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

254 ORTHOKERATOLOGY

advice. During consulting hours this will be by telephoning the practice. In the event that the orthokeratology practitioner is not in attendance, most queries can be dealt with by any contact lens practitioner or trained member of staff. However, such nonqualified staff need to be aware of the danger signs associated with contact lens-induced symptoms. These are no different from any other type of contact lens.

These are:

•pain

•discharge

•redness

•poor vision.

Although in most cases a "false alert" will be raised, avoidance of microbial keratitis should be uppermost in any contact lens practitioner's mind. Therefore practitioners must ensure that they are contactable wherever they might be during daylight hours. This may be via mobile telephone or pager. If overseas, a colleague experienced in contact lens management (and ideally orthokeratology) should be asked to provide cover.

When the situation arises outside consulting hours, then patients should have an emergency number to seek guidance. Again, this might be a pager or mobile telephone number. Naturally clear advice in what constitutes an emergency is prudent to avoid unnecessary calls. Increasing pain, blurring of vision, redness, or discharge following lens removal are the signs the patient must be aware of.

Only if contact with the practitioner is impossible should the patient have to resort to attending an emergency department. It is recommended that as well as getting patients to sign a statement of informed consent (detailed in Ch. 5), they be given a card indicating that they wear rigid contact lenses and bearing an emergency out-of- hours number. In several years of issuing patients with such information, the authors have been troubled on only very few occasions.

PROBLEM-SOLVING

Orthokeratology should not be considered a "routine" optometric procedure. The time to achieve the correct fitting response requires

considerable experience in the interpretation of sodium fluorescein patterns and a reasonable understanding of corneal topographic changes.

However, of greater importance is the management of fitting complications. Although many of these complications are similar to those observed with conventionally designed RGP lenses, the fact that the design is radically different means that modification of lens parameters requires a different approach. One key difference in both fitting and modification of lens parameters is the manner in which one considers the fit of the lens. Rather than simply think of the lens as being flat or steep with regard to the BOZR, practitioners should consider the relationship between the sag of the lens and the sag of the cornea. The principal reason for using this approach is because the fit of the lens is not primarily controlled by the BOZR as with conventional RGP lenses; the secondary and sometimes tertiary reverse curves also have a significant impact on the fit of the lens.

Previous chapters have dealt with strategies designed to minimize the risk of adverse outcomes with respect to patient selection and trial lens fitting. However, as with all forms of contact lens practice, problems will occur after the lenses have been dispensed. In orthokeratology, these take the form of corneal and conjunctival staining, dimple veiling, contact lens papillary conjunctivitis (CLPC), lack or loss of refractive response, and unwanted corneal distortion and poor visual acuity.

The single most common cause of a poorly performing lens is practitioner misinterpretation of the results of an overnight trial, where the final lens is ordered on the basis of incorrect information. This problem is compounded if the lens is empirically fitted, as the practitioner is then dependent on the laboratory consultant (who has never seen the patient) to resolve the issues. In the authors' opinion, the critical factor in the correct interpretation of a trial wear period is a comprehensive knowledge of topography maps and the information they present. The majority of problems can be resolved simply by doing a trial overnight wear and interpreting the results correctly.

In this chapter, the common causes of the events that occur postlens fitting will be described, as well as the means of rectification.

CORNEAL STAINING

Several types of sodium fluroescein stain may be seen with day wear or overnight orthokeratology. These varied stains have different etiologies and are listed below:

•central stain

•3 and 9 0' clock staining

•dimpling

•Fischer-Schweitzer polygonal mosaic

•conjunctival stain

•fenestration imprint

•lens binding

•deposit-induced staining.

As with any type of ocular abnormality, practitioners should grade the severity using a standardized scale.

Central staining

With overnight wear, the most common form of staining seen is central superficial punctate erosions (SPE). At most aftercare visits, the patient will have removed their lenses several hours previously. Generally, the degree of staining observed will be very low. It can range from insignificant grade 1 staining to unacceptable grade 2 staining and above. The CCLRU scale is ideal for recording staining changes, as both the severity and location can be recorded.

Localized central staining is shown in Figure 9.20. This is grade 2 staining and is unac-

Figure 9.20 Unacceptable grade 2 confluent superficial stain present 2 h after the removal of an orthokeratology lens.

ceptable. Grade 1 or less staining is clinically insignificant, and is usually totally resolved within an hour of lens removal. The staining can occur at the first aftercare or any time afterwards, and if consistently present indicates that the cause is mechanical, in that the lens back surface is coming into direct and heavy contact with the epithelium. The primary cause for this is a lens that has insufficient apical clearance, or alternatively, too much peripheral clearance, thereby allowing the lens to settle back into direct contact with the corneal surface. The aim of remedying localized central staining would therefore be to increase the sag of the lens to relieve the excessive apical bearing. This can be done by:

1.steepening the BOZR (not possible if the Jessen factor is used)

2.steepening the alignment curvets)

3.increasing the TD (three-zone lenses)

4.steepening the reverse curve.

