corresponds to approximately 0.25 D of refractive error. Therefore, a myope of 1.00 DS with a habitual VA of 6/9 (three-line loss in VA from 6/4.5 or 20/15) is likely to need a change in refractive correction of about 0.75 DS and an updated prescription to approximately 1.75 DS. An older hyperope of 1.00 DS with a habitual VA of 6/9 (two-line loss in VA from 6/6 or 20/20) is likely to need a change in refractive correction of 0.50 DS and an updated prescription to approximately1.50 DS. Astigmatism in adults changes with age from with-the-rule in young adults to against-the- rule in older patients. However, these changes in astigmatism are negligible over the typical 1–3- year period between eye examinations, so that habitual distance VA reductions in spectacle wearing myopic astigmats and older hyperopic astigmats indicate the increase in spherical power required. Therefore, a myope of 1.00/ 0.50 180 with a habitual VA of 6/12 or 20/40 (four-line VA loss from 6/4.5 or 20/15) is likely to need a change in refractive correction of 1.00 DS and an updated prescription to approximately 2.00/ 0.50 180. Similarly, an older hyperope of 1.50/ 0.75 90 with a habitual VA of 6/12 or 20/40 (three-line loss from 6/6 or 20/20) is likely to need a change in refractive correction of 0.75 DS and an updated prescription to approximately 2.25/ 0.75 90.

4.2.4 Near VA

An estimate of myopia can be determined from the patient’s far point. Ask the patient to remove any spectacles and bring in the near VA card until they can just see it. The far point provides an estimate of the mean sphere refractive correction. For example, patients with far points of 33 cm, 25 cm and 20 cm have mean sphere refractive corrections of approximately 3.00 DS, 4.00 DS and 5.00 DS respectively.

4.3 KERATOMETRY

4.3.1 Corneal topography

The topography of the anterior corneal surface can be assessed using computerised videokeratoscopes and the curvature of the corneal cap can be estimated using keratometry. Videokeratoscopes illuminate the cornea with a Placido disc series of concentric rings and the reflections from the cornea are captured and analysed. The results are displayed as colour coded topographic maps, with areas with the same radius of curvature displayed in the same colour. Steep areas of the cornea are typically shown as red, average areas as yellow and green and flat areas as blue. Different shades of these colours are used to provide smaller step sizes. Keratoscopy is mainly used:

■Before and after refractive surgery. Prior to surgery, it is used to screen for otherwise sub-clinical disorders such as early keratoconus. Postoperatively, it is used to determine the outcome of surgery, particularly in relation to the centration and symmetry of the ablation zone. When patients have postoperative irregular astigmatism or asymmetry, keratoscopy is an essential part of the ‘refinement’ process.

■For the early detection and better management of keratoconus and other corneal disorders.

■For fitting contact lenses, particularly to abnormal corneas, such as keratoconic and post-refractive surgery (although here it is working to low accuracy). The limited range of parameter variation (radius and diameter) of soft lenses means that the use of videokeratoscopes for the determination of fit of these lenses is an overkill.

■To assess contact lens induced corneal change. Keratoscopes are more sensitive to such changes than keratometry because they assess most of the corneal surface whereas a keratometer only measures a single radius of curvature around 1.5 mm off the instrument axis.

This is an optical instrument that measures the radius of curvature of the anterior corneal surface along the two principal meridians.

■In the management of patient postpenetrating keratoplasty.

■As part of orthokeratology.

There are many different systems available that use different approaches and there is insufficient space to describe them in this text. Suffice to say that all of them require an accurately focused image before image acquisition by the computer. The algorithms for image analysis vary with the manufacturer and rely on assumptions that may or may not be valid. The main differences in the various instruments are associated with how an accurate focus is achieved and how the data are finally displayed.

The procedure that is much more commonly used to measure the anterior curvature of the cornea, keratometry, will be described. In contact lens fitting, keratometer measurements are used to indicate the corneal curvature, thus allowing an appropriate contact lens curvature to be deduced for the first trial contact lens. In contact lens aftercare, keratometer readings can be used to monitor lens-induced corneal surface changes. For example, distortion of the keratometer mires can indicate unwanted mechanical action of the lens on the cornea, and a steepening cornea can indicate oedema. The keratometer can also be used to measure the base curve of rigid contact lenses, and provide an indication of tear film quality. Non-invasive tear break-up time can be measured as the number of seconds between a blink and the first indication of distortion of the mire images (section 6.7). Keratometry can also be used to detect keratoconus and other disorders of the cornea that change its shape, but it is much less sensitive to these changes than videokeratoscopy.

horizontal mires are imaged at the same time and in higher degrees of corneal toricity this can mean the vertical mire is not in focus at the same focusing position as the horizontal mire. Of course, if required the keratometer can always be turned to the second principal meridian and the second reading taken in a ‘two-position’ style. Lastly, the Bausch and Lomb keratometer tends to use a shorter working distance than other instruments and this can lead to larger measurement errors.

