Focal Weakening/Thinnest Corneal Point Proximity in Normal, Subclinical, and Keratoconic Corneas Using Motion-Tracking Brillouin Microscopy.

Purpose: To determine the proximity between the thinnest corneal point (TCP) and focal corneal weakening in normal, subclinical keratoconus (SKC), and manifest keratoconus (KC) eyes using motion-tracking Brillouin microscopy.

Design: Prospective cross-sectional study.

Participants: Ninety-five eyes from 95 patients were evaluated: 40 from bilaterally normal patients (controls), 40 from patients with SKC, and 15 from patients with manifest KC.

Methods: All patients underwent Scheimpflug tomography, anterior segment OCT (AS-OCT), and custom-built motion-tracking Brillouin (MTB) imaging. Mean and minimum MTB shift values were calculated within the anterior plateau region. Euclidean distances between the TCP (identified by AS-OCT) and the minimum Brillouin shift (MTB-Min) were determined. Motion-tracking Brillouin minimum values within 10 MHz of the absolute minimum were considered equivalent (MTB-Min(e)). Receiver operating characteristic curves were generated for both metrics to determine the area under the curve (AUC), sensitivity, and specificity.

Main Outcome Measures: Distance between MTB-Min and TCP; group discrimination based on area under the receiver operating characteristic curve, sensitivity, and specificity.

Results: No significant differences were found between groups for age, sex, KMean, or KMax. Subclinical keratoconus and KC eyes were significantly thinner than controls. Motion-tracking Brillouin minimum values were significantly lower in SKC and KC eyes compared with controls. Average distances between MTB-Min and TCP were 0.31 ± 0.16 mm (0.04-0.69 mm) in controls, 0.53 ± 0.28 mm (0.11-1.19 mm) in SKC, and 0.54 ± 0.35 mm (0.10-1.36 mm) for KC ( < 0.001). Motion-tracking Brillouin minimum (e) values were within 1 mm of the TCP in 100% of control eyes, 92.5% of SKC eyes, and 86.7% of KC eyes ( = 0.1). Motion-tracking Brillouin minimum values (e) performed nearly equivalently (AUC = 0.999) to the absolute MTB-Min shift (AUC = 1.0) in differentiating SKC from controls.

Conclusions: Focal corneal weakening occurs in close proximity to the thinnest corneal point in SKC. This study provides the first experimental confirmation of this relationship and demonstrates focal mechanical localization in subclinical and early KC.

Financial Disclosures: Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.