**Background:** Hereditary transthyretin amyloidosis (ATTRv amyloidosis) is a rare, life-threatening autosomal-dominant disorder caused by mutations in the TTR gene, leading to extracellular deposition of misfolded transthyretin fibrils. The most common manifestations are cardiomyopathy (ATTRv-CM) and polyneuropathy (ATTRv-PN), with early small nerve fiber involvement. Timely diagnosis and treatment are critical to limit disease progression, but diagnostic delay is common due to genetic and phenotypic heterogeneity. Corneal confocal microscopy (CCM) is a non-invasive imaging technique that quantifies corneal small nerve fibers and immune cell infiltrates in vivo, and has proven diagnostic value in various peripheral neuropathies. This cross-sectional study assessed the utility of CCM for early diagnosis in a genetically heterogeneous cohort of ATTRv patients and presymptomatic carriers.
**Methods:** The study was conducted at the Department of Neurology, University Hospital Essen between January 2020 and December 2021. Twenty patients with confirmed amyloidogenic TTR mutation (6 ATTRv-CM, 14 ATTRv-PN), 5 presymptomatic carriers (relatives with normal nerve conduction studies, echocardiography, and clinical examination), and 20 age- and sex-matched healthy controls from the University of Manchester were included. Exclusion criteria were known ophthalmologic disease or ocular symptoms. CCM was performed using a Heidelberg Retina Tomograph (HRT III, Rostock Cornea Module). At least six images per patient meeting quality criteria were analyzed using ACCMetrics software for automated quantification of corneal nerve fiber density (CNFD, major nerves/mm²), corneal nerve fiber length (CNFL, mm/mm²), and corneal nerve branch density (CNBD, major branches/mm²). Corneal immune cells were manually counted by a blinded investigator using ImageJ and classified into four subtypes: dendritic cells with fiber contact (DCF), dendritic cells in the periphery without fiber contact (DCP), non-dendritic cells with fiber contact (NCF), and non-dendritic cells in the periphery without fiber contact (NCP). Statistical analyses used Kruskal-Wallis test with Dunn's post hoc test and Spearman's rank correlation; p<0.05 was considered significant.
**Key Results:** Patients with ATTRv amyloidosis (mean age 64.1±2.9 years, 15 male, 5 female) had significantly lower CNFD (21.05±1.59 vs. 30.75±1.58 fibers/mm², p=0.0003) and CNFL (12.95±0.79 vs. 17.40±0.93 mm/mm², p=0.0032) compared to healthy controls (mean age 62.7±1.7 years, 15 male, 5 female). CNBD did not differ significantly (29.05±4.03 vs. 39.02±4.55 branches/mm², p=0.119). A CNFD threshold of <24 fibers/mm² (two standard deviations below control mean) yielded a relative risk of 3.75 for symptomatic amyloidosis (15/20 with CNFD<24 vs. 5/25 with CNFD≥24 were symptomatic). Presymptomatic carriers (mean age 40.4±2.8 years, 2 male, 3 female) also had significantly lower CNFD compared to controls (19.00±4.11 vs. 30.75±1.58 fibers/mm², p=0.025), comparable to symptomatic patients. In subgroup analysis, ATTRv-PN patients showed significantly lower CNFD (20.36±1.77 vs. 30.75±1.58, p<0.001) and CNFL (12.71±0.87 vs. 17.40±0.93, p=0.008), while ATTRv-CM patients had significantly lower CNFD (22.67±3.51 vs. 30.75±1.58, p=0.038) but not CNFL. CNBD did not differ in either subgroup. Regarding immune cells, NCF was significantly higher in ATTRv patients compared to controls (11.80±3.98 vs. 2.30±0.68 cells/mm², p=0.021), but not in presymptomatic carriers (4.20±2.43 vs. 2.30±0.68, p>0.999). Total cell count, DCF, DCP, and NCP did not differ between groups. NCF correlated inversely with CNFD in ATTRv patients (r=-0.50, p=0.026). The proportion of corneal cells with fiber contact was 44% in ATTRv patients, 28% in presymptomatic carriers, and 26% in controls.
**Clinical Implications:** This study demonstrates that CCM can detect small nerve fiber damage in both symptomatic ATTRv patients and presymptomatic carriers, even decades before expected disease onset. The significant reduction in CNFD and CNFL across diverse genotypes and phenotypes suggests CCM may serve as a predictive surrogate marker to identify carriers at risk of developing symptomatic amyloidosis, potentially enabling earlier intervention. The finding of increased non-dendritic immune cells (NCF) only in symptomatic patients, correlating with nerve fiber loss, supports an immune-mediated mechanism in amyloid neuropathy and may help differentiate presymptomatic from symptomatic stages. CCM could complement current monitoring algorithms (e.g., PADO concept) and provide a non-invasive, reiterative tool for early diagnosis and disease progression assessment. Limitations include the small sample size due to the rarity of ATTRv amyloidosis outside endemic areas, and the need for longitudinal studies to validate predictive value.