In the case of fourand five-zone lenses, the most common approach is to steepen the alignment curve. As shown in Chapter 4, steepening the alignment curve by 0.15 mm (0.75 D) will lead to an increase of 10 urn in lens sag. The Corneal Refractive Therapy (CRT) lens requires that the return zone depth be increased or the cone angle steepened, whilst the BE program requests that the practitioner supply information about the error of the instrument so that corneal sag can be recalculated, leading to changes in the BOZR, reverse curves, and tangent.

In practice, the practitioner will also utilize findings from topography patterns to decide whether the lens is "steep" or "flat". However, in some instances topography can be misleading. For example, in the case of central corneal staining, as shown in Figure 9.20, the reflection of the topography mires can be slightly distorted (often this cannot be discerned by observation of the Placido mires). Because of the subtle distorting effects of the centrally stained area, a topographic pattern that would indicate a "flat" fit could lead to the practitioner making an incorrect decision. Figure 9.21 shows a topographic difference map obtained from a patient who had localized central staining caused by a flat lens. This represents a "fake" central island, and occurs due to the inability of the

instrument's reconstruction algorithm to represent the changes correctly.

Alternatively, gross disruption of the mires will result in a central divot topography map, wherein the "ring jam" occurring as a result of the staining leads to the appearance of gross central flattening (Fig. 9.22). The important differential diagnostic tool in these instances is the tangential subtractive map, which would show the centration of the lens. If the lens was decentered superiorly, it could be considered to be too flat. However, if the lens is well-centered, the cause of the central staining may be due to three other factors: lens binding, surface deposits, or an incorrectly made lens (see later).

Three and 9 o'clockstaining

Three and 9 o'clock staining is a complication exclusive to RGP lenses (Fig. 9.23). It occurs as a result of corneal exposure at the nasal and temporal regions at the edge of a contact lens extending to the corneal periphery (Korb & Korb 1970). Principally, the etiology lies in the fact that the

cornea does not wet in the 3 and 9 0' clock areas. Reduced corneal wetting is exacerbated by the fact that the lid cannot wet the cornea because of the edge of the contact lens (Fig. 9.24). Thus, excessive edge lift is a major cause of RGP 3 and 9 0' clock staining. However, insufficient edge lift has also been implicated in 3 and 9 o'clock staining in extended RGP wear (Andrasko 1991) as a result of reduced tear flow under the edge of the lens. Most orthokeratology lenses are fitted in order to have an axial edge clearance (AEC) of 70-80 urn and a width of approximately 0.5 mm.

Three and 9 o'clock staining has been shown to be more prevalent with extended wear (Schnider et al 1991). However, it is the authors' experience that, in contrast to previous studies (Schnider et al 1991), modern-day orthokeratology lenses rarely cause 3 and 9 o'clock staining and the staining is never seen in overnight wear, presumably because the lenses are removed on awakening. Another possible explanation may lie in the large-diameter lenses used (generally exceeding 10 mm).

Figure 9.23 Three and 9 o'clockstaining in a daytime wearer of small-diameter low-riding orthokeratology lenses.

If 3 and 9 o'clock staining occurs in daily wear, then it is recommended that the peripheral curves are altered such that, if there is too much edge lift, it is reduced, and if there is too little, then it is increased in order to increase tear flow. Additionally the practitioner should ensure that lens

Figure 9.22 A central divot caused by marked central staining and distortion of the topographer mires (seen in the top right-hand image). Notethe large false refractive change and the small area overwhich it occurs, as shown in the tangential map (bottom left-hand image).

Figure 9.24 The bridging theory is often used to explain the etiology of 3 and 9 o'clockstaining. During blinking, the eyelid cannot wet the entire Cornea outside the boundary of the contact lens (in the temporal and nasal areas). As a result, drying occurs, leading to localized punctate staining in these areas.

centration is optimal and low-riding lenses avoided by attention to the fit and the addition of a negative carrier or Korb edge to increase lens-lid interaction.

