Tear biomarkers for Alzheimer’s disease screening and diagnosis (the TearAD study): design and rationale of an observational longitudinal multicenter study

Sande van de et al. BMC Neurology (2023) 23:293 https://doi.org/10.1186/s12883-023-03335-y S T U DY P R OTO CO L BMC Neurology Open Access Tear biomarkers for Alzheimer’s disease screening and diagnosis (the TearAD study): design and rationale of an observational longitudinal multicenter study Nienke van de Sande1*, Inez H. G. B. Ramakers2, Pieter Jelle Visser2,3, Frans R. J. Verhey2, Frank D. Verbraak4, Femke H. Bouwman3, Tos T. J. M. Berendschot1, Rudy M. M. A. Nuijts1, Carroll A. B. Webers1 and Marlies Gijs1 Abstract Background Alzheimer’s disease (AD) is the most common cause of dementia, and due to increasing life expectancy the number of patients is expected to grow. The diagnosis of AD involves the use of biomarkers determined by an amyloid PET scan or cerebrospinal fluid analyses that are either invasive or expensive, and not available in each hospital, thus limiting their usage as a front-line screener. The TearAD study aims to use tear fluid as a potential source for AD biomarkers. In previous reports, we demonstrated that AD biomarkers amyloid-beta and tau, are measurable in tear fluid and are associated with disease severity and neurodegeration. This study aims to validate previous results in a larger cohort and evaluate the diagnostic accuracy of tear biomarkers to discriminate between individuals with and without neurodegeneration as determined by hippocampal atrophy. Methods The TearAD study is an observational longitudinal multi-center study that will enroll 50 cognitively healthy controls, 50 patients with subjective cognitive decline, 50 patients with mild cognitive impairment and 50 patients with AD dementia from the memory clinic. Participants will be examined at baseline, after one year, and after two years follow-up. Study assessments include neuropsychological tests and ophthalmic examination. All participants will receive a MRI scan, and a subset of the study population will undergo cerebral spinal fluid collection and an amyloid PET scan. Tear fluid will be collected with Schirmer strips and levels of Aβ38, Aβ40, Aβ42, t-tau and p-tau in tear fluid will be determined using multiplex immunoassays. Blood samples will be collected from all participants. Images of the retina will be obtained with a standard, hyperspectral and ultra-wide field fundus camera. Additionally, macular pigment optical density will be measured with the macular pigment reflectometer, and cross-sectional images of the retina will be obtained through optical coherence tomography imaging. Discussion The TearAD study will provide insight into the potential diagnostic use of tear biomarkers as a minimally invasive and low cost tool for the screening and diagnosis of AD. Trial registration Retrospectively registered at clinicaltrials.gov (NCT05655793). *Correspondence: Nienke van de Sande Full list of author information is available at the end of the article © The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Sande van de et al. BMC Neurology (2023) 23:293 Page 2 of 7 Keywords Alzheimer’s disease, Tear fluid, Retinal nerve fiber layer, Amyloid beta, Neurofibrillary tangles, Cognition, Dementia, Biomarkers Background Approximately 57 million people are affected by dementia globally, and due to the ageing of the global population this number is expected to increase to 152 million cases by the year 2050 [1]. Patients with dementia experience memory problems, disorientation and difficulties with performing routine tasks. Alzheimer’s disease (AD) is the most common cause of dementia. AD is associated with the accumulation of senile plaques (deposits of amyloidbeta (Aβ)) and neurofibrillary tangles (deposits of tau protein (NFT)) in the brain [2] as well as brain atrophy in the cerebral cortex and hippocampus. Other pathological processes can be observed in AD patients alongside or caused by the accumulation of Aβ-plaques and NFT, such as inflammation, oxidative damage, and blood-brain barrier dysfunction [3]. Timely diagnosis is important to promote early and optimal management of the disease. Furthermore, this will be essential for future therapies as they may be more effective when started earlier in the disease process. Patients are diagnosed with subjective cognitive decline (SCD), mild cognitive impairment (MCI) or AD dementia based on clinical symptoms, cognition and daily functioning [4–7]. Additionally, the etiology of the disease can be determined with the use of biomarkers. These biomarkers are classified into three categories: Aβ deposition, neurofibrillary tangles and neurodegeneration (ATN) [8]. Cerebral spinal fluid (CSF) analysis or amyloid positron emission tomography (PET) imaging are currently used for biomarker determination [5, 6]. However, these diagnostic tests are invasive, expensive and not widely available, thereby limiting their potential as a front-line screener. Considering the expected increase of people affected by AD, there will be a growing need for non-invasive and cost-effective tools allowing identification of patients in the earlier phases of the disease [1]. In this study, we propose using tear and retinal biomarkers to mirror changes in the brain. The eyes and the brain are closely connected through the optic nerve and both the retina and the central nervous system share a common origin from the developing neural tube. Given this relation, many brain diseases have ocular manifestations [9]. Retinal abnormalities are observed in AD patients and with the advent of high resolution imaging modalities these retinal measures can be quantified in vivo. Decreased peripapillary retinal nerve fiber layer (RNFL) and macular thickness is observed in patients with AD dementia and MCI [10]. Specifically, in patients with AD type dementia a decreased choroidal thickness is observed [11]. Another interesting parameter is the macular pigment optical density (MPOD). MPOD is a biomarker for the amount of pigment in the brain, mainly lutein, (...truncated)