Ординатура / Офтальмология / Учебные материалы / Orthokeratology Principles and Practice 2004

The choice of lens design and supplier is a very important one. Lens designs have been described in Chapter 4. It is important to use one design and become very familiar with it rather than pick and choose from a range of designs. As important as the design of the orthokeratology lens used is the quality of its manufacture. Inaccuracies as small as 0.02 mm make a measurable difference to the corneal response. Poor-quality surfaces may lead to reduced wetting, more rapid spoilation of the lens, tendency to parameter instability, and reduced comfort and possibly corneal staining. As well as verifying the accuracy of ordered lenses, it is recommended that the practitioner check the fitting set carefully upon receipt and then at regular intervals. If the ordered lens departs unexpectedly from the fitting set lens, then a scenario for frustration and disaster is set!

To arrive at the desired outcome, the ability of the fitter and quality of the lenses used are not the only prerequisites. Good-quality instrumentation is also required.

In terms of instrumentation, the minimum desirable list is as follows:

•Reverse geometry fitting set of lenses made in high-oxygen-permeability material. The manufacturer should be chosen with great care. As a general principle, if the practitioner does not find the conventional RGP lenses second to

none, then the reverse geometry lenses will certainly be disappointing!

•Slit-lamp microscope incorporating high-quality optics, up to 40x magnification, and barrier filter to enhance fluorescence of sodium fluorescein. Extremely careful examination of the cornea is necessary before fitting commences to rule out any subtle dystrophies or corneal anomalies. The use of a yellow (Wratten 12) filter is incredibly useful to enhance the fluorescence of sodium fluorescein in blue light. The filter is placed in front of the microscope objective lens and removes the blue background light and thus enhances the contrast between the fluorescent and nonfluorescent areas. Both corneal staining and lens-fitting patterns are dramatically enhanced using this filter. All the images taken using fluorescein that appear in this book have been captured using a barrier filter.

•Computerized video keratoscope with the following features:

o Manual editing, to remove artefacts. If this is not done the determination of the corneal eccentricity will be grossly inaccurate.

o Accurate and repeatable determination of apical radius and eccentricity.

0)Able to give tangential topographic maps as these more closely indicate shape changes on the cornea than sagittal maps.

oPresentation of difference maps, preferably based on both sagittal and tangential analysis.

Without a difference display it is difficult to know what the topographical change actually is and to categorize it as "bull's eye," "smiley face," or "central island."

•Test chart. This should incorporate several lines corresponding to the higher acuities (6/6 or 20/20, etc.) to prevent memorization of the chart. Ideally, log minimum angle of resolution (logMAR) charts in high and low contrast to allow data analysis and evaluate acuity at different contrasts. Illumination should be controlled during use, ideally by prior measurement using a light meter and elimination of daylight from the examination room. Alternatively, projector or computer-generated charts are very useful in that the presentation of only one line at a time helps reduce memorization.

•Focimeter. Generally, lenses for night therapy are left plano-powered. This has the advantage of minimizing thickness and therefore maximizing oxygen permeability. However, patients on day therapy will require effective refractive correction. The optical quality and accuracy of the power are obviously then as important as with any RGP lens.

•Radiuscope. It is vital for the success of any orthokeratology treatment that lenses are used that have been manufactured to a very high degree of accuracy using state-of-the-art computerized lathes. It is possible to see significant differences in treatment outcome using lenses that differ in base curve by as little as 0.02 mm. Given that the International Standards Organization (ISO) tolerance for the manufacture of RGP lenses is only 0.05 mrn, then the quality standards of the laboratory itself become all-important. It is essential that the practitioner check the lens base curve before issuing the lens to the patient. In addition, trial lens parameters must be carefully evaluated and lenses may need to be verified again at aftercare visits.

•Diameter gage. Just as small errors in radius cause unexpected outcomes in orthokeratology, errors in diameter of the optic zone produce similar alterations to the expected outcome. Whilst errors of 0.03 in radius are certainly significant, larger errors, of the order of 0.20 mm, are required in total diameter or

optic zone diameter to cause alterations to treatment outcome.

