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Ronald H Silverman, Raksha Urs, Harriet O Lloyd, Timothy J Archer, Marine Gobbe, Dan Z Reinstein; Bowman’s Layer Acoustic Impedance in Normal and Keratoconus Corneas. Invest. Ophthalmol. Vis. Sci. 201657(12):.
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© ARVO (1962-2015); The Authors (2016-present)
Focal disruptions and lamellar thinning in Bowman’s layer may contribute to the mechanical weakness of corneas in keratoconus (KC). We have long used ultrasound to measure and map epithelial thickness, but now consider the acoustic reflectivity of Bowman’s layer, a function of its acoustic impedance (density x speed-of-sound), which is in turn related to its elastic modulus.
We scanned both eyes of 15 normal subjects with Artemis 2. We measured acoustic impedance of the central epithelium and determined and mapped the impedance of Bowman’s layer at each point. We next examined data previously acquired with Artemis 1 on both eyes of 12 normal and 12 KC subjects and compared Bowman's layer impedance values in the central cornea.
Central epithelial impedance of normal subjects averaged 1.606±0.011 MRayl. Centrally, Bowman’s impedance ranged from 1.618 to 1.634 MRayl, averaging 1.625 MRayl. Maps were quite uniform in most cases, even in the presence of epithelial thickness variation. While ANOVA demonstrated significant variation in central Bowman's impedance between subjects (p<0.001), left and right eye values within subjects were strongly correlated and paired T-tests showed no significant difference between eyes of the same subject. Bowman's impedance maps of KC subjects showed significantly greater variability (p=0.004) and maximum values than normals (p=0.012) paracentrally.
The analysis and mapping of Bowman’s acoustic impedance is distinct from other topographic and tomographic methods in that maps depict a parameter related to biomechanical properties rather than anatomy. Given that elastic changes in Bowman’s layer may be a precursor to development of KC, this technique offers a new approach towards early diagnosis, which will be a subject of future investigation.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
Map depicting mean difference in Bowman's layer acoustic reflectivity in keratoconus versus normal corneas (24 eyes of 12 subjects in each group).
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