Keratoconus

Overview

Keratoconus is a bilateral and typically asymmetric corneal disorder characterized by progressive thinning and steepening (ectasia) of the cornea, leading to irregular astigmatism and decreased visual acuity.¹ It normally develops in the second and third decades of life and can progress until roughly the fourth decade, affecting all ethnicities and both sexes, though with variable prevalence.¹

Epidemiology & Risk Factors

Reported prevalence and incidence vary widely by population and diagnostic criteria, with generally higher rates reported in Middle Eastern and Asian populations and in younger age groups (20s–30s).¹ Recognized risk factors include a family history of keratoconus, chronic eye rubbing, atopy/allergic eye disease, eczema, and asthma.¹ Down syndrome is associated with a markedly higher prevalence of keratoconus than the general population, compounded by diagnostic challenges from poor imaging cooperation.

Sociodemographic factors also influence disease severity and access to care: in one tertiary-center study, non-White race, lower income, and Medicaid/Medicare coverage were each independently associated with more severe keratoconus at presentation, and Medicaid/Medicare coverage was specifically associated with delay in receiving corneal cross-linking, highlighting disparities in access to timely treatment.²

Pathophysiology

Progressive stromal thinning, rupture of the anterior limiting (Bowman's) membrane, and subsequent ectasia of the central or paracentral cornea are the most commonly observed histopathological findings.¹ Although traditionally described as a noninflammatory disease, keratoconus has more recently been associated with localized ocular surface inflammation, which may interact with mechanical factors (eye rubbing) to drive progressive biomechanical weakening of the stroma.¹

Clinical Presentation

Early keratoconus may be entirely asymptomatic or present with mild, progressively worsening blur and glare not fully correctable with spectacles due to irregular astigmatism. As disease progresses, patients may report frequent changes in glasses prescription, monocular diplopia/ghosting, and glare/halos, particularly at night. Acute hydrops — sudden stromal edema from a Descemet membrane break — presents with acute pain, photophobia, and marked vision loss, usually in advanced disease.

Exam Findings

• Scissoring reflex on retinoscopy
• Munson's sign (V-shaped conjunctival indentation on downgaze) in advanced disease
• Vogt's striae (fine vertical stress lines in the deep stroma)
• Fleischer ring (iron deposition at the base of the cone, best seen with cobalt blue light)
• Corneal thinning and steepening on slit lamp and confirmed on topography/tomography
• Acute hydrops: sudden stromal/epithelial edema, sometimes with a visible Descemet membrane break

Differential Diagnosis

• Pellucid marginal degeneration (inferior peripheral thinning with a characteristic "crab-claw" topographic pattern)
• Post-refractive surgery ectasia
• Keratoglobus
• Simple irregular astigmatism from other causes
• Contact lens warpage (can transiently mimic topographic irregularity)

Management

Diagnosis relies primarily on corneal topography and tomography (e.g., Scheimpflug imaging), which detect characteristic anterior and posterior corneal steepening and thinning patterns; a single parameter is often insufficient in early/subclinical disease, so pachymetry and higher-order aberration data are increasingly used alongside topography.¹ Machine learning-based algorithms analyzing multiple corneal parameters are an active area of research aimed at improving detection of subclinical keratoconus, though no consensus yet exists on the optimal parameter set or algorithm design.³

Severity and progression may be graded using morphological, ocular sign-based, or index-based classification systems (e.g., Amsler-Krumeich).¹ Treatment is stepped by severity¹:

• Mild disease: spectacles
• Moderate disease: specialty contact lenses (rigid gas-permeable, scleral lenses)
• Progressive disease (any severity): corneal cross-linking (CXL), which halts or slows progression and is most effective when performed early
• Severe disease not manageable with scleral lenses: corneal surgery — deep anterior lamellar keratoplasty or, in advanced/scarred cases, penetrating keratoplasty

Complications

• Progressive irregular astigmatism and worsening best-corrected vision
• Acute hydrops with associated pain and marked acute vision loss
• Corneal scarring, particularly after hydrops or with longstanding advanced disease
• Contact lens intolerance in advanced disease, sometimes necessitating surgical intervention

Clinical Pearls

• Chronic eye rubbing is a modifiable risk factor — counsel all keratoconus patients, especially those with concurrent allergic eye disease, to stop rubbing
• Corneal cross-linking is most effective at halting progression when performed early, making prompt referral for documented progression important
• A near-normal topography in a young patient with unexplained irregular astigmatism should still prompt tomography, since early/subclinical keratoconus can be missed by topography alone³
• Down syndrome patients have markedly higher keratoconus prevalence and unique diagnostic/management challenges — maintain a low threshold for screening
• Recognize disparities in access to cross-linking; insurance status and socioeconomic factors have been associated with treatment delay²

Common Pitfalls

• Attributing progressively worsening astigmatism in a young patient to simple refractive error without considering keratoconus
• Delaying cross-linking referral until disease is already advanced, missing the window where CXL is most effective
• Failing to counsel patients on eye-rubbing cessation as part of management
• Missing early/subclinical keratoconus by relying on topography alone without pachymetry or tomography
• Overlooking keratoconus risk in patients with Down syndrome due to imaging/cooperation challenges