Dimple staining

Dimple staining is a misnomer. There is no actual staining present, but fluorescein pooling. In order to face the challenge of correcting greater degrees of myopia, manufacturers have altered lens designs by increasing the width or depth of the

Figure 9.25 Dimple veiling underneath a double reverse geometry lens. Note the large width of the tear reservoir and the degree of dimpling, particularly in the central cornea.

tear reservoir (TR). These factors will often promote bubble formation within the TR. With time (often as little as 30 min) and repetitive blinking, the bubbles break down and froth. These smaller bubbles can spread to encompass the central cornea (Fig. 9.25). This is an unfortunate but completely innocuous complication of advanced orthokeratology. So-called dimple staining or dimple veiling simply represents multiple indentations within the corneal epithelium caused by the bubbles. The staining usually recovers after 1-2 h. Dimple staining prevents the practitioner from recording accurate or valid acuity measurements upon lens removal if the "staining" involves the central cornea. Furthermore, it also prevents accurate corneal topographic measurements from being taken as the "dimples" distort the reflected Placido mires. This degree of dimpling would certainly need to be managed.

In the authors' experience, dimpling may also occur haphazardly as an isolated incident. When dimpling is seen as an isolated incidence, patients should be advised to fill the back surface of the lens with wetting or conditioning solution, as this will usually resolve the problem. The true etiology of dimpling with regards to orthokeratology has not yet been discovered, although it appears that it is likely to be associated with the design of the TR.

All fourand five-zone reverse geometry lenses function optimally with a tear layer thickness (TLT) at the BOZO of between 45 and 65 um. If the TLT exceeds this amount and approaches 90/-Lm, bubbles will occur. TLT of this magnitude will occur if a lens design that has fixed parameters is fitted to a high-eccentricity cornea, or where the corneal topographer has underestimated the eccentricity of the cornea, resulting in a "steep" lens fit. If the eccentricity is underestimated, so the apical clearance will be too great and the reverse curve and alignment curves or tangent effectively be too steep. This not only increases the apical clearance, but also the TLT at BOZO, to the extent that the formation of bubbles becomes possible.

Since the apical clearance is excessive, the refractive change will be minimal, although this can be difficult to determine if the dimpling invades the pupil zone. Certainly, the incidence of dimple veiling is greater with orthokeratology lenses compared to conventional rigid designs. The management strategy is to reduce clearance of the lens from the cornea in order to minimize bubble formation. This is usually accomplished by one of the following:

1.Perform a retrial with the next flattest lens in the trial set so that a better match to the eccentricity becomes possible.

2.Cease lens wear and repeat topography.

3.Flatten the alignment curve(s).

4.Flatten the reverse curvets).

It is not uncommon to have dimple veiling occur on the overnight trial. However, this usually occurs as the trial lens parameters are primarily based on the mean corneal eccentricity, and if the individual eccentricity is higher, the clearance factor or TLT depth at the BOZO will be greater than that of the prescription lens. The bubbles do not usually occur with the correct lens.

Fischer-Schweitzer polygonal mosaic

The Fischer-Schweitzer polygonal mosaic is occasionally noted on lens removal. It comprises fluorescein pooling in a polygonal fashion over the central region of the cornea. Furthermore, there is no break in the corneal epithelial layer (Fig. 9.26). The etiology of the polygonal mosaic

was originally described by Bron et al (1978), who suggested that the fluorescein pattern occurs as a result of external pressure on the cornea causing ridges to form in Bowman's layer. As the external force is removed and Bowman's layer returns to its original shape, grooves form where epithelial cells have been compressed into the ridges. The mosaic effect is usually present for up to 10 min, after which it disappears. The presence of the Fischer-Schweitzer polygonal mosaic is generally considered to be of no clinical significance in orthokeratology patients. It is rarely seen; however, this may be as a result of the fact that most patients are reviewed without lenses in situ following completion of the treatment and additionally the forces acting on the central cornea are modest.

Conjunctival staining

The use of localized central SPE stain in providing evidence of a flat-fitting reverse geometry lens has already been discussed. Inferior conjunctival stain may also provide a clue to the fit of a reverse geometry lens (although it is not as diagnostic a sign as central staining). Inferior conjunctival staining may be present in a steep-fitting lens, a bound lens (Fig. 9.27), or even occasionally

Figure 9.26 Fischer-Schweitzer polygonal mosaic. This is rarely seen following orthokeratology lens wear.

Figure 9.27 Indentation in the inferior limbal conjunctiva arising from a low-riding lens that has bound.

with a flat-fitting lens. Superior conjunctival stain is indicative of a flat-fitting lens.

