Retinal biomarkers for early Alzheimer’s detection: a systematic review of optical coherence tomography (OCT) findings

Objective: Retinal biomarkers accessible via non-invasive optical coherence tomography (OCT) could facilitate early detection of Alzheimer's disease (AD), complementing current invasive or costly diagnostic methods. This review evaluates the evidence for spectral-domain OCT (SD-OCT) and OCT angiography (OCT-A) in identifying retinal changes associated with preclinical and early AD. Methods and analysis: We conducted a systematic review registered in PROSPERO and aligned with Preferred Reporting Items for Systematic Reviews and Meta-Analyses 2020 guidelines. PubMed/MEDLINE was searched up to April 2025, complemented by reference list screening and citation tracking. Eligible studies assessed SD-OCT and/or OCT-A in biomarker-defined preclinical or early AD, mild cognitive impairment or mild AD. Data were synthesised narratively by disease stage, and methodological quality was appraised with the Newcastle-Ottawa Scale. Results: 22 studies met inclusion criteria. Reported alterations included thinning of the peripapillary retinal nerve fibre layer and retinal ganglion cell layer, macular and choroidal thickness changes and microvascular alterations on OCT-A. However, findings were heterogeneous: some studies observed early thickening or increased vascular density, possibly reflecting inflammatory or compensatory mechanisms, while others reported thinning and rarefaction more consistent with neurodegeneration. Most studies were of moderate quality, limited by small sample sizes, cross-sectional designs and incomplete control for ocular/systemic confounders. Conclusion: SD-OCT and OCT-A hold promise as candidate biomarkers of early AD, but current evidence remains variable, non-specific and methodologically constrained. Further research is needed to standardise imaging protocols, validate findings in biomarker-confirmed longitudinal cohorts and compare OCT-based measures across dementia subtypes. Integration with other biomarkers (eg, plasma or metabolomics) may improve diagnostic specificity and support translation of OCT/OCT-A into clinical practice. Prospero registration number: CRD42024600456.