Brillouin Spectroscopy of Normal and Keratoconus Corneas

doi:10.1016/j.ajo.2019.02.010

American Journal of Ophthalmology

Volume 202, June 2019, Pages 118-125

https://doi.org/10.1016/j.ajo.2019.02.010 Get rights and content

Purpose

To investigate the age dependence of Brillouin spectroscopy of the cornea and to compare normal and keratoconus corneas.

Design

Retrospective case-control study.

Methods

Study Population: Healthy patients and patients suffering from keratoconus seen at the Institut für Refraktive und Ophthalmo-Chirurgie (IROC) between December 2016 and March 2017. Brillouin frequency shifts of patients of 2 different groups were examined with Brillouin spectroscopy perpendicular to the corneal surface. Group 1 consisted of 47 healthy eyes, whereas Group 2 included 36 eyes with keratoconus of unclear progression. Besides Brillouin examinations, corneal tomographies were acquired so that correlations and comparisons between geometric parameters and Brillouin frequency shifts could be evaluated. Main Outcome Measures: Corneal Brillouin frequency shifts averaged over full corneal thickness.

Results

A significant correlation between age and central Brillouin frequency shift was identified (P = .011) with an increase in Brillouin frequency shift of 4 MHz per decade in normal corneas. Keratoconus corneas have a significantly reduced Brillouin frequency shift at the thinnest point compared to normal corneas (5.7072 ± 0.0214 vs 5.7236 ± 0.0146 GHz, P < .001). The Brillouin frequency shift at the point of maximum posterior elevation showed best correlation with geometry-derived keratoconus indices. The receiver operating characteristic curve analysis of Brillouin frequency shift showed substantially worse sensitivity and specificity compared to K_max and thinnest pachymetry for keratoconus detection.

Conclusion

Noninvasive Brillouin spectroscopy adds clinical information about the biomechanical state of the cornea perpendicular to the surface. An age-dependent stiffening of the cornea has been found and keratoconus corneas are statistically significantly different from normal corneas, but for precise differentiating of keratoconus stages (including normal corneas) the method is currently neither specific nor sensitive enough. Further development including standardized mapping and establishment of new indices may increase the potential of Brillouin spectroscopy substantially.

Section snippets

Patients

In this retrospective case-control study, 2 groups of patients were examined: (1) patients with normal eyes, to establish a correlation of age and Brillouin frequency shift (n = 47); and (2) patients with KCN, irrespective of progression (n = 36). The patients were asked not to use contact lenses during the 2 weeks before the examination. The study was performed at the Institut für Refraktive und Ophthalmo-Chirurgie (IROC) in Zürich, Switzerland following approval from the Institutional Review

Group 1: Central Brillouin Frequency Shift in Normal Corneas

The age of subjects included ranged from 18 to 82 years with an average of 39.0 years and a sex distribution of 22:25 (female:male). The central Brillouin frequency shift (B_central) ranged from 5.6977 to 5.7582 GHz with an average of 5.7242 ± 0.0160 GHz. Age and B_central were correlated with a correlation coefficient of 0.369 and a 2-sided P value of .011, which declares a statistical significance (Figure 2). Regression analysis using linear, polynomial, and exponential fits yielded the highest

Discussion

The main findings of this study are as follows: (1) the Brillouin frequency shift (and consequently the bulk modulus) of normal corneas increases with age, with an average rate of approximately 4 MHz per decade; (2) ectatic areas of keratoconus corneas have a highly significantly smaller Brillouin frequency shift than central normal corneas; and (3) at the point of maximum posterior elevation, Brillouin frequency shift correlates significantly with geometric keratoconus indices.

The stiffness of

References (31)

J. Liu et al.
Influence of corneal biomechanical properties on intraocular pressure measurement: quantitative analysis
J Cataract Refract Surg
(2005)
G. Scarcelli et al.
In vivo measurement of age-related stiffening in the crystalline lens by Brillouin optical microscopy
Biophys J
(2011)
T.T. Andreassen et al.
Biomechanical properties of keratoconus and normal corneas
Exp Eye Res
(1980)
J.S. Friedenwald
Contribution to the theory and practice of tonometry
Am J Ophthalmol
(1937)
T.R. Friberg et al.
A comparison of the elastic properties of human choroid and sclera
Exp Eye Res
(1988)
K. Biniek et al.
Understanding age-induced alterations to the biomechanical barrier function of human stratum corneum
J Dermatol Sci
(2015)
N.S. Malik et al.
Ageing of the human corneal stroma: structural and biochemical changes
Biochim Biophys Acta
(1992)
M. Kohlhaas et al.
Biomechanical evidence of the distribution of cross-links in corneas treated with riboflavin and ultraviolet A light
J Cataract Refract Surg
(2006)
D.M. Maurice
The cornea and the sclera
J.M. Vaughan et al.
Brillouin scattering, density and elastic properties of the lens and cornea of the eye
Nature
(1980)

G. Scarcelli et al.

Brillouin optical microscopy for corneal biomechanics

Invest Ophthalmol Vis Sci

(2012)

L. Brillouin

Diffusion de la lumière et des rayons X par un corps transparent homogène - Influence de l’agitation thermique

Ann Phys

(1922)

S. Reiß et al.

Spatially resolved Brillouin spectroscopy to determine the rheological properties of the eye lens

Biomed Opt Express

(2011)

G. Lepert et al.

Assessing corneal biomechanics with Brillouin spectro-microscopy

Faraday Discuss

(2016)

T.G. Seiler et al.

