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#Keratoconus vs pellucid marginal degeneration how to

Orbscan II topography anterior elevation 3 Orbscan II topography posterior elevation 3 Here are some examples of various systems, but these are not necessarily the only criteria by which to rule in or rule out keratoconus.

There are many systems to grade keratoconus.

Differences in steepness between the two corneas of a given patient.Įxpected topography: inferior steepening on anterior axial map and corresponding thinning on pachymetry map.

Large difference between the power of the corneal apex and periphery.

Topographic diagnosis of keratoconus is suggested by:

K Max (Front): Steepest point over anterior corneal surfaceĭ) Values used in IOL calculations (out of scope of this article) 5 Clinical Uses.

Thinnest Location: Thinnest point over anterior corneal surface.

Pachy apex: Corneal thickness at the apex.

The same variables described for the front of the cornea are used to characterize the back of the cornea.Ĭ) Pupil center: Calculated by finding the center point based on edge detection on the iris then the distance is calculated in mm

R min: Smallest radius of curvature in entire field measurement.

R per: Average radius of curvature between the 6mm and 9mm zone center.

Axis: The meridian that requires no cylinder power to correct astigmatism.

More negative values may suggest keratoconus or hyperopic correction whereas positive values may suggest myopic correction.

Q-val: Describes the corneal shape factor, or eccentricity of the cornea.

“OK,” “Data gaps,” “Fix,” “Model”) may alert the technician to retake the exam due to suspect quality

R f, R s, R m: Radii corresponding with K 1, K 2, and K m, respectively.

Red corresponds with the steep meridian whereas blue corresponds with the flat meridian.

K 1, K 2, K m: The two major meridians ( K 1, K 2), determined using the 3mm ring, are 90 degrees from each other.

Useful for identifying forme fruste keratoconusĮxpected topography: Progressive flattening from center to the periphery by 2-4D, with the nasal area flattening more than the temporal area.

4) Posterior elevation map (bottom right).

Warmer colors indicate where the cornea is elevated above the best fit sphere cooler colors indicate where the cornea is depressed below the best fit sphere.

Useful for assessing regularity of astigmatism, location of astigmatism and surgical planning for AK, toric planning.

Coolcolors = thick (think “in the cold wear thicker layers”).

Warmcolors = thin (think “in the heat wear thinner layers”).Displays distribution of corneal thicknesses across the entire measured area.2) Corneal thickness, aka pachymetry map (bottom left).Warmcolors = steep (think “ steeping warm tea”).Useful for assessing irregularity of astigmatism and planning suture removal after PK.On our representative Pentacam images below, you will see four different types of maps. These range from warm colors ( red, orange, yellow), to neutrals ( green) to cool colors ( blue, purple). Guiding suture removal and placement of corneal relaxing incisionsĬolored Maps: You will see a rainbow of colors on every topographic map.Determining visual significance of corneal and conjunctival lesions, such as pterygia and Salzmann’s nodular degeneration.Detection of ectatic disorders such as keratoconus, pellucid marginal degeneration and post-LASIK ectasia.Screening candidates for refractive surgery by identifying irregular astigmatism and helping estimate postoperative ectasia risk.Management of astigmatism in cataract surgery and after corneal transplant.Scheimpflug imaging (tomography): Evaluates the cornea using a camera that captures cross-sections of the cornea as it rotates.Nowadays, tomography is most commonly used. Placido disc (topography): Evaluates the cornea based on the reflection of concentric rings (mires).Ģ) Corneal to mography computes a 3-D image of the cornea and assesses the entire cornea, anterior and posterior surfaces.This is the technical distinction between topography and tomography:ġ) Corneal to pography is a non-invasive imaging technique for mapping the surface curvature and shape of the anterior corneal surface. We will also review 5 clinical uses for topography that will prepare you well for cornea clinic.

#Keratoconus vs pellucid marginal degeneration how to

In this article, we will review what corneal topography and tomography are, why they are useful, and how to interpret a normal Pentacam scan. Except for our section differentiating between them, we will also refer to both as topography. However, both are colloquially referred to as topography. *Note:* Technically, topography and tomography are different imaging modalities (explained below). Angela Chen, B.S., Benjamin Lin, M.D., Shawn Lin, M.D.