Two position variable mire keratometers include the Javal Schiotz and copies. Measurements of corneal radii are achieved by the physical movement of the mires along an arc. The main criticism of this instrument is that unlike the variable doubling keratometer, where measurements are made on a linear scale, radii that fall at the extreme ends of the arc are non-linear and this can lead to measurement inaccuracy. Although two-position variable doubling instruments do require a second adjustment of the instrument to find and measure the second principal meridian, they tend to be more accurate because of a longer working distance. Perhaps the best instrument is the two-position variable doubling keratometer, especially if it is of telecentric design. The advantage of telecentricity is that focusing of the eyepiece of the telescope is not necessary and therefore there are no inaccuracies due to focusing errors. Unfortunately, these telecentric instruments can be expensive and are not commonly encountered.

One-position variable doubling keratometers (Bausch and Lomb type) or two-position variable doubling keratometers or copies of them are widely used in optometric practice. The one-posi- tion instrument is said to be quicker to use since once aligned on one of the corneal principal meridians, both principal meridians can be measured without further adjustment, hence the ‘one-posi- tion’ name. Disadvantages include that the instrument design assumes that the principal meridians lie at exactly 90° to each other, which is not always the case, especially when the cornea has been subjected to contact lens wear. Also, since it is a oneposition instrument, both the vertical and

1.Seat the patient comfortably in front of the keratometer, and ask them to remove any spectacles. Sit opposite the patient, across the instrument table, and dim the room lighting.

2.Explain the procedure to the patient: ‘I am going to measure the shape of the front of your eye/cornea’. You may add: ‘. . . so that I will know which contact lens to fit’ or ‘. . .

so that I can tell whether the contact lens is changing the shape of your cornea.’

3.Adjust the eyepiece of the instrument by directing the telescope to a distant object

(such as the cubicle wall), turning the eyepiece anti-clockwise as far as it will go and then turning the eyepiece clockwise until the black cross hair just comes into sharp focus.

4.Adjust the height of the patient’s chair and the instrument to a comfortable position for both you and the patient. Ask the patient to lean forward and place their chin in the chin rest and forehead against the headrest. Occlude the eye not being tested by swinging the instrument’s occluder into place. Then adjust the chin rest so that the outer canthus aligns with the headrest marker.

5.Ask the patient to look at the image of their own eye in the centre of the instrument and to open the eye wide after a full blink. If a high refractive error prevents the patient seeing their own eye, then ask them to look down the centre of the instrument. Make vertical adjustments of the instrument if the patient is unable to see into the centre.

6.Align the instrument so that the lower right mire image is centred on the crosshairs, and lock the instrument into place.

7.Adjust the focusing of the instrument by turning the focusing knob until the mires are clear and the lower right mire is no longer doubled.

8.If you cannot focus the mires, check to see that the patient’s head is firmly against the headrest. If the patient is in the correct position, and the mires are still out of focus, adjust the position of the headrest forward or backward while continuing to focus until the mires are in focus.

9.Measure the principal meridian that is closest to the horizontal first. Rotate the instrument so that the plus signs are set ‘in step’ (Fig. 4.1b). This ensures that the instrument is aligned precisely on a principal meridian. Use one hand to adjust the focusing knob to ensure a single, clear

Fig. 4.1 Alignment of the mires on a Bausch and Lomb keratometer.

(a) The view when the mires are off the principal meridians. (b) The view when the mires are on the principal meridians.

(c) The view when the plus and minus signs are overlapping to measure the ‘horizontal’ and ‘vertical’ radii of curvature.

plus sign, and adjust the horizontal alignment wheel to superimpose the two plus signs with the other hand (Fig. 4.1c). Note that you will need to constantly adjust the instrument position to maintain image focus, so keratometric measurements are always a two-handed operation. Note the radius of curvature (or dioptric power) and orientation of this meridian.

10.Measure the second principal meridian, which is theoretically 90° to the primary one. Adjust the focusing knob to give the best focus for the minus signs and then adjust the vertical alignment wheel until the minus signs are superimposed (Fig. 4.1c). Note the radius of curvature (or dioptric power) and orientation of this meridian. On a toric cornea, the plus signs will be out of focus and not superimposed, but this does not matter as you have completed your measurement of the near horizontal principal meridian.

11.Repeat the measurements on the other eye.

4.3.4Procedure for two-position variable doubling type keratometer

1.Set up the patient and the instrument as described in steps 1–8 above.

