Keratoconus

Keratoconus is a bilateral, usually asymmetric, progressive ectatic disorder in which the central or paracentral cornea thins and steepens into a cone, producing irregular astigmatism, myopia, and visual loss that glasses cannot fully correct.

• Typical onset is at puberty to the third decade; progression is most rapid in adolescents and young adults and usually slows by the fourth decade as natural corneal collagen cross linking increases.
• Mechanism: biomechanical weakening of stromal collagen with increased matrix metalloproteinase activity and oxidative stress; chronic eye rubbing is the strongest modifiable risk factor.
• Associations: atopy (allergic conjunctivitis, eczema, asthma), vigorous eye rubbing, Down syndrome, Ehlers-Danlos and Marfan syndromes, floppy eyelid syndrome and obstructive sleep apnoea, Leber congenital amaurosis, mitral valve prolapse, and a positive family history (about 10 percent).
• Prevalence is higher in people of Middle Eastern, South Asian, and African descent in many series.

• Progressive blurring and distortion, monocular diplopia or ghosting, glare, haloes, photophobia, and frequent changes of spectacle prescription (worsening myopia and oblique astigmatism).
• Itching and eye rubbing.
• Signs on examination: scissoring reflex on retinoscopy, an oil droplet reflex on ophthalmoscopy, Munson sign (the lower lid bulges on downgaze), Vogt striae (vertical stress lines in the deep stroma that vanish on gentle pressure), Fleischer ring (iron deposition at the cone base), Rizzuti sign (conical light reflection on the nasal cornea when lit from the temporal side), prominent corneal nerves, and apical scarring.
• Acute hydrops: sudden pain, redness, and marked visual loss from a break in Descemet membrane with stromal oedema; it resolves over weeks to months with a stromal scar.

• Corneal tomography and topography (Scheimpflug or anterior segment OCT): inferior or inferotemporal steepening, an asymmetric bowtie pattern, maximum keratometry above about 47 to 48 dioptres, a thin corneal apex (often under 500 micrometres), posterior surface elevation, and an abnormal progression index. Screen for subclinical (forme fruste) disease before any laser refractive surgery.
• Staging by keratometry, thickness, and refraction guides management; progression is documented by an increase in maximum keratometry of 1 dioptre or more, a fall in thickness, or an increase in refractive astigmatism over 6 to 12 months.
• Slit lamp for scarring, striae, Fleischer ring, and hydrops.
• Differential diagnosis: pellucid marginal degeneration (inferior band like thinning), keratoglobus, post-refractive-surgery ectasia, contact lens induced warpage, and Terrien marginal degeneration.

• All patients: stop eye rubbing, treat allergy (olopatadine 0.1 percent drops 1 drop twice daily or ketotifen 0.025 percent 1 drop twice daily, with lubricants), treat blepharitis, and screen for and treat obstructive sleep apnoea when a floppy eyelid is present.
• Optical correction:
  • Early disease: spectacles or soft toric contact lenses.
  • Moderate to advanced disease: rigid gas permeable lenses, which mask irregular astigmatism; piggyback systems, hybrid lenses, and scleral lenses (vault the cornea, the best option for advanced disease and intolerance of rigid lenses).
• Corneal collagen cross linking is the only treatment that stops progression and is indicated for documented progression in children and adults, and often at diagnosis in young patients:
  • Epithelium off protocol: remove 8 to 9 mm of central epithelium, soak with riboflavin 0.1 percent (dextran or hypromellose) every 2 to 3 minutes for 30 minutes until riboflavin is in the anterior chamber, then irradiate with ultraviolet A at 365 nm, 3 mW/cm2 for 30 minutes (5.4 J/cm2 surface dose); accelerated protocols (for example 9 mW/cm2 for 10 minutes) are used with comparable results.
  • A stromal thickness of at least 400 micrometres after epithelial removal is needed; hypotonic riboflavin is used to swell thin corneas.
  • Contraindications: thickness under 400 micrometres despite swelling, active infection, pregnancy, herpetic keratitis history without antiviral cover, and severe scarring.
  • After the procedure: bandage contact lens for 3 to 5 days, topical antibiotic (moxifloxacin 0.5 percent four times daily until epithelium heals), topical corticosteroid (fluorometholone 0.1 percent four times daily for 1 to 2 weeks), lubricants, and oral analgesia. Complications include infectious keratitis, haze, delayed healing, and endothelial damage.
  • Epithelium on techniques have less pain and quicker recovery but a smaller, less consistent effect.
• Intrastromal corneal ring segments flatten the cone and improve contact lens tolerance and vision in selected cases with clear central cornea; often combined with cross linking.
• Keratoplasty for advanced disease with scarring, intolerance of lenses, or poor corrected vision: deep anterior lamellar keratoplasty is preferred (preserves host endothelium, minimal rejection risk), penetrating keratoplasty when Descemet is scarred or ruptured. Graft survival in keratoconus exceeds 90 percent at 10 years.
• Acute hydrops: cycloplegia (atropine 1 percent once or twice daily), hypertonic saline 5 percent drops four times daily and ointment at night, topical corticosteroid (prednisolone acetate 1 percent four times daily with a taper) if inflammation, bandage lens for comfort, and observation; the scar often flattens the cornea. Intracameral gas (perfluoropropane 14 percent or air) can speed resolution.
• Follow up every 6 months in progressive disease; yearly in stable disease; family members should be screened.