Section ONE Preliminaries

•The lids should be examined subsequent to the general routine as manipulation of the lid margins can release debris and lipids into the tears which may mask other signs.

•The lower lid margin and inferior conjunctival sac are examined with the eyes looking up. The meibomian glands are gently squeezed with finger pressure and their openings checked for patency. The lipid secretions expressed can then be graded (see 6.2.3).

•The upper lid is everted either with fingers or the help of a cotton bud. The patient looks down and the bud is placed along the natural fold in the upper lid. The lid margin is slightly raised by pressing on the fold while the other hand pulls the now upturned lid edge upwards; the bud pushes downwards so that the tarsal plate everts. The bud can now be removed and the lid held in position to stop it flipping back before it has been fully examined. The lid should finally be allowed to return to its natural position.

PRACTICAL ADVICE

•Correct operation of the slit lamp requires the coordinated use of both hands – one to control the joystick and the other the slit beam.

•The slit lamp routine at an aftercare visit may be carried out in a different order. If the patient wears rigid gas-permeable lenses and fluorescein has already been instilled to check the fitting, the blue light assessment can be carried out first, followed by white light and lid eversion.

•Lid eversion is usually left to the end of the slit lamp routine as many patients find it mildly distressing and may need a few moments to recover.

•With some very sensitive patients, it may be impossible to evert the lids fully. In these cases, some view of the papillary conjunctiva can be obtained by gently pulling the upper lid margin forwards while the head is tilted back.

2.2Keratometers and autokeratometers

Keratometers measure the curvature of the central cornea over an area of approximately 3–6 mm to determine:

•The radii of curvature.

•The directions of the principal meridians.

•The degree of corneal astigmatism.

•The presence of any corneal distortion.

2.2.1 Types of keratometer (ophthalmometer)

Most keratometers are calibrated using a refractive index for the cornea of 1.3375. There are two types of instrument, depending on the system of doubling

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employed to measure the separation of the mire images. Doubling helps in taking the reading since rapid eye movements would otherwise make the measurement extremely difficult.

PRACTICAL ADVICE

The same cornea measured with more than one instrument can result in a variety of readings because different keratometers employ:

•Different mire separations, so that the area of cornea used for reflection varies.

•Different refractive indices for calibration, so that the same radius could give a variety of surface powers.

Variable doubling

The mires have a fixed separation. The separation of the mire images is found by varying the doubling power. The Bausch & Lomb keratometer, for example, has two variable doubling devices and two sets of fixed mires (Figure 2.6). Both principal meridians can therefore be measured simultaneously and it is called a one position instrument.

Figure 2.6  Bausch & Lomb mires

Fixed doubling

The doubling is fixed for a particular separation of the mire images. It is only by altering the separation of the mires themselves that a reading can be taken. These keratometers are called two position instruments and are based on the Javal–Shiötz design (Figure 2.7).

Figure 2.7  Javal–Schiötz mires

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2.2.2 Focusing the eyepiece

Most keratometers have a graticule incorporated in the eyepiece. This should be focused prior to taking a reading to prevent accommodation giving an inaccurate result.

2.2.3 Taking a measurement

The patient should be comfortably seated, with the forehead positioned firmly against the headrest. Fixation should be accurate, with the other eye occluded. To help line up the optical system and locate the patient’s cornea use:

•The sights attached to the instrument.

•The light from a pen torch directed through the eyepiece, looking for the corneal reflection.

The instrument is positioned initially at a greater distance from the cornea than necessary and slowly moved forward until the mire images come into view and are sharply focused. With a Javal-Schiötz type there should be four images, two of each mire either side of the centre. The middle pair are brought together until lined up in the correct position to take a reading (see Figure 2.7).

Similar images need to be superimposed (Figure 2.8), whereas dissimilar mires are required to touch (see Figure 2.7). In each case, the instrument is rotated to orientate with the first principal meridian. The measurement is recorded and the second principal meridian found by rotating the instrument through 90°.

Figure 2.8  Superimposed images – Zeiss (Oberkochen) mires

PRACTICAL ADVICE

•The principal meridians are not at 90° with an irregular cornea.

•Instruments usually show the corneal radius in both millimetres and dioptres.

