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

one design of single RGL (Contex OK704T). He demonstrated that a reduction of approximately every 0.20 of the corneal eccentricity or e gives a 1.00 D reduction in myopia. The relation is not exactly linear, as shown in Figure 5.11, but this is still a useful rule of thumb.

Figure 5.11 The relationship between the change in refractive errorand the change in corneal eccentricity occurring after an orthokeratology treatment (from Mountford 1997b).

Therefore, if e is equal to the mean in the population of approximately 0.45 (Guillon et al 1986) then a likely reduction in myopia of the order of 2.25 D is possible. Mountford's analysis of the refractive changes in 60 patients showed a mean change of 2.19 D (Mountford 1997b). This must correlate reasonably well with patients' spherical refraction for them to be free of the lenses for most of the day.

If the patient has a prefitting refraction of -5.00 D, then reducing it by 2.50 D will not help greatly, unless the intention is merely to reduce the myopia, perhaps for occupational reasons. If it is -2.50 D then it is worth going ahead with a fitting if the desired outcome is emmetropia. Obviously, the higher the corneal eccentricity, the greater potential there is for reduction of myopia. The endpoint of any orthokeratology procedure has traditionally been described as a cornea with an eccentricity of zero, i.e., spherical (Kerns 1976, Binder et al 1980). Mountford (l997b) has shown that this holds true for the central 4-5 mm of the cornea. Outside this zone, there is a steepening of the cornea, so that the concept of a simple conic section breaks down.

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This concept of the patient's corneal eccentricity defining the final outcome of the procedure is challenged by the latest double reverse geometry lens (DRGL) designs now available (see Ch. 4). Using the more sophisticated examples of these designs it is possible to program the required refractive change into the lens geometry to produce a unique lens for the individual topography and refractive characteristics. In these instances, the measurement of the corneal eccentricity serves to define the potential outcome using a conventional treatment zone. The extra refractive change required will reduce the size of this treatment zone. This makes measurement of the patient's pupil size important to predict the likelihood that flare will influence the quality of the visual result.

The departure from a prolate ellipse-type geometry prefitting to a more complex shape postorthokeratology means that it is not possible to use the concept of a single eccentricity value to describe the corneal shape. Whilst the new corneal shape will have an eccentricity of a over the central 4-5 mm, outside this, in the region of steepening, the use of the e-value becomes unworkable as there is no longer a prolate geometry. A negative e-value has no basis in mathematics. The use of a p-value can be used, but again this assumes a simple conical geometry, which may not be appropriate. Outside this zone of steepening, the cornea flattens again, in a similar way to its prefitting form.

Thus we have a three-zone surface: a central near-spherical zone of flattening, a concentric, narrow zone of steepening, surrounded by a relatively unchanged periphery. Higher-order mathematics is required to describe this changed geometry. It is for this reason that the apical radius and eccentricity values produced by the videokeratoscope do not adequately serve to establish the radius of any second lens in the same way that they can predict the initial lens.

Kero tometry

The changes in corneal topography that occur in orthokeratology are not accurately detected or measured using a keratometer. Joe et al (1996) showed that the predictive expression of myopia reduction K t ) + LOa, derived by

Wlodyga & Harris (1993), linking the relation between a temporal and central keratometry reading, is not well correlated to outcome in accelerated orthokeratology. It has been shown (Ericson & Thorn 1977, Mountford 1997b) that approximately 0.75 0 of refractive change can occur before any alteration in the keratometry reading will arise. Thus there is little benefit in using a keratometer at the initial appointment. The instrument also has very little place in the aftercare of patients, as shown in Chapter 9.

However, if adapted using the method of Wilms & Rabbetts (1977) it is possible to derive a measure of the corneal eccentricity from the keratometer. Central and peripheral readings are taken from precisely defined corneal locations and a conic section fitted. Whilst this would serve adequately as a means of deriving the initial eccentricity, thereby aiding in the selection of the initial lens, it is really in follow-up of the patient that the absence of a means of determining the topography would be felt. None of the principal topographic determinants of a substandard fit is measurable using a keratometer. These are, of course, central islands, inferior and superior zones of steepening (smiley faces), and decentered treatment zones. Thus, one would be reliant on the retinoscopy reflex and subjective comments regarding visual quality as a means of defining the quality of the treatment outcome. The authors feel that this would not be commensurate with ethical practice.

