Corneal nerve imaging in systemic neuropathy: A minireview of experimental and precision medicine applications

Abstract

Small-fiber neuropathy is a common complication of metabolic, neurodegenerative, and treatment-related disorders. Early detection can be challenging because established diagnostic approaches may be invasive, resource-intensive, or relatively insensitive to early small-fiber injury. Corneal confocal microscopy (CCM) is a rapid, noninvasive imaging technique that enables quantitative assessment of the corneal subbasal nerve plexus and has been proposed as a biomarker of peripheral small-fiber damage. This minireview critically evaluates the role of CCM for early detection, diagnostic testing, prognostic stratification, longitudinal follow-up, and treatment response assessment in systemic neuropathies, particularly diabetic peripheral neuropathy (DPN). Evidence is strongest in DPN, in which CCM consistently demonstrates group-level corneal nerve loss; however, individual diagnostic performance is only moderate and varies according to patient spectrum, reference standard, sampling protocol, analysis method, and diagnostic threshold. Evidence in other neurologic diseases, including Parkinson’s disease, multiple sclerosis, and chemotherapy-induced peripheral neuropathy, is less robust. Advancements in automated image analysis and artificial intelligence have improved reproducibility and efficiency of corneal nerve quantification in recent years. Nevertheless, widespread implementation in the clinics is still hampered by methodological heterogeneity, lack of full standardization, and absence of universally validated diagnostic and prognostic cut-off points. CCM is therefore a promising tool for translational research and precision medicine, but population-level differences in corneal nerve parameters should not be interpreted as establishing an individual diagnosis. At present, CCM should complement rather than replace established clinical, neurophysiological, functional, and pathological assessments, including quantitative sensory testing and skin biopsy when clinically indicated. Before integration into clinical practice can be recommended, multi-center prospective studies utilizing standardized acquisition and analysis protocols, validated thresholds, and clinically meaningful patient outcomes are required.

Key Words: Peripheral neuropathy; Corneal nerves; Small fiber neuropathy; Confocal microscopy; Precision medicine

Core Tip: Corneal confocal microscopy provides a fast, non-invasive quantitative assessment of corneal small-nerve morphology and has shown reproducible group-level abnormality in diabetic peripheral neuropathy and a few other systemic neuropathies. Nevertheless, population-based distinctions do not mean a reliable diagnosis in a single patient. Research shows that when it comes to diabetic neuropathy, the best studied condition, the variability in diagnostic performance can be attributed to the studied population, the reference standard employed, the sampling protocol used, the analysis chosen and the threshold selected. Also, there is considerable overlap and variation between those affected and not affected.

INTRODUCTION

The cornea is densely innervated and provides an accessible window into peripheral small-fiber integrity. In individuals with impaired glucose tolerance, those who later developed type 2 diabetes showed reductions in corneal nerve fiber density, branch density, and length, whereas those who reverted to normal glucose tolerance showed improvement[1]. In established diabetes, corneal confocal microscopy (CCM) abnormalities generally become more pronounced with increasing neuropathy severity and may precede abnormalities on conventional clinical or neurophysiological testing[2-5]. Reduced corneal nerve parameters have also been reported in Parkinson’s disease, particularly in association with autonomic involvement[6-8], and in early relapsing-remitting multiple sclerosis[9]. These findings support investigation of CCM as a noninvasive marker of systemic small-fiber involvement.

Neurotoxic chemotherapy can also alter corneal nerve morphology. An initial clinical report described CCM abnormalities in a patient with chemotherapy-induced sensory neuropathy[10]. A subsequent prospective study found longitudinal worsening of corneal nerve parameters alongside neuropathy measures after chemotherapy, supporting the potential of CCM as a noninvasive structural marker of chemotherapy-induced peripheral neuropathy[11]. These observations remain exploratory and require validation in larger disease- and treatment-specific cohorts.

CCM may also be useful for longitudinal assessment of nerve repair. In type 1 diabetes, simultaneous pancreas-kidney transplantation was followed by increases in corneal nerve fiber density and length within 6 months[12]. A subsequent systematic review and meta-analysis reported improvement in corneal nerve parameters after several pharmacological and surgical interventions, supporting CCM as a potential research endpoint for nerve regeneration[13]. Automated and artificial intelligence (AI)-based approaches may further improve the speed and reproducibility of quantification, and early classifiers have shown potential for diabetic neuropathy classification[14-17].

Overall, these data support further evaluation of CCM as a rapid, noninvasive, objective, and reproducible adjunctive biomarker of small-fiber injury, particularly in diabetic peripheral neuropathy (DPN); however, the available evidence does not establish disease-specific patient-level diagnostic validity. The growing literature underscores its clinical relevance, although broader translation into routine practice still depends on standardized protocols for image acquisition, sampling, selection, and analysis, as well as the availability of age-adjusted normative reference values[18-22]. Consequently, this review is located predominantly within experimental and translational medicine and appraises CCM as a quantitative biomarker of systemic small-fiber injury. Corneal nerve morphology may offer a in vivo window on peripheral nerve involvement in systemic and neurologic disease because the cornea contains a dense and readily accessible network of small sensory nerve fibers (Figure 1). Primary corneal neuropathy disorders which affect the cornea or ocular surface as the primary site of disease are beyond the scope of this review, and shall only be considered when relevant to anatomic context, methodologic interpretation or risk of confounding. The aim of this minireview is to discuss on the state of the evidence surrounding CCM for the detection, diagnosis, risk stratification, longitudinal assessment, response-to-treatment evaluation and so forth of systemic small-fiber neuropathies, while alluding to the methodological pitfalls that ought to be resolved prior to implementation in practice.

