Keratoconus

INTRODUCTION Keratoconus is a bilateral, non-inflammatory but asymmetric disease of the cornea, in which progressive corneal thinning and steepening lead to irregular astigmatism and vision loss.[1-4] It presents in early adolescence and progresses into the second or third decades of life; however, it may be seen earlier in childhood or continue to progress beyond the third decade.[3] Major risk factors include eye rubbing, history of atopy (allergy, asthma and eczema) and family history of keratoconus.[3] Pointers to Diagnose/Suspect Keratoconus Defective vision that is not easily corrected with spectacles. Best corrected visual acuity ≤ 6/9 Asymmetric refractive error with high or progressive astigmatism of > 1.5 D Distortion of vision and glare Frequent change of spectacles Scissoring reflex in retinoscopy History of frequent eye rubbing and atopy Family history of keratoconus Oil droplet sign in distant direct ophthalmoscopy Syndromic associations like Down’s syndrome, Leber’s congenital amaurosis and Ehlers-Danlos/connective disorders.[2] Classical Signs of Keratoconus[1,4] Munson sign—a V-shaped indentation observed in the lower eyelid when the patient is looking downwards. Rizutti’s sign—a conical reflection on the nasal cornea when a penlight is shone from the temporal side Conical cornea with thinning mainly in the inferior/paracentral cornea. Fleischer ring—an iron deposit present within the epithelium around the base of the cone. It is brown in colour and best visualized with a cobalt blue filter. Vogt’s striae—fine, roughly vertically parallel striations in the stroma which generally disappear with firm pressure applied over the eyeball and re-appear when pressure is discontinued. Rabinowitz/Mcdonnell Criteria for Diagnosing Keratoconus[5] Central corneal power K > 47.2 Dioptres Inferior-Superior (I-S) dioptric asymmetry: >1.4 D Sim K astigmatism: >1.5 D Skewing of radial axis: >21 degree. Keratoconus Progression Indicators[4][Table 1] Increase in K max ≥ 1.0 D Increase in astigmatism ≥ 1.0 D Significant changes in the orientation of refractive axes Increase of 1.0 D or more in the optical power of the steepest corneal meridian Decrease of 25 μm or more in corneal thickness. Table 1: Amsler-Krumeich Classification for Keratoconus[ 4 , 8 , 9 ]Management of Keratoconus A. Visual improvement: Those for stable refraction with low or moderate risk with no progression [Figure 1a] Spectacles Contact lens Soft or soft toric contact lens for early keratoconus (Less than 1.5 – 2.0 D astigmatism)[6,7] Rigid gas permeable lens[6,7] Corneal: Rose-K lens, Limbal RGP lens Corneoscleral lens Scleral: Mini-scleral and Full scleral lens. Piggyback contact lens (PBCL) is indicated for those with RGP discomfort or intolerance, unstable RGP on the eye, popping out of lens, 3 and 9 o’clock staining with RGP fit and presence of scar or sometimes for comfort[7] Hybrid lenses. Intrastromal corneal ring segments (ICRS)Those with unsatisfactory visual acuity or intolerant to lenses can opt for ICRS followed by repeat contact lens trial. If still intolerant, they will require corneal transplantation. B. Preventing Progression of Disease: Corneal Collagen Cross-Linking (CXL) [Figure 1b] CXL is a minimally invasive procedure in which riboflavin, a photosensitizer and ultraviolet—A light is used to induce the formation of strong chemical bonds between collagen fibrils in the cornea, thereby rendering it to be stiffer and less susceptible to ectatic changes. Conventional Isotonic CXL—Standard Dresden Protocol[10] It was formulated by Wollensak et al. for corneas with minimal thickness of 400μm.[11] After epithelium debridement, 0.1% riboflavin 5-phosphate drops with 20% dextran solution are instilled every 5 minutes for 30 minutes. This is followed by UVA (370 nm, 3 mw/cm2) irradiation for 30 minutes with simultaneous instillation of riboflavin drops every 5 minutes. Hypotonic CXL[10] This is done for patients who have corneal thickness less than 400 microns and preferably more than 380 microns. Hypotonic riboflavin (0.1% riboflavin with 0.9% sodium chloride) is used to increase the corneal hydration and thickness intraoperatively. Accelerated CXL The idea behind accelerated CXL is to shorten the irradiation time by increasing the illumination intensity, while maintaining a constant radiant exposure of 5.4 J/cm2, based on Bunsen–Roscoe law of reciprocity. It uses high energy up to 30 mW/cm2 for shorter time (3–10 min).[12] An irradiation dose of 10 mW/cm2 for 9 minutes has a better therapeutic and safety profile than higher irradiation doses for shorter periods.[10] Figure 1: (a) Mild keratoconus. (b) Moderate keratoconus. (c) Severe keratoconus with apical scarring. (d) Acute corneal hydropsPrecautions for CXL In the conventional CXL method, a minimum de-epithelialized corneal thickness of 400 μ is recommended to avoid potential irradiation damage to the corneal endothelium cells.[11] When corneal thickness is <400 microns, preferred treatment options include Hypotonic CXL or Epithelium on–Transepithelial CXL. It is important to verify riboflavin saturation by checking for flare in the anterior chamber using a slit lamp before initiation of UVA irradiation. A limbal guard is used during irradiation to protect limbal stem cells from damage. Any concomitant inflammation, such as allergic conjunctivitis or blepharitis, must be treated and controlled before proceeding with the CXL procedure. C. High-risk keratoconus or with advanced keratoconus with corneal scar needs surgical intervention.[13] [Figure 1c] Deep anterior lamellar keratoplasty (DALK) or femtosecond DALK for those with anterior stromal scar Penetrating keratoplasty (PKP) for those with full-thickness scar D. Acute Hydrops-Cycloplegics [Figure 1d] Hyperosmotic agents like sodium hydrochloride drops/ointment Patching/bandage contact lenses (or) Intracameral air (SF6, C3F8) injection. Financial support and sponsorship Aravind Eye Hospital, Pondicherry. Conflicts of interest There are no conflicts of interest.