This will take you through a step by step interpretation of 4 composite refractive map of Pentacam. For interpretation of other maps of Pentacam, like EKR/Holladay report, we will cover in a separate guide. In the video presentation 'Understanding Corneal Topography and Pentacam (Part 1)' we did cover the basic challenges with topography or curvature based maps. Recall in those maps, the basic challenge was that such maps derived elevation from curvature based maps. In pentacam, as with other Scheimflug based maps, elevation of the cornea is measured first, and then curvature of cornea is derived.
The 4 maps refractive consists of the following(fig1):
Sagittal Curvature Front or also called axial curvature of the anterior surface of cornea.
Elevation of Front (anterior cornea).
Elevation of Back ( posterior cornea)
Corneal Thicknes (pachymetry)
Key Points to remember before you interpret Pentacam 4-maps Refractive:
A. Note that Quality Specification (QS) should be OK and the box is in colour white. Do not interpret map if QS is in yellow and repeat the measurement.
B. Ensure you have set the map to 8 or 9 mm display.
C. Check if there are extrapolated data in the central 8 mm display. If there are black dots or white areas, then discard the map and take a fresh measurement (fig2)
D. Check the Kmax. If Kmax is inferiorly displayed such that it is in the periphery, then repeat the measurement. Check if there is any lid interference.
E. It is important to make sure that you have not measured on a mis aligned or non fixating eye. To rule out that apply the following equation:
X+X <.2 ; Y-Y <.2
That is compare the X and Y cordinates of pupil centre of two eyes. The X of pupil centre of RE plus the X of pupil center of LE should be less than 200 microns or .2. Similarly, the Y of pupil center of RE minus the Y of pupil center of LE should be less than 200 microns or .2. If it is more than 200 microns or .2mm, expect misalignment of eye. This is important to rule out false angle kappa.
Once you have determined that there is no fixation related issues by applying the above technique, you can check for angle kappa (chord μ) through Holladay Report map, that will be discussed in a separate edition.
Interpreting Pentacam 4-maps refractive (fig3):
The Km or mean K of the anterior corneal surface should be less than 48 diopters. Note this value as shown in Fig3. Consider moderate risk when values of Km between 48-50 D and high risk beyond 50 D.
Consider thinnest location of Cornea below 470 microns as high risk. Values of more than 500 microns may be considered low risk. (refer fig3)
The Y coordiate of Thinnest location should be less than negative .5 mm. This is because keratoconus is mostly associated with an inferiorly displaced thinnest location. A value of more than -.5mm is a yellow flag and more than -.1 mm is high risk. (refer fig3)
In the sagittal curvature (axial) map, note the difference in power of two opposite hemi meridians, that is the inferior-superior asymmetry. To know this you may calculate the difference between the two opposite points on the vertical axis within the central 4 mm zone. The inferior value is less than 1.5 diopter to superior values, then it is normal. A value greater than 1.5 diopter however indicates abnormality. If the superior is more than the inferior by more than 2.5 diopters, consider abnormal. Therefore, I-S<1.5 D and S-I<2.5 D are normal.
The difference between pachymetry apex and thinnest location should not be more than 10 microns.
SRAX or skewed radial axis is another indicator. In regular astigmatism, there is a symmetry in astigmatism axis between the superior and inferior axis. If the superior and inferior axis of astigmatism is skewed, this is called skewed radial axis (SRAX). Consider abnormality if SRAX is beyond 22 deg.
Elevation maps: Interpreting the elevation maps are key to diagnosing Keratoconus or determining forme fruste keratoconus. The elevation maps are considered abnormal or normal based on its comparison with a reference sphere, often the Best Fit Sphere (BFS) or the Best Fit Toric Ellipsoid (BFTE). Any of the two reference spheres may be applied, but do note that the indices for interpretation change according to your choice of BFS or BFTE. For cut off values see Fig 4
Note: The above interpretation is based on experience with map readings and interpretation are subjective in nature and not definitive. The article is of informative value only and the author requests to consider applying such information based on individual clinical judgement and findings.