Pupil diameter andtreatment zone

Swarbrick et al (1998) have shown that Munnerlyn's formula can be used to account accurately for the refractive changes occurring in orthokeratology due to epithelial thinning. The maximum degree of central epithelial thinning with orthokeratology appears to be approximately 20 urn. If Munnerlyn's formula is expressed in orthokeratology terms, the relationship between refractive change and treatment zone diameter can be calculated.

The formula is:

Epithelial thinning = RO 2 3

where R is refractive change in diopters and 0 the treatment zone diameter.

Figure 5.12 The results for a range of refractive changes plotted against change in corneal sag. Notethat, as the refractive change required increases, the treatment zone (TxZ) diameter decreases.

If, as stated above, the assumed maximum thinning is 20 urn, the refractive change required can be substituted into the formula and the TxZ diameter determined. The results for a range of refractive changes are shown in Figure 5.12. Note that, as the refractive change required increases, the TxZ diameter decreases. For a nominal pupil zone of 3.00 mm diameter, a refractive change of up to 6.00 0 may be feasible as the TxZ diameter and pupil diameter are the same. However, if it is assumed that the limit of the refractive change is determined by the pupil diameter in dim illumination, there is a marked reduction in effective refractive change possible. Assuming that the pupil diameter increases to 5.00 mm in dim illumination, the refractive change that gives an effective TxZ is reduced to 2.50 O. This point is of vital importance when considering low-contrast vision, as pupil dilation in dim light associated with large refractive error reductions will lead to flare and haloes, and a reduction in visual quality (Applegate & GanseI1991).

Oavidorf (l998) drew attention to this fact in regard to refractive surgery (Figure 5.13). Small treatment zones combined with large pupil diameters and deep anterior chamber depths lead to excessive flare, haloes, and decreased low-contrast vision post-Lasik. Refractive surgeons are able to compensate to an extent for these effects by manipulating both the TxZ and the method of laser ablation. This may also be possible to some

Figure 5.13 The effect of a treatment zone (TxZ) diameter being the same size asthe pupil in dim illumination.The ray Yis within the TxZand will be blocked by the pupil border. More peripheral rays (l) will be refracted to a greater degree by the sudden change in curvature at the edge of the TxZ, and be refracted through the pupil, leading to flare and haloes.

extent with different designs of RGL. The difference in TxZ for two different lens designs is shown in Figure 5.14. The refractive changes are similar, but the TxZ of one is greater than the other. Alterations to the lens back optic zone diameter (BOZO) can lead to increased TxZ diameters without compromising the refractive change but require careful calculations (see Ch. 9).

Lens design does play an important role in the TxZ diameter. Mountford fitted 14 patients with a generic 6.00 mm BOZO lens in one eye and an IDEAL lens in the other (7.30 mm BOZO). The refractive change was a mean of 1.86 ± 0.56 0 for all patients. Subtractive axial power maps were used to measure the TxZ diameters following lens wear. The results are shown in Figure 5.15. Although the difference in TxZ diameter between the two designs when taken in isolation is not dramatic, it does become quite significant when compared to the percentage of pupil coverage under different conditions (Fig. 5.16). All of the currently available designs use a 6.00 or 6.50 mm BOZO, but future design improvements may allow for a greater control over the TxZ diameter.

From a clinical viewpoint, refractive changes of up to 3.00 0 rarely cause symptoms of poor vision in dim illumination, but greater changes can. If a patient requires a greater refractive change, the practitioner is well advised firstly to determine the likely TxZ using Munnerlyn's formula, and then determine the patient's dilated pupil diameter. There are dedicated instruments that can be used to do this, like an infrared pupillometer, but

Treatment zone diameters

Figure 5.15 The mean treatment zone (TxZ) diameters for two different lens designs. The four-zone lenshas a back optic zone diameterof 6.00 mm,and the IDEAL 7.30 mm.

TXZ diameter asapercentage of pupildiameter

Figure 5.16 The percentage of pupil coverage of the different treatment zone (TxZ) diameters for different pupil sizes. Smaller TxZ diameters areacceptable for normal pupil sizes, but the efficacy diminishes asthe pupil size increases.

the HVID is 12 mm, but 120 mm on screen and the pupil measures 50 mm on screen, then the "real" pupil diameter is 5 mm. If the calculated TxZ is smaller than the pupil zone in dim illumination, then it is highly probable that the patient will report symptoms of flare, haloes, and poor low-contrast vision.