•Thickness gage. As stated previously, in night therapy, the lenses used are generally planopowered. This has the advantage of reducing the overall oxygen transmissibility by virtue of the even thickness profile. If the practitioner wishes to check the manufactured lens thickness, then a thickness gage is useful. Rigidity of the lens is important and therefore the minimum central thickness is of the order of 0.16mm.

•Appropriate fitting software. A variety of programs are available, as set out in Chapter 8.

FACTORS AFFECTING PATIENT SUITABILITY FOR ORTHOKERATOLOGY

There are several key considerations to address when advising prospective patients as to their suitability for orthokeratology: refractive, anatomical, occupational and recreational, physiological and psychological. These are set out below. However, these considerations should be seen as augmenting the normal contact lens preliminary examination.

Refractive

Visual acuity improvement

The studies show a mean improvement in highcontrast visual acuity (VA) of approximately 5.5 lines of Snellen acuity. Thus, if a patient presents with unaided VA of 6/60 (20/200), the reduction in myopia is likely to be associated with an improvement in VA to 6/9 (20/30). The patient can then be shown the residual error and decide whether the improvement in vision is acceptable. However, it is wise to remember that the posttreatment VA is usually better than that predicted by the refractive error, and it not uncommon for patients who have a 1.00 0 residual error still to see 6/6 (20/20) comfortably (Lui & Edwards 2000, Cho et al 2002). Low-contrast improvements tend to be less than those of high-contrast vision, and are more difficult to demonstrate. As far as low-contrast vision is concerned, other factors, such as pupil diameter in low illumination, treat-

ment zone diameter, and refractive change come into the equation, and will be discussed in a later section.

Myopia

At present, single and double reverse geometry lens designs can only effectively eliminate mild to moderate amounts of myopia and are currently not effective at reducing hypermetropia. The use of high-eccentricity aspheric back surface RGP lenses fitted to give vaulting over the corneal apex may produce some hypermetropic or thokeratology, but such designs have not been validated and are outside the scope of this book. Effective reduction is taken as being a stable reduction of refractive error, which does not compromise best-corrected VA.

Topographic sagittal change maps will indicate the quality of the refractive change in the cornea (see Ch. 2). An optimal result using current lens designs is a well-centered zone of flattening in a corneal topography difference map that is approximately 4-5 mm across (Fig. 5.2). If this is accepted as the ideal result then, at present, the mean documented refractive change is of the order of 2.25 D, with a standard deviation of approximately 0.75 D (Mountford 1997b, 1998, EI Hage et al 1999, Swarbrick & Alharbi 2001, Lui & Edwards 2000, Nichols et al 2000).

However, refractive change is also determined by the initial corneal eccentricity, with the cornea changing from a prolate towards either a spherical or oblate surface (Mountford 1997a, Mountford & Noack 1998, EI Hage et al 1999). The higher the initial eccentricity, the greater the refractive change possible (see Ch. 6). This finding is held in some dispute (Day et al 1997), but to date there have been no published reports to substantiate the claims.

Both the Mountford-Noack model and Day kappa function provide a means of determining the refractive change possible from prefit corneal shape data. Also, both methods are in close agreement with one another (see Ch. 8). Using the Mountford-Noack model, Swarbrick & Alharbi (2001) found a mean final refraction of +0.02

± 0.18 D from that predicted. If the prefit corneal data are used to determine the expected refractive

change for an individual, the degree of residual error can be demonstrated, and the patient given the choice to proceed. The ideal cases are those where the predicted change is greater than that required, as a small (0.50 D) degree of overcorrection can be incorporated into the treatment in order that the patient can be ultimately moved to wear every second night.

Mountford (1998) found a mean regression of 0.38 D / day, whilst Swarbrick & Alharbi (2001) and Nichols et al (2000) found approximately 0.25 D regression by day 10 of their studies. If a patient is overcorrected by 0.50 D in the morning, the regression at the end of day 1 will mean that he or she is still approximately 0.25 D overcorrected. In this situation, there is no need for the patient to wear the lens that night, as the vision and refraction are stable. This should drop to approximately 0.25 D undercorrected by the end of the second day, with some associated blur, necessitating lens wear. However, if the predicted refractive change possible is less than that required, the patient will always be undercorrected by that amount, and nightly lens wear will be required to maintain the change. The ability to limit wear to every second night has advantages in that the already low risks associated with overnight wear are probably further reduced, and the patient is more independent of the lenses.

