2020–2021 BCSC Basic and Clinical Science Course™
8 External Disease and Cornea
Chapter 7: Corneal Dystrophies and Ectasias
Keratoglobus is a rare, bilateral, noninflammatory condition that is typically present at birth, unlike keratoconus and PMD. It is usually not hereditary. Keratoglobus manifests as a globular rather than a conical deformation of the cornea (Fig 7-35).
Figure 7-34 Power map shows the typical “crab claw” pattern seen in pellucid marginal degeneration, which can also be seen in keratoconus with inferior apical decentration.
(Courtesy of Robert S. Feder, MD.)
Figure 7-35 The presence of corneal thinning and the type of contour abnormality can be helpful to the clinician in recognizing the type of ectatic disorder. A, Keratoconus. B, Pellucid marginal degeneration. C, Keratoglobus.
(Reproduced with permission from Feder RS, Neems LC. Noninflammatory ectatic disorders. In: Mannis MJ, Holland EJ, eds. Cornea. Vol 1. 4th ed. Philadelphia: Elsevier; 2017:822.)
Keratoglobus is strongly associated with blue sclerae and Ehlers-Danlos syndrome type VI (see Chapter 8), and it may represent a defect in collagen synthesis. Histologically, it is characterized by an absent or fragmented Bowman layer, thinned stroma with normal lamellar organization, and a thin Descemet membrane. Unlike keratoconus, keratoglobus is not associated with atopy or hard contact lens wear.
The corneas have a globular shape with a deep anterior chamber. The corneal curvature may be as steep as 50–60 D, and generalized thinning appears, especially in the periphery. This is in contrast to keratoconus, which typically has inferior paracentral thinning and protrusion with thinning at the apex. Spontaneous rupture of the Descemet membrane and corneal hydrops can occur, but iron lines, stress lines, and anterior scarring are not typically seen. The corneal diameter may be slightly increased. Fleischer rings are usually not present, but prominent folds and areas of thickening in Descemet membrane are common.
Contact lenses, especially scleral lenses, may be of benefit. The prognosis for PK is much poorer in keratoglobus than in the other corneal ectasias. Tectonic lamellar keratoplasty followed by PK could be considered in cases requiring intervention to maintain functional vision. Spontaneous corneal rupture has been reported; thus, patients must be counseled regarding the importance of protective eyewear. High myopia is treated with spectacles to prevent amblyopia in children.
Feder RS, Neems LC. Noninflammatory ectatic disorders. In: Mannis MJ, Holland EJ, eds. Cornea. Vol 1. 4th ed. Philadelphia: Elsevier; 2017:820–843.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.