Transcript Slide 1
ANATOMY OF THE EYE Normal cornea: • prolate shaped • > 20% thinner in the center than the periphery Corneal Layers OPTICS OF THE EYE OPTICS OF THE EYE Abnormal corneal shape distorts vision ABERRATIONS REDUCE VISION Normal 20/16 Mild KC 20/25 Moderate KC 20/32-1 CORNEAL TOPOGRAPHER Cone target, fine mire Different topographers Large target, coarse mire Cone target, fine mire Different topographers Large target, coarse mire Different topographers, similar result NORMAL CORNEAL TOPOGRAPHY • Average corneal power 40.7 – 46.5 diopters. • Uniform central corneal powers with regular contours and flattening toward the periphery. • Often a symmetrical bow tie pattern is present, an indication of natural astigmatism. NORMAL CORNEAL ASTIGMATISM ABNORMAL CORNEAL TOPOGRAPHY • Corneal diseases produce abnormal topography and reduce vision. • Examples of pathology: – Keratoconus and Keratoconus Suspect – Pellucid Marginal Degeneration – Basement Membrane Dystrophy PELLUCID MARGINAL DEGENERATION Typical KERATOCONUS • Keratoconus describes a condition of the cornea that causes thinning and protrusion. • Diagnosis is by observation of corneal steepening on corneal topography (usually inferior), corneal thinning, and certain biomicroscope findings. • Treatment: when eye glasses or contact lenses no longer provide good vision, a corneal transplant may be recommended. ADVANCED KERATOCONUS KERATOCONUS KERATOCONUS DEVELOPMENT OVER 6 YEARS KERATOCONUS SUSPECT • Keratoconus suspect describes a condition of the cornea that may lead to keratoconus with thinning and protrusion. • Identification is by observation of a subtle localized corneal steepening on corneal topography; this may be accompanied by findings on retinoscopy. • Management: repeat eye exams every 6 months to a year to watch for progression to keratoconus. Keratoconus Suspect Keratoconus Suspect Keratoconus Suspect MANUAL PRE-OPERATIVE SCREENING VERTICAL POWER GRADIENT (I-S, RABINOWITZ, 1989) > 1.4 D = KCS OR ABNORMAL > 1.9 D = KC OR ABNORMAL + Δ = 1.58 D possible KCS + REFRACTIVE SURGERY COMPLICATIONS • Kerectasia: protrusion of the cornea following refractive surgery. Associated with: – Pre-operative signs of keratoconus – Too thin a residual stromal bed thickness. • Poor vision – Irregular astigmatism from LASIK flap complications (for example: button hole, free cap, partial flap). – Dry eye. Pre-op LASIK ON KCS 3 mon 18 mon Basement membrane dystrophy DALE ROBINSON PRE-OPERATIVE SUMMARY • Normal corneal findings: – Pre-operative average corneal power was within the range for normals (40.7 – 46.5 diopters). – Central corneal thicknesses (on calibrated Orbscan II 555, 538 microns) within normal range. DALE ROBINSON PRE-OPERATIVE SUMMARY • Abnormal corneal findings: – Topography: • Central corneal powers were not uniform, but exhibited a localized, inferior area of corneal steepening, a classic sign of keratoconus. • The gradient in corneal power was 8 D in the left eye and 6 D in the right eye, 300-400% greater than the Rabinowitz criterion (>1.9 D). – Pachymetry • 18 Micron difference in thickness between right and left eyes. • Thin inferior corneas. ORIGINAL ORBSCAN PRINTOUT Inferior steepening ORIGINAL ORBSCAN PRINTOUT Inferior steepening THICKNESS CORRECTED ORBSCAN Inferior steepening THICKNESS CORRECTED ORBSCAN Inferior steepening DALE ROBINSON SUMMARY FINDINGS • Dale Robinson was not a good candidate for LASIK surgery. – Pre-operative corneal topography revealed the presence of keratoconus in both eyes. – When LASIK is performed on eyes with keratoconus, the eyes are at risk for keratectasia which severely impairs vision and often leads to corneal transplantation. – The standard LASIK procedure is a contraindicated in patients with keratoconus.