Keratoectasias (keratoconus, keratoglobus, and pellucid marginal degeneration) are predominantly sporadic, typically bilateral noninflammatory disorders that share a unifying feature: stromal thinning. See also BCSC Section 8, External Disease and Cornea.
Keratoconus, which is typically diagnosed during adolescence or young adulthood, is characterized by inferior or inferotemporal corneal stromal ectasia (Fig 6-16A). Although it is frequently sporadic, familial inheritance and association with other ocular and systemic conditions, including atopy, Down syndrome, and Ehlers-Danlos syndrome, have been described. The basic pathophysiologic change is loss of stromal structural integrity, leading to keratoectasia, or stretching and thinning of the corneal stroma. The alteration in the normal corneal contour produces irregular astigmatism, occasionally requiring deep anterior lamellar keratoplasty (DALK). Advanced cases with significant apical scarring are managed with PK.
Figure 6-16 Keratoconus. A, Clinical photograph showing inferior conical deformity of the cornea. B, Low-magnification photomicrograph showing apical stromal thinning (arrow).C, Masson trichrome stain demonstrating focal disruption of Bowman layer (arrow).D, Prus sian blue stain demonstrating focal intraepithelial iron deposition (Fleischer ring). E, In a patient with prior hydrops, PAS stain highlights the rupture of Descemet membrane, with rolled edges on either side (arrows).
(Part A courtesy of Sander Dubovy, MD; part C courtesy of Hans E. Grossniklaus, MD; part D courtesy of Tatyana Milman, MD; part E courtesy of George J. Harocopos, MD.)
Histologic findings in keratoconus include central stromal thinning and small focal discontinuities in the Bowman layer. Apical anterior stromal fibrosis is often present (Fig 6-16B, C). Iron deposition in the epithelium at the base of the cone (Fleischer ring) can sometimes be demonstrated with Prussian blue stain (Fig 6-16D). In patients with a history of corneal hydrops, a focal break in Descemet membrane may be seen (Fig 6-16E).
In 2016, the US Food and Drug Administration (FDA) approved collagen crosslinking using UVA light and riboflavin (vitamin B2) to slow or arrest the progression of the ectasia in keratoconus. There are no human studies describing the histologic and cellular changes following the procedure.
Pellucid marginal degeneration
Pellucid marginal corneal degeneration (PMCD) is a rare, bilateral, noninflammatory, ectatic peripheral corneal disorder, usually involving the inferior portion of the cornea; the band of thinning is separated from the limbus by a zone of normal corneal thickness. The cornea protrudes above the thinned area, resulting in high irregular astigmatism. Histologic examination demonstrates that the epithelium of the thinned region is intact, with microscopic disruptions in the Bowman layer similar to those seen in keratoconus. The pathogenesis of PMCD is poorly understood.
Excerpted from BCSC 2020-2021 series: Section 4 - Ophthalmic Pathology and Intraocular Tumors. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.