Ординатура / Офтальмология / Английские материалы / Step by Step Reading Pentacam Topography (Basics and Case Study Series)_Sinjab_2010

32 Step by Step Reading Pentacam Topography

Fig. 3.2: Negative spherical aberration. This occurs when the peripheral part of the refracting surface has lower power of refraction than its central part (prolate cornea).

dpt means that the anterior surface of the cornea became very steep (prolate) and the patient is suffering from negative spherical aberrations (Fig. 3.2). Fig. 3.3 shows the quality of vision in positive and negative spherical aberrations.

•Corneal astigmatism: We have to calculate algebraic sum of astigmatism of the anterior and posterior corneal surfaces, then we should compare with the manifest refraction to exclude causes of incongruence, such as lenticular astigmatism, subtle posterior subcapsular cataract, tear film disturbance,…etc.

•Q-value: This value describes the slope of the cornea. It is an average value, we should refer to the topometric map to estimate this value with its many clinical interpretations and applications, some examples will be discussed later in the case studies.

Reading Pentacam Topography 33

Fig. 3.3: Quality of vision in spherical aberrations.

•Thinnest location: This category gives us an idea about corneal thickness, but we should refer to the thickness map to have full picture about the case. It is very important to study the relationship between the thinnest location and the pachy apex according to the thickness and according to the location.

•Pupil center location: It is important when doing decentration of the ablation profile especially when treating hyperopia.

Studying the Maps

The Anterior Sagittal Curvature Map

The normal cornea: When considering the topography of a normal cornea, we feel the need to remember that there is a wide spectrum of normality. No human cornea demonstrates the kinds of regularity found in the calibration spheres of the topographer: the eye is not molded glassmade. Normal corneal topography can take one of the following:

36 Step by Step Reading Pentacam Topography

Fig. 3.7: Symmetric bow tie with oblique astigmatism, still normal but less common.

When the bow tie is diagonal, it represents a cornea having an oblique astigmatism (Fig. 3.7).

In the normal eye, nasal cornea is flatter than temporal. The nasal side of a healthy corneal map becomes blue more quickly, indicating that the nasal cornea is flatter than temporal.

Generally, the two eyes of the same normal subject are very similar, and present a mirror image of each other (Fig. 3.8). This phenomenon is called enantiomorphism. The

Reading Pentacam Topography 37

Fig. 3.8: Enantiomorphism. The anterior sagittal curvature map of the right eye is a mirror to that of the left eye.

knowledge of this fact is useful to decide whether a cornea is normal or not, by comparing with the map of contralateral eye.

P.S. When studying the pattern of corneal curvature, it is important to study the single enlarged map choosing the option of projected circles and the two major axes of curvature, in order to easily compare values in the same eye and between both eyes (Fig. 3.9).

38 Step by Step Reading Pentacam Topography

Fig. 3.9: Projected circles and the two major axes of curvature on the curvature map. This is very important for quantification of this map.

Topographic Shape Patterns which Characterize Irregularity (Fig. 3.5)

Please refer to my book “Corneal Topography in Clinical Practice” published by Jaypee Brothers 2009, to study these patterns carefully. But in general, the most concerning here are the steep K-readings, inferior-superior asymmetry and skewing of the steep axis.

Look at the central 4 mm circle:

•Calculate I-S Rabinovich ratio, it should be <+2.

•Beware of Rowsey’s rule of 2’s.

•The curvature power of the lower point in the circle should be no more than 1.5D from the upper point and no more than 2.5D vice versa.

Reading Pentacam Topography 39

Important figures:

•Previously, it was recommended—when taking the decision—that any anterior K readings should not be more than 47D on the front sagittal curvature map. Recently, with the availability of thin flap technology, the power 49D became acceptable. More than 49D is risky regardless of the patient’s refractive error.

•Corneal astigmatism on either surface should not be higher than 6D; otherwise it is a risk factor.

•Against the rule astigmatism is considered suspicious.

The Elevation Maps

We look at the values within the central 4 mm circle using the best fit sphere float reference body (BFS). That is because the following normative data depend on this particular kind of reference bodies:

1.Theelevation valueson the frontsurfacemapshould not exceed +12 µ. Values between +13 µ and +15 µ are suspected, and any value > +15 µ is considered a risk factor.

2.The elevation values on the back surface map should not exceed +17 µ. Values between +18 µ and +20 µ are suspected, and any value >+20 µ is considered a risk factor.

3.The difference between the back and front surfaces (back-front) should not exceed +5 µ at the same point. For example: if the back is +12 µ and the front is +4 µ at the same point, it is suspicious although both values are within the normal limits.

4.If there is any isolated island on either front or back surfaces, it would be suspected, even with values within the normal limits (Fig. 3.10).

40 Step by Step Reading Pentacam Topography

Fig. 3.10: Nearly isolated island on the posterior surface of the cornea. We should pay attention to this sign and quantify it carefully to exclude any risk factor.

P.S. 1: Be careful when any value of the central 4 mm of the elevation maps is more than +15 µ for the anterior surface, and more than +20 µ for the posterior surface. P.S. 2: Be careful when the “back-front” difference is more than +5 µ at the same point.

P.S. 3: Be careful when there is an isolated island on either surface.

Corneal Thickness Map

1.Look at the shape of the thickness map, it gives an idea about the shape of the cornea (Fig. 3.11).