Figure 1 Clinical role of corneal confocal microscopy in systemic neuropathies. Corneal confocal microscopy has been investigated in several systemic and neurologic conditions, including prediabetes and diabetic neuropathy, Parkinson’s disease, multiple sclerosis, chemotherapy-induced neuropathy, and small-fiber neuropathy. Across these settings, the most consistent findings are reductions in corneal nerve fiber density, branch density, and length, consistent with early corneal nerve loss. These alterations may support early detection of small-fiber damage, identification of subclinical involvement, patient stratification, longitudinal monitoring, and adjunctive assessment of neuropathy. These findings primarily describe associations observed at the group level; no disease-specific corneal confocal microscopy pattern or universally validated threshold currently establishes systemic neuropathy in an individual patient. Image(s) provided by Servier Medical Art (https://smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/Licenses/by/4.0/). CNFD: Corneal nerve fiber density; CNBD: Corneal nerve branch density; CNFL: Corneal nerve fiber length.

Existing reviews have largely emphasized disease-specific diagnostic performance, technical aspects of CCM, or selected applications such as nerve regeneration. The present minireview instead organizes the evidence around five clinically distinct questions: Early detection, patient-level diagnostic evaluation, prognostic stratification, longitudinal monitoring, and treatment-response assessment. Its incremental contribution is not a new diagnostic meta-analysis, but an integrated translational framework that combines quantitative patient-level performance, methodological standardization, ocular confounding, and AI-related bias to define the boundary between group-level association and individual clinical utility. In this framework, translational medicine refers to the evaluation of CCM as a candidate biomarker moving from research toward clinical validation, whereas precision medicine refers specifically to the potential use of validated patient-level CCM information for stratification or decision support. CCM is therefore considered complementary when it adds structural small-fiber information to established testing, unsuitable as a substitute when conventional diagnostic confirmation remains indicated, and inappropriate for stand-alone decision-making when thresholds are unvalidated, ocular confounding is unresolved, or disease-specific evidence is insufficient.

SEARCH STRATEGY

The objective of this minireview was not to conduct a systematic review or meta-analysis, but rather to conduct a structured narrative review. The primary bibliographic database was PubMed/MEDLINE because the goal was to develop a clinically focused synthesis of biomedical evidence across heterogeneous disease settings and study designs, rather than an exhaustive systematic evidence search. Two searches were conducted, with the final search performed on June 1, 2026. The following set of terms was combined for the search: (“corneal confocal microscopy” OR “in vivo corneal confocal microscopy” OR “corneal nerve imaging” OR “corneal nerve fiber”) AND (“small fiber neuropathy” OR “small-fiber neuropathy” OR “diabetic neuropathy” OR “diabetic peripheral neuropathy” OR “Parkinson disease” OR “Parkinson’s disease” OR “multiple sclerosis” OR “chemotherapy-induced peripheral neuropathy” OR “peripheral neuropathy”) AND (“diagnosis” OR “early detection” OR “prognosis” OR “longitudinal monitoring” OR “treatment response” OR “nerve regeneration” OR “artificial intelligence” OR “automated analysis” OR “precision medicine”). To mitigate the risk of failing to detect important studies, we screened reference lists of eligible reviews, systematic reviews, meta-analyses, and key original studies. Two authors screened and selected the studies and extracted relevant data independently, with disagreements resolved by consensus. Since the review was narrative rather than systematic, PRISMA reporting and a formal PRISMA flow diagram were not applied; the structured search is reported to enhance transparency and reproducibility of study identification only and does not imply systematic-review completeness. However, the single-database strategy remains a limitation and a potential source of selection bias.

Eligible publications were original human clinical or translational study, systematic review, meta-analysis or a clinically relevant narrative review evaluating quantitative corneal nerve imaging relevant to systemic small-fiber or peripheral neuropathy. Studies that reported any of the following: Corneal nerve fiber density, corneal branch density, corneal nerve fiber length, corneal nerve fiber tortuosity, inferior whorl measurements, diagnostic performance, longitudinal change and treatment-related nerve regeneration, were included in the review. We included studies of the imaging methodology, normative values, reproducibility, automated analysis, or artificial intelligence that were directly relevant to interpreting or clinically translating measurements. We excluded reports that dealt exclusively with large-fiber neuropathy and primary corneal disease or an ocular surface disorder without relevant systemic neuropathy. Only those case reports and small case series that provide historical importance or mechanistic or hypothesis-generating information were considered. We also considered whether studies reported or controlled clinically relevant ocular factors capable of independently modifying corneal nerve measurements, particularly ocular surface disease, contact lens wear, previous ocular surgery, and primary corneal disease. Incomplete reporting or control of these variables was considered a potential source of confounding when interpreting associations between CCM findings and systemic neuropathy.

We screened titles and abstracts for relevance and then full texts of possibly eligible papers. A narrative synthesis was conducted across five a priori clinical domains: Early detection, diagnostic evaluation, prognostic stratification, longitudinal monitoring, and treatment-response assessment. Due to considerable disparity in study populations, imaging protocols, outcome definitions, reference standards, and analysis methods, quantitative pooling was not undertaken.

Due to the variability of the included designs, no single formal risk-of-bias instrument was applied to all studies. Instead, evidence was evaluated according to prespecified characteristics relevant to clinical credibility. The features considered were study design, whether data were collected prospectively or retrospectively, sample size, single-center or multicenter setting, external validation, appropriateness of the clinical/neurophysiological/pathological reference standard, and control of relevant ocular confounders. Also considered was consistency with independent studies. Greater interpretive weight was given to systematic reviews, meta-analyses, multicenter studies, and prospective longitudinal cohorts. By contrast, small cross-sectional studies, case series, and case reports were viewed as exploratory or hypothesis-generating; these sources were not used alone to support patient-level diagnostic claims.

