Corneal Ectasias

corneal ectasia

progressive eye condition characterized by distortion of corneal curvature due to associated thinning

primary corneal ectasias

• keratoconus

• pellucid marginal degeneration

• keratoglobus

• posterior keratoconus

secondary corneal ectasias

• iatrogenic ectasias

• post-laser refractive surgery ectasia

• post-penetrating keratoplasty ectasia

corneal topography

non-invasive imaging technique that creates detailed color-coded maps of the cornea’s surface curvature & elevation

corneal tomography

• non-invasive 3D imaging technique which provides understanding of corneal structure & internal geometry

• specifically acquires information related to anterior corneal surface, posterior corneal surface & corneal thickness

axial/sagittal curvature map

provides steepness of anterior surface of different points in dioptric values (warm = steep; cool = flat)

corneal thickness (pachymetry)

displays distribution of corneal thicknesses across entire measured area (warm = thin; cool = thicker)

anterior elevation map

displays height difference between anterior surface & reference surface (warm = elevated; cool = depressed)

posterior elevation map

displays height difference between posterior surface & reference surface (warm = elevated; cool = depressed)

K1 & K2

represent major meridians 90 degrees apart

Km

average of K1 & K2

QS (quality score)

provides alert that scan needs to be retaken due to suspect quality

Q-val

describes corneal shape factor or eccentricity of cornea

Pachy apex

corneal thickness @ apex

thinnest location

thinnest point over anterior corneal surface

K max (front)

steepest point over anterior corneal surface

normal anterior corneal curvature

• means is between 42 D - 45 D

• max below 47 D

• symmetric/regular curvature

normal pachymetry

• ranges between 500 - 600 microns

• thinnest point usually slightly inferotemporal to corneal apex

normal anterior & posterior elevation map

symmetric, regular, & relatively smooth

normal corneal thickness distribution

regular concentric progression from thin center toward periphery

Anterior Segment OCT (AS-OCT)

non-contact imaging technique that helps visualize anterior structures of eye & increase diagnostic confidence; generates pachymetry maps & epithelial thickness maps; provides cross-sectional structural explanation for abnormalities seen on other tests

Epithelial thickness map

imaging maps that show thickness & spatial distribution of corneal epithelium across corneal surface; utilizes anterior segment optical coherence & high resolution corneal OCT; measures at multiple points

common use of epithelial thickness map

• keratoconus detection (most likely to reveal early disease)

• corneal ectasia screening

• refractive surgery evaluation

• monitoring corneal disease /healing

• differentiating corneal irregularities

• dry eye disease (tx monitoring)

normative results of epithelial thickness map

• minimal epithelial thickness: central thickness m. commonly falls between 50 & 55 um

• min-max difference: should not exceed 6 microns

• superior-inferior (S-I) Asymmetry: slightly thicker inferiorly d/t blinking mechanics impacting superior aspect

corneal biomechanical testing

measurement of corneal response secondary to application of corneal force; used in tandem w/ other diagnostics

what are common uses of corneal biomechanical testing?

• diagnosing corneal ectasia

• determining risk of post-refractive surgery ectasia

• monitoring corneal cross linking

• glaucoma

Ocular Response Analyzer (ORA)

• utilizes air pulses to flatten cornea & then measures response as it returns toward original configuration

• corneal hysteresis

• corneal resistance

corneal hysteresis

measures energy cornea dissipates as it deforms & then returns toward original shape (assessment of shock absorbing behavior)

corneal resistane

measures cornea’s overall resistance to deformation w/ greater emphasis on cornea’s elastic/stiffness component

corvis ST

• utilizes air pulses coupled w/ ultra-high-speed Scheimpflug camera to film dynamic rxn of the cornea

• major outputs: IOP, pachymetry, corneal biomechanical characteristics

normal corneal hysteresis results

• 10 - 11 mmHg; less than 9mmHg considered suspicious

• high value > cornea can absorb more energy

• low value > cornea has difficulty w/ energy absorption

normal corneal resistance results

• 10 - 11 mmHg; les than 9mmHg considered suspicious

• high value > greater resistance to deformation

• low value > lower resistance to deformation