There are patients who may feel that a reduction of 2.00 D for 3.00 D of myopia is acceptable, but others for whom the outcome may be unacceptable due to poor unaided VA. The patient should always be shown the likely improvement, leaving the final decision as to whether to proceed to the individual. For many western practitioners, the major aim of orthokeratology is a total or near-total reduction of the myopia so that the patient can experience clear unaided vision without lens wear during the day. However, in Asia, the main use of orthokeratology is to reduce the degree of myopia. By showing the patient what is possible prior to treatment, expectations can be kept realistic, and unhappy outcomes avoided.

As a simple general rule, the following formula can be used to determine the refractive change possible. This is considered in more detail below.

Eccentricity / 0.21 = Refractive change

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There have been anecdotal reports of much greater refractive changes occurring, but to date no studies have been published. Cho et al (2002) reviewed the results of 59 patients fitted with reverse geometry lenses (RGLs) from an orthokeratology practice in Hong Kong. The mean prefit refractive error was -3.97 ± 2.28 D. Of the 49 patients who wore their lenses on an overnight schedule only, 44 (89%) had unaided vision of 6/6. Four subjects had less than 6/9, two having 6/36 in one eye and 6/6 in the other, one had 6/12 in both eyes, whilst the remaining subject had 6/12 in the right eye and 6/6 in the left eye. There was no statistically significant correlation between prefit refractive error and postwear unaided vision (Pearson correlation -0.21 < r < - 0.05, P > 0.14).

Reim has kindly made available, prior to publication, data from a long-term study of myopia reduction with the Dreimlens. Four hundred and five subjects with refractive errors between - 0.50 0 and - 4.00 0 were fitted and wore the lenses purely on an overnight-wear schedule. The relationship between the initial refraction and the mean difference between the original and achieved change is shown in Figure 5.3. In most cases, the mean error tends towards mild overcorrection, except for the 4.000 group, where mild undercorrection is the norm. Note

Difference between refraction and mean errorn=409

Figure 5.3 The mean error of the achieved refractive change versus the aimed-for change in a large sample group. Note that, at both extremes, the low and high myopes were undercorrected. Courtesy ofTom Reim.

also the range of results, which tend to indicate poorer predictability for the higher attempted changes. Reim states that the maximum attempted change should be 4.00 D.

The neophyte orthokeratologist is strongly advised to gain experience by correcting the lower degrees of myopia first, and then proceeding to the higher errors. Claims of consistently high refractive change should be treated with some suspicion until independent proof is forthcoming. There are practical limitations to the refractive changes possible with orthokeratology. If it were possible to correct 6.00 0 of myopia, the treatment zone (TxZ) size would have to be reduced and would be less than 3.00 mm in diameter, leading to poor low-contrast vision and haloes. The aim should therefore be quality of unaided vision and corneal optics rather than large refractive changes.

Astigmatism

RGL designs do not appear to reduce against-the- rule or oblique astigmatism and may, in fact, increase it (Mountford 1997a). Therefore, a significant degree of this form of astigmatism is a relative contraindication for orthokeratology using these designs. With-the-rule astigmatism is reduced by the use of RGLs and experience to date indicates that a modest reduction will occur. Soni & Horner (1993) found a 60% reduction in the amount of astigmatism, whilst Mountford & Pesudovs (2002) found that a reduction in prefitting astigmatism of approximately 50% occurred providing the axis was with ± 30° of the horizontal. An upper limit of approximately 1.500 for the total change has been reported (Mountford & Pesudors 2002).

Needless to say, orthokeratology can only reduce corneal astigmatism. The general principles that apply to the calculation of residual astigmatism with rigid lens fitting also apply to orthokeratology, with the added complication of a 50% reduction in corneal astigmatism. It is vital therefore to ascertain the degree of initial corneal astigmatism to the total amount in the refraction, and the influence that any lenticular astigmatism will have on the final outcome.

