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

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 5.7: Two corneal cross-sections

at 55D, the cornea was of borderline thickness, and the preoperative prescription high.

The final map to study is the pachymetry map. This is map four of our quad map in Figure 5.4. Traditionally, pachymetry has been measured using ultrasound, which provides a reading of corneal thickness from Bowman’s membrane to Descemet’s membrane. Through slit scanning technology, Orbscan provides us with a pachymetry reading from the precorneal tear film to the endothelium, therefore slightly thicker readings can be expected.10 The Orbscan can, however, be calibrated to take this into consideration when comparing readings. The true

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CHAPTER 5: ORBSCAN CORNEAL MAPPING IN REFRACTIVE SURGERY

advantage of the pachymetry map is that it provides us with thickness information across the cornea from limbus to limbus, not just in single points as with ultrasound. This once again gives the opportunity to detect areas of weakness, thinning or scarring. Auffarth et al11 state that the relationship between the highest point on anterior and posterior elevation maps, and the thinnest point (shown by a yellow dot) is an indicator of kerataconus.

The relationship between pachymetry readings can be looked at, and it has been suggested that 100 µm should be a cut-off criteria between thickness regions on the map. Figure 5.8 shows the relationship between the central reading in the white circle, and the four peripheral readings, indicated by the arrows. Once again these criteria would be used alongside other information, but alone would not exclude a patient. The readings within the circles are averages of measurements within the area, but the Orbscan also flags the thinnest point, indicated by a yellow dot.

In conclusion, it can be seen that much information can be obtained from analysis of Orbscan maps, and this information does not have the scope to cover it all. The most important message is that the criteria does not stand alone, and by looking at all the maps together along with other information, an informed decision can be made as to whether it is safe to proceed to surgery.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 5.8: The relationship between the central reading in the white circle, and the four peripheral readings

Selection Criteria

The diagnosis of keratoconus proper, seldom proves to be problematic, and represents the undisputed black end of the spectrum, as would be the case for the patient with simple with-the-rule astigmatism who would be on the white end of the spectrum. However, in between a large gray area exists, consisting of aginst-the-rule astigmatism, asymmetric astigmatism, non-orthogonal astigmatism, irregular astigmatism and forme fruste keratoconus.

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CHAPTER 5: ORBSCAN CORNEAL MAPPING IN REFRACTIVE SURGERY

Over the past number of years since true elevation

corneal topography became available, a set of criteria (Tables 5.1 and 5.2)6,8,9 were developed to distinguish

among these entities. Altough corneal topography provides us with the most clues for the diagnosis of early keratoconus, other clinical criteria also need to be considered.

These would include the patient’s age, a family history of keratoconus, history of systemic or local pathology, asociated with keratoconus, refractive stability, as well as wheter a good and crisp endpoint could be achieved on refractive testing of the subject.

No single corneal topographic sign is in its own right diagnostic of fruste keratoconus, but rather a combination of a set of criteria. One might look at each of these criteria as a “alarm sign” noted, with the porobability for early keratoconus propotionate to the number of alarms present.

These criteria can be divide into the following categories:

a.Power map changes

b.Posterior elevation maps

c.Pachymetry

d.Composite/integrated topography information.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

Table 5.1: Rousch’s Orbscan criteria for subclinical keratoconus detection

1.Elevation difference superior of 100 mm at the central optical zone of 7 mm.

2.Clinical difference superior of 100 mm at the central optical zone of 7 mm.

3.Anterior elevation superior of 40 mm at the central optical zone of 7 mm from BFS.

4.Posterior elevation superior of 50 mm at the central optical zone of 7 mm from BFS.

5.Posterior BFS reference > 55D.

Table 5.2: Efkarpides’s Orbscan criteria for subclinical keratoconus detection

1.Anterior/posterior BFS of reference difference in mm superior to 1.25 to 1.27.

2.Morphological difference between anterior and posterior face (warpage).

3.Remarkable convergence of points (highest point on anterior elevation, highest point on posterior elevation, thinnest point in pachymetry, steepest curvature on the power map).

4.Inferotemporal displacement of these remarkable points.

5.Color code statistical analysis (Normal band scale). Elevation values, curvature, pachymetry of more than 2 standard deviations from controls.

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CHAPTER 5: ORBSCAN CORNEAL MAPPING IN REFRACTIVE SURGERY

Power Maps

Mean corneal curvature > 45 diopters: Mean corneal curvature measuring in excess of 45 diopters is a well established feature of keratoconus.

Bow-tie/broken-tie pattern: In addition to steep corneal curvatures, the bow-tie or broken bow-tie appearance of astigmatic pattern might be indicative of early keratoconus, and also a well known criteria.

