Keratoconus: Signs, Indices, Staging, Progression and Treatment
Looking for patient-friendly information on treatment instead? See our page on C3R treatment for keratoconus.
Last updated: October 2026
Key points in 60 seconds
- Bilateral, asymmetric, non-inflammatory corneal ectasia starting around puberty; eye rubbing is the main modifiable risk.
- Signs: scissoring reflex, oil-droplet reflex, Vogt striae, Fleischer ring, Munson and Rizzuti signs.
- Tomography (posterior elevation, pachymetry, Belin-Ambrosio display) detects it earlier than curvature alone.
- Document progression before cross-linking, which halts it.
- Vision: glasses, then RGP or scleral lenses, ring segments, and DALK for advanced scarring.
Keratoconus is a bilateral, asymmetric, non-inflammatory ectasia with central or paracentral thinning, protrusion and irregular astigmatism. This page covers the essentials a resident needs: who gets it, signs, tomography indices, staging, progression and treatment.
Key facts
| Item | Detail |
|---|---|
| Name | Greek kerato (cornea, horn) + konos (cone); first described in detail by John Nottingham (1854) |
| Prevalence | Older studies: 50–230 per 100,000. Tomography studies: about 1 in 375 to 1 in 2000, higher in South Asia and the Middle East |
| Onset and course | Starts around puberty and progresses for 10–20 years, usually slowing in the 30s; faster in children |
| Inheritance | Mostly sporadic; family history in about 6–23%; genetics are complex (VSX1, ZNF469 and others) |
| Risk factors | Eye rubbing (the strongest modifiable factor), atopy, Down syndrome, floppy eyelid syndrome, family history |
| Protective | Diabetes (natural glycation cross-links collagen) |
| Pathology | Fewer collagen lamellae (about 41% inside the cone), breaks in Bowman layer, Fleischer iron ring at the cone base, keratocyte apoptosis and protease imbalance |
Symptoms and signs
| Stage | Findings |
|---|---|
| Symptoms | Blurring, ghosting, glare and haloes, frequent spectacle changes; contrast sensitivity falls before Snellen acuity |
| Early signs | Scissoring reflex on retinoscopy; oil-droplet (Charleux) red reflex; irregular mires on keratometry |
| Slit lamp | Vogt striae (vanish with gentle pressure); Fleischer ring; prominent corneal nerves; central thinning; apical scarring |
| External signs | Munson sign (V-shaped lower lid on downgaze); Rizzuti sign (light focused at the nasal limbus) |
| Cone types | Nipple (small, central), oval (sagging, inferior, harder to fit), globus (most of the cornea) |
| Complications | Acute hydrops (Descemet rupture) causing sudden stromal oedema, especially with eye rubbing, Down syndrome and VKC; scarring (risk with K over 52 D, contact lens wear, staining, age under 20) |
Topography and tomography indices
| Index | Abnormal value | Notes |
|---|---|---|
| Central K (Rabinowitz) | > 47.2 D | Rabinowitz–McDonnell criteria with I–S |
| I–S value (inferior–superior asymmetry) | > 1.4 D (1.2–1.4 suspect) | Measured 3 mm above and below centre |
| SRAX (skewed radial axes) | > 21° | Skewed asymmetric bow tie |
| KISA% | > 100% keratoconus; 60–100% suspect | Combines K, I–S, AST and SRAX |
| Belin/Ambrósio Enhanced Ectasia Display (BAD-D) | > 1.6 suspicious; > 2.6 abnormal (approx.) | Pentacam; includes posterior elevation and pachymetry progression |
| Posterior elevation (best-fit sphere, thinnest point) | > 15–20 µm suspicious | Earliest sign of subclinical disease |
| Klyce–Maeda KPI and Smolek–Klyce indices | Device-specific | Older Placido-based indices |
Forme fruste keratoconus (Amsler) describes an abortive, subclinical form, usually the fellow eye of a patient with keratoconus. Screen every refractive surgery candidate with tomography.
More: Topography and keratoconus screening card
Staging
| Stage (Amsler–Krumeich) | Refraction | Mean central K | Thickness / scarring |
|---|---|---|---|
| I | Eccentric steepening; myopia and/or astigmatism under 5 D | Under 48 D | — |
| II | Myopia and/or astigmatism 5–8 D | Under 53 D | Over 400 µm; no scarring |
| III | Myopia and/or astigmatism 8–10 D | Over 53 D | 300–400 µm; no scarring |
| IV | Refraction not measurable | Over 55 D | Under 200 µm; central scarring |
The Belin ABCD system adds the posterior surface (A = anterior radius, B = posterior radius, C = thinnest pachymetry, D = best-corrected acuity, each staged 0–4) and is better for documenting progression: see the ABCD table.
Keratoconus Severity Score (McMahon, 2006)
| Grade | Stage | Average corneal power | Higher-order RMS (anterior surface) |
|---|---|---|---|
| 0 | Normal topography | ≤ 47.75 D | ≤ 0.65 µm |
| 1 | Atypical topography | ≤ 48.00 D | ≤ 1.00 µm |
| 2 | Suspect topography | ≤ 49.00 D | 1.00–1.50 µm |
| 3 | Mild disease | ≤ 52.00 D | 1.50–3.50 µm |
| 4 | Moderate disease (scarring up to CLEK grade 3.0) | 52–56 D | 3.50–5.75 µm |
| 5 | Severe disease (scarring CLEK 3.5 or more) | > 56 D | > 5.75 µm |
Slit-lamp signs are required from grade 3 upwards; the worst additional feature decides the grade.