Lens modification is usually successful in treating the staining. For example, inferior conjunctival stain associated with lens binding would be treated by one of the following:

•Reduce total diameter by 0.5 mm and retrial.

•Try a flatter lens in 0.02 mm steps (remember that small changes in BOZR have a radical effect on the fit of the lens).

Table 9.6 The likely nature of the lensfit with the presence of corneal and conjunctival staining

Steep Flat

Central SPE stain

Inferior conjunctival stain

Superior conjunctival stain

SPE, superficial punctate erosion.

•Steepen the alignment curve to improve centration.

Superior corneal staining may also be accompanied with a topographic smiley-face pattern (see Ch. 2, Fig. 2.23A). The accuracy of topographer and lens design will dictate the magnitude of the change in base curve. Generally, increments of 0.02 or 0.05 mm are made when refining lens fit.

Table 9.6 shows the likely causes of conjunctival stain.

Fenestration imprint

Some practitioners feel that fenestrations at the center of the TR reduce the incidence of dimpling (as a way for bubbles to leave) whist others believe that fenestrations may act as an entrance site for bubbles. The fact is that bubbles form under both fenestrated and nonfenestrated lenses. There are also some who believe that fenestrations reduce the likelihood of lens adherence; however, this is an unlikely scenario, as the fenestrations are sealed once the lids are closed over the lens surface. From a purely logical perspective, fenestrations may work to help loosen up a bound lens by allowing enhanced tear exchange with resulting dilution of the mucous layer under the lens. There is much dispute regarding these statements. Moreover, there is no evidence to support or reject the use of fenestrations in orthokeratology. From the authors' experience, if the lens is marginally too steep, fenestrations will exacerbate the problem. The negative pressure under a "steep" lens may draw tears through the lens fenestration, and in so doing will also allow

Figure 9.28 Practitioners should notethe position of' fenestrations prior to removal in case of corneal staining. This figure also illustrates the effect of the fenestration in facilitating the flow of tears out of the fenestration (superior fenestration),

air into the postlens tear film if the TLT is excessive. Thus, the problem may not be the fenestrations, but excessive apical clearance. If fenestrated lenses are the preferred option, always ensure that the trial lenses are also fenestrated, so that easier detection of lens clearance at the BOZD is made by the appearance of bubbles in the postlens tear layer.

Occasionally, one may see a fine local area of staining on the corneal surface on removal of the lens. The practitioner should always make a note of the position of the fenestrations before lens removal as often these areas of staining correspond to the location of the fenestration (Fig. 9.28).

The fenestration should always be placed at the point of maximal tear layer depth of the lens, at the BOZD / reverse curve interface. If staining occurs as a result of the fenestration, the following are the most likely causes:

1.the fenestration is in the incorrect position (on the reverse curve / alignment curve junction)

2.the fenestration is poorly finished

3.the fenestration is clogged with debris.

Corneal staining from fenestrations is a rare occurrence.

Lens binding

During a morning aftercare, slight central staining may occur in a patient who has, until that time, always shown an ideal response to the lens and no staining. This is usually due to an episode of lens binding, and the associated minimal trauma induced by an incorrect attempt at freeing up the lens. Another common cause of this binding-associated staining is the simple act of requiring the patient to present in the morning with the lens in situ. Patients usually report discomfort and mild photophobia and offer the comment that this-usually only occurs when they have to wear the lenses into the practice. A simple means of verifying that the staining occurred as a result of the extended period of open-eye wear is to have the patient represent a few days later having removed the lenses at home in the usual manner. If the staining is still present, it is either due to incorrect removal following binding or an incorrect fit.

The etiology and appearance of lens adherence are discussed in Chapter 3. Although most subjects suffer from adherence on waking, most lenses tend to become mobile after a short period of eye opening. The incidence of lens binding following overnight wear with either reverse geometry or standard gas-permeable lenses ranges from 90% to zero, depending on the author and anecdotal reports. Those claiming a zero rate of binding are obviously not assessing the lens fit prior to lens removal, and basing their assessment on patient reports.

The important factor to remember with lens binding is that alterations to the lens fit do not resolve the problem (Swarbrick & Holden 1996). Lens binding occurs as a direct result of the alterations to corneal shape and the postlens tear film induced by the lens, and is, in some ways, a patient-dependent phenomenon. Some patients, due to their tear viscosity, are chronic binders, whilst others will have episodic events that appear to be random.

