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Ingo Schmack, Gabriel Shajari, Miad PourSadeghian, Matthias Remy, Fritz Hengerer, Thomas Kohnen; Characteristics of corneal astigmatism of anterior and posterior surface in healthy individuals and keratoconus patients. Invest. Ophthalmol. Vis. Sci. 2016;57(12):2913.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate and compare power and axis orientation of anterior and posterior astigmatism in eyes with keratoconus to healthy eyes.
This retrospective study examined 861 eyes of 494 patients diagnosed with keratoconus with a mean age of 35±11 years and 500 eyes of 500 healthy individuals serving as a control group. Using a Scheimpflug device we measured magnitude and axis orientation of anterior and posterior corneal astigmatism, as well as, corneal thickness and conus location. The results were compared between the different stages of the disease according to Amsler Krumeich classification (124 subclinical; 233 Stage1; 327 Stage2; 146 Stage3; 31 Stage4) and to eyes of healthy individuals.
Magnitude of corneal astigmatism was 3.47±2.10 D on anterior surface and 0.69±0.40 on posterior surface in eyes with keratoconus. We found significant increase of anterior and posterior corneal astigmatism with progression of disease (p<0.01, one-way ANOVA) and significant correlation between anterior and posterior corneal astigmatism (r=0.77, p<0.01). In contrast to eyes of healthy individuals where posterior corneal axis alignment is vertical in majority of cases independent of anterior alignment, we found, in eyes with keratoconus, a match between anterior and posterior alignment when alignment was vertical in 97% of eyes, 46% when oblique and 61% when horizontal (Cohen kappa coefficient κ = 0.55, p=0.00). With progression of the disease, alignment of anterior and posterior corneal astigmatism became increasingly vertical. Logistic regression analysis showed a significant increase in vertical alignment with decrease in corneal pachymetry (p=0.02 for anterior surface and p=0.01 for posterior surface). Vertical alignment was found significantly more often if cone was central compared to paracentral location (p<0.01 for anterior and posterior surface).
In difference to healthy individuals in eyes with keratoconus posterior axis alignment of corneal astigmatism is in line with alignment of anterior surface in majority of cases. With progression of the disease and decrease in pachymetry corneal resistance to vertical forces, most probably by eyelids, decrease and axis orientation becomes increasing vertical. Axis alignment can be used in algorithm to support diagnosis and staging of keratoconus.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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