The following examples are used to point out the various factors that need to be considered.

1.Consider a refraction of -2.00/ -1.50 x 180. The keratometry shows 1.50 0 corneal astigmatism at the same axis. Therefore, all the astigmatism is corneal and the initial astigmatism will be reduced by approximately 50% to 0.75D@ 180.

2.If the same refraction is encountered, and keratometry shows the cornea is spherical, the residual astigmatism will be a minimum of 1.500 @ 180. In practice, the horizontal meridian is always flattened to a greater extent than the vertical, with the possibility of increased with-the-rule astigmatism to 2.00 D. This patient is a poor candidate for orthokeratology.

Figure 5.4 (A) The effect of orthokeratology on "wedge" or limbus-to- limbusastigmatism. Note that in the prefit plot on the right the astigmatism extends to the periphery. The postwear plot shows greaterflattening of the flat meridian compared to the steep, leading to an overall increase in astigmatism. These cases are not suitable for current orthokeratology strategies.

(8) Anothercase of limbus- to-limbus astigmatism. In this instance, the lens decentered superiorly, resulting in induced irregularastigmatism. This type of corneal topography is currently contraindicated for orthokeratology.

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3.Assume the refraction is now -2.00/-0.50 x 180. If keratometry shows 1.50 0 of corneal astigmatism at 180°, it follows that the likely outcome will be an induced against-the-rule astigmatism of approximately 1.00 D. The difference between the initial spectacle astigmatism and the corneal astigmatism is due to lenticular astigmatism, which is unaffected by orthokeratology.

4.If the refraction is -0.50/-1.50 x 180 and keratometry shows that the astigmatism is all corneal, then in theory the final Rx could be + 0.50/-0.75 x 180. This, however, is rarely the case, as the low degree of initial myopia will nearly always be overcorrected, leaving the patient unacceptably hypermetropic. In general, the astigmatic component of the refraction should never exceed the sphere if current-generation RGLs are to be fitted.

Additionally, the topographical appearance of the astigmatism is of vital importance. Clinical experience indicates that there are three distinct types of corneal topographical astigmatism: central, limbus-to-limbus, and irregular. Currently both limbus-to-limbus and irregular astigmatism are

almost impossible to treat effectively with reverse geometry designs. In both of the above cases, the lenses consistently decenter superiorly, with either a smiley-face topography (and increased with-the-rule astigmatism) or, in the case of limbus-to-limbus astigmatism, greater flattening of the superior cornea than the inferior, and induced irregular astigmatism (Fig. 5.4).

Central astigmatism responds more favorably, but the 50% reduction in astigmatism occurs only over the central 2.00 mm chord (Fig. 5.5). In most cases of relatively high central astigmatism, the TxZ diameter difference between the steep and flat meridian is quite marked, and the lens does not succeed in "pushing" the astigmatism out past the pupil zone. This can result in an increase in with-the-rule astigmatism, as the flat meridian undergoes a greater change than the steep meridian, leading to a reduction in the myopic error but an increase in the astigmatism (Fig. 5.6). In general, only the classical "bowtie" type central astigmatism responds well to orthokeratology, with aberrant forms responding poorly. The current spherically based lens designs do not work well or predictably for astigmatism. In future, however, toric designs may lead to

Figure 5.5 Simple central bowtie astigmatism and an ideal response to orthokeratology. Note that the steeper meridian is flattened to a greater extent than the flatter, allowing for correction of the astigmatism. Also note that the major change occurs within the central 2.00 mm chord, whilst the keratometer chord

(3.00 mm) shows only a

0.70 D reduction in astigmatism.

B

Figure 5.6 Axial (top)and tangential (bottom) postwear topography on a patient with high central astigmatism. Note the "island"of irregular astigmatism inferiorly. This has a detrimental effect on unaided visual acuity, and, in effect, is an increase in prefit astigmatism.

greater success. In the meantime, it is vital that not only the degree of refractive astigmatism compared to corneal astigmatism be properly assessed prior to fitting, but also the type of topographical astigmatism.