The standardized terminology and measurement units used to describe corneal nerve morphology are summarized in Table 1. To enhance comparability across studies, we paid particular attention to the imaging device and acquisition mode used, anatomical region examined, number and area of images analyzed, image-selection strategy, image quality criteria, analysis software, and use of manual, semiautomated, or fully automated quantification.

Table 1 Standardized terminology and reporting conventions for corneal confocal microscopy nerve metrics.

Metric	Abbreviation	Standardized definition	Unit	Reporting convention
Corneal nerve fiber density	CNFD	Number of main nerve fibers within the analyzed corneal area	fibers/mm2	Main nerve trunks should be distinguished from secondary branches
Corneal nerve branch density	CNBD	Number of primary branches arising from the main nerve fibers within the analyzed area	branches/mm²	Only branches originating directly from main nerve fibers are counted
Corneal total branch density	CTBD	Total number of branch points within the analyzed area, including primary and higher-order branching	branch points/mm²	The branch-order convention and analysis software should be reported
Corneal nerve fiber length	CNFL	Cumulative length of all visible main nerve fibers and branches divided by the analyzed area	mm/mm2	The analyzed area, anatomical location, and analysis method should be specified
Corneal nerve fiber tortuosity	CNFT	Degree to which the course of a nerve fiber deviates from a straight path	Unitless coefficient, software-specific index, or ordinal grade	The calculation algorithm, grading method, and software should be specified
Inferior whorl length	IWL	Cumulative length of visible nerve fibers and branches within the defined inferior-whorl region divided by the analyzed area	mm/mm2	The location and dimensions of the inferior-whorl region of interest should be reported

CNBD: Corneal nerve branch density; CNFD: Corneal nerve fiber density; CNFL: Corneal nerve fiber length; CNFT: Corneal nerve fiber tortuosity; CTBD: Corneal total branch density; IWL: Inferior whorl length.

Anatomy of corneal nerves and its pathophysiological connection to systemic neuropathy

Corneal sensory innervation arises predominantly from the ophthalmic division of the trigeminal nerve. Stromal nerve trunks enter radially from the limbus, lose their myelin sheaths and perineurium, branch anteriorly, penetrate Bowman’s layer, and form the dense subbasal nerve plexus between Bowman’s layer and the basal epithelium. Most quantitative CCM studies of systemic neuropathy focus on this plexus because its relatively planar architecture is well suited to reproducible imaging and morphometry. Corneal sensory nerves consist mainly of small Aδ and C fibers and contribute to sensation, blink reflexes, tear secretion, epithelial turnover, and wound healing[23-27]. Through release of neuropeptides, growth factors, and other mediators, these nerves also support epithelial trophism and ocular-surface homeostasis[28]. This anatomical accessibility and small-fiber composition provide the rationale for evaluating corneal nerve morphology as a marker of peripheral nerve integrity[25,29,30]. In this review, the cornea is therefore considered an accessible tissue for assessing systemic small-fiber integrity rather than the primary site of a corneal neuropathic disorder.

Chronic hyperglycemia, in fact, activates several alternative glucose metabolic pathways: For example, the metabolism of polyols, protein kinase C, hexosamines, and advanced glycation products, which determine alterations and redox imbalances with mitochondrial activity and accumulation of toxic metabolites[31-34]. These processes are accompanied by microvascular impairment, reduced endoneuria perfusion, and ischemic stress, all of which further compromise axonal and Schwann cell integrity[32-34]. At the same time, low-grade intraneural inflammation, marked by macrophage activation and increased cytokine signaling, appears to sustain ongoing degeneration, particularly in small nerve fibers[34,35]. In type 2 diabetes, dyslipidemia, obesity, and insulin resistance may intensify these mechanisms, reinforcing the idea that diabetic neuropathy arises from converging rather than isolated pathogenic pathways[33,34].

Neurodegenerative disorders share several mechanisms relevant to axonal injury, including mitochondrial dysfunction, oxidative stress, altered proteostasis, and impaired axonal transport[36,37]. Because neuronal survival depends on sustained bioenergetics and long-distance intracellular trafficking, distal axons and small sensory fibers may be particularly vulnerable. CCM studies have reported corneal nerve loss in several peripheral neuropathies and selected neurodegenerative diseases, supporting investigation of the subbasal plexus as an accessible marker of broader neuroaxonal injury[29,30].

Accumulating evidence indicates that corneal nerve abnormalities can be detected early in dysglycemia and diabetic neuropathy, in some cases before neuropathy becomes evident according to conventional clinical criteria[1,5]. In individuals with impaired glucose tolerance, reduced corneal nerve fiber density and length have been documented, particularly in those who later progressed to type 2 diabetes, despite largely comparable nerve conduction findings[1]. Together with meta-analytic evidence showing corneal nerve loss even in diabetic patients without clinically established neuropathy, these observations suggest that corneal imaging can reveal subclinical small-fiber damage that may be missed by traditional diagnostic approaches[4,5,18]. Longitudinal studies further indicate that corneal nerve parameters are dynamic markers of small-fiber injury rather than static descriptors. In subjects with impaired glucose tolerance, lower baseline corneal nerve fiber density, branch density, and length were observed in those who later developed type 2 diabetes, whereas significant improvements in these measures were observed in individuals who reverted to normal glucose tolerance[1]. In people with diabetic neuropathy, loss of corneal nerves associated with central and inferior whorl is reported more[38].

