Ophthalmology – Keratoconus

Basics

Description

Keratoconus is a progressive, noninflammatory disorder of the cornea characterized by corneal thinning, steepening, and protrusion. As the cornea becomes increasingly irregular, it develops a cone-like shape.

The resulting optical distortion commonly causes:

• Progressive astigmatism
• Increasing myopia
• Reduced best spectacle-corrected visual acuity
• Glare and visual distortion
• Monocular diplopia
• Difficulty tolerating contact lenses
• Corneal scarring in advanced disease

The condition is usually bilateral, although one eye may be much more severely affected.

Epidemiology

Keratoconus often becomes clinically apparent during puberty or early adulthood.

The prevalence varies considerably between populations and studies, but it is generally considered an uncommon-to-moderately common corneal ectatic disorder.

Progression is usually faster in younger patients and may slow with increasing age.

Risk Factors

Important risk factors include:

• Eye rubbing
• Chronic ocular allergy
• Atopy
• Family history of keratoconus
• Repetitive mechanical pressure on the eye
• Floppy eyelid syndrome
• Sleep apnea

A first-degree relative with keratoconus increases the likelihood of developing the disease.

Genetics

Keratoconus has a genetic component, but there is no single inheritance pattern in most patients.

Familial clustering occurs, and some families show patterns compatible with incomplete autosomal dominant inheritance.

The disease is likely multifactorial, involving both genetic susceptibility and environmental or mechanical influences.

General Prevention

The most important modifiable measure is to avoid eye rubbing.

Patients with ocular allergy should receive appropriate treatment to reduce itching and the urge to rub.

Avoiding direct pressure on the cornea during sleep may also be reasonable in susceptible individuals.

Pathophysiology

Keratoconus produces progressive biomechanical weakening of the cornea.

The anterior and posterior corneal surfaces become increasingly irregular, causing:

• Irregular astigmatism
• Higher-order aberrations
• Reduced image quality

As the cornea thins and protrudes, the cone may become more pronounced.

Breaks in Bowman layer can lead to fibrosis and stromal scarring.

A rupture in Descemet membrane can allow aqueous humor to enter the corneal stroma, producing sudden severe edema known as acute corneal hydrops.

Etiology

The exact cause remains incompletely understood.

Likely contributing factors include:

• Genetic predisposition
• Abnormal corneal biomechanics
• Oxidative stress
• Enzymatic alterations within the corneal stroma
• Chronic mechanical trauma from eye rubbing

Commonly Associated Conditions

Keratoconus has been associated with:

• Floppy eyelid syndrome
• Obstructive sleep apnea
• Atopy
• Allergic conjunctivitis
• Down syndrome
• Leber congenital amaurosis
• Aniridia
• Retinopathy of prematurity
• Ehlers-Danlos syndrome
• Marfan syndrome
• Osteogenesis imperfecta

Some corneal dystrophies may also coexist.

Diagnosis

History

Patients may complain of:

• Gradually worsening vision
• Increasing astigmatism
• Frequent changes in glasses prescription
• Glare
• Halos
• Ghost images
• Monocular diplopia
• Reduced contrast sensitivity
• Difficulty driving at night
• Contact lens intolerance

The disease is commonly asymmetric.

A history of habitual eye rubbing should be specifically sought.

Some patients report pressing or rubbing one eye more than the other or sleeping predominantly on the more affected side.

Physical Examination

Refraction

Refraction may show:

• Increasing myopia
• Increasing astigmatism
• Irregular astigmatism
• Fluctuating refractive measurements

Vision may eventually become poorly correctable with spectacles.

Retinoscopy

A classic finding is a scissoring reflex due to irregular corneal curvature.

The red reflex may appear distorted or irregular.

External Signs

Advanced disease may produce visible corneal protrusion.

Munson Sign

When the patient looks downward, the cone-shaped cornea may indent the lower eyelid in a V-shaped configuration.

This is known as Munson sign.

It is generally a late finding.

Slit-Lamp Findings

Important signs include:

Fleischer Ring

An iron deposition ring around the base of the cone.

It is located in the basal epithelium and may be partial or complete.

Vogt Striae

Fine vertical lines in the deep stroma and Descemet membrane.

They often disappear temporarily when gentle external pressure is applied to the globe.

Corneal Thinning

The cornea is thinnest near the apex of the cone, which is often displaced inferiorly or inferotemporally.

Corneal Scarring

Progressive stromal stress and breaks in Bowman layer may produce:

• Apical haze
• Stromal scarring
• Dense central opacity in advanced cases

Prominent Corneal Nerves

Corneal nerves may appear unusually visible on slit-lamp examination.

Acute Corneal Hydrops

Acute hydrops occurs when Descemet membrane ruptures.

Aqueous rapidly enters the corneal stroma and causes:

• Sudden severe corneal edema
• Marked reduction in vision
• Pain or photophobia
• Epithelial bullae

Hydrops usually resolves gradually over weeks to months but may leave significant scarring.

Diagnostic Tests and Interpretation

Corneal Topography

Corneal topography is one of the most important diagnostic tests.

Typical findings include:

• Inferior or inferotemporal steepening
• High keratometric values
• Asymmetric bow-tie pattern
• Skewed radial axes
• Irregular astigmatism

Topography can detect disease before obvious slit-lamp findings appear.

Corneal Tomography

Modern tomography evaluates both the anterior and posterior corneal surfaces.

Scheimpflug-based systems such as Pentacam can demonstrate:

• Anterior elevation
• Posterior elevation
• Abnormal corneal thickness distribution
• Decentered thinnest point
• Progressive ectatic change

Posterior corneal elevation may become abnormal relatively early in the disease.