It therefore follows that, in general, to be considered suitable patients should have:

•low to moderate myopia (less than about 4.00 D if full correction is expected)

•mild « 1.50 DC) with-the-rule astigmatism or no astigmatism

•no significant against-the-rule astigmatism or oblique astigmatism.

Patients with unstable refractiveerrors

There will be instances where the refractive error measured by the optometrist will not be representative of the true underlying error and a patient must be advised accordingly. Such instances may include:

•Long-term polymethyl methacrylate (PMMA) wearers. In many cases, such patients have either clinically significant corneal distortion or unintentional orthokeratology. Good alignment fitting of PMMA lenses results in an approximate reduction in myopia of 0.25-1.50 D on rising following daytime lens wear (Saks 1966, Rengstorff 1970a). In addition, there may be significant variation in refraction during the day, with myopia typically being lowest on rising and increasing throughout the day (Rengstroff 1970b). Furthermore, removal of PMMA lenses may result in a significant variation in the degree of myopia and astigmatism for up to approximately 3 weeks (Rengstorff 1965, Harris et aI1973), with least myopia being recorded within 2-3 days of removal (Rengstorff 1967). The average decrease is 1.32 D followed by increases for up to 3 weeks back to the level found on lens removal (Rengstorff 1967). Since there may well be corneal distortion as well as this refractive variation, it is wise to avoid performing orthokeratology on an existing PMMA wearer. If it is considered desirable to do so, then the patient should first be refitted with conventional RGP lenses and the cornea allowed to normalize for several months. Sequential topography measurements should be made until these appear stable. Since the cornea may never completely return to its prefitting state in these patients, it is probably best to avoid long-term PMMA wearers as candidates for orthokeratology.

•Existing RGP wearers. Even the best-fitted RGP lenses seem to produce some degree of sphericalization of the cornea, particularly in extended wear (Rivera & Polse 1991, Young & Port 1992). Since, as is shown below, the corneal eccentricity is a major factor to be taken into account when predicting the outcome of a period of orthokeratology treatment, it is best to allow the cornea to normalize

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completely by removing the RGP lenses until sequential corneal maps show no significant change. This may take between 3 and 4 weeks. Patients wearing spherical RGP lenses on corneas with moderate astigmatism may, in addition, show a reduction in the corneal cylinder on removal of the lens. This may lead a practitioner to attempt orthokeratology when it is not really an appropriate treatment for the patient. Previous clinical records containing prefitting data, as well as removal of lenses for a period, may be helpful when advising the patient. Given the possible variability in the final refractive error and the time that it will take for the cornea to normalize, it is probably better to avoid existing RGP wearers, particularly in the early stages of one's experience with the technique.

•Existing wearers of thick, low-water soft lenses. Although evidence is sparse in the literature, clinical experience suggests that such patients can also have some degree of corneal warpage. Once again, this needs to be monitored by sequential topography, until normal corneal topography returns. Anecdotally, this appears to occur quicker than with rigid lenses. In addition, the presence of chronic

edema can lead to the phenomenon of "myopic creep" where the refractive error continues to increase in adult patients, when it should have stabilized (Dumbleton et al 1999). Subsequent refitting of the patient with lenses having a higher oxygen transmissibility will lead to a reduction in myopia (Dumbleton et al 1999). Once again, it is probably wise to ensure that the cornea has completely normalized prior to embarking on a course of orthokeratology.

•Spasm of accommodation. Whilst not a particularly common clinical entity, this must always be considered when the refractive endpoint is variable and where there is not the usual close correlation between objective and subjective findings. More appropriate treatment, such as binocular vision investigation and a reading addition, should be instigated, rather than attempt orthokeratology.

Anatomical

Prefitting topographical assessment

Accurate corneal topography measurement is a vital part of the prefitting assessment. Topography has three main uses in orthokeratology:

Figure 5.7 The map on the right is the initial prefitting mapand is inaccurate. The mapon the left was taken 2 weeks after cessation of lens weardue to an unacceptable result. The difference in apical poweris 1.60 D, but the difference in sag was greaterthan 50 ILm.