Progress in corneal confocal microscopy and imaging technologies

Over the past two decades, in vivo CCM has evolved from a predominantly research technique into a rapid, noninvasive method for high-resolution assessment of the cornea[18,39,40]. Modern laser-scanning systems, particularly the Heidelberg Retina Tomograph-Rostock Corneal Module, enable quantitative analysis of subbasal nerve morphology. The principal CCM metrics and reporting conventions used in this review are defined in Table 1 and are not repeated here. Manual and automated approaches can provide reproducible quantification, with automated methods reducing analysis time and observer dependence[14,15,41]. Wide-field composite imaging expands spatial sampling, while emerging techniques such as micro-optical coherence tomography may further improve visualization of corneal nerves[42-44].

Technical challenges and unresolved issues in corneal nerve image analysis

The measurement of CCM is still sensitive to acquisition and sampling. The areas covered by single frames of the spatially heterogeneous subbasal plexus are small, and measurements may vary across central, paracentral, and inferior-whorl areas. Many other factors like eye movement, focus, depth of imaging, illumination, contrast, and compression of tissue can also affect image quality. Longitudinal studies also face a challenge in determining biological change due to the difficulty of relocating the same microscopic region, making biological change difficult to distinguish from spatial sampling variability[21,39,42,45]. Strategies for image selection also differ, as in representative, random, consecutive nonoverlapping, central, inferior-whorl, and wide-field sampling. Observer bias can be introduced by manual selection, while overlapping frames can duplicate nerve segments. Wide-field mosaics allow sampling error to be reduced, although extra registration, de-duplication, and analysis steps are required[21,22,39,42,45].

The software packages also differ in their analytical definitions, like how to distinguish between nerve trunks and branches, how to identify crossings and branch points, how to deal with fibers that are cut off at the image edge, and how to determine tortuosity. Low-contrast or discontinuous nerves can easily be missed by such algorithms. Reflective cells and imaging artifacts can also be classified as nerve fibers. Thus, measurements obtained through different software, preprocessing steps, segmentation thresholds, and parameter definitions may not be interchangeable[15,39,45,46].

Clinical applications and evidence domains in systemic neuropathies

To clarify, available evidence is discussed in relation to five key clinical applications, namely, early detection of neuropathy, diagnostic work-up, prognostic stratification, disease course monitoring, and assessment of response to therapy. To distinguish the maturity of evidence across these applications, we use three descriptive categories rather than formal Grading of Recommendations Assessment, Development and Evaluation ratings: Relatively established, emerging, and exploratory. Evidence is relatively established for group-level detection and complementary diagnostic assessment in diabetic neuropathy; emerging for longitudinal monitoring and treatment-response assessment; and exploratory for prognostic prediction and most disease-specific applications outside diabetes. These categories reflect study design, sample size, external validation, reference standard, and independent replication and should not be interpreted as formal certainty-of-evidence grades.

Early detection of neuropathy

Using traditional clinical or neurophysiological criteria, corneal nerve abnormalities may be detectable before neuropathy. In people with impaired glucose tolerance, reductions in the density of corneal nerve fibers, corneal nerve branch density, and length have been reported, particularly in those who later developed type 2 diabetes[1]. Evidence from meta-analyses and cohorts also shows that corneal nerve loss may occur in patients with diabetes without clinically evident neuropathy[5,47-49]. Previous studies reported similar early abnormalities in relapsing-remitting multiple sclerosis, but the evidence in that setting is more limited[9]. These findings suggest that CCM may detect subclinical small-fiber involvement, although the abnormalities observed are not disease-specific.

Diagnostic evaluation

The diagnostic evidence for diabetic neuropathy is strongest, but separation at the group level does not necessarily translate into accuracy at the individual level. A systematic review and meta-analysis of 38 studies with almost 4000 participants showed that corneal nerve fiber density, branch density, fiber length, and inferior whorl length were decreased in a graded manner across controls, diabetes without clinically established neuropathy, and established DPN. For corneal nerve fiber length, the pooled mean difference between established neuropathy and diabetes without neuropathy was -3.08 mm/mm² (95%CI: -3.58 to -2.58)[5]. A meta-analysis of 52 studies in non-diabetic neuropathies similarly found lower corneal nerve fiber length in clinical and subclinical neuropathy groups, but disease heterogeneity precluded a common diagnostic threshold[30].

Patient-level performance is more variable. In the study with 998 participants from the multinational consortium, automated corneal nerve fiber length yielded an area under the curve (AUC) of 0.77 in type 1 diabetes and an AUC of 0.68 in type 2 diabetes; in the overall cohort, a threshold of 12.3 mm/mm² yielded an AUC of 0.71, with a sensitivity and specificity of 67% and 66%, respectively, with a considerable intermediate range remaining unclassified[48]. Performance also depended on the comparator: One cohort reported an AUC of 0.84 when abnormal peroneal motor nerve amplitude was used as the comparator, but approximately 0.67-0.69 when warm-detection thresholds were used[14].

In more recent or mild type 2 diabetic polyneuropathy, the sensitivity of corneal nerve fiber length was 14.4%, but specificity was high at 95.7% (AUC 0.55), and it did not improve diagnostic performance over other small-fiber measures[47]. Wide-field CCM likewise exhibited limited correlation with concurrent skin biopsy or functional tests[50]. In a study of 680 unselected neurological patients, sensitivity and specificity of CCM were 44% and 75%, respectively, with an AUC of 0.63, while skin-biopsy specificity was 99% and its AUC was 0.74[51]. CCM must be viewed as a complementary structural measure that serves as neither a stand-alone diagnostic test nor a direct substitute for established small-fiber assessments.

