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

Reading Pentacam Topography 41

Fig. 3.11: The thinnest location is severely displaced giving the thickness map the shape of a cone.

42 Step by Step Reading Pentacam Topography

2.The relation between the apex of the cornea and the center of the pupil: It is important in the issue of “decentration” or “offset pupil”.

3.The relation between the apex of the cornea and the thinnest location: This relationship could be according to thickness or to coordinates.

•The relationship according to thickness: Let us look at Fig. 2.13 to compare the thickness at the apex with the thickness at the thinnest location. The difference should be no more than 10 µ and not increasing with follow up.

•The relationship according to coordinates (Fig. 3.12): When the value exceeds -500 µ, it is a risk factor, especially when the difference is increasing with follow up at one to two years intervals. When this value exceeds -1000 µ, it is an important sign of an advanced keratoconus (see Fig. 2.13).

4.Look for the thickness on the 4 mm circle: There should be no more than 30 µ difference between the superior and inferior points (we will call this from now as superior-inferior difference).

Fig. 3.12: The relationship between pachy apex and thinnest location coordinates. The ‘y’ coordinate is more important than ‘X’; because the thinnest location is generally displaced vertically in the development of keratoconus or keratoectasia.

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Fig. 3.13: The topometric map. Q-value measured and displayed in the four main sectors of the cornea.

5.Compare the thinnest location between the examined cornea and its fellow, the difference should be no more than 30 microns.

Topometric Map

Fig. 3.13 illustrates Q-value in four sectors of the cornea: nasal, temporal, upper and lower sectors. It also shows the average vertical and horizontal values. Fig. 3.14 shows Q-values in the four sectors of the cornea within circles centered on the apex of the cornea with 6, 7, 8, and 9 mm diameters (or in angles of 20°, 25°, 30° and 35° respectively).

The most important values are the vertical in general and the inferior in particular, as in Fig. 3.15. The normal

Fig. 3.14: Q-values according to different concentric rings and in the four main sectors.

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Fig. 3.16: Mean corneal thickness values on rings concentrically to the thinnest location. The black dotted lines are the normal range and average in normative data. The red line is for the measured cornea.

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Fig. 3.17: Progression of corneal thickness. The black dotted lines are the normal range and average in normative data. The red line is for the measured cornea.

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Fig. 3.18: The progression of corneal thickness of a keratoconic cornea. The measured red line is out of the normal range, and it deviates before 6 mm indicating a rapidly increasing thickness from the thinnest location towards periphery.

Table 3.1: The normal, suspected and abnormal findings in corneal topography

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