# Keratoconus: Diagnosis, Progression, and Corneal Cross-Linking

## Definition and Epidemiology

Keratoconus is a progressive, bilateral (though often asymmetric), non-inflammatory ectatic disorder in which the cornea thins and protrudes into a conical shape, producing irregular astigmatism and visual impairment. Its prevalence is approximately 1 in 2,000 in the general population, though it is significantly higher in Middle Eastern and South Asian populations, where rates may reach 1 in 400. Onset typically occurs around puberty, with the most active progression during the teens and twenties. Males are slightly more affected than females, and the condition may stabilize by the fourth decade in many cases. There is a strong association with eye rubbing, atopic conditions (allergies, eczema, asthma), Down syndrome, connective tissue disorders such as Ehlers-Danlos and Marfan syndromes, and sleep apnea with floppy eyelid syndrome.

## Pathophysiology

The pathogenesis of keratoconus is multifactorial, involving genetic predisposition, biomechanical weakening, and environmental triggers. The corneal stroma thins, particularly in the inferior and central regions, due to a loss of collagen cross-links and abnormal orientation of collagen lamellae. Keratocyte density decreases while proteolytic enzyme activity (including MMP-9 and cathepsin) increases, further degrading the stromal architecture. Iron from the tear film is deposited at the base of the cone, forming the Fleischer ring. As the cone develops, breaks occur in Bowman layer, leading to anterior stromal scarring.

## Clinical Features

### Symptoms

Patients typically notice progressive blurring and distortion of vision, which may include monocular diplopia or ghosting. Frequent changes in spectacle prescription -- particularly an increasing cylinder component -- are common. As the disease advances, patients find they cannot achieve satisfactory best-corrected visual acuity with glasses alone. Photophobia and glare are additional complaints.

### Slit Lamp Signs

The most fundamental finding is corneal thinning, most pronounced in the inferior-central or paracentral region, with visible conical protrusion best appreciated in profile on downgaze. Vogt striae are fine, vertical stress lines in the deep stroma and Descemet membrane that characteristically disappear with gentle digital pressure on the globe. The Fleischer ring is a partial or complete iron (hemosiderin) ring at the base of the cone, best visualized with a cobalt blue filter. In advanced cases, the Munson sign -- a V-shaped indentation of the lower eyelid on downgaze -- becomes visible. The Rizzuti sign is a bright conical light reflex on the nasal limbus observed when light is directed from the temporal side. Apical scarring represents anterior stromal scarring at the cone apex resulting from Bowman layer breaks. Acute hydrops is a dramatic event in which sudden rupture of Descemet membrane allows aqueous humor to flood the stroma, producing severe corneal edema with pain and decreased vision. While self-limiting, acute hydrops may leave residual scarring.

### Retinoscopy

On retinoscopy, keratoconus produces a characteristic "scissors reflex" -- alternating light and dark reflexes resembling the blades of scissors -- which indicates the presence of irregular astigmatism.

## Diagnostic Imaging

### Corneal Topography (Placido-Disc Based)

Corneal topography remains the gold standard screening tool for keratoconus. The classic pattern is inferior steepening with an asymmetric bowtie and skewed radial axes (SRAX). The inferior-superior (I-S) keratometry difference is a key diagnostic metric: values greater than 1.4 D are suspicious, while values exceeding 1.9 D are considered diagnostic. Additional computed indices such as KISA%, KCI, and the Klyce-Maeda classifier aid in automated detection.

### Corneal Tomography (Scheimpflug -- Pentacam)

Scheimpflug-based tomography provides a more comprehensive evaluation by imaging both the anterior and posterior corneal surfaces along with a full pachymetry map. Posterior corneal elevation is often the earliest detectable sign of forme fruste keratoconus, making it particularly important for identifying subclinical disease. Key parameters include anterior and posterior elevation maps referenced to a best-fit sphere, pachymetry maps showing the thinnest point displaced inferiorly, and the Belin-Ambrosio Enhanced Ectasia Display (BAD-D), a composite deviation index where values greater than 1.6 standard deviations are suspicious and values exceeding 2.6 are highly suspicious. Tomographic screening is considered essential before any refractive surgery procedure.

### Corneal Biomechanics

Corneal biomechanical assessment provides additional diagnostic information. The Ocular Response Analyzer (ORA) measures corneal hysteresis (CH) and corneal resistance factor (CRF), both of which are reduced in keratoconus. The Corvis ST combines Scheimpflug imaging with an air puff to measure deformation amplitude, stiffness parameter (SP-A1), and the tomographic-biomechanical index (TBI). TBI is currently considered the most sensitive single parameter for detecting subclinical keratoconus.