The influence of hydration on different mechanical moduli of the cornea

Graefes Arch Clin Exp Ophthalmol

(2018)

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To characterize focal biomechanical alterations in subclinical keratoconus (SKC) using motion-tracking (MT) Brillouin microscopy and evaluate the ability of MT Brillouin metrics to differentiate eyes with SKC from normal control eyes.
Prospective cross-sectional study.
Thirty eyes from 30 patients were evaluated, including 15 eyes from 15 bilaterally normal patients and 15 eyes with SKC from 15 patients.
All patients underwent Scheimpflug tomography and MT Brillouin microscopy using a custom-built device. Mean and minimum MT Brillouin values within the anterior plateau region and anterior 150 μm were generated. Scheimpflug metrics evaluated included inferior–superior (IS) value, maximum keratometry (K_max), thinnest corneal thickness, asymmetry indices, Belin/Ambrosio display total deviation, and Ambrosio relational thickness. Receiver operating characteristic (ROC) curves were generated for all Scheimpflug and MT Brillouin metrics evaluated to determine the area under the ROC curve (AUC), sensitivity, and specificity for each variable.
Discriminative performance based on AUC, sensitivity, and specificity.
No significant differences were found between groups for age, sex, manifest refraction spherical equivalent, corrected distance visual acuity, K_max, or KISA% index. Among Scheimpflug metrics, significant differences were found between groups for thinnest corneal thickness (556 μm vs. 522 μm; P < 0.001), IS value (0.29 diopter [D] vs. 1.05 D; P < 0.001), index of vertical asymmetry (IVA; 0.10 vs. 0.19; P < 0.001), and keratoconus index (1.01 vs. 1.05; P < 0.001), and no significant differences were found for any other Scheimpflug metric. Among MT Brillouin metrics, clear differences were found between control eyes and eyes with SKC for mean plateau (5.71 GHz vs. 5.68 GHz; P < 0.0001), minimum plateau (5.69 GHz vs. 5.65 GHz; P < 0.0001), mean anterior 150 μm (5.72 GHz vs. 5.68 GHz; P < 0.0001), and minimum anterior 150 μm (5.70 GHz vs. 5.66 GHz; P < 0.001). All MT Brillouin plateau and anterior 150 μm mean and minimum metrics fully differentiated groups (AUC, 1.0 for each), whereas the best performing Scheimpflug metrics were keratoconus index (AUC, 0.91), IS value (AUC, 0.89), and IVA (AUC, 0.88).
Motion-tracking Brillouin microscopy metrics effectively characterize focal corneal biomechanical alterations in eyes with SKC and clearly differentiated these eyes from control eyes, including eyes that were not differentiated accurately using Scheimpflug metrics.
Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.
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The layer has assumed considerable importance in anterior and posterior lamellar corneal transplant surgery by improving our understanding of the behaviour of corneal tissue during these procedures, improved techniques and made the surgery safer with better outcomes. It has led to the innovation of new surgical procedures namely, pre-Descemet's endothelial keratoplasty, suture management of acute hydrops, DALK-triple and Fogla's mini DALK.
The discovery and knowledge of the layer has introduced paradigm shifts in our age old concepts of Descemet's membrane detachment, acute corneal hydrops in keratoconus and Descemetoceles, with impact on management approaches. It has been shown to contribute to the pathology and clinical signs observed in corneal infections and some corneal dystrophies. Early evidence suggests that it may have a role in the pathogenesis of keratoconus in relation to its elastin content. Its contribution to corneal biomechanics and glaucoma are subjects of current investigations.
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Induced corneal collagen crosslinking and mechanical stiffening via ultraviolet-A photoactivation of riboflavin (UVA CXL) is now a common treatment for corneal ectasia and Keratoconus. Some effects of the procedure such as induced mechanical stiffening, corneal flattening, and cellular toxicity are well-known, but others remain more controversial. Authors report a variety of contradictory effects, and provide evidence based on individual results and observations. A full understanding of the effects of and mechanisms behind this procedure are essential to predicting its outcome. A growing interest in modifications to the standard UVA CXL protocol, such as transepithelial or accelerated UVA CXL, makes analyzing the literature as a whole more urgent. This review presents an analysis of both the agreed-upon and contradictory results reported and the various methods used to obtain them.
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To characterize focal biomechanical differences between normal, keratoconic, and post–laser vision correction (LVC) corneas using motion-tracking Brillouin microscopy.
Prospective cross-sectional study.
Thirty eyes from 30 patients (10 normal controls [Controls], 10 post-LVC, and 10 stage I or II keratoconus [KC]) had Scheimpflug and motion-tracking Brillouin microscopy imaging using a custom-built device. Mean, maximum (max) and minimum (min) Brillouin shift, spatial standard deviation, and max-min values were compared. Min values were correlated with local Brillouin values at multiple Scheimpflug imaging locations.
Mean (P < .0003), min (P < .00001), spatial standard deviation (P < .01), and max-min (P < .001) were significantly different between the groups. In post hoc pairwise comparisons, the best differentiators for group comparisons were mean (P = .0004) and min (P = .000002) for Controls vs KC, min (P = .0022) and max-min (P = .002) for Controls vs LVC, and mean (P = .0037) and min (P = .0043) for LVC vs KC. Min (area under the receiver operating characteristic = 1.0) and mean (area under the receiver operating characteristic =  0.96) performed well in differentiating Control and KC eyes. Min values correlated best with Brillouin shift values at the thinnest corneal point (r² = 0.871, P = .001) and maximum keratometry value identified in the tangential curvature map (r² = 0.840, P = .002).
Motion-tracking Brillouin microscopy effectively characterized focal corneal biomechanical alterations in LVC and KC and clearly differentiated these groups from Controls. Primary motion-tracking Brillouin metrics performed well in differentiating groups as compared with basic Scheimpflug metrics, in contrast to previous Brillouin studies, and identified focal changes after LVC where prior Brillouin studies did not.

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Original articleBrillouin Spectroscopy of Normal and Keratoconus Corneas