Parkinson’s Disease
According to the World Health Organization (WHO), the prevalence of Parkinson’s Disease (PD) has doubled in the past 25 years. Global estimates from 2019 reported over 8.5 million individuals with PD, and it caused 329 000 deaths, an increase of over 100% since 2000 (1).
To date, PD diagnosis remains primarily clinical and is based on the three cardinal motor manifestations of parkinsonism: rest tremor, bradykinesia and rigidity. Supportive diagnostic criteria include a therapeutic response to levodopa, olfactory testing, or cardiac MIBG scintigraphy (2).
The clinical diagnostic criteria do not fully capture the broad spectrum of symptoms experienced by PD patients (2–4). Oculomotor impairment has long been recognized in PD, affecting up to 75% of patients (5), but for decades its evaluation remained largely confined to research due to the lack of accessible eye-tracking devices. Recent technological advances and the development of modern eye-tracking systems have renewed interest in visual dysfunction in PD (6). Visual impairment in PD includes abnormalities in complex tasks processed by the brain (motion perception, visuospatial construction, face and emotion recognition…) but also in eye movement (saccades, fixation, smooth pursuit, pupillary response), and in ophthalmic visual processing (contrast sensitivity, visual acuity...) (7, 6). Both eye movements and ophthalmic parameters can be assessed with BulbiCAM®.
The International Parkinson and Movement Disorder Society (MDS) proposed a definition for three stages in early PD: preclinical PD (neurodegenerative processes have commenced, but there are no evident symptoms or signs); prodromal PD (symptoms and signs are present, but are yet insufficient to define disease); and clinical PD (diagnosis of PD based on presence of classical motor signs) (8).
Prodromal PD diagnosis remains very challenging, based on risk markers and non-motor clinical symptoms, potentially supported by neurophysiology, biology, or neuroimaging (3,9). The ability to detect pre-clinical or prodromal stages may prompt the pursuit of disease-modifying treatments and lower the risk of enrolling a very heterogeneous group of participants (10,11).
Visual impairment has been reported as early as during prodromal PD stages (12,13), making eye-tracking a valuable non-invasive tool for early diagnosis.
Diagnostic accuracy in early PD, has been reported to be as low as 58% (14) and a meta-analysis from 2016 showed no significant improvement over the preceding 25 years (15). Accurate diagnosis of PD is important for prognostic and therapeutic reasons but also for clinical, pharmacologic, and epidemiologic studies. Eye-tracking diagnostic capabilities have been demonstrated in several studies (6), and BulbiCAM® has been shown to identify validated diagnostic biomarkers in PD (16).
While clinical diagnosis and prodromal criteria can be supported by neurophysiology, biology or imaging, the MDS-UPDRS score used to rate and follow patients is purely clinical (17). Oculomotor dysfunctions (saccades, fixation, smooth pursuit, pupillary response) have been correlated with cognitive function and decline (6,18), making oculomotor evaluation an objective tool for patient follow-up and treatment assessment.