W4:Topography

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Contents

Purpose and uses

Scales

Maps

Indices

Pachymetry

Meibomian gland evaluation

Uses instruments to accurately measure, assess and record the corneal curvature and regularity

Correctly interprets the information gathered

What are the purposes and clinical uses of corneal topography?

• Study normal corneal topography

• Study effects of disease

• Pre- and post-surgical comparisons

• Assess effects of contact lenses

• Compare changes with refractive surgery

• Document changes with orthokeratology

• Aid design of customised contact lenses

• Check whether the topographer measures anterior only or both anterior and posterior surfaces

What are the features of absolute scales used in corneal topography?

• Same colour assigned to a given dioptric interval

• Compared with a computerised reference eye

• Allows comparison between eyes

• Useful for screening

• Uses large step increments, so fine detail is lost

What are the features and limitations of relative scales in corneal topography?

• Smaller dioptric range per colour than absolute scales

• Colours of 2 diff maps cannot be compared directly

  • Provides a more detailed description

  • Removes smoothing effect

  • Cannot compare btwn eyes

What does a sagittal (axial) corneal topography map measure?

• Measures the curvature at a point on cornea in axial direction relative to the centre

• Make the assumption that the centre of the radius of curvature is always on the central axis

• Gives a global view of corneal curvature

• Usually used in conjunction with an absolute scale

What does a tangential corneal topography map measure?

• Measures curvature at a point on cornea in meridional direction relative to the other points on a particular ring

• Does not assume:

  • eye is spherical

  • refracted rays converge to central point on visual axis: each data point is in relation to surrounding data points

  • where the centre of radius might be

• Simply look at a certain part of the cornea + measure the radius of that point under 90◦ (as a tangent)of which the centre of radius could be anywhere

What does an elevation map show in corneal topography?

• Shows the measured height from which the corneal curvature varies

  • from a computer generated reference sphere,

    • which best fits the measured corneal topography

How are steep and flat meridians represented on an elevation map?

Red dot:

• steepest meridian, above

• How many microns space between lens + corneal surface

Blue dot:

• flattest meridian, below

• How many microns needed between lens + corneal surface

What is the clinical use of elevation maps in contact lens practice?

Most useful in predicting fluorescein patterns w/ RGP lenses

What information is provided by a refractive map, and when is it useful?

Insight into:

• ∆s in refractive power across a pt’s cornea

• the cornea’s contribution to the eye’s dioptric power

• the magnitude of cylinder in an astigmatic eye

When is a refractive map useful?

• When estimating values for LASIK refractive surgery

• or selecting an IOL for cataract extraction

What are difference maps used for in corneal topography, and how are they interpreted?

• Often used in orthokeratology fitting

  • Compare before and after lens fitting

  • Identify what has changed

• Can be used pre- and post-surgery

• Use only one eye in all comparisons

• Blue indicates flattening

• Red indicates steepening

• Represented as A − B = C

What is an aberration map and who is it associated with?

• Developed from work by Frits Zernike

  • Dutch physicist + Nobel Prize winner (1888–1966)

• Uses a mathematical representation

• Describes deviations of a real wavefront from an ideal wavefront

• Expressed as a sum of polynomials

What factors influence correction of aberrations in the eye?

• Spectacles correct lower-order aberrations

• Tear film can mask corneal irregularities

• Masking can improve higher-order aberrations caused by irregularities

What does the Index of Surface Variance (ISV) measure?

• Deviation of individual corneal radii from the mean value of a predetermined eye curvature

• Displayed by two lines of the principal meridians on an x:y plotted graph

How is the Index of Surface Variance (ISV) interpreted clinically?

• Elevated in all types of corneal surface irregularity

  • Normal + pathological

• Examples incl:

  • Scars

  • Astigmatism

  • CL–induced deformities

  • Keratoconus

• Red = pathological

• Yellow = out of normal range

Which topography indices are used for keratoconus diagnosis with Orbscan and Atlas?

• Irregularity Index (SRI)

• Surface Asymmetry Index (SAI)

Which indices are used for keratoconus diagnosis with the Pentacam?

• KC index (KI, CKI) on a 0–4 scale

• Index of Surface Variance (ISV)

• Asymmetry degree (IVA, IHA)

• Index of Height Asymmetry (IHD)

• Aberration coefficient (ABR > 1 = KC)

• Pachymetry progression index

• All based on number of standard deviations from the average

How is a skewed radial axis interpreted in corneal topography?

• Difference in axis angle > 15° in one meridian = abnormal

• Difference > 21° = diagnostic indicator for keratoconus

What comparisons are required when assessing a skewed radial axis map?

• Compare ring pairs:

  • 3 mm to 5 mm

  • 5 mm to 7 mm

  • 3 mm to 7 mm

• Be critical of the map

• Extrapolated data is not excluded

What factors can affect the analysed area and extrapolated data in ocular imaging?

Analysed area / extrapolated data / reflective device issues:

• Lids

• Blinking / ptosis

• Nose

What reliability checks should be applied when interpreting ocular imaging results?

• Use as much information as possible to identify an abnormality

• Ensure the scan is centred

• Use the right maps + scales together

How does imaging assist in contact lens design and fitting, and what is its limitation?

• Helps predict pooling patterns

• Does not account for lid involvement

What are the key features of non-invasive pachymetry using Orbscan?

• Non-invasive pachymetry method (Orbscan)

• Measures CT perpendicular to the cornea

• Ultrasonic pachymetry also measures perpendicularly

• Orbscan thickness measurements are greater than manual ultrasonic pachymetry

  • 7–10% thicker (≈ 30 μm)

Meibomian gland evaluation