Documenting progression
The Global Consensus (2015) defines progression as consistent change in at least two of: steepening of the anterior surface, steepening of the posterior surface, and thinning or a faster rate of thickness change towards the thinnest point. Common practical thresholds:
| Parameter | Change suggesting progression |
|---|---|
| Kmax | Increase of 1 D or more over 6–12 months |
| Kmean | Increase of 0.75 D or more |
| Astigmatism (Kmax − Kmin) | Increase of 1 D or more |
| Thinnest pachymetry | Decrease of 2% or more (or about 10 µm) |
| Manifest refraction | Myopic or cylinder shift of 0.5 D or more |
| ABCD | Change in the A, B or C parameter beyond the device's repeatability |
Associations
| Group | Conditions |
|---|---|
| Atopy | Asthma, eczema, hay fever, vernal and atopic keratoconjunctivitis (eye rubbing) |
| Connective tissue | Marfan syndrome, Ehlers–Danlos, osteogenesis imperfecta; mitral valve prolapse is more common |
| Syndromes | Down syndrome (about 5–15%; hydrops is common), Turner syndrome |
| Eye conditions | Leber congenital amaurosis, retinitis pigmentosa, floppy eyelid syndrome (about 17%), aniridia, ROP, essential iris atrophy |
Management
| Step | Options | Points |
|---|---|---|
| Stop progression | Corneal cross-linking (CXL) | Offer as soon as progression is shown, earlier in children; stop eye rubbing and treat allergy |
| Correct vision | Spectacles; soft toric or keratoconus soft lenses; RGP (apical clearance or three-point touch); hybrid; scleral lenses | "No cone that cannot be fitted": try a true scleral lens (over 18 mm) before calling contact lens failure |
| Reshape the cornea | Intracorneal ring segments (Intacs, Ferrara; 5.0–6.8 mm optical zones); CAIRS (donor-tissue rings, newer) | For contact lens intolerance, clear central cornea, mean K under about 53 D |
| Refractive add-ons | Topography-guided PRK with CXL; phakic toric IOL in stable eyes | Only in selected, stable corneas |
| Transplant | DALK (preferred: keeps the host endothelium, lower rejection) or penetrating keratoplasty | For central scarring or intolerance of all lenses; size the graft to the Fleischer ring |
| Acute hydrops | Hypertonic saline, cycloplegia, topical steroid; intracameral air or gas to speed healing | Usually resolves in 2–4 months with a scar |
Cross-linking protocols
| Protocol | Method | Evidence |
|---|---|---|
| Standard (Dresden) | Epithelium off, riboflavin, UVA 3 mW/cm² for 30 minutes (5.4 J/cm²) | Longest record; the benchmark |
| Accelerated | Epithelium off, higher irradiance (9–30 mW/cm²) for a shorter time, same total energy | Similar flattening in most series, sometimes slightly less; shorter treatment |
| Transepithelial (epi-on) | Epithelium left intact, with modified riboflavin or iontophoresis | Cochrane 2021: older epi-on methods were less effective than epi-off. A newer oxygen-supplemented epi-on system (Epioxa) was FDA-approved in October 2025 after phase 3 trials |
| Thin corneas (under 400 µm) | Hypo-osmolar riboflavin to swell the cornea, contact-lens-assisted CXL, or the individualised Sub400 protocol | Protects the endothelium |
Where residents slip
- Relying on anterior topography alone: posterior elevation and pachymetry pick up early disease.
- Calling the McMahon Keratoconus Severity Score "ABCD": they are different systems.
- Waiting for vision to fall before cross-linking: treat when progression is documented.
- Forgetting eye rubbing and allergy control, which drive progression.
Key sources
- Gomes JAP, Tan D, Rapuano CJ, et al. Global consensus on keratoconus and ectatic diseases. Cornea. 2015;34(4):359-369
- Belin MW, Duncan JK. Keratoconus: the ABCD grading system. Klin Monbl Augenheilkd. 2016;233(6):701-707
- McMahon TT, Szczotka-Flynn L, Barr JT, et al. A new method for grading the severity of keratoconus: the Keratoconus Severity Score (KSS). Cornea. 2006;25(7):794-800
- Ng SM, Ren M, Lindsley KB, et al. Transepithelial versus epithelium-off corneal crosslinking for progressive keratoconus. Cochrane Database Syst Rev. 2021;3:CD013512
- Godefrooij DA, de Wit GA, Uiterwaal CS, et al. Age-specific incidence and prevalence of keratoconus: a nationwide registration study. Am J Ophthalmol. 2017;175:169-172
Related: C3R treatment (patient page), Corneal topography tips, Refractive surgery for residents, Contact lens quick reference.
Revise with flip cards
📘 Want the exam-ready deep dive? This topic is covered in iNotes 2027: Cornea and Refractive (Ophthalmology Postgraduate Exam Notes) — available on Kindle. See all my books.
Want to know more about treatment? Read about C3R Treatment for Keratoconus at Sadbhaav or book a consultation with Dr. Dhaval Patel.