Neuron J is a rapid and reliable open source tool for evaluating corneal nerve density in herpes simplex keratitis

In vivo confocal microscopy (IVCM) demonstrates reduction in corneal sub-basal nerve density in herpes simplex keratitis (HSK). Image J is an open source image-analysis platform that can be combined with a nerve tracer, Neuron J. We sought to compare the reliability and speed of corneal nerve density quantification between these modalities and their relation to clinical damage.

A total of 16 eyes (14 patients) with chronic HSK was assessed clinically and by IVCM. Randomly ordered triplicate, representative images from the central cornea were presented to two masked observers and corneal sub-basal nerve density was measured using Image J/Neuron J. Agreement was quantified using intraclass correlation coefficients (ICC), Bland-Altman plots together with mean difference, and level of agreement (LoA).

The median nerve density was measured at 7.1 mm/mm(2) (quartiles, 3.3-11.2), with Neuron-J demonstrating good intra-/interobserver agreement (ICC, 0.96-0.99; P < 0.001; mean difference, 0.1-1.4; LoA, <±3.3). Intraeye reliability was less consistent (mean difference, 1.7-2.3; LoA, ±8.8-9.8). Neuron J was highly comparable to Image J for both observers (ICC, 1.0; P < 0.001; mean difference, <0.2; LoA, ±<1.2) and significantly faster than Image J (median, 49 vs. 102 seconds, P < 0.001). Diminished nerve density was associated with corneal opacification and reduction in visual acuity (both P = 0.03).

The IVCM combined with Neuron J affords objective, user-friendly, and fast quantification of corneal nerve damage in HSK. It provides semiobjective phenotyping of the sequelae of neurotrophic corneal damage and offers a potential tool for measuring vulnerability to relapse or additional infections. Further exploration in a larger longitudinal cohort is warranted.