Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard Mini Atlas Series CORNEALTOPOGRAPHY_Agarwal, Jacob_2009

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

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CHAPTER 4: CORNEAL ECTASIA POST-LASIK: THE ORBSCAN ADVANTAGE

FIGURE 4.4: The posterior elevation map shows multiple red flags as described in the text. The highest point on the posterior elevation coincides with the highest point on the anterior elevation, the thinnest point on pachymetry, and the point of steepest curvature on the power map

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CHAPTER 4: CORNEAL ECTASIA POST-LASIK: THE ORBSCAN ADVANTAGE

could be indicative of early keratoconus. This criterium is not diagnostic as a sign of early ectasia per se, as this may also be seen in small corneas (WTW<11mm), very steep corneas or in Asian eyes.

A power between 53 and 54 diopters can be suspicious, and needs to be correlated with other signs and/or symptoms.

3.Roush criterion: Indicative of early keratoconus is a relative difference > 100 microns between the highest and lowest point on the posterior elevation map (Fig. 4.4). A relative difference >70 microns is a yellow flag, except when the cornea is very symmetrical and when it is caused by a regular astigmatism.

POWER MAP

1.Steep corneal curvatures are always suspicious. Keratometric mean power map > 46 diopters or total mean power map > 45 diopters are definitely red flags (Fig. 4.6).

2.Bow-tie/broken bow-tie pattern. The so-called “lazy- C” on the axial power map is very suspicious when the astigmatism shifts > 20° from a straight line (Fig. 4.7).

3.Central corneal asymmetry. A change within the central 3 mm optical zone of the cornea of more than 3 diopters from superior to inferior (yellow flag) can be correlated

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CHAPTER 4: CORNEAL ECTASIA POST-LASIK: THE ORBSCAN ADVANTAGE

FIGURE 4.7: Lazy-C associated with central corneal asymmetry

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with the present of vertical coma (Fig. 4.7). However, this may be merely a sign of asymmetric astigmatism, and is not necessarily indicative of pathology.

COMPOSITE/INTEGRATED INFORMATION

1.Correlation of signs with the highest point on the posterior elevation: This is probably the strongest topographic sign indicative of early keratoconus. If the highest point on the posterior elevation coincides with the highest point on the anterior elevation, the thinnest point on pachymetry, and the point of steepest curvature on the power map, never perform laser refractive surgery. This implies that the thinnest point represents a structural weakness, which causes a forward bending on the cornea (Fig. 4.4).

2.Efkarpides criteria: The ratio of the radius in mm of the anterior BFS divided by the radius in mm of the posterior BFS. Surprisingly in normal corneas this ratio will be around 1.21. Between 1.23 and 1.27 we should be suspicious and look for other abnormalities. But when this ratio is 1.27 or higher this cornea should never be treated with laser (Fig. 4.8).

3.Bent/warped cornea: Similarity between the anterior and posterior profiles implies a forward bending of those areas shown above BFS. If this bending is in association with the

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thinnest point on the cornea, it could relate to a structural weakness in the cornea. This sign needs to be evaluated within the context of the other parameters (Fig. 4.9).

4.Infero-temporal displacement of the highest point: On the anterior as well as the posterior elevation profile can be indicative of early keratoconus (Fig. 4.10).

5.Nature is surprisingly often very symmetrical, also our corneas. When a difference of more than 1D of astigmatism between two eyes is detected, there exists a higher risk of ectasia postoperatively.

6.Never forget to look at the information in the center of the quad maps. An irregularity of >1.5 D in the 3 mm central zone and of > 2.0 D in the 5 mm central zone should alert us (R. Lindstrom, MD). This sign is probably the weakest of all, but should not be ignored (Fig. 4.11).

7.Normal band scale: For the anterior and posterior elevation maps a normal band means an elevation within +/- 0.25 microns of the best fit sphere (BFS). The normal band for the total corneal power map is 40-48 diopters and for corneal thickness is between 500 and 600 microns. One pop-up means caution, two means concern and three pop-ups means a no-go situation (Figs 4.12 to 4.14).

8.If one eye fails on the indices but the other eye doesn’t: Never

Treat either eye.

The above mentioned indices can become useful in the armamentarium of preoperative evaluations for potential

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CHAPTER 4: CORNEAL ECTASIA POST-LASIK: THE ORBSCAN ADVANTAGE

FIGURE 4.9: Bent/warped cornea: asymmetry of anterior and posterior elevation

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