If patients do present with symptoms of flare and poor vision in low illumination, the problem is easily detected with simple retinoscopy in a darkened room. Ineffective TxZ diameters will appear exactly like the retinoscopy reflex through a concentric bifocal contact lens. The approximate

Figure 5.17 An infrared retinal camera is used to measure the pupil in dim illumination. The large circle is equal to the horizontal visible iris diameter (HVID). Simple proportional analysis of the pupil zone with respect to the HVID gives the pupil size in dim illumination.

increase in TxZ diameter required to resolve the problem can be determined by simply comparing the central treatment area to the pupil diameter.

Cho et al (2002) reported on the patient responses to a questionnaire about the incidence of various symptoms following overnight wear of orthokeratology lenses. Approximately 33% (21/61) reported poor vision in dim or dark conditions, so the problem does have a relatively high incidence in this patient group. The majority of the subjects (50/61) were less than 16 years of age, so the effects of the poor vision for night driving could not be gaged.

Hile & Marsden (2001) studied the effects of pupil dilation on vision and refraction in a group of eight subjects. Mean low-contrast VA decreased with increased pupil size (0.015, P = 0.036) and refractive error (autorefraction) increased by a mean of 1.12 D (P = 0.01). This is similar to the effects of small ablation zones in refractive surgery (Lohman et al 1993, Pop 1996), with a common level of complaint (30%), as found by Cho et al (2002) in a group of orthokeratology patients.

Future developments in lens design, as well as preliminary measurement of the pupil diameter, may help to reduce the incidence of the problem. The inclusion of an estimation of the pupil size in dim illumination is a valuable prefitting aid in being able to detect problem cases before they occur.

Lidposition and tension

There are two main reasons for considering rejecting patients with loose lids or deep-set eyes, or those with low upper lids and narrow apertures. The first is that it can be difficult to get accurate or repeatable topography data, especially with small Placido instruments, as the cone tends to contract the orbital bones before the corneal apex can be properly located. If large Placido instruments are used, the focusing problems are not as great, but a greater surface area of the cornea is lost due to lid, nose, and brow shadow, leading to inaccuracies in the estimation of eccentricity. This will mean that potentially more trials will have to be completed until the fitting is refined.

Secondly, just as with the fitting of conventional RGP lenses, a low lower lid can result in more difficulty in attaining good centration, due to inadequate support to the lens (Carney et al 1997). This is especially the case if the low lower lid is combined with a low top lid. However, this may only be the case when daily wear is being undertaken. The authors feel that for night therapy this should not be seen as a relative contraindication and a trial should still be carried out. It is a fairly common observation that patients with lenses that sit inferiorly when the eyes are open, despite a classic fluorescein pattern, can have a perfect topographical response to overnight wear of the same lens. Presumably, with the eyes closed the centration is improved.

It may be that the lens position adopted in the consulting room, with the patient sitting upright in the examination chair and the lids and gravity exerting their customary effects on centration, is quite different from that arising when the patient is lying down with the lids closed. The effect of Bell's reflex may be significant, as the eyes may adopt a higher position relative to the primary position in which the lens is checked in

the consulting room. Therefore inferior centration, but with a classic fluorescein fit, should be seen as a relative contraindication to the night treatment form of orthokeratology and an overnight trial should still be conducted to measure the quality of the corneal response. However, for daytime orthokeratology it is a defini te contraindication.

In a similar way, there are instances of nasal and superior decentration, which also do not lead to decentered treatment zones following an overnight trial. The practitioner is advised to check the fluorescein pattern in a centered position. If it is thought to fulfill the attributes set out in Chapter 6 and the lens in not too small, then it is worth proceeding with an overnight trial. The position of the resultant TxZ, determined in the difference map the next morning, would serve to define the quality of the fitting. However, if the difference map indicates that decentration has occurred, it may be that fitting will have to be discontinued.

Low lid tension seems, anecdotally, to reduce the effectiveness of orthokeratology, presumably due to lowered lid forces acting on the lens. However, like abnormal lid position, it should be seen as a relative contraindication. It is still worthwhile performing a daytime or overnight trial to assess the corneal response to a well-fitted lens. The hydrostatic forces behind the lens are far stronger than the lid force and the authors have had several successful elderly patients with low lid tension achieve good outcomes with orthokeratology.

Occupational and recreational

A significant percentage of patients attending an orthokeratology practice do so in order to meet the visual standards for a particular employer or occupation. The standards vary from country to country and will not be listed here. Practitioners are advised to consult their professional body for a list of the standards that apply in their own country. In the UK the Association of Optometrists publishes these on its website at www.optometry.co.uk.

