Corneal confocal microscopy as a paraclinical test in neurodegenerative disease: a scoping review

Corneal confocal microscopy (CCM) is a non-invasive imaging technique that enables quantification of the corneal sub-basal nerve plexus and has emerged as a potential surrogate biomarker for peripheral neurodegeneration. This scoping review evaluated current evidence on the use of CCM in assessing corneal nerve fibre changes across neurodegenerative diseases (NDDs) and explored its potential as a paraclinical diagnostic and monitoring tool. A comprehensive search of PubMed and Scopus was conducted in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews guidelines to identify studies reporting quantitative CCM metrics, including corneal nerve fibre density (CNFD), corneal nerve branch density (CNBD) and corneal nerve fibre length (CNFL). Both cross-sectional and longitudinal studies of patients with NDDs were included, and findings were narratively synthesised. 50 studies were included: Parkinson's disease (n=13), multiple sclerosis (n=11), cerebrovascular accidents (n=7), post-COVID-19 neuropathy (n=5), amyotrophic lateral sclerosis (n=4), chronic inflammatory demyelinating polyneuropathy (n=4), Alzheimer's disease (n=3), Fabry disease (n=2) and neurofibromatosis type 1 (n=1). CNFL and CNFD were consistently reduced in Parkinson's disease, multiple sclerosis, cerebrovascular accidents, amyotrophic lateral sclerosis, chronic inflammatory demyelinating polyneuropathy and post-COVID-19 neuropathy, whereas CNBD results were inconsistent. The strongest evidence supported the role of CCM in Parkinson's disease and multiple sclerosis. CNFL and CNFD emerged as the most reliable CCM-derived metrics across NDDs, supporting their potential as objective biomarkers for neurodegeneration. While findings support the potential of CCM as a paraclinical diagnostic tool, methodological heterogeneity in image acquisition, analysis software and study design limited comparability. Standardised imaging and analysis protocols are needed to enable broader clinical application and validation across NDDs.