Pachymetry

Corneal thickness mapping shows:

• Localized thinning
• An abnormally displaced thinnest point
• Abnormal progression of thickness from center to periphery

Serial pachymetry is useful in monitoring progression.

Anterior-Segment OCT

Anterior-segment OCT can provide detailed maps of:

• Corneal thickness
• Epithelial thickness
• Stromal abnormalities

It can also help distinguish keratoconus from other forms of corneal irregularity.

Wavefront Aberrometry

Keratoconus produces increased higher-order aberrations, particularly vertical coma.

This helps explain why patients may have poor visual quality even when standard refractive error appears reasonably corrected.

Pathological Findings

Microscopic changes may involve all corneal layers.

Possible findings include:

• Epithelial thinning
• Degeneration of basal epithelial cells
• Iron deposition
• Breaks in Bowman layer
• Altered stromal collagen arrangement
• Stromal thinning
• Endothelial pleomorphism and polymegathism

The exact pathologic appearance varies with disease severity.

Differential Diagnosis

Important differential diagnoses include:

• Physiologic astigmatism
• Pellucid marginal degeneration
• Keratoglobus
• Post-refractive surgery ectasia
• Contact lens-induced corneal warpage
• Traumatic corneal scarring

Treatment

Management depends on:

• Severity
• Evidence of progression
• Age
• Visual needs
• Presence of scarring
• Contact lens tolerance

The goals are to improve vision and, when possible, stop progression.

Spectacles

Glasses may provide satisfactory vision in early disease when astigmatism remains relatively regular.

As irregularity increases, spectacles become less effective.

Contact Lenses

Soft Contact Lenses

Soft toric lenses may work in mild disease with relatively regular astigmatism.

Rigid Gas-Permeable Lenses

RGP lenses are commonly used because the rigid front surface provides a more regular refractive interface over the irregular cornea.

They can significantly improve vision.

Piggyback Systems

A soft lens may be worn underneath an RGP lens to improve comfort.

Hybrid Lenses

Hybrid lenses combine a rigid center with a soft peripheral skirt.

Scleral Lenses

Scleral lenses vault over the cornea and rest on the sclera.

They are particularly useful for:

• Advanced irregularity
• Contact lens intolerance
• Corneal scarring
• Severe visual distortion

Corneal Cross-Linking

Corneal collagen cross-linking is now a major treatment for progressive keratoconus.

It uses riboflavin and ultraviolet-A light to strengthen corneal collagen and increase biomechanical stability.

The primary goal is to halt or slow progression, not necessarily to restore normal corneal shape.

Cross-linking is particularly important in:

• Young patients
• Documented progression
• Increasing steepness
• Increasing refractive error
• Progressive thinning

Early treatment can reduce the likelihood of later transplantation.

Intrastromal Corneal Ring Segments

Intrastromal ring segments may be considered in selected patients.

They can:

• Flatten the cone
• Reduce irregularity
• Improve spectacle or contact lens tolerance

They do not reliably stop progression, so cross-linking may still be necessary when the disease is progressing.

Treatment of Acute Hydrops

Management is mainly supportive.

Options may include:

• Cycloplegic drops
• Hypertonic sodium chloride drops or ointment
• Lubrication
• Bandage contact lens in selected cases
• Pain control

More advanced procedures may be considered by a corneal specialist in severe hydrops.

Patients require close follow-up until the edema resolves.

Corneal Transplantation

Corneal transplantation may be necessary when there is:

• Severe corneal scarring
• Contact lens intolerance
• Advanced ectasia
• Poor functional vision despite other measures

Deep Anterior Lamellar Keratoplasty

DALK replaces the diseased corneal stroma while preserving the patient’s own endothelium.

It is often preferred when technically feasible because it avoids endothelial rejection.

Penetrating Keratoplasty

Full-thickness transplantation may be required in selected advanced cases.

Keratoconus generally has a good graft prognosis compared with many other indications for corneal transplantation.

Referral

Referral to an experienced contact lens specialist is appropriate when conventional correction is inadequate.

Referral to a corneal specialist is indicated for:

• Documented progression
• Consideration of cross-linking
• Acute hydrops
• Significant scarring
• Contact lens intolerance
• Consideration of ring segments or transplantation

Follow-Up

Stable patients should generally have periodic examinations, often at least annually.

Younger patients and those with documented progression require more frequent monitoring.

Follow-up may include:

• Refraction
• Visual acuity
• Corneal topography
• Tomography
• Pachymetry

Patients with acute hydrops may need review every few weeks until resolution.

Patient Education

Patients should be strongly advised:

• Do not rub the eyes
• Treat ocular allergy appropriately
• Attend regular corneal imaging appointments
• Report sudden visual deterioration promptly
• Understand that progression can occur even when vision seems relatively stable

Family members may also benefit from screening when there is a strong family history.

Prognosis

The clinical course is highly variable.

Some patients remain mild and stable for years, while others progress rapidly.

Progression is generally more likely in:

• Younger patients
• Eyes with steeper corneas
• Patients with significant eye rubbing
• Patients with documented serial changes

Modern cross-linking has substantially improved the ability to stabilize progressive disease.

Complications

Potential complications include:

• Progressive irregular astigmatism
• Severe visual distortion
• Contact lens intolerance
• Corneal scarring
• Acute hydrops
• Rare corneal perforation
• Need for corneal transplantation

The key clinical pearl is: keratoconus should be suspected in a young patient with progressive asymmetric myopia and astigmatism, reduced spectacle-corrected vision, and corneal topography showing inferior or inferotemporal steepening.