Ординатура / Офтальмология / Английские материалы / LASIK A Handbook for Optometrists_Hanratty_2005

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Figure 3.1 Keratometry mires as viewed with a Bausch & Lomb keratometer

Topography is used in the pre-operative assessment to screen for anterior corneal surface anomalies that may interfere with or contra-indicate treatment. It is also useful in monitoring the progression of a patient who has had corneal surgery. Any changes to the corneal shape after treatment and as it recovers from surgery will be evident from topographical analysis.

Types of topographic displays

Topography instruments have become increasingly sophisticated and most are able to present several displays.

Axial map This is a simple display which assumes that the eye is spherical. It displays the corneal surface as a topographical map where the colours of the map indicate the dioptric power of the cornea. The flatter areas are shown in ‘cool’ colours such as blue and green and the steeper areas are shown in ‘hot’ colours such as red and orange. Although easy to interpret, this map tends to ‘smooth out’ some of the smaller areas of curvature variation. Figure 3.2a shows an axial map of an astigmatic eye. There is a vertical bow-tie appearance which is typical of rule astigmatism.

Tangential map This map uses colours in the same way as the axial map to display dioptric power but uses a different algorithm to calculate the corneal

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(a)

(b)

Figure 3.2 (a) Axial map of a cornea exhibiting with the rule astigmatism; (b) tangential display of the same eye (continued on next page)

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(c)

Figure 3.2 (c) Elevation map of the same eye (continued from previous page)

curvature and is able to map the peripheral curves of the cornea more accurately. It does not assume that the eye is spherical but bases its calculations on the local curvature at a given meridian. This display represents the ‘true’ corneal profile and is able to show sharp power transitions more easily than the axial map. It is very useful for showing the changes that occur as the cornea heals postoperatively. Figure 3.2b shows the same eye using a tangential display which shows the true curvature of the eye.

Elevation map Figure 3.2c shows the measured height from which the corneal curvature varies from a computer-selected reference surface. Warm colours indicate points above the reference line and cool colours represent points below. This display is useful in evaluating corneal irregularities and post-surgical abnormalities.

Photokeratoscopic view This is an actual image of the eye that is captured by the video camera. The tear film acts like a convex mirror and reflects the illuminated placido rings (Figure 3.3a). The spacing between the concentric rings can indicate the steepness of the cornea if there are gross changes. Any irregularity of the mires can indicate corneal distortion or where the tear film has evaporated. This information can be useful in ruling out artefacts on maps caused by factors such as dry eye. In Figure 3.3b, the axial map of the cornea looks slightly unusual nasally. The reason for this can be seen in the photokeratoscopic image (Figure 3.3c) which shows corneal distortion that is caused by a pterygium encroaching the cornea.

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Topography scales

When interpreting maps, it is imperative that the practitioner refers to the scale of the map before drawing conclusions. There are generally two types of scale that are commonly used.

(a)

(b)

Figure 3.3 (a) Photograph of the placido rings that are projected onto the cornea; (b) nasal corneal irregularity with apparent flattening (continued on next page)

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(c)

Figure 3.3 (c) Photograph of the projected placido rings show that the irregularity is due to the presence of a pterygium (continued from previous page)

Absolute scale This is a fixed numerical scale where each colour represents a specified dioptric power. The range of the scale is the same for each map. This scale is useful in the general screening for anomalies as only features of clinical significance will be visible. Although specific corneal anomalies appear more subtle with an absolute scale, the practitioner will be able to interpret the maps more easily with experience. Figure 3.4a shows the same eye as in Figure 3.2 but uses an absolute scale rather than a normalised one. The plots look quite different, but the actual data are the same. The benefit of a fixed scale is the easy comparison of different maps. Comparison between maps from the eyes of a patient is straightforward using an absolute scale and it can be seen in Figure 3.4b that the eyes are fairly symmetrical and both have oblique astigmatism.

Normalised scale This is a floating scale where the computer selects the numerical range from the highest and lowest data points. Therefore, the colours of the map will show a very detailed display of the cornea. Every small curvature change will be detectable; as a result, it may introduce a significant amount of ‘noise’ which could mislead the practitioner. As the scale is relative to that particular cornea, it can be difficult to compare multiple maps or to track the progress of corneal changes. However, if a condition is detected with the absolute scale, the detail given on a normalised can be useful for further analysis of that condition. Figure 3.5a shows a suspect keratoconus that is detectable on an absolute scale, but more detailed information is seen with the normalised scale (Figure 3.5b).

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(a)

(b)

Figure 3.4 (a) Map of an astigmatic cornea presented with an absolute scale; (b) comparison of multiple maps is facilitated by using an absolute scale (continued on next page)

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(c)

Figure 3.4 (c) Against the rule astigmatism (continued from previous page)

(a)

Figure 3.5 (a, b) A keratoconus suspect – the use of different scales can be used to screen or investigate conditions (continued on next page)

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(b)

(c)

Figure 3.5 (c) Moderate keratoconus where the corneal curvature is greater than 55 D at the apex of the cone (continued from previous page)

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Map interpretation

The colours of a topography map can be misleading and to avoid misinterpretation of the data it is important to have a systematic approach.

1.The first and most obvious step is to confirm that the patient details correspond with the patient that is being examined.

2.Check the type of map being shown.

3.Check the type of scale being used and the step interval used between colours.

4.Check the dioptric powers on the map and see if they fall with the normal range. Readings of 47 D or greater are suspicious and could indicate keratoconus.

5.Look of areas of unusual flattening or steepening as well as any asymmetry.

6.Look for features that would identify specific conditions.

Chart 3.1 shows a decision-making flow chart to aid interpretation of topography maps and to reach a clinical decision.

The following section describes the features that are commonly seen in the pre-operative screening of patients for corneal abnormalities. There are other corneal disorders that are not mentioned here and further information is available from other texts on topography.

The normal cornea The normal cornea is prolate in shape, meaning it is steeper in the centre and flatter towards the periphery. It should be reasonably uniform in appearance with a dioptic power of 42–46 D at the centre. As the cornea flattens towards the periphery, the dioptric power declines and the nasal cornea shows a greater degree of change than the temporal cornea. Normal corneas may also have a small astigmatic component and there is usually a degree of mirror image symmetry between the two eyes.

The astigmatic eye In most cases the astigmatism is regular, with the steepest and the flattest meridians at 90° to each other. Astigmatism typically looks like a ‘bow-tie’ or a figure-of-eight on a topography map. Where the steeper meridian is vertical, it is called ‘With the Rule’ (see Figures 3.2a and 3.4a) as it the most common form of astigmatism. ‘Against the Rule’ astigmatism has the steeper meridian in the horizontal position (Figure 3.4c). An example of oblique astigmatism can be seen in Figure 3.4b, where the axis of the cylinder is near to 45° or 135°.

Keratoconus This is characterised by inferior localised steepening of the cornea. The example shown in Figure 3.5a,b is a mild form of keratoconus where the maximum dioptric power of the cornea is less than 55 D. The apex of the cone is usually found slightly to the left or right of the 6-o’clock position as in Figure 3.5c, which is also a more significant degree of keratoconus. The map is displayed using a normalised scale as the standard scale is not large enough to cover the dioptric range of the cornea. Keratoconus can also be classified according location and shape of the cone.