Prognostic stratification

Prognostic evidence remains limited. In a 3-year cohort of 30 participants with impaired glucose tolerance and 17 controls, 10 progressed to type 2 diabetes, 15 remained with impaired glucose tolerance, and five reverted to normal glucose tolerance. Progressors had lower baseline corneal nerve fiber density, branch density, and length, whereas those reverting to normal glucose tolerance showed improvement[1]. However, only 10 participants progressed, no externally validated prediction model was developed, and the outcome was progression to diabetes rather than incident or worsening neuropathy. These findings are therefore hypothesis-generating for metabolic trajectory rather than validation of CCM as a prognostic biomarker.

Evidence in Parkinson’s disease is predominantly cross-sectional. A study of 98 patients and 26 controls found lower corneal nerve fiber density, branch density, total branch density, and fiber length, without consistent associations with disease duration, motor severity, stage, subtype, cognition, or quality of life[6]. Another study found greater corneal nerve loss with increasing autonomic involvement, with AUCs of 0.872 and 0.915 for discrimination between current autonomic phenotypes[8]. These findings describe cross-sectional phenotyping rather than prediction of future progression.

Longitudinal disease monitoring

CCM is well suited to serial assessment because it is noninvasive and repeatable[21,22]. Longitudinal changes have been reported in impaired glucose tolerance[1] and after neurotoxic chemotherapy[11]. However, serial CCM findings should be interpreted alongside symptoms, neurological examination, neurophysiology, skin biopsy, and other small-fiber measures rather than as an isolated marker[30,50,51].

Assessment of therapeutic response

CCM may also detect morphological nerve regeneration after intervention. In type 1 diabetes, corneal nerve fiber density and length increased within 6 months of simultaneous pancreas-kidney transplantation[12], and a subsequent systematic review and meta-analysis reported improvement in corneal nerve parameters after several pharmacological and surgical interventions[13]. These changes support CCM as a potential research endpoint, but short-term morphological improvement is not a validated surrogate for long-term neurological recovery or clinical benefit.

The available evidence therefore varies considerably according to the clinical use. The strongest support is for early detection and assessment of diabetic neuropathy with complementary diagnostic tests. Prognostic stratification, longitudinal monitoring of disease, and assessing treatment response require further evaluation in prospective studies. Table 2 summarizes representative quantitative evidence and patient-level diagnostic performance across the principal clinical settings.

Table 2 Representative quantitative evidence and patient-level diagnostic performance of corneal confocal microscopy.

Clinical setting	Representative evidence	Sample size	Reference standard/comparator	Key quantitative findings	Diagnostic performance and interpretation
Impaired glucose tolerance/prediabetes[1]	3-year longitudinal cohort	30 participants with IGT + 17 controls; 10 progressed to T2D, 15 remained with IGT, and 5 reverted to normal glucose tolerance	3-year glycemic trajectory (progression to T2D, persistent IGT, or reversion to normoglycemia); healthy controls	Progressors showed lower baseline CNFD (20.0 ± 2.2 fibers/mm2 vs 30.7 ± 1.5 fibers/mm2), CNBD (25.6 ± 5.2 branches/mm2 vs 37.0 ± 2.7 branches/mm2), and CNFL (13.7 ± 1.2 mm/mm2 vs 20.4 ± 3.2 mm/mm2) than controls	Exploratory association with metabolic trajectory. No externally validated prediction model or neuropathy-specific prognostic threshold was developed
Diabetic peripheral neuropathy[5]	Systematic review and meta-analysis	38 studies; approximately 4000 participants	Study-specific clinical and/or neurophysiological definitions of DPN across included studies	CNFL was lower in established neuropathy than in diabetes without clinically established neuropathy by a pooled MD of -3.08 mm/mm2 (95%CI: -3.58 to -2.58; 34 studies; n = 3868). IWL was lower by -4.11 mm/mm2 (95%CI: -5.10 to -3.12; 6 studies; n = 459)	Strong evidence for group-level differences, but the meta-analysis did not establish a pooled patient-level diagnostic threshold
Diabetic sensorimotor polyneuropathy[48]	Pooled multinational multicenter cross-sectional study	998 participants with diabetes: 516 with type 1 diabetes and 482 with type 2 diabetes	Toronto consensus criteria incorporating lower-limb electrophysiological abnormality	Automated CNFL showed an AUC of 0.77 in type 1 diabetes and 0.68 in type 2 diabetes	For the overall cohort, a CNFL threshold of 12.3 mm/mm2 yielded AUC 0.71, sensitivity 67%, specificity 66%, PPV 59%, and NPV 74%. A lower threshold of 8.6 mm/mm2 provided 88% specificity, whereas CNFL > 15.3 mm/mm2 provided 88% sensitivity for exclusion. A substantial intermediate range remained unclassified
Predominantly mild/recent type 2 diabetic polyneuropathy[47]	Comparative cohort	374 participants: 214 with DPN, 63 with diabetes without DPN, and 97 controls	Toronto criteria for DPN; IENFD and thermal thresholds were not included in the DPN case definition	CNFL was significantly lower in patients with DPN than in participants without DPN and controls	AUC 0.55, sensitivity 14.4%, and specificity 95.7%. IENFD showed higher sensitivity (51.1%). These findings demonstrate that significant group-level differences do not necessarily translate into useful individual screening performance
Small- or mixed-fiber neuropathy[51]	Prospective unselected neurological cohort	680 patients assessed; 244 with small- or mixed-fiber neuropathy were included in the primary sensitivity analysis, and 179 patients with established alternative diagnoses were used for specificity calculations	Predefined clinical criteria for SFN/MFN; comparison with IENFD and cold-detection threshold	Limited concordance was observed between CCM and skin biopsy. Among the 244 affected patients, only 41 were abnormal on both tests, whereas 66 had abnormal CCM alone and 63 had abnormal skin biopsy alone	CCM sensitivity 44% (95%CI: 38%-51%), specificity 75% (95%CI: 69%-81%), and AUC 0.63. Skin biopsy showed sensitivity 43%, specificity 99%, and AUC 0.74. CCM therefore cannot be considered a direct substitute for skin biopsy
Parkinson’s disease with autonomic involvement[8]	Cross-sectional phenotyping study	71 patients with PD and 30 healthy controls: 14 without autonomic symptoms, 14 with single-domain autonomic involvement, and 43 with multiple-domain autonomic involvement	SCOPA-AUT autonomic-domain classification; healthy controls	CNFD decreased from 30.88 ± 2.42 fibers/mm2 in patients without autonomic symptoms to 23.63 ± 3.93 fibers/mm2 in those with multiple-domain autonomic involvement. CNFL decreased from 17.54 ± 2.03 mm/mm2 to 12.85 ± 2.55 mm/mm2	The combination of CNFD, CNBD, and CNFL yielded an AUC of 0.872 for distinguishing single-domain autonomic involvement from no autonomic involvement (n = 14 vs n = 14; sensitivity 85.7%, specificity 92.9%) and an AUC of 0.915 for distinguishing multiple-domain from single-domain autonomic involvement (n = 43 vs n = 14; sensitivity 79.1%, specificity 92.9%). These values represent cross-sectional phenotype discrimination rather than diagnosis of Parkinson’s disease or prediction of future progression
Chemotherapy-induced peripheral neuropathy[11]	Prospective longitudinal study	95 patients recruited; 73 included in the post-treatment analysis, 32 completed paired clinical Total Neuropathy Score assessments, and 14 underwent paired skin-biopsy and CCM assessment	Longitudinal clinical Total Neuropathy Score; paired skin biopsy available in a subgroup	Longitudinal reductions in corneal nerve density and density-to-tortuosity measures were reported following neurotoxic chemotherapy, whereas CNFL did not uniformly decrease	No validated disease-specific AUC, sensitivity/specificity threshold, or diagnostic cutoff is currently available. Findings support potential sensitivity to longitudinal treatment-related nerve changes but remain exploratory