•The power and radius scales have their maximum and minimum values at opposite ends.

•The axis of the meridian is usually obtained from an external protractor scale.

•Keratometry readings are expressed as along a particular meridian.

•Patients fitted in the USA generally have their keratometry and contact lenses specified in dioptres. Technically, this is not accurate for the contact lenses since their refractive index is not the same as that of the cornea.

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2.2.4 Extending the range

Radii steeper than the range of the instrument (e.g. in keratoconus) can be obtained by placing a +1.25 D trial lens in front of the keratometer objective. At the flatter end, the range can be similarly extended with a −1.00 D lens. Prior calibration is necessary using steel balls of known radius.3

PRACTICAL ADVICE

•For keratoconus, the Javal-Schiötz instrument is particularly useful, because the range at the steep end extends as far as 5.50 mm.

•Keratometers with circular mires (e.g. Bausch & Lomb) give a

qualitative assessment of corneal distortion.

•Keratometers with circular mires can be used for measurement of NIBUT (see Section 6.3.3).

2.2.5 Topographical keratometer

The keratometer can also be used to explore the paracentral and peripheral areas of the cornea by means of a graduated fixation attachment.4

2.2.6 Autokeratometers

Autokeratometers determine the radii and principal meridians along the visual axis.5 They can also measure peripheral radii at predetermined positions away from the corneal apex (e.g. at 23° and 30° with the Bon Speedy K). Some instruments use computerized image processing to determine the flattest and steepest corneal radii with corresponding meridians and powers.

Autokeratometers achieve the measurement by calculating the distance between the reflected images from light-emitting diodes. The mires are usually circular and designed to reveal any corneal distortion. They also include distance indicators which enable the reading to take place. Despite the speed of measurement, steady fixation by the patient is essential. This is often assisted by a light-emitting diode, but the practitioner should carefully observe the eye during measurement as well as ensuring that the patient maintains a wide palpebral aperture.

Hand-held models (e.g. Nidek KM-500, Alcon Renaissance) allow singlehanded operation and offer benefits in dealing with infants and young children.6 They can also be employed in the operating theatre and where disabilities make conventional instruments impossible to use. The practitioner must be at the same height as the patient. Some models have a levelling device that automatically corrects the measured axis even if the instrument is held up to 15° off axis.

2.3 Corneal topographers

The development of corneal topographers in the late 1980s exerted considerable influence on the diagnosis and treatment of corneal disease. Instruments

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Section ONE Preliminaries

Apical radius 7.68 mm

3.0 mm chord

Figure 2.9  Topographical apical radius compared with keratometry

measure the corneal shape accurately along all meridians by generating a topographical map based on thousands of data points. There are two forms of topographer currently available: reflective or Placido based devices (e.g. Eyesys, Medmont, Scout, Tomey Technology, Zeiss Humphrey Systems) and slit scanning devices (e.g. Orbscan).

It is important to note that the value for the apical radius provided by the topographer differs from any reading obtained by keratometry because these instruments measure the cornea over an area approximately 3 mm around the apex (see Figure 2.9).

Placido based

The assessment of corneal topography is based on a series of annular rings projected onto the cornea and reflected by the tear film to form a virtual image behind the cornea. With the use of reverse ray tracing, the reflecting surface parameters are calculated to capture a two-dimensional image. The Placido rings can be edited for image analysis. Complex algorithms are applied to reconstruct a three-dimensional corneal profile from the two dimensional image captured by the instrument.

Advantages

•Placido based systems are most commonly used in clinical practice.

•Their measurements are accepted as being accurate and repeatable.

Disadvantages

•They measure spherical and aspherical surfaces very accurately but have been shown to produce errors when measuring surfaces with marked curvature changes.7

•Assumptions in formulae are made in relation to where the corneal surface lies in space, which can result in inaccuracies for the corneal periphery and elevation maps.7

•Some inaccuracies have also been reported in the estimation of the location of the corneal apex.8

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•Scarring in the central part of the cornea (e.g. in keratoconus) affects the captured image.

•Some of these errors are minimized with sophisticated software enhancements.

