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Spectral-domain optical coherence tomography is useful for diagnosing early keratoconus because it detects characteristic changes in epithelial and stromal thickness.
TAKE HOME MESSAGE: Spectral-domain optical coherence tomography is useful for diagnosing early keratoconus because it detects characteristic changes in epithelial and stromal thickness.
By Cheryl Guttman Krader; Reviewed by Aleksandar Stojanovic, MD
He proposed that the non-invasive imaging technology could become part of the routine diagnostic evaluation for early keratoconus based on the findings of a study comparing features of SD-OCT epithelial and stromal thickness distribution maps between eyes with keratoconus and healthy eyes with corneal astigmatism ≥2.0 D.
Dr. Stojanovic performed the study with Wen Zhou, MD, MSc, who conceived the study and is the first author of the online published article [PLoS One. 2014;9(1):e85994]. The analyses showed significant between-group differences in epithelial and stromal thickness at sites along the steepest meridian that were due to pathologic changes in the keratoconic eyes.
“As the keratoconus progresses, stromal bulging will initially be manifested only by the protrusion of the posterior corneal surface, since the corresponding protrusion of the anterior stromal surface will be masked by compensatory thinning of the epithelium covering the protrusion,” said Dr. Stojanovic, senior consultant in charge of refractive surgery and keratoconus, Department of Ophthalmology, University Hospital North Norway, Tromsø, Norway.
“This is due to the ability of the epithelium to remodel itself in an attempt to keep the ocular surface smooth,” Dr. Stojanovic added. “This situation will last until the epithelial compensatory ability has reached its limits and during that stage the early keratoconus could be diagnosed only by imaging the posterior cornea by mainly Scheimpflug-based corneal tomography, or by corneal epithelial mapping, which would reveal the shape of the underlying anterior stroma.”
SD-OCT allows mapping of the epithelial and stromal thickness and the pattern of changes that occurs early in keratoconus, he noted.
The SD-OCT study included 20 eyes in each group. The keratoconic eyes were seen on referral for corneal crosslinking (CXL) at the University Hospital North Norway, and the healthy eyes with ≥2D corneal astigmatism presented for refractive surgery to SynsLaser Kirurgi AS, Tromsø, Norway, where Dr. Zhou is chief of research and development and Dr. Stojanovic is medical director. The two groups were matched by age (mean ~29 years), sex, and anterior corneal cylinder (mean ~3.5 D).
Corneal topography was used to identify the steepest and flattest meridians. Thicknesses of the epithelium and stroma were compared between groups at 11 locations, which included the corneal vertex and the points 1.5 and 2.5 mm from the corneal vertex along the flattest and steepest meridians. Locations of maximum thickening and thinning were also identified. The mean location of the steepest meridian was at 67.8° in the keratoconic eyes and at 90.3° in the healthy eyes.
Looking along the steepest meridian, the keratoconic eyes exhibited maximum epithelial thinning inferotemporally at about 1.2 mm from the vertex and maximum thickening supranasally at about 1.4 mm from the vertex. Along the flattest meridian the epithelium was getting thicker towards the periphery along both the steepest and flattest meridians in the keratoconic eyes.
Comparisons between the two groups showed statistically significant differences in epithelial thickness along the steepest meridian. Specifically, compared with the healthy eyes, the epithelium in eyes with keratoconus was significantly thinner inferiorly to the cone and at its center and significantly thicker superiorly to the cone. Looking along the flattest meridian, the only significant difference between groups was seen at the corneal vertex, where the epithelium was thinner in the keratoconic eyes.
“As expected, the stromal thickness distribution maps showed thinning over the cone in the keratoconic eyes compared with the healthy eyes, and the stroma was also thinner in general in the keratonic group, although mostly temporally and inferiorly,” Dr. Stojanovic said.
Dr. Stojanovic noted that as a result of the epithelial remodeling that occurs in eyes with keratoconus, a “concave lens” is created over the cone that induces negative refractive power. Hence, he proposed that epithelial mapping is crucial for accurate planning when performing topography-guided surface ablation in combination with CXL as treatment for keratoconus, as well as in any type of surface ablation where the epithelium is removed before the refractive ablation.
“Planning the custom ablation based only on the topography and failing to take into account the significant difference between the stromal surface and the epithelial surface that is present in eyes with keratoconus can lead to an erroneous result,” he said.
“Not taking into account the epithelial remodeling, which always occurs with any stromal surface irregularity, will result in treatment of an unknown surface if epithelium is removed prior to the laser ablation,” Dr. Stojanovic said.
Aleksandar Stojanovic, MD
This article was adapted from Dr. Dr. Stojanovic’s presentation at Refractive Surgery Subspecialty Day during the 2014 meeting of the American Academy of Ophthalmology. Dr. Stojanovic and Dr. Zhou have no relevant financial interest in the technology used in the study.