AUC: Area under the receiver operating characteristic curve; CCM: Corneal confocal microscopy; CNBD: Corneal nerve branch density; CNFD: Corneal nerve fiber density; CNFL: Corneal nerve fiber length; CNF: Corneal nerve fiber; DPN: Diabetic peripheral neuropathy; IGT: Impaired glucose tolerance; IWL: Inferior whorl length; MD: Mean difference; NPV: Negative predictive value; PPV: Positive predictive value.

Precision medicine and AI in corneal nerve evaluation

The integration of corneal nerve imaging into precision medicine is supported by its potential to provide objective, noninvasive, and repeatable biomarkers of small-fiber injury and repair, particularly in diabetic neuropathy and in longitudinal studies of treatment response[18,19,48]. Precision medicine depends on reliable patient-level biomarkers, and CCM-derived measures can quantify corneal nerve morphology with good repeatability when image acquisition and analysis are performed using standardized protocols[21,22]. AI-based methods may further strengthen this approach by enabling faster and potentially more consistent image classification. Even so, broader clinical adoption still depends on external validation, technical standardization, and a clearer definition of how these tools should complement established diagnostic tests rather than simply replicate them[41,47,48].

AI has increasingly been applied to CCM analysis to reduce reliance on labor-intensive manual assessment and improve consistency[15,16,41]. Automated and deep-learning approaches have shown good agreement with expert annotation[14-16]. In a cohort of 222 participants, including 90 without and 132 with neuropathy according to the Toronto criteria, a deep-learning classifier achieved an AUC of 0.83, sensitivity of 0.68, and specificity of 0.87[16]. A subsequent three-class study tested the algorithm in 40 participants, including 15 healthy volunteers, 13 participants without peripheral neuropathy, and 12 with peripheral neuropathy. For the peripheral-neuropathy class, the model achieved a recall of 0.83 (95%CI: 0.58-1.00), precision of 1.00 (95%CI: 1.00-1.00), and F1 score of 0.91 (95%CI: 0.74-1.00)[17]. These results are encouraging but derive from selected datasets and require larger prospective external validation. Current AI evidence, so far, does not yet support autonomous clinical deployment, despite these results. Most CCM AI studies to date are based on selected datasets with limited diversity in centers, devices, acquisition protocols, and patient populations. Not all published literature lacks external validation; for instance, Williams et al[16] included an external image-validation dataset. However, prospective clinical validation across independent centers, different CCM devices, operators, acquisition settings, and demographic groups remains limited[16,17,41,46]. It is important to make this distinction because good performance within an external image dataset does not necessarily establish transportability across real-world clinical domains.

Another crucial methodological risk is data leakage. A common scenario in CCM is obtaining multiple frames from the very same participant. Hence, splitting by random image-level assignment can lead to placing highly correlated images from the same subject into both training and test datasets, which may inflate apparent performance. Training, validation, and test sets should therefore be separated at the participant level, and preferably at the center level when considering true external generalizability. Class imbalance, selection of images based on quality, preprocessing procedures, annotation variability, and device differences might introduce additional bias or domain shift[41,46].

Interpretability stays unresolved too. A classifier’s high accuracy does not mean that the algorithm has learned biologically meaningful neuropathological features. Deep-learning systems can use acquisition artifacts, illumination, focus, image quality, and other non-causal image characteristics that correlate with study labels. Attribution or saliency maps may help visualize model behavior, but they do not demonstrate biological or clinical validity on their own. Systems used in the clinic should therefore provide transparent image-quality control, interpretable morphometric outputs when possible, patient-level predictions, calibration and uncertainty estimates, and external validation rather than an isolated categorical diagnosis[41,46]. AI must show added value beyond traditional CCM morphometry and existing neuropathy assessments before claims of clinical implementation can be justified.