1.It provides corneal data to facilitate lens design.

2.It allows comparative analysis of the effects the lens has on corneal shape in order to optimize the lens fit and refractive outcome.

3.It acts as a permanent record of a course of treatment (Mountford et al 2002).

The single most important factor is the reliability and repeatability of the prefit topography. This not only gives the information required in order to fit the lens, but also the primary topography plot from which all postwear assessments of the lens effects will be judged. If the initial maps are inaccurate, the subsequent decisions on remedial actions will be invalid, leading to unnecessary lens reorders and more visits.

Figure 5.7 shows an invalid prefit map. The patient had three unsuccessful lens fits to the left eye, shown in the figure, compared to none for the right. Lens wear was ceased for 2 weeks and the topography repeated. The second map (lefthand side) gave totally different lens parameters to the initial lens. In the initial fitting, the lens design was based on an Ro of 7.39 mm and a corneal sag of 1.6310 mm. The second reading gave values for Ro and sag as 7.57 mm and

PATIENT SELECTION AND PRELIMINARY EXAMINATION 119

1.5794 mm respectively. The corneal response to the first trial is shown in Figure 5.8. It appears to be an ideal response, with a 2.00 0 refractive change. However, the VA was 6/9 with an overrefraction of -1.00. The important thing to note is that the whole surface is flatter than the original cornea. However, the peripheral cornea is little affected by orthokeratology. So in this case, the initial map was invalid, but the postwear map was valid, giving an erroneous difference map.

The repeated reading done a few weeks later is approximately 1.50 0 flatter than the original. The sag difference was 51.6 urn less than the original sag. Recalculation of the lens gave an ideal response and is shown in Figure 5.9.

Most corneal topographers derive elevation values first, and then construct the curvature values using their specific reconstruction algorithms. The crucial aspect of the whole process is the instrument's ability to determine the corneal apex. The topography plot shown in Figure 5.7 is a prime example of an inaccurate detection of the corneal apex. This can occur due to either eyelash interference or patient movement between the time taken for the instrument to detect the apex and then capture the image. The errors can be

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quite large, as in the example shown. Indeed, once topography plots are taken, comparisons between the rightand left-eye results should be made. Both eyes in patients free of pathology should always be very similar in terms of Ro and e-values. If the discrepancies are large (» 0.50 D), the readings should be retaken.

In order to maximize the rate of first-fit success, repeated readings in order to assess the mean values for Ro and elevation and the standard deviation of error of the instrument are an invaluable tool, and should be considered an integral part of the prefitting assessment.

The technique used by the authors is set out in Chapter 6. Four repeated readings are taken and compared. Any clearly anomalous measurement is rejected and the remaining readings averaged. Reliance on one assessment of the corneal topography as a basis for designing the lens will lead to a low rate of first-fit success.

There are distinct corneal shapes that simply do not respond to orthokeratology. These are:

1.limbus-to-limbus or "wedge" astigmatism, as described above

2.off-center corneal apex (Fig. 5.10).

As stated previously, limbus-to-limbus astigmatism usually leads to an increase in astigmatism. In those cases where the corneal apex is decentered to a significant degree, the same problems that occur with some patients with deep-set eyes will arise: chronic smiley-face postwear plots and haloes and flare. Until better lens designs are developed for these cases, they should be considered unsuitable for orthokeratology.

The final factors to consider with the prefit topography are the quality of the tear film and the stability of the patient during the capture process. A lot of time and expense can be saved by including good topography information prior to fitting. Clearly a patient on whom one cannot obtain reliable topographic data is a poor patient to select for orthokeratology.

Eccentricity

The key anatomical consideration that arises from the prefitting topography (other than the measurement of the corneal apex) is the corneal eccentricity. Mountford (l997b) has shown that the corneal eccentricity is intimately linked to the outcome of accelerated orthokeratology using

Figure 5.9 The same eye as that in Figure 5.8 following an overnight trial based on the second set of corneal data. The change in apical corneal power(2.20 DJ was equal to the measured refractive change of 2.25 D. The peripheral cornea is not now flattened by the lens.