Posterior Keratoconus

Key points at a glance

Key Points at a Glance

• A rare congenital corneal disease characterized by increased curvature of the posterior corneal surface.
• There are two subtypes: generalis and circumscriptus.
• Usually unilateral and sporadic, often accompanied by corneal stromal opacity.
• A form of anterior segment dysgenesis, considered the mildest type of Peters anomaly.
• Generally non-progressive, and visual prognosis is often good.
• In children, prevention of amblyopia is most important; in adults, refractive correction is the main treatment.

1. What is Posterior Keratoconus?

Posterior keratoconus (PKC) is a rare corneal disease characterized by increased curvature of the posterior corneal surface. In most cases, it is congenital, unilateral, and sporadic. Many cases are accompanied by corneal stromal opacity.

PKC has the following two subtypes:

• Keratoconus posticus generalis: The entire posterior corneal surface is affected.
• Keratoconus posticus circumscriptus: The abnormality is localized. It is subclassified into central, paracentral, or peripheral types.

Several familial cases have been reported, often showing an autosomal dominant inheritance pattern. Familial PKC may be associated with systemic abnormalities such as cleft lip/palate, webbed neck, limb defects, and genitourinary anomalies.

Q How is posterior keratoconus different from typical keratoconus?

A

Typical keratoconus is a progressive disease in which the anterior corneal surface protrudes forward in a conical shape, usually starting in adolescence and stabilizing around age 30. Posterior keratoconus is a congenital disease with increased curvature of the posterior corneal surface, typically non-progressive and unilateral. In typical keratoconus, the anterior corneal surface is abnormal, whereas in PKC, the anterior corneal surface often appears clinically normal.

2. Main Symptoms and Clinical Findings

Subjective Symptoms

Often asymptomatic. Some patients may notice long-term vision loss in the affected eye. In adults, it may be discovered incidentally during routine eye exams. If corneal opacity is visible to the naked eye from childhood, it is detected early.

Clinical Findings

• Posterior corneal depression: Localized or global increase in posterior curvature with corneal thinning
• Corneal stromal opacity: May be the most clinically prominent feature
• Posterior corneal pigmentation: Associated with the area of depression
• Normal anterior corneal findings: The anterior surface appears clinically normal, which is characteristic of PKC
• Ocular complications: May include optic disc coloboma, congenital cataract, etc.
• Abnormal corneal topography: Localized central PKC shows steepening of the anterior cornea, while peripheral PKC shows flattening of the anterior cornea

3. Causes and Risk Factors

Congenital PKC

Congenital PKC is thought to result from anterior segment dysgenesis. It may represent the mildest form of Peters anomaly. Since it is sporadic, no known risk factors have been identified.

Familial PKC shows an autosomal dominant inheritance pattern, but no specific causative gene locus has been identified. It may be associated with systemic abnormalities such as cleft lip/palate, webbed neck, limb defects, and genitourinary anomalies, suggesting involvement of widespread developmental abnormalities during embryogenesis.

Acquired PKC

In patients with a history of ocular trauma, acquired PKC may occur. It is thought that disruption of the inner corneal layers leads to localized damage to Descemet’s membrane and the corneal endothelium, and subsequent tissue remodeling results in acquired PKC.

About your child’s vision

Posterior keratoconus is often unilateral and can cause amblyopia in children. If you notice a white opacity in your child’s eye or a difference in vision between the eyes, please consult an ophthalmologist early. Early detection and appropriate treatment can prevent amblyopia.

4. Diagnosis and Examination Methods

Basic Evaluation

• Slit-lamp microscopy: Confirms the posterior corneal depression and stromal opacity. The anterior cornea often appears normal.
• Anterior segment optical coherence tomography (OCT): Observes the cross-sectional structure of the cornea. Can quantitatively evaluate paracentral thinning and posterior steepening.
• Corneal topography / Scheimpflug tomography: Evaluates the curvature of the anterior and posterior cornea. Abnormal posterior curvature is important for diagnosing PKC¹⁾.
• Corneal pachymetry: Quantifies the degree of thinning.
• Ultrasound biomicroscopy (UBM): Used adjunctively to evaluate deep corneal and angle structures.
• Gonioscopy: Checks for the presence of iridocorneal adhesions.

If systemic abnormalities are suspected, consider systemic evaluation and clinical tests such as thyroid function tests.