Overall, AI has the potential to reinforce the role of corneal nerve imaging within precision medicine, but wider clinical implementation will still require larger external validation studies, standardization, and effective regulatory translation[17,41]. Figure 2 summarizes the main strengths, current limitations, and future priorities of corneal nerve imaging.

Figure 2 Corneal nerve imaging: From promise to practice. Corneal confocal microscopy offers several practical strengths for evaluating small-fiber damage, including noninvasiveness, repeatability, quantification, and clinical utility. However, wider clinical implementation is still limited by operator dependence, methodological heterogeneity, incomplete validation, and the lack of a stand-alone diagnostic role. Future progress will depend on stronger standardization, multicenter studies, artificial intelligence validation, normative cutoffs, and closer integration into clinical practice.

DISCUSSION

There is current evidence that supports CCM as a non-invasive structural marker of small-fiber injury, with the strongest data in diabetic neuropathy[1,5,18,30,48]. However, correlations with skin biopsy, functional measures, and neuropathy severity are inconsistent, especially in cases of milder or recently diagnosed type 2 diabetes[47,50,51]. Thus, CCM must be considered complementary rather than a stand-alone surrogate of the status of the peripheral nerve. This review considers the corneal subbasal plexus to be an accessible surrogate tissue for systemic small-fiber injury, differentiating it from primary corneal neuropathic diseases in which the cornea is the target of disease[18,40].

Because of the fact that CCM directly measures the cornea, local ocular conditions can also modify the same morphometric endpoints that are interpreted as markers of systemic small-fiber injury. Dry eye disease (DED) is relevant. A recent systematic review and meta-analysis found central corneal nerve length and nerve density to be significantly reduced in DED compared to healthy controls (pooled mean differences approximately -4.0 mm/mm² and -7.2 nerves/mm², respectively[52]).

The emergence of a CCM phenotype overlapping with systemic neuropathy can be caused by ocular surface disease that is unidentified or poorly described. Primary corneal diseases such as keratoconus or ectasia, infectious or inflammatory keratitis, corneal dystrophies, neurotrophic keratopathy, and previous corneal injury might also influence corneal innervation[24,25]. Another important modifier is prior eye surgery. Corneal refractive surgeries transect or disrupt corneal nerves directly and are associated with procedure-dependent patterns of reinnervation[53]; cataract surgery has also been associated with changes in the subbasal nerve plexus[54]. It is necessary to document contact lens wear too. Evidence on conventional soft and rigid contact lenses is varied. However, nerve distribution and corneal sensitivity show more pronounced alterations with orthokeratology and contact-lens-related ocular surface disease[55].

It is possible that these factors have research and clinical implications. In studies of CCM as a marker of systemic neuropathy, ocular history and ocular-surface assessment should be prespecified. The study should document clinically relevant DED, contact lens type and use, previous ocular surgery and time since surgery, topical medication, previous corneal trauma or infection, and primary corneal disease. Patients with active primary corneal disease or who have had recent procedures that are likely to substantially alter corneal innervation should generally be excluded from diagnostic-accuracy analyses or analyzed separately. In the case of a more common condition such as DED, contact lens wear, or remote ocular surgery, exclusion may not always be appropriate. In such circumstances, prespecified stratification, adjustment, or sensitivity analyses may be more useful as they are better at preserving external validity. In CCM neuropathy studies, there is no universally validated washout interval for these factors; thus, arbitrary time-based exclusions should be avoided. In the clinical context, abnormal CCM values should not be attributed to systemic neuropathy in the absence of an ocular history and a slit-lamp/ocular-surface examination. When ocular exclusion criteria are poorly defined, the association of corneal nerve abnormalities with systemic disease should be viewed more cautiously. As a methodological limitation, this narrative synthesis relied on a single bibliographic database and did not apply design-specific formal risk-of-bias instruments, which could lead to selection or appraisal bias.

Table 3 summarizes the main strengths, current limitations, and future priorities of corneal nerve imaging.

Table 3 Strengths, current limitations, and future priorities in corneal nerve imaging.