## Staging Systems

### Amsler-Krumeich Classification

The traditional Amsler-Krumeich system divides keratoconus into four stages.

| Stage | Myopia/Astigmatism | Mean K | Pachymetry | Scarring |
|-------|-------------------|--------|------------|----------|
| I | < 5 D | < 48 D | Normal | None |
| II | 5-8 D | < 53 D | > 400 um | None |
| III | 8-10 D | > 53 D | 200-400 um | None |
| IV | Unmeasurable | > 55 D | < 200 um | Central scarring |

### ABCD Grading System (Belin-Duncan)

The more comprehensive ABCD system incorporates anterior curvature (A), posterior curvature (B), thinnest pachymetry (C), and best-corrected distance visual acuity (D), along with corneal scarring. Each parameter is graded from 0 to 4, providing a more nuanced framework that is better suited for tracking progression over time.

## Management

### Spectacles

Spectacles are adequate for early keratoconus when the astigmatism is still predominantly regular. As the irregular component increases, glasses become insufficient for achieving satisfactory visual acuity.

### Contact Lenses

Contact lenses are the primary mode of visual rehabilitation for moderate keratoconus. Rigid gas-permeable (RGP) lenses mask the irregular corneal surface with a smooth tear lens, providing significantly better vision than spectacles. Scleral lenses vault over the entire cornea and rest on the sclera, making them an excellent option for advanced cones, patients with concurrent dry eye, or those who cannot tolerate RGP lenses. The piggyback system -- an RGP lens worn over a soft contact lens -- improves comfort while maintaining the optical benefits of a rigid surface. Hybrid lenses (such as SynergEyes) combine an RGP center with a soft peripheral skirt. Custom soft toric lenses like KeraSoft IC are suitable for mild to moderate disease.

### Corneal Collagen Cross-Linking (CXL)

#### Principle

Corneal collagen cross-linking uses riboflavin (vitamin B2) combined with ultraviolet-A light (370 nm wavelength) to create new covalent bonds between collagen fibrils in the stroma. This process biomechanically stiffens the cornea by approximately 300%. The primary goal is to halt progression rather than reverse the ectasia, though some flattening of the cone and visual improvement are commonly observed.

#### Dresden Protocol (Standard Epi-Off CXL)

The Dresden protocol is the original and best-studied CXL technique. After mechanical removal of the epithelium, riboflavin 0.1% in dextran solution is applied to the exposed stroma for 30 minutes. The cornea is then irradiated with UVA light at 3 mW/cm2 for 30 minutes, delivering a total energy dose of 5.4 J/cm2. A critical safety requirement is that the stromal thickness must be at least 400 micrometers after epithelial removal to protect the endothelium from UV-induced damage. A bandage contact lens is placed postoperatively until the epithelium heals.

#### Accelerated CXL

Accelerated protocols use higher irradiance (9, 18, or 30 mW/cm2) for correspondingly shorter durations, maintaining the same total energy of 5.4 J/cm2 based on the Bunsen-Roscoe reciprocity law. While offering a practical convenience advantage, whether accelerated CXL achieves equivalent efficacy to the standard protocol remains under active investigation. The choice between pulsed and continuous irradiation is also debated, as pulsed delivery may improve oxygen replenishment, which is important given that the cross-linking reaction is oxygen-dependent.

#### Epi-On (Transepithelial) CXL

In transepithelial CXL, the epithelium is left intact, and riboflavin penetration is enhanced using permeability agents such as benzalkonium chloride (BAK), EDTA, or sodium iodide iontophoresis. This approach offers less postoperative pain, faster recovery, and a lower infection risk. However, it remains controversial because reduced riboflavin penetration through the intact epithelium may lead to inferior biomechanical strengthening. Some studies demonstrate equivalent halting of progression, while others show less effect on Kmax flattening. Newer formulations and enhanced delivery methods continue to close this gap.

#### Indications for CXL

CXL is indicated when there is documented progression, defined as an increase in Kmax greater than 1.0 D, increase in astigmatism, decrease in thinnest pachymetry, or decrease in best-corrected visual acuity over a 6-12 month period. Patients under 35 years of age are at higher risk of progression and are prioritized for treatment. The minimum corneal thickness at the thinnest point must be 400 micrometers after epithelial removal. Active infection, inflammation, and significant central scarring are contraindications.

#### Outcomes of CXL

CXL achieves stabilization of ectasia in more than 95% of treated eyes. The mean reduction in Kmax is 1-2 D at one year, and modest improvement in best-corrected visual acuity is observed in many patients. Potential complications include corneal haze (usually transient but occasionally persistent), infection, sterile infiltrates, endothelial damage (if the minimum thickness requirement is not met), and delayed epithelial healing.

