Keratoglobus: Diffuse Corneal Thinning Beyond Keratoconus

Keratoglobus is a rare, non-inflammatory corneal ectasia in which the entire cornea thins diffusely and balloons into a globular shape, extending out to the limbus rather than staying confined to a central or paracentral cone as in keratoconus. The thinning is most pronounced peripherally, which is the opposite pattern from keratoconus and is what gives the cornea its rounded, oversized appearance.

Most cases are congenital and associated with connective tissue disorders, particularly brittle cornea syndrome and, less consistently, Ehlers-Danlos syndrome. An acquired form exists too, linked to chronic eye rubbing and, historically, to vernal keratoconjunctivitis.

Pathogenesis

Histologically, the corneal stroma in keratoglobus shows thinning to roughly a third of normal thickness, with a reduced number of collagen lamellae and disorganized fibril architecture. Descemet’s membrane is often normal or even thickened, which helps distinguish it from acute hydrops in keratoconus.

In brittle cornea syndrome, mutations in ZNF469 or PRDM5 disrupt collagen and extracellular matrix regulation throughout the body, so the corneal finding is one manifestation of a systemic connective tissue defect rather than an isolated eye disease.

Associated Systemic Findings

• Blue sclerae, from scleral thinning that lets the underlying uvea show through
• Joint hypermobility and skin hyperextensibility
• Hearing loss (in brittle cornea syndrome)
• Hip dysplasia and other skeletal fragility features
• Mitral valve prolapse in some connective tissue overlap syndromes

Recognizing keratoglobus should prompt a search for these features, since the ocular finding is frequently the presenting sign that leads to the systemic diagnosis, especially in a child.

Clinical Presentation

Vision loss develops gradually from the high, irregular astigmatism that comes with a globally distorted corneal shape. Because the whole cornea is thin, these eyes are also unusually fragile — minor blunt trauma that would be trivial in a normal eye can cause spontaneous corneal or scleral rupture.

Some patients present acutely with hydrops, a sudden break in Descemet’s membrane that lets aqueous flood into the stroma and causes acute pain, photophobia, and marked corneal edema. This is less common than in keratoconus but tends to be more severe when it happens, given the extent of pre-existing thinning.

Differential Diagnosis

• Keratoconus — central/paracentral cone, relatively preserved peripheral cornea
• Pellucid marginal degeneration — inferior peripheral thinning band, “kissing dove” topography
• Megalocornea — enlarged but structurally normal-thickness cornea
• Congenital glaucoma (buphthalmos) — enlarged globe from elevated IOP, Haab’s striae
• Terrien’s marginal degeneration — peripheral thinning, usually superior, in older adults

Diagnostic Evaluation

Slit-lamp examination shows the diffusely thin, domed cornea extending to the limbus without the sharp demarcation typical of keratoconus. Corneal topography and tomography (Scheimpflug imaging) quantify the pattern of thinning and steepening across the whole cornea rather than a focal apex, and pachymetry documents just how thin the tissue is — often under 400 microns, sometimes far less.

Genetic testing is useful when brittle cornea syndrome is suspected, both to confirm the diagnosis and to guide counseling for the family, since it is autosomal recessive. Axial length and corneal diameter measurements help distinguish keratoglobus from buphthalmos, where intraocular pressure — not just corneal shape — is the primary abnormality.

Management

Rigid gas-permeable or scleral contact lenses can improve vision by masking irregular astigmatism, but fitting is difficult given the globular shape and fragility of the cornea, and lens wear itself carries a real risk of mechanical trauma to tissue this thin. Protective eyewear is worth discussing early, since these patients are at genuine risk of globe rupture from impacts that most people would shrug off.

Corneal transplantation is technically demanding in keratoglobus because there is so little healthy peripheral tissue to support graft-host apposition; large-diameter penetrating keratoplasty or epikeratoplasty has been used, but outcomes are less predictable than in keratoconus and rejection or wound dehiscence risk is higher given the abnormal collagen. Some surgeons favor staged approaches — tectonic patch grafts to reinforce weak areas before attempting an optical procedure.

Complications and Prognosis

Spontaneous or trauma-related perforation is the complication that most changes management, since these patients need to understand that even modest impact — a fall, a poorly aimed ball, rubbing the eye vigorously — can rupture the globe. Acute hydrops, when it occurs, usually resolves over weeks to months as the endothelium re-establishes its pump function, though it can leave residual scarring.

Visual prognosis depends heavily on the severity of the ectasia and whether the systemic disorder is otherwise well managed. Genetic counseling matters for families with brittle cornea syndrome given its recessive inheritance and multisystem impact.

References

• Al-Hussain H, et al. Brittle cornea syndrome and its delineation from the kyphoscoliotic type of Ehlers-Danlos syndrome. American Journal of Medical Genetics.
• Abu A, et al. Mutation in PRDM5 in brittle cornea syndrome. American Journal of Human Genetics.
• Meire FM. Keratoglobus. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea.
• Cameron JA. Keratoglobus. Cornea.
• Rathi VM, Vyas SP, Sangwan VS. Keratoglobus. Indian Journal of Ophthalmology.