The use of visual standards in a selection process has caused many of the involved bodies

significant problems in finding sufficient appropriate entrants given the rise in the incidence of myopia in the young population globally. There have been several examples where they are prepared to accept applicants who are known to use orthokeratology as a means of reducing their refractive error, whilst maintaining their corrected visual acuity.

For example, in Spain the Madrid Police Authority refers otherwise suitable applicants to an orthokeratologist for treatment with a view to allowing them to meet the vision standards (F Hidalgo personal communication). From a medicolegal perspective, the practitioner should ensure that the patient achieves excellent corrected vision after orthokeratology and realizes that it is the patient's responsibility to declare or not declare the usage of orthokeratology lenses as a device to meet the appropriate standards. Examples of patients who may seek the services of an orthokeratology practitioner are airline pilots, police officers, and firefighters. Practitioners who are concerned to be totally protected from a medicolegal point of view would be wise to write to the appropriate authority and seek clarification as to whether orthokeratology is appropriate.

There are also many patients who are active in sport who find conventional contact lenses inconvenient. The possibility of being free from optical aids whilst participating in the sport is attractive to them. Examples are swimmers who find that rigid lenses are likely to be lost from the eye and that soft lenses alter their parameters and stick to the eye whilst swimming; divers and surfers who worry about losing lenses whilst being underwater; those involved with contact sports who have had lenses displaced, and any sports people who find that lenses become uncomfortable in the environmental conditions they participate in, such as hot, dry climates or dusty, windy places.

Patients who wear lenses in night clubs can find they get dry and uncomfortable and seek an alternative. Actors and television personalities may find that having a contact lens in the eye causes them to alter their blink rate and appear less at ease than if they are free of contact lenses. Practitioners should also remember that not all patients find contact lenses convenient or comfortable in their particular situation and be receptive to the positive benefits

that orthokeratology can bring to these individuals. A particularly large group are those patients who find that they are unable to tolerate soft contact lenses all day due to dry-eye symptoms when lenses are worn (see below).

Physiological

Just as is the case for conventional RGP wear, there are physiological considerations to make when advising patients as to their suitability for orthokeratology. In essence, the contraindications to orthokeratology are either absolute, leading to immediate rejection of the patient, or relative, indicating that a cautious approach is required.

Absolute contraindications

Keratoconus Just as with refractive surgery, this must be excluded by videokeratoscopy before attempting any fitting. The keratoconic cornea would respond quite abnormally to a RGL and there is also evidence that flatly fitted lenses cause scarring on keratoconic corneas (Korb et al 1982). Identification of keratoconus from a topography map (Fig. 5.18) can be made by consideration of five factors:

1.The apical radius. If this is less than 7.20 mm then keratoconus is more likely. However, keratoconus can occur with apical radii as flat as 7.8 mm (Woodward 1980), therefore other features must be present to confirm the diagnosis.

2.A greater eccentricity than normal. Typically, 95% of the normal population have an eccentricity in the flattest corneal meridian between 0.0 and 0.88 (Guillon et al 1986). Douthwaite's study using the EyeSys 2000 instrument (Douthwaite et al 1999) showed a narrower range of 0.0-0.71. However, in keratoconus eccentricities are typically from 0.6 to over 1.0.

3.Variation in interocular apical radius greater than 0.20 mm. Typically there is close association between both the apical radii and eccentricity between the two eyes.

4.Loss of symmetry either side of the horizontal midline. If corresponding points equidistant from the apex are considered then, in the

normal eye, there are seldom variations greater than 0.20 mm (l D). However, in the keratoconic eye there are, typically, greater than 0.60 mm (3 D) differences between these two regions on the cornea.

5.A localized area of corneal steepening, either located inferiorly (typically in 75% of cases) or centrally.

Corneal dystrophies Nothing is known about the effect of orthokeratology on a dystrophic cornea. In the absence of such knowledge, it would be foolhardy knowingly to fit a type of lens that is thought to bring about redistribution of epithelium and possibly alter the shape of stromal tissue. Careful slit-lamp examination is essential to exclude any sign of abnormality, since many dystrophies are subtle. Direct focal, indirect, retro, and specular illumination techniques are required to exclude corneal dystrophy.

Corneal edema present without contact lenses would obviously be a contraindication to orthokeratology as even the highest-transmissibility lenses would add to the cornea's load. Signs of such metabolic stress might be centrally located, vertically oriented striae or folds in the stroma of the cornea. Other signs might be multiple epithelial microcysts or, in extreme cases, vacuoles in the epithelium.

If there is any doubt as to the presence of subclinical dystrophy, it is wise to refer the patient for ophthalmological opinion before embarking on a course of orthokeratology.

