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Keratoconus

Looking for patient-friendly information on treatment instead? See our page on C3R treatment for keratoconus.

✦     ABCD Classification: The ABCD classification is measured at the cone.

✤     A: Anterior radius of curvature from a 3.0-mm zone centered on thinnest point

✤     B: Posterior (back) radius of curvature from a 3.0-mm zone centered on the thinnest point

✤     C: Minimal corneal thickness (not apical)

✤     D: Best spectacle-corrected visual acuity

✦     Systemic Association:

✤     ATOPY

▪     Asthma

▪     Atopic  keratoconjunctivitis

▪     Hay fever

▪     Eczema

✤     CONNECTIVE TISSUE DISORDERS

▪     Marfan’s syndrome: An increased prevalence (38%[20] to 58%) of mitral valve prolapse has been found in keratoconus patients

▪     EDS

▪     Osteogenesis imperfecta

✤     MISCELLANEOUS

▪     Down’s: 5.5% and 15%

‣       structural or biochemical changes

‣       habitual eye rubbing

▪     Turner’s syndrome

✤     Diabetes offered a protective effect regarding keratoconus. (also smoking?? As they cause C3R like effect)

✦     Ocular Associations:

✤     RP

✤     Infantile tapetoretinal degeneration (Leber's congenital amaurosis) is frequently complicated by keratoconus and cataract.

✤     retinopathy of prematurity, progressive cone dystrophy, aniridia, iridoschisis, and essential iris atrophy

✤     VKC: 26.8%.

✤     17% in a group of patients with floppy eyelid syndrome.

✦     Complications:

✤     High Refractive errors:  Intolerance to glasses

✤     Acute Hydrops : Rupture Descemet’s membrane → Aqueous influx →  Corneal edema →  Sudden drop in vision / Opacity

✦     Keratoconus Progression

✤     K-max (steepest keratometry) ≥ 1 D increase

✤     K-max – K-min ≥ 1 D increase (K-min, flattest keratometry)

✤     Kmean ≥ 0.75 D increase (Kmean = average of K-max K-min)

✤     Pachymetry ≥ 2% decrease in central corneal thickness (CCT)

✤     Corneal apex power ≥ 1 D increase (measured with cone location and magnitude index)

✤     MRSE change ≥ 0.5 D

✤     Several established decision trees exist based on combinations of the above, such as the Klyce indices of Surface Asymmetry Index (SAI) and Surface Regularity Index (SRI) and KISA% Index.

Management

✦     The management of keratoconus begins with spectacle correction.

✦     Once glasses fail to provide adequate visual function, contact lens fitting is required. Contact lens wear improves visual function by creating a new anterior refractive surface. Contact lenses do not prevent progression of corneal ectasia. While they seem to be associated with the development of keratoconus in some cases, this important mode of therapy should never be withheld for fear of causing progressive disease.

✤     RGP: three-point touch technique, remain the mainstay of contact lens treatment for keratoconus. apical clearance fitting technique is also commonly used.

✤     Other options include soft toric lenses, standard bicurved hard lenses, custom-back toric lenses, piggyback systems, hybrid lenses made of combined hard lens with a soft skirt, scleral lenses, and mini-scleral lenses.

✤     Silicone-hydrogel (Si-Hy) lenses, keratoconus designs

✤     Hybrid lenses, such as the SoftPerm lens (CIBA Vision Corp., Duluth, GA) and the newer SynergEyes KC lens (SynergEyes, Inc., Carlsbad, CA) may be more comfortable for patients who cannot tolerate an RGP alone.

✤     Mini-scleral lenses have a diameter of 14–17 mm compared to scleral lenses with a diameter of 20–24 mm.

✤     PROSE (prosthetic replacement of the ocular surface ecosystem): A medical model

✤     Outcomes

▪     Bigger is better. Size matters.

▪     There is no cone that cannot be fit.

▪     Scleral lens is an option after hydrops.

✤     New Paradigm for Contact Lens in Keratoconus

▪     Not a “contact lens failure” without trial of “true” scleral lens > 18 mm

▪     Penetrating or lamellar keratoplasty only for axial opacity limiting vision (in specialty lens)

▪     No regraft for cylinder or recurrence of ectasia without trial of specialty lens

▪     New Si-Hy lenses with keratoconus designs have extended the use of soft lenses in keratoconus.

▪     New hybrid materials and designs address past failures from lens fragility and hypoxia.

▪     Scleral lenses are in the repertoire of an increasing number of specialty lens fitters.

▪     Scleral lenses are a useful option in cases of RGP corneal lens failure due to instability or tight lens syndrome.

▪     The definition of scleral lenses is evolving. “Miniscleral,” corneoscleral, and intralimbal lenses may not perform as well as scleral lenses.

▪     PROSE treatment is a good option for contact lens and even scleral lens failures and can accommodate any cone.

▪     PROSE treatment has favorable 1-year outcome in comparison to keratoplasty for moderate to severe keratoconus.

✦     Contact lens-intolerant keratoconus patients without central scarring, who have mild or moderate disease, may be candidates for intrastromal ring segment insertion. The ideal candidates also have low spherical equivalents and average keratometry readings of less than 53 D.

✤     Ferrara rings (Ferrara Ophthalmics, Belo Horizonte, Brazil) and Intacs (Addition Technology Inc, Des Plaines, IL, USA), commonly used ring segments, are made of rigid polymethyl methacrylate. Ferrara rings have a fixed inner diameter of 5.0 mm and a triangular anterior contour. Intacs have an inner diameter of 6.8 mm, a flat anterior surface, and are available in thicknesses of 0.25–0.45 mm, in 0.05 mm increments.

✦     C3R

✦     While penetrating keratoplasty has traditionally been the surgery of choice, lamellar surgery is becoming more popular for patients with mild to moderate disease.

✤     The iron ring, found at the base of the cone, should be used as a reference when planning graft size.

✤     Postkeratoplasty myopia can be reduced by using the same-sized donor and host corneal buttons.

✦     Lamellar Keratoplasty

✤     Deep anterior lamellar keratoplasty (DALK): host endothelium is preserved, thus reducing the risk of rejection. The risk of endophthalmitis is theoretically less because this is largely an extraocular procedure.

✦     Corneal Allogenic Intrastromal Ring Segments (CAIRS) Combined With Corneal Crosslinking for Keratoconus

✤     Under study

✤     CAIRS trephined from donor cornea using a double-bladed trephine were implanted into mid-depth femtosecond laser– dissected channels in the cornea of patients with keratoconus in the 6.5-mm optic zone, followed by accelerated corneal crosslinking (A-CXL)—either conventional or contact lens–assisted CXL (A-CACXL), depending on minimum corneal thickness.

Cross-linking protocols: current comparative evidence

Three main CXL protocols are in clinical use, and residents should be able to compare them:

For very thin corneas where standard epi-off CXL risks endothelial toxicity, individualized protocols (e.g., the Sub400 protocol using hypo-osmolar riboflavin to first swell the cornea) allow CXL to still be performed safely below the traditional 400-micron cutoff.

Key references: Belin MW, et al. Global Consensus on Keratoconus and Ectatic Diseases. Cornea. 2015;34:359-369.  |  Ng SM, et al. Transepithelial versus epithelium-off CXL for progressive keratoconus. Cochrane Database Syst Rev. 2021;3:CD013512.  |  Alzahrani M, et al. Assessment of CXL effect using the ABCD grading system. Clin Ophthalmol. 2024;18:2349-2355.