Domain	Strengths	Current limitations	Future priorities
Imaging technique[18,39,40]	Rapid, noninvasive, and repeatable visualization of the corneal subbasal nerve plexus	Requires dedicated equipment and trained operators; image quality may be affected by focus, illumination, motion, tissue compression, and limited sampling area	Wider access to the technique; standardized operator training, acquisition protocols, image-quality criteria, and anatomical sampling strategies
Biological significance[24,25,29,30]	Direct quantitative evaluation of small sensory nerve fibers in an easily accessible tissue	Corneal nerve abnormalities are not disease-specific and similar morphometric changes may occur across different systemic, neurological, and ocular conditions	Interpretation of CCM findings within the appropriate systemic, neurological, and ocular clinical context
Quantitative analysis[15,21,22,39,45]	Objective morphometric assessment of CNFL, CNFD, CNBD, CTBD, IWL, and other structural nerve parameters	Considerable heterogeneity in devices, sampling strategies, image-selection procedures, parameter definitions, and manual, semiautomated, or automated analysis methods	Harmonization of acquisition, sampling, parameter definitions, image selection, analysis software, and reporting standards
Clinical utility[5,47,48,51]	Potential role in detecting subclinical small-fiber abnormalities, longitudinal follow-up, patient phenotyping, and assessment of nerve regeneration	Group-level differences do not necessarily translate into accurate classification of individual patients; correlations with neuropathy severity and established reference tests are variable	Prospective multicenter diagnostic-accuracy and longitudinal studies using prespecified thresholds, representative populations, and clinically meaningful patient-level outcomes
Relationship with established tests[47,50,51]	May provide complementary structural information alongside clinical examination, quantitative sensory testing, neurophysiological assessment, and skin biopsy	Agreement with skin biopsy and functional small-fiber assessments is incomplete; CCM may capture partially overlapping but distinct aspects of nerve injury and cannot currently replace established diagnostic tests	Define the incremental clinical value of CCM and its optimal position within multimodal and multidisciplinary diagnostic pathways
Ocular confounding factors[24,25,52-55]	Ocular history, slit-lamp examination, and ocular-surface assessment allow many local modifiers of corneal nerve morphology to be identified	Dry eye disease, primary corneal disease, contact lens-related changes, previous ocular surgery, corneal trauma or infection, and topical treatments may independently alter CCM measurements and confound attribution to systemic neuropathy	Prespecified ocular eligibility criteria; standardized ocular-surface assessment; documentation of contact lens use and previous ocular procedures; exclusion of major active corneal disease when appropriate; and stratified, adjusted, or sensitivity analyses for relevant ocular confounders
Automation and artificial intelligence[16,17,41,46,56,57]	Faster image analysis, reduced dependence on manual quantification, improved repeatability, and encouraging diagnostic-classification performance	Current evidence is derived largely from selected datasets; limited cross-device and multicenter validation, potential patient-level data leakage, class imbalance, domain shift, insufficient calibration, and limited model interpretability restrict clinical generalizability	Participant-level data separation; multicenter and cross-device external validation; diverse populations; transparent reporting; calibration and uncertainty estimation; interpretable outputs; and demonstration of incremental clinical value beyond conventional CCM morphometry
Reference standards and diagnostic thresholds[20,47,48,51]	Growing availability of normative datasets and quantitative reference values for major CCM parameters	Diagnostic thresholds remain insufficiently validated for broad patient-level clinical use and may vary according to population, device, anatomical region, analytical method, and reference standard	Expansion of representative normative datasets and prospective validation of clinically meaningful, device- and population-appropriate thresholds

CCM: Corneal confocal microscopy; CNBD: Corneal nerve branch density; CNFD: Corneal nerve fiber density; CNFL: Corneal nerve fiber length; CTBD: Corneal total branch density; IWL: Inferior whorl length.

Research supports CCM as a marker of DPN. According to[1,5,48], the evidence base is clearest for DPN. It is supported by a number of systematic reviews, meta-analyses, multicenter diagnostic studies, and longitudinal cohorts. Patient-level performance and agreement with established small-fiber measures still vary between populations in this setting[47,50,51], and there is currently no evidence of CCM-guided management independently improving clinical outcomes. Evidence in Parkinson’s disease, multiple sclerosis, and chemotherapy-induced peripheral neuropathy is mostly observational and exploratory[6-11]. CCM is best viewed as a complementary biomarker. Its routine implementation would require standardized acquisition and analysis, validated thresholds, appropriate reference standards, and clinically meaningful longitudinal endpoints.

A highly relevant application of CCM may be longitudinal and interventional studies. CCM parameters might serve as responsive research endpoints, owing to corneal nerve regeneration following pancreas transplantation and other interventions[12,13]. Nevertheless, morphological improvement must not be considered a validated surrogate for neurological recovery; future studies should further couple CCM with symptoms and findings from neurological examination, skin biopsy, and neurophysiology, along with patient-reported outcomes[13,47,51].

CCM adoption in standard clinical practice also depends on practical factors including dedicated instrumentation, operator training, acquisition time, image-quality control, automated analysis, interoperability, and reimbursement. These barriers may be particularly relevant in settings with limited resources where dedicated CCM hardware, trained operators, standardized analysis software, and reimbursement pathways may be restricted[18,39,40]. The use of AI may lessen manual burdens and improve repeatability, but improved classification performance alone is not enough. In fact, algorithms should be validated externally across devices, populations, and disease contexts. In addition, there should be transparency in reporting, interpretability, and regulatory assessment[16,17,41]. Future CCM-AI research should also follow contemporary principles of prediction-model reporting and validation, such as transparent partitioning of participant-level data, model-performance and calibration reporting, external validation, and explicit assessments of risk of bias and applicability, consistent with TRIPOD+AI and PROBAST+AI[56,57].

Corneal nerve imaging will most likely achieve clinical value when it is incorporated into clearly defined multidisciplinary pathways in which it answers a specific diagnostic or monitoring question and is interpreted in conjunction with established assessments of small-fiber disease[18,30]. Operationally, CCM is best positioned after clinical evaluation as a noninvasive structural adjunct: Concordant findings may strengthen phenotyping or longitudinal monitoring, whereas discordant or diagnostically consequential findings should be interpreted alongside nerve-conduction studies and/or quantitative sensory testing and, when confirmation of small-fiber neuropathy is required, skin biopsy[18,30,47,51].

CONCLUSION

Corneal nerve imaging is a promising non-invasive method to evaluate small-fiber injury in systemic neuropathies. Evidence has been strongest in diabetic neuropathy and more limited in neurodegenerative or treatment-related conditions. The cornea can be considered a surrogate tissue, not the primary site of systemic neuropathy in this regard. Quantification of corneal nerve morphology may allow earlier objective identification of structural small-fiber changes, longitudinal assessment, and multimodal phenotyping of peripheral nerve injury. An abnormal CCM result is not currently sufficient for establishing a diagnosis of systemic neuropathy and does not replace established clinical assessment, quantitative sensory testing, neurophysiological testing, or skin biopsy where indicated. In order to enable broader application in a clinical setting, acquisition and analysis must be standardized, patient-level thresholds validated, multicenter longitudinal validation achieved, and integration with existing diagnostic pathways established. This process may be assisted by automated analysis and AI, but these remain adjunctive pending robust external validation.