9 Optical Coherence Tomography in Corneal Ectasia
Keratoconus is the most common primary ectatic corneal disorder. It is characterized by progressive corneal thinning, irregular astigmatism, and corneal protrusion that may eventually result in scarring and loss of vision. This disease is associated with stromal thinning, a decrease in keratocyte density, various amounts of stromal haze, Bowman layer disruption and splitting in the region of the cone, and epithelial changes. 1 Optical coherence tomography (OCT) provides an accurate assessment of corneal layers changes during keratoconus and should be performed systematically in association with topographic examinations in the evaluation of keratoconus eyes.
9.2 Keratoconus Screening
The diagnosis of moderate to advanced keratoconus is not difficult because of the characteristic topographic patterns. However, identification of forme fruste keratoconus (FFK) in patients with minimum or no clinical signs can be challenging. Preoperative accurate detection of keratoconus among refractive surgery candidates is crucial; FFK is the main cause of postoperative corneal ectasia after LASIK (laser-assisted in-situ keratomileusis) surgery.
Li and colleagues conducted a study using a time-domain OCT system to map pachymetry for keratoconus detection. 2 They determined the criteria for a diagnosis of keratoconus based on the first percentile or 99th percentile cutoff points of the normal range. These criteria include the following:
Asymmetric parameters: superonasal-inferotemporal (SN-IT) or superior-inferior values greater than 45 μm
Minimum corneal thickness < 470 μm
Focal thinning parameter minimum: Maximum value < –100 μm
Early epithelial changes are present in subclinical cases of keratoconus. The epithelium is able to compensate fully for the subsurface cone, which is topographically evident on the back surface, resulting in an apparently normal anterior-surface topography. 3 Analysis of the corneal epithelial thickness profile may aid in the interpretation of corneal topography, improving the detection of FFK. In a recent study of 38 keratoconus cases, we demonstrated that the thinnest epithelial point was located inferiorly compared with the normal cornea and corresponded to the location of the thinnest corneal point on OCT pachymetry and the maximal posterior corneal elevation zone on corneal topography. The epithelial thickness of the thinnest point was thinner in FFK compared with that of normal corneas. A pattern of thin epithelium surrounded by a zone of epithelial thickening (“doughnut pattern”) as described by Reinstein et al is suggestive of mild keratoconus (Fig. 9.1). 3
9.3 Keratoconus Evaluation
Many classifications of keratoconus based on the location of the cone, slit-lamp appearance, and indirect topographic patterns have been proposed in the literature; however, these classifications may have not taken into account direct corneal microstructure and histologic changes occurring during keratoconus evolution. Indeed, a microstructural corneal analysis directly reflects abnormalities of corneal layers occurring in keratoconus and is more informative than the corneal topographic changes in the assessment of corneal architecture.
Stage 1: Thinning of epithelial and stromal layers at the conus. Corneal layers have a normal aspect.
Stage 2: Hyperreflective anomalies occurring at the Bowman layer level (varying from a barely visible hyperreflective line to a hypertrophic scar) and epithelial thickening at the conus (3a, clear stroma; b, stromal opacities)
Stage 3: Posterior displacement of the hyperreflective structures occurring at the Bowman layer level with increased epithelial thickening and stromal thinning (a, clear stroma; b, stromal opacities)
Stage 4: Pan-stromal scar. In stage 4, when the residual stroma is thin, it acquires an hourglass-shaped scar with increased epithelial thickening.
Stage 5. Represents the acute form of keratoconus (hydrops): 5a, acute onset, characterized by the rupture of the Descemet membrane with dilacerations of collagen lamellae, large fluid-filled intrastromal cysts, and the formation of epithelial edema; b, healing stage, pan-stromal scarring with a remaining aspect of Descemet membrane rupture.
Clinical and paraclinical characteristics of keratoconus eyes, including visual acuity, corneal epithelium and stromal thickness changes, corneal topography, biomechanical corneal characteristics, and microstructural changes observed on confocal microscopy were concordant with the OCT classification.
This OCT grading supports the original idea, stated by Chi, Katzin, and Teng, that the earliest ultrastructural changes in keratoconus occur at the epithelial basement membrane and Bowman layer. 1 These changes induce a complex imbalance between proinflammatory and anti-inflammatory cytokines, leading to keratocyte activation and corneal scar formation in advanced cases.
Vogt striaes were predominantly observed in stages 2 and 3. They have the aspect of dark parallel lines running through the stromal thickness between the Descemet and Bowman layers. Their appearance was similar to those observed in confocal microscopy, suggesting that these lines actually represent collagen lamellae under stress rather folds in the Descemet membrane (Fig. 9.3).