Differential Diagnosis

Disease	Key differentiating points
Keratoconus	Anterior protrusion of the cornea. Progressive.
Pellucid marginal degeneration	Band-like thinning of the inferior peripheral cornea
Peters anomaly	Central disc-shaped corneal opacity. Severe.
Corneal scar	History of trauma or infection

5. Standard Treatment

Management in Children: Amblyopia Prevention

PKC is often unilateral and can cause anisometropic amblyopia due to astigmatism or form deprivation amblyopia due to corneal opacity. Therefore, amblyopia prevention is paramount in pediatric cases.

• Occlusion therapy of the healthy eye: Basic treatment for amblyopia
• Refractive correction glasses or hard contact lenses: Correction of astigmatism
• Cycloplegic eye drops: Attempt to secure the optical pathway when corneal opacity obstructs the visual axis

Management in Adults

In adults, the mainstay of treatment is correction with refractive glasses and/or hard contact lenses. Since PKC is generally non-progressive, adults with isolated PKC can be followed up with annual regular visits.

Surgical Treatment

Surgery is considered when there is visually significant corneal opacity that does not improve with mydriatic agents.

• Optical iridectomy: Creates an optical pathway by bypassing the corneal opacity.
• Rotational autokeratoplasty: Rotates the opaque area to place the clear portion in the visual axis.
• Penetrating keratoplasty (PKP): Indicated when corneal opacity is severe.

In adults with long-standing amblyopia, visual improvement may be limited even after surgery, and careful judgment is required.

Cautions during Cataract Surgery

When PKC patients undergo cataract surgery, the total corneal refractive power may be overestimated due to the abnormal curvature of the posterior corneal surface. Measure both anterior and posterior corneal curvatures preoperatively and include them in intraocular lens power calculations. This is important to avoid unexpected postoperative hyperopia.

Q Can posterior keratoconus in children cause amblyopia?

A

Yes. PKC is often unilateral and can cause anisometropic amblyopia due to astigmatism or form deprivation amblyopia due to corneal opacity. Early detection and occlusion therapy/refractive correction are important to prevent amblyopia. Monitor amblyopia progression with frequent follow-ups.

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6. Pathophysiology and Detailed Mechanisms

PKC as an Anterior Segment Dysgenesis

The exact pathophysiology of PKC is unknown, but histological and clinical findings suggest it results from anterior segment dysgenesis. It has been suggested that it may be the mildest form of Peters anomaly, and two embryological mechanisms have been proposed.

• Delayed separation of the lens from the surface ectoderm: Failure of the lens vesicle to separate normally from the surface ectoderm leads to abnormal formation of the posterior cornea.
• Abnormal migration of mesoderm (neural crest cells): Abnormal migration of neural crest cells that form the corneal endothelium and Descemet’s membrane results in structural abnormalities of the posterior cornea.

Systemic complications such as cleft lip/palate and genitourinary abnormalities have been reported in both familial and sporadic cases, suggesting a possible origin from widespread defects in embryonic or early fetal development.

Mechanisms of Acquired PKC

In patients with a history of ocular trauma, disruption of the inner corneal layers is thought to lead to localized damage to Descemet’s membrane and the corneal endothelium, with subsequent tissue remodeling resulting in abnormal curvature of the posterior cornea.

Histological Findings

In thinned areas, various abnormalities of Descemet’s membrane have been reported.

• Descemet’s membrane: Disorganized, widely spaced collagen, abnormal banding, and multilayered structure are observed. Descemet’s membrane is generally intact, but small breaks or hyaline elevations may be seen.
• Corneal endothelium: Usually intact, but occasionally guttae are observed.
• Bowman’s layer: Areas replaced by irregularly arranged collagen and fibroblasts have been confirmed by electron microscopy.
• Epithelial basement membrane: May become disorganized, with abnormalities in the basal cell layer.

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8. References

1. American Academy of Ophthalmology Corneal/External Disease Preferred Practice Pattern Panel. Corneal Ectasia Preferred Practice Pattern. San Francisco: AAO; 2024.
2. Silas MR, Hilkert SM, Reidy JJ, Farooq AV. Posterior keratoconus. Br J Ophthalmol. 2018;102(7):863-867. PMID: 29122822.
3. Williams R. Acquired posterior keratoconus. Br J Ophthalmol. 1987;71(1):16-7. PMID: 3814563.