurodegenerative diseases appear in retinal blood vessels. “The inner retina is neural tissue, and research has demonstrated changes in oxygen saturation in retinal blood vessels of people with multiple sclerosis, or MS, mild to severe dementia because of Alzheimer’s disease and people with mild cognitive impairment which in some cases develops into dementia like Alzheimer’s,” Þórunn explains.

The connection between the retina and the brain is based on that during fetal development the retina develops from the diencephalon (posterior part of the forebrain), and the vascular structure of the retinal blood vessels closely resembles the cerebral microvasculature, as well as cerebral blood flow and its regulation.

Results from studies on patients with Parkinson’s disease show that neurodegeneration in the brain is reflected in changes in retinal function, partly due to dopamine deficiency,” says Þórunn, continuing: “Both imaging studies and electrophysiological measurements have demonstrated degeneration of the neural tissue of the inner retina and reduced neuronal responses on retinal recordings (pattern electroretinography), as well as changes in the amplitude of visual evoked potentials from the visual cortex of the brain.”

A collaborative project between the University of Iceland, Landspítali, and Reykjalundur

The study is threefold and involves measurements of oxygen saturation in retinal vessels, imaging of retinal tissue, and electrophysiological measurements of neuronal responses in the retinal neurons of the eye and in the visual cortex of the brain in response to repeated light stimulation. “The results of the oxygen saturation and vessel diameter measurements are then compared with the thickness of the retinal nerve layer and the neuronal responses in the retina of the eye and in the visual cortex of the brain to repeated light stimulation,” Þórunn explains.

In addition to Þórunn, the research team includes Ólöf Birna Ólafsdóttir, associate professor at the Faculty of Medicine at the University of Iceland, and Marianne E. Klinke, professor at the Faculty of Nursing and Midwifery. The study was also part of the master’s thesis of Freyja Húnfjörð Jósepsdóttir, who played a leading role in data collection and measurements, both in participants with Parkinson’s disease and in a healthy control group. Freyja graduated with a master’s degree in biomedical science from the University of Iceland in spring 2024.

As previously mentioned, the study is a collaborative project between the University of Iceland, Landspítali – University Hospital, and Reykjalundur. Co-investigators include Sóley Guðrún Þráinsdóttir, chief physician of the Neurology and Rehabilitation Team at Reykjalundur; Anna Bryndís Einarsdóttir, chief physician at the Department of Neurology at Landspítali; Sveinn Hákon Harðarson, associate professor at the Faculty of Medicine, University of Iceland; and Einar Stefánsson, professor emeritus at the Faculty of Medicine, University of Iceland, and a specialist in ophthalmology.

“I have worked with Ólöf and Sveinn on measurements of oxygen saturation in retinal vessels for over a decade, for the most part under the leadership of Einar Stefánsson, chief physician and professor, and we were all his doctoral students. The retinal oximeter is the result of collaboration by a multidisciplinary team of specialists in cooperation with the University of Iceland and is based on the work of numerous scientists around the world. Anna Bryndís was also part of this group, and our current collaboration with her and Sóley Guðrún involves their expertise in neurology. Marianne is a specialist in neurological nursing and director of academic affairs in neurological nursing. I, together with Ólöf, Marianne, and Sveinn, supervised Freyja and served on her master’s thesis committee,” Þórunn says about the research group.

Results could influence disease diagnosis

The study is in its final stages, and Þórunn is eager to see the results. “The results will primarily provide information on the usefulness of retinal oxygen saturation measurements for diagnosing Parkinson’s disease in retinal tissue, and positive findings could prove important for early diagnosis of the disease and thereby for improving quality of life and monitoring disease progression in individuals diagnosed with Parkinson’s disease.”