### CXL-Plus Procedures

CXL can be combined with other interventions to achieve both stabilization and visual rehabilitation. The Athens protocol (or Cretan protocol) involves simultaneous topography-guided PRK to regularize the corneal surface followed immediately by CXL. CXL can also be combined with intracorneal ring segments (ICRS) for mechanical flattening plus cross-linking. A sequential rather than simultaneous approach is often preferred to allow safety monitoring between procedures.

### Intracorneal Ring Segments (ICRS)

ICRS devices -- including Intacs, Ferrara rings, and Kerarings -- are PMMA arc segments inserted into deep stromal tunnels created manually or with a femtosecond laser. They flatten the central cone by shortening the arc length of the cornea, improving both contact lens fit and visual acuity. However, ICRS do not halt disease progression, so CXL should be performed adjunctively.

### Corneal Transplantation

Transplantation is reserved for advanced keratoconus with contact lens intolerance, significant apical scarring, or failure of other interventions. DALK is preferred over PK because it preserves the host endothelium and eliminates the risk of endothelial rejection. Outcomes for keratoconus are excellent, with graft survival exceeding 90% at 10 years. Postoperative contact lens fitting is often still required to manage residual astigmatism.

<image>Pentacam tomography display of a keratoconus eye showing four maps. Top left: anterior sagittal curvature map with inferior steepening and a red/orange cone. Top right: posterior elevation map relative to a best-fit sphere showing a posterior elevation island at the cone location. Bottom left: corneal thickness (pachymetry) map with the thinnest point displaced infero-temporally marked with a crosshair. Bottom right: Belin-Ambrosio Enhanced Ectasia Display (BAD-D) showing deviation values exceeding normal limits in red. Include numerical values for Kmax, thinnest pachymetry, and BAD-D score.</image>

<image>Slit lamp photograph montage of keratoconus clinical signs. Six panels: (1) Fleischer ring visible as a partial golden-brown iron ring at the base of the cone under cobalt blue illumination; (2) Vogt striae — fine vertical stress lines in the deep stroma visible on direct illumination; (3) Munson sign — V-shaped indentation of the lower lid on downgaze seen in profile; (4) Rizzuti sign — bright conical light reflex on the nasal limbus; (5) Apical scarring at the cone apex; (6) Acute hydrops with dramatic central corneal edema from Descemet membrane rupture. Label each sign.</image>

<image>Step-by-step illustration of the corneal collagen cross-linking procedure (Dresden protocol epi-off). Four sequential panels: (1) Epithelial debridement with a blunt spatula exposing bare stroma; (2) Riboflavin 0.1% solution being applied every 2 minutes for 30 minutes with the stroma appearing yellow-green; (3) UVA light (370 nm, 3 mW/cm2) directed at the cornea for 30 minutes with continued riboflavin application; (4) Bandage contact lens placed after the procedure. Include a cross-sectional inset showing new collagen cross-links forming between stromal fibrils, with the 400 um minimum thickness safety zone marked and the endothelium protected below.</image>

## Key Clinical Pearls

Eye rubbing is the single most important modifiable risk factor for keratoconus progression, and all patients must be counseled to stop rubbing their eyes. Posterior elevation change on Pentacam tomography is the earliest detectable sign of subclinical keratoconus and forme fruste disease. CXL should be offered early when progression is documented, because the goal is to treat before significant visual loss occurs. The minimum stromal thickness of 400 micrometers after epithelial removal must always be confirmed to protect the endothelium from UV damage. While the epi-on versus epi-off debate continues, the epi-off Dresden protocol remains the gold standard with the strongest supporting evidence. Acute hydrops should be managed conservatively with cycloplegia, hypertonic saline, and IOP control -- it is not an indication for urgent keratoplasty. Keratoconus is an absolute contraindication to LASIK, and all refractive surgery candidates must be screened with topography and tomography. When corneal transplantation is needed, DALK is preferred over PK for keratoconus, with excellent long-term outcomes.

## References

- Godefrooij DA, et al. Age-specific incidence and prevalence of keratoconus: a nationwide study. Am J Ophthalmol. 2017;175:169-172.
- Wollensak G, Spoerl E, Seiler T. Riboflavin/ultraviolet-A-induced collagen crosslinking for the treatment of keratoconus. Am J Ophthalmol. 2003;135(5):620-627.
- Randleman JB, et al. Randomized controlled trial of corneal collagen cross-linking with riboflavin and UVA. Ophthalmology. 2017;124(9):1251-1257.
- Belin MW, Duncan JK. Keratoconus: the ABCD grading system. Klin Monbl Augenheilkd. 2016;233(6):701-707.
- AAO BCSC Section 8: External Disease and Cornea. 2023-2024.