Central corneal asymmetry: A change within the central 3 mm optical zone of the cornea of more than 3 diopters from superior or inferior can be correlated with the presence of vertical coma. However, this may be merely a sign of asymmetrical astigmatism, and is not necessarily an indicative of pathology.

Posterior Elevation

The elevation map displays corneal height or elevation relative to a reference plane, which may be a spherical or aspherical surface depending on the topographer. It is important to note that the elevation display depends on reference surface size, shape, alignment, and fitting zone. This map shows the three-dimensional shape of the cornea and is useful in measuring the amount of tissue removed by a procedure, assessing postoperative visual problems, or planning/monitoring surgical procedures.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

Many surgeons think the first sign of keratectasia appers on the posterior surface of the cornea, not on the anterior topography map. Considering this, the importance of recognizing a change in the posterior surface deserves special emphasis.1-3 While one would not perform corneal lasersurgeryoneyeswithkeratoconus,keratoconussuspects or posterior ectasia defined by technologies with the capability of posterior corneal float analysis, it might prove useful to look at the criteria for early form of keratoconus in order to define those cases, and distinguish them from eyes that would be suited to laser refractive surgery.

Laser clinics had shown that 5 to 8 % of patients screened for refractive procedure are not good candidates because ketatoconus detection by simple axial topography, however, a Mexican study demostrated that 3.13% of population screened for Laser eye surgery had posterior ectasia criteria by Orbscan, despite having axial topography clasified as normal (unpublished data).3

We have found four different posterior float pattens in patients screened for Laser eye surgery: complete positive band 71.87%, ncomplete postitive band 18.75%, butterfly wings cummon in patients with high astigmatism 6.25%, and central island 3.13% (Fig. 5.9). In other words, if we don´t know posterir float features from every case, we have at least 3% risk of unstable Lasik result or iatrogenic ectasia in the worst case.3

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CHAPTER 5: ORBSCAN CORNEAL MAPPING IN REFRACTIVE SURGERY

FIGURE 5.9: Orbscan posterior float, 4 different types in patients screened for Laser eye surgery

Best fit sphere >55 diopters. The most common reference surface for viewing elevation maps is the “best fit sphere”. This geometric surface is constructed by fitting a spherical spline with the least square of difference values though the three-dimensional elevation data from the cornea,

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

whether it be the anterior or posterior profile. The sphere can thus be employed to judge the average profile of the surface in question. A best fit sphere (BFS) with the power of 55 diopters or more on the posterior profile, could be indicative of posterior ectasia. This criterion is not diagnostic as a sign of early keratoconus per sé, as this sign may also be seen in small diameter corneas.

Posterior high point >50 microns above BFS: Early keratoconus is often seen first on the posterior corneal profile. Whenever

a localized elevation above BFS on the posterior surface measures more than 50 microns in elevation, this might be indicative of an

early posterior ectasia.

Many authors have review the posterior surface’s response to Lasik, and Orbscan is an unique technology to evaluate this changes.12-15 Increased forward shift of the posterior corneal surface is common after myopic Lasik and correlates with the residual corneal thickness and ablation percentage per total corneal thikness.12

An excessively thin residual corneal bed or a large ablation percentage may increase the risk of iatrogenic complications, such ectasia. Others have considered that even if a residual corneal bed of 300 mm or thicker is preserved, anterior bulging of the cornea after Lasik can occur. Eye with thin corneas and high myopia requiring

greater laser ablation are more predisposed to an anterior shift of the posterior central cornea.13,14

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CHAPTER 5: ORBSCAN CORNEAL MAPPING IN REFRACTIVE SURGERY

Roberts15 with the help of Orbscan proposes a new theory to explain the increased posterior elevation postLASIK, she suggests that the mild ectasia appearance may be due to a backward swelling of the peripheral redistributed cornea rather than a pathological forward bulging of the central cornea.

Pachymetry

Thinnest point <470 microns: This constitutes an absolute contraindication to corneal refractive surgery.In pathological corneas, this thinnest point is often displaced inferotemporal.

Difference of > 100 microns at 7 mm optical zone: A difference of more than 100 microns from the thinnest point to the values at the 7 mm optical zone implies a steep gradient of thinning from the midperiphery towards the thinnest point. These, in conjuntion with other signs, can be indicative of early pathology.9

As a default, four corneal maps are routinely presented by the Orbscan II (Bausch and Lomb, Orbtek, Salt Lake City, Utha).

Elevation topography system: This includes the anterior elevation profile, posterior elevation profile, power map, and a total pachymetry map. Through integration of the information provide on these maps one is able to detect subtle, but powerful signs not present on any individual map. These signs include the following:

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