Slit scan topography

An example of slit scan topography is the Orbscan IIz (Bausch & Lomb, Rochester, NY). It uses a calibrated video and scanning slit-beam system to measure the x, y and z locations of several thousand points on the cornea’s anterior and posterior surfaces as well as providing measurement for the anterior chamber. The data derived from the scan are used to construct topographical maps of both corneal surfaces. In addition, pachymetric and keratometric measurements are provided. Altogether 40 slit scan images, each 240 pixels in height, produce up to 9600 measured data points. The sections of the cornea are recorded in two scans, each of which moves across the cornea obtaining 20 frames at the different locations. Information on elevation and corneal curvature is made by direct measurement with a suitable placido disc attachment.7

Advantage

• Accuracy and reproducibility is accepted as good.7,9

Disadvantages

•Measurement is not instantaneous

•Results can be affected by ocular micro movements so that a   tracking system has been incorporated in order to eliminate potential inaccuracies.

Scheimpflug based topography

Another device, which has moved away from the idea of Placido technology, uses a rotating Scheimpflug camera Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany). The relative tilt of the optic section allows the lens and image plane to maintain focus along the sections viewed.

2.3.1 Analysis of corneal topography (Oculus Keratograph)

Contour maps

Topographical data are conveyed in colour-coded contour maps where each colour designates a particular value for dioptric curvature. The red and blue ends of the scale represent respectively steepness and flatness; green and yellow denote curvatures in the mid range. The scale applied can be relative (also called normalized) or absolute (Figure 2.10). A relative scale is usually preferred for very steep or very flat areas, as the dioptric values and colours are related to the average corneal curvature (Figure 2.11).

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Section ONE Preliminaries

Figure 2.10  Absolute

Absolute maps

An absolute map has the same colour corresponding to a specific dioptric  value (or radius in millimetres) regardless of the range of values for a particular cornea.

PRACTICAL ADVICE

•A system with a large range of curvatures uses larger intervals, which may mask clinically significant irregularities.

•Systems with a smaller range may not have this disadvantage. However, for an unusually steep cornea, for example, the map may lack interval definition since all the radii may fall below the steepest radius value on the scale.

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Figure 2.11  Relative

Axial maps

Corneal curvature can be displayed either in an axial or tangential repre­ sentation. The axial map (also called sagittal) gives the true refractive power  at each point of the surface (Figure 2.12). This is helpful in lens radius selection and is therefore more useful in contact lens fitting. Keratometers measure axial  radius.

Tangential maps

Tangential maps give a better representation of the overall corneal shape by giving the true radius at each point of the surface (Figure 2.13). Steep areas therefore appear steeper and flat areas flatter. They are much more sensitive to local changes in the corneal surface and are able to show transitions in curvature with much greater sensitivity than axial maps. The tangential display measures the radius of curvature of the corneal surface along the normal to the

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Figure 2.14  Symmetrical ‘bow-tie’ (with-the-rule)

Shape factor (SF)

The shape factor is a measure of eccentricity and has a value 0.55 for the average cornea. The value is based on the mathematical description of an ellipse where zero represents a circle with no flattening and 1.0 represents the maximum flattening in the periphery.

Elevation maps

Elevation maps are very useful for contact lens fitting. They describe the difference in ‘height’ or ‘elevation’ of the cornea (measured in microns) by superimposing a reference surface. In this way, the most elevated part of the cornea can be located.

Difference maps

Difference maps compare sequential topographical images and display localized changes in corneal contour. They are helpful in monitoring the healing rate of

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Section ONE Preliminaries

Figure 2.15  Asymmetrical ‘bow-tie’

the cornea after surgery and trauma. They also show contact lens-induced changes, for example with orthokeratology (see Chapter 14) and corneal warpage (see Chapter 30).

Fourier analysis

Fourier and Zernicke algorithms provide higher order analysis of corneal topography maps. Fourier analysis represents the two-dimensional conversion of the original image into single concentric rings; the curvature of each ring is displayed in sine curves. The first order designates decentration. Only relative values are provided as the median value is zero. For a normal cornea, the value is rarely higher than 0.45 mm and the axis of decentration is usually towards the horizontal meridian.

Zernicke analysis

Zernicke analysis uses polynomials to describe high order three-dimensional aberrations of circular lens systems where, ideally, the focal point is found at its

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