Ectatic Conditions of the Cornea
KERATOCONUS
Keratoconus is a fairly common condition characterized by corneal thinning, protrusion, and irregularity. It is almost always bilateral, although the severity of involvement is often asymmetric.
Etiology
• Sporadic or autosomal dominant with incomplete penetrance
Symptoms
• Gradually decreasing vision, typically beginning in adolescence and progressing into adult life
• Patients often relate a history of not being able to attain good vision despite multiple changes of glasses or soft contact lenses.
• May have a history of eye rubbing
• Can develop acutely decreased vision and pain due to hydrops with advanced disease
Signs
• Earlier
▪ Progressive myopia and astigmatism
▪ Scissors reflex on retinoscopy
▪ Irregular mires on keratometry
▪ Inferior steepening on computerized corneal topography (Fig. 4-1A) and tomography (Fig. 4-1B and C). Eyes with “low sagging cones” can demonstrate a mild crab-claw topographic pattern (Fig. 4-1D), which is similar to the pattern seen in pellucid marginal degeneration.
▪ Central or paracentral stromal thinning of the cornea with the greatest protrusion at the apex of the thinning (Fig. 4-1E, eFig. 4-1E)
▪ Fleischer ring: epithelial iron deposits at the base of the cone (Fig. 4-1F)
▪ Prominent corneal nerves (Fig. 4-1G)
• Later
▪ Vogt striae: fine vertical deep stromal tension lines that disappear temporarily with digital pressure applied to the limbus (Fig. 4-1H)
▪ Abnormal “oil droplet” red reflex
▪ Rizzuti sign: conical light reflection on the nasal limbus when light is shone from the temporal side
▪ Variable corneal scarring, depending on severity (Fig. 4-1I-N). May develop an elevated apical nodule (Fig. 4-1O, eFig. 4-1I-L)
▪ Munson sign: bulging of the lower eyelid in downgaze
▪ Acute hydrops: severe corneal edema resulting from a tear in Descemet membrane (Fig. 4-1P-T, eFig. 4-1P and Q)
Associations
• Ocular: vernal disease, blue sclera, retinitis pigmentosa, Leber congenital amaurosis, floppy eyelid syndrome
• Systemic: Down syndrome, sleep apnea, Ehlers-Danlos syndrome, Apert syndrome, ocular allergies, osteogenesis imperfecta
Differential Diagnosis
• Pellucid marginal degeneration: inferior peripheral corneal thinning with protrusion of the cornea above the area of maximal thinning
Treatment
• Mild cases: glasses and soft contact lenses
• Moderate cases: rigid gas-permeable contact lens (RGPCL), hybrid lens, or scleral lens
• Severe and contact lens-intolerant cases:
▪ Lamellar keratectomy with a blade or excimer laser for anterior nodules
▪ Placement of intracorneal ring segments
▪ Deep anterior lamellar keratoplasty
▪ Penetrating keratoplasty
▪ Epikeratoplasty and thermokeratoplasty are rarely performed.
▪ Refractive surgery in patients with keratoconus is unpredictable and generally
not recommended except under special protocols.
• Acute hydrops: generally resolves on its own over weeks to months
▪ Sodium chloride 5% drops q.i.d. and/or sodium chloride 5% ointment one to four times a day
▪ Photophobia may be helped by cycloplegia (e.g., cyclopentolate 1% t.i.d.).
▪ Consider an aqueous suppressant if intraocular pressure is somewhat elevated (e.g., brimonidine 0.1% b.i.d. to t.i.d.).
▪ Intracameral air, or nonexpansile concentrations of SF6 or C3F8, may be helpful in resolving edema more quickly than medical therapy.
• Corneal collagen cross-linking: indicated for progressive disease in eyes with sufficient corneal thickness and minimal to no corneal scarring with the objective of halting progression. Generally performed by placing riboflavin drops on the cornea and then treating the cornea with ultraviolet light to “strengthen” the cornea to prevent worsening of keratoconus. Recently approved by the Food and Drug Administration to treat progressive keratoconus and post-refractive surgery ectasia. Short- and medium-term results are promising for stopping progression. Best results are obtained when treating keratoconus at a relatively early stage.
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