Overnight wear of reverse geometry lenses leads to an increase in viscosity of the tear layer, and this, associated with the molding effect of the lens, increases the viscous adhesion between the lens and the ocular surface. The fluid forces

induced by the lenses are modeled in Chapter 10, and it appears that binding is to be expected from overnight wear. The majority of lenses will automatically free up following active blinking and induced tear exchange. It is of vital importance that patients should be properly instructed on the correct method of freeing up a bound lens.

All orthokeratology patients should be shown how to loosen bound lenses prior to removal so as not to cause or to minimize any possible epithelial damage. Patients must also be taught the difference in awareness between a bound lens and one that is mobile. Bound lenses are usually quite comfortable, with the main symptom being a feeling of dryness. Mobile lenses, on the other hand, are always a source of mild discomfort to the patient due to the lens movement. Orthokeratology patients never really adapt to the feeling of the lens in the open-eye situation, simply because they never Wear them long enough to become adapted. Once the patient can appreciate the difference in sensation between a bound and mobile lens, the techni.que for removal can be taught. If the lens is mobile when removal is about to occur, no special steps need to be taken. If the lens is bound and immobile, the following procedure should be followed:

1.Instill one or two drops of lens lubricant into the eye and blink a few times.

2.Look upwards, and press gently but firmly against the inferior limbus with the edge of the lower eyelid three times using the index finger.

3.Look downwards and repeat the procedure with the upper lid against the superior limbus.

4.Once the patient becomes aware of the feeling of lens movement, the lens can be safely removed in the usual manner. It is vital that the patient be made aware of the difference in feeling between a free-moving lens and a bound lens. This is usually taught at the trial fit, and then reinforced at the first and second morning aftercare visits.

It is vital to make sure that the patient does not push the edge of the lens with the lid. This will induce a large shear stress that can exacerbate the potential for trauma.

262 ORTHOKERATOLOGY

Figure 9.29 shows the sequence of steps required to free up a bound lens.

Once the lens is free, it is not uncommon to see a layer of thick mucus in the postlens tear layer.

This will sometimes stain with fluorescein and give the appearance of staining (Fig. 9.30). If the lens is removed and a lubricant instilled, the "staining" will no longer be present.

Patients who do not have a previous history of lens binding, but present with the signs and symptoms of the condition, should be assessed for any changes in the ocular surface that could lead to increased mucus production or other tear

Figure 9.30 A layer of thick mucus is seen that stains with fluorescein. If the lens is removed and a lubricant instilled, the "staining" will no longer bepresent after a few firm blinks.

Figure 9.31 Heavy deposit build-up on the back surface of an orthokeratology lens within the optic zone.

abnormalities, and treatment initiated. Also, a heavily coated lens is prone to binding, and the lens must be inspected and cleaned if this is the cause.

In summary, binding should be considered a relatively normal aspect of overnight wear, with a major emphasis placed on the correct removal of the lens.

Surface deposit-induced staining

Central diffuse grade 1 type staining will sometimes occur after 9-12 months of successful lens wear. It is commonly associated with symptoms of blurred vision at night and a loss of retention. This usually results in the lenses being worn on a nightly schedule instead of every second night. The onset of the effect is slow, but usually gets to the stage that it becomes the main motivation for the visit. The single most common cause of this loss of effect is a heavy build-up of deposits in the back optic zone of the lens (Fig. 9.31). It is invariably associated with increased discomfort and corneal staining.

In a personal communication, the late Roger Kame reported that microscopic analysis of the deposits showed them to be a mixture of exfoliated epithelial cells and thick mucus. The average thickness of the films analyzed was 15 p.m, This layer of debris has an enormous effect on the mode of action of the lens. The

surface layer on the back of the lens has a dramatic effect on the dynamics of the squeeze film forces, and reduces them to the extent that the cornea does not attain the correct shape following lens wear. The corneal staining often seen in these instances is easily explained when one considers that the apical clearance in orthokeratology is of the order of 5-10 urn. The added thickness of surface deposits of 15 urn will effectively result in contact and thus insult to the corneal epithelium. Corneal staining is resolved by sending the lens back to the laboratory for professional repolishing, or manually cleaning the debris off the lens by using Boston Advanced Cleaner and a cotton bud. This job is best performed by the practitioner as the patient may be too enthusiastic and warp or damage the lens. A thorough clean and polish of the lens is all that is usually required to resolve this type of staining.

CONTACT LENS PAPILLARY

CONJUNCTIVITIS

CLPC is a rare complication of orthokeratology lens wear. It is associated with episodes of itching, mucus discharge, and heavily deposited lenses (Fig. 9.32). The treatment regimen is the same as for standard cases of CLPC; special care is taken to reinforce the correct maintenance procedures.