2.Move the telescope forward by adjusting the focusing knob appropriately. You may need to make minor adjustments both horizontally and vertically to centre the mire images and achieve a view as depicted in Figure 4.2. If the blocks and staircase are in step (Fig. 4.2a), then the orientation of the instrument arc is aligned to one of the two principal meridians and you can now proceed to step 4.

3.If the picture you see is similar to Figure 4.2c, where the blocks and staircase are out of step, then the angle of the instrument arc is not aligned along a principal meridian. Move the arc slowly until the staircase and block mires are in step and are able to be brought into contact by turning the knurled knob situated below the arc as in Figure 4.2d.

4.Ask the patient to blink and then keep their eyes as wide open as possible. Turn the knurled knob situated below the arc until the staircase and block mires are just touching. You must simultaneously adjust the instrument position to maintain focus of the mire images. If you turn the knob too much and the mires overlap, a white area of overlap will be seen. Adjust the position of the mires until they are just touching with no overlap. If the hair wire does not cross through the middle of the touching mires, make final horizontal and vertical adjustments to achieve this.

5.Read off the angle of the arc from the degree scale of the instrument and the radius of curvature along this meridian from the mm scale.

6.Turn the arc through 90° and make adjustments as in steps 4 and 5 to achieve a picture similar to 4.13b or 4.13e. Note the reading off the scales. This is the corneal radius along the other meridian.

4.3.5 Recording

The results can be recorded with the radius of curvature of the horizontal meridian first followed by the vertical as follows:

R 7.75 @ 175 / 7.60 @ 85

L 7.70 @ 180 / 7.60 @ 90.

The @ nomenclature can be replaced by ‘along’. A degree sign (°) should not be used after the axis direction. It is possible for the ° to be confused with a 0, so that 15 degrees could become 150 degrees. If the mires are distorted, this must be recorded.

Alternatively, the results can be recorded in dioptres, in which case the amount of corneal astigmatism is usually calculated and recorded. Note that this is the total corneal astigmatism due to the anterior and posterior corneal surfaces and is usually derived by assuming a corneal refractive index of 1.3375. The difference between the two powers equals the approximate total corneal astigmatism and the meridian with the lower power corresponds to the corneal cylinder axis.

OD: 42.00 @ 175 / 43.75 @ 85, 1.75 175, mires distorted

OS: 43.50 @ 180 / 44.25 @ 90, 0.75 180, mires clear.

4.3.6 Interpretation

The power of the anterior corneal surface (Fc) is estimated from the radii measurements (r) using the equation Fc (n 1)/r. A small radius means

a steep corneal surface, which is more powerful and more myopic (or less hyperopic). Larger radii mean flatter surfaces, which are less powerful and more hyperopic (or less myopic). The refractive index (n) of the cornea is about 1.376, but most keratometers use a value for n of 1.3375. The lower value for n is intended to compensate for the negative power of the posterior corneal surface. It is assumed that the posterior surface reduces the overall corneal power by about 10%. This also assumes that the two surfaces have the same proportion of astigmatism. Other factors also lead to slight errors in keratometry readings: keratometers assume that the cornea is spherical (most are elliptical) and that the visual axis runs through the corneal apex, which it usually does not.

The anterior radii of curvature of the cornea are usually between 7.25 mm and 8.50 mm, with myopes having steeper (smaller) radii and hyperopes having flatter (larger) radii. Dioptric powers range between 46.50 D and 40.00 D, and the estimated corneal astigmatism is usually less than 2.00 D. It is most common to find the flattest corneal meridian lying near the horizontal (with-the-rule astigmatism, WTR) in younger patients. This is likely due to lid tension steepening the vertical meridian. Little change in curvature occurs between the mid-teens and the late 40s. Over the age of 40 there is a significant shift towards against-the-rule astigmatism (the flattest meridian is along the vertical, ATR), likely due to relaxed lid tension. The corneal astigmatism is called oblique when the principal meridians are between 30° and 60° and 120° and 150°. Unusually steep readings with irregular principal meridians can be indicative of keratoconus. Large changes in the degree of astigmatism within a short time can be indicative of keratoconus, lid neoplasms, pterygium, or a chalazion. Large changes in spectacle astigmatism without corneal astigmatic changes in the elderly are likely to be due to cortical cataract. Keratometer readings can also be used to help indicate whether ametropia is refractive or axial. For example, a patient with increasing myopia but no change in keratometry readings probably has axial myopia. An anisometrope with different keratometry readings probably has refractive anisometropia, while an anisometrope with similar keratometry readings probably has axial anisometropia.

The amount of corneal astigmatism determined using keratometry can be used to estimate spectacle