Making Corneal Sensation Testing Accessible: Bench Validation of a Low-Cost Adjustable Monofilament Aesthesiometer

Corneal sensation testing is essential for diagnosing and monitoring neurotrophic keratopathy, yet the gold-standard Cochet-Bonnet (COBO) aesthesiometer (>£1000) is unavailable in most ophthalmology departments. The cotton wisp alternative is qualitative and insensitive to early sensory loss. This study evaluated whether a generic adjustable monofilament corneal aesthesiometer (GAMCA) fabricated from commodity materials could replicate COBO performance.

The GAMCA comprised a 0.12 mm nylon monofilament of adjustable length on a millimetre-scaled sliding carrier, constructed for £10.28. Two independently fabricated devices were validated across 12 lengths (5-60 mm, 20 measurements each) against published humidity-corrected COBO reference pressures using precision balance and corneal phantom testing. Agreement was assessed by standard-error weighted log-transformed Bland-Altman analysis with supporting regression and correlation statistics.

Both devices produced monotonically increasing pressure-length relationships consistent with beam mechanics. Coefficient of variation ranged from 11.0% to 19.9%. Rank-order correspondence with COBO was near-perfect (Pearson's r = 0.998-0.999; Spearman's ρ = 1.00; Lin's concordance correlation coefficient 0.849-0.869). Geometric mean bias was ×1.22 (95% limits of agreement ×0.939 to ×1.576) at 60% humidity and ×1.41 (×1.177 to ×1.680) at 50% humidity.

A corneal aesthesiometer replicating COBO mechanical principles can be constructed for approximately 1% of its cost, with preserved rank-order fidelity and acceptable repeatability across clinically relevant pressures. This device could be suitable for community screening, ward-based settings and resource-scarce environments. Prospective clinical validation of reproducibility and threshold detection in normal and hypoaesthetic corneas are now warranted.