Any active anterior segment disease The contraindications to conventional lens wear apply

just as much to orthokeratology treatment. Where overnight wear is to be employed, it is more important than ever to be certain that the anterior segment is free of any active pathology which would contraindicate lens usage. It is known, for example, that blepharitis is a risk factor for infection in conventional contact lens wear (Holden et al 2000, Jalbert et al 2000, Willcox et al 2000). Therefore, all cases of significant blepharitis should be treated before a program of orthokeratology begins. Recurrent conjunctivitis is equally a contraindication until the cause is identified and treated.

Whilst it is most unlikely that a patient with active keratitis would present for fitting, careful slit-lamp examination is required to evaluate the extent and depth of any corneal staining and identify any areas of infiltrate in the asymptomatic patient. Although there is little knowledge of the potential effects, it would be unwise, for example, to fit a patient with adenoviral infiltrates until they have fully resolved. Chronic anterior uveitis may be exacerbated by lens wear, particularly if this is overnight.

Severe dry eye is equally an absolute contraindication to lens wear. The presence of gross corneal stain (grade 3 and above) or corneal filaments is indicative of pathological dry eye.

Relative contraindications

Marginal dry eye In normal soft-lens practice, the commonest symptom reported by patients is ocular dryness, affecting 75% of wearers (Brennan & Efron 1989). This does not always correlate

with the typical minor inferior desiccation stain that is commonly observed in wearers of soft contact lenses (Pritchard & Fonn 1995). This type of stain does not appear to be observed in orthokeratology patients on night therapy.

Particularly during closed-eye conditions, RGP lenses are better suited to patients with slightly reduced tear volumes than hydrogel lenses. Obviously, less evaporation occurs during closedeye conditions, making this modality of wear more suited to marginally dry eyes. When the lipid layer is absent, the rate of evaporation from the open eye is approximately four times the normal value. Thus wearing lenses during a closed-eye situation would not be expected to lead to increased evaporation.

However, it should be remembered that truly dry eyes are a contraindication to all contact lens wear. Any patient who suffers from ocular dryness without any contact lenses and who exhibits inferior desiccation stain greater than grade 2 (mild stain) should probably be rejected as an orthokeratology patient for either modality. For other patients, it is well worth giving them an overnight or daytime trial and checking the cornea carefully using fluorescein and a barrier filter both before and after lens wear. Any increase in staining beyond grade 1 or greater than grade 2 staining (Cornea and Contact Lens Research Unit (CCLRU) grading scale) would be regarded as an adverse sign.

When there is reduced tear volume there is always the possibility of a reduced tendency for lenses spontaneously to unbind in the morning after wakening. Greater attention should be given in aftercare to identifying those patients with persistent binding. The use of artificial lubricants and the lens-loosening procedure, as detailed in Chapter 10, should be actively encouraged, as well as self-monitoring of lens adherence before removal.

Generally, those patients with substandard tears tend to show a higher level of deposition on RGP lenses with time. Given that deposits on the back surface of RGLs fitted for orthokeratology seem to cause a greater degree of corneal insult than those on conventional lenses, it is wise to ask patients demonstrating increased deposition to attend frequently for in-office inspection and

cleaning. The authors find that Progent (Menicon Co. Ltd) is particularly useful at removing protein films. In addition, cleaners containing polymeric beads tend to be more effective at loosening adherent deposits and are advised for daily patient use.

Three and 9 o'clockstaining with existing RGP lenses This relatively common complication of conventional RGP lens wear has not been observed by the authors in overnight RGL wear with removal of lenses on wakening. RGP lenses are remarkably comfortable with the eyes closed and this type of desiccation stain simply does not seem to occur, perhaps because there is no problem in resurfacing the tear film when the eyes are closed.

Additionally, it appears to be relatively uncommon in daytime users of the lenses, possibly because of the larger diameter than conventional lenses. Thus, it is unlikely to be a significant problem, particularly with night therapy. However, patients who are RGP lens failures on the grounds of unresolvable 3 and 9 o'clock stain should be warned that this may still be a problem if daytime wear of orthokeratology lenses is required. However, this should not preclude scheduling a trial wearing period.

Psychological

Perhaps more important than any other factor is patient motivation. It is vital that patients understand what orthokeratology is about, how it works, what their responsibility is, and that it is reversible. This last point is crucial. Time and time again one needs to reinforce the message that this is a reversible technique. This is comforting to the patient worried about the irreversible complications of refractive surgery, but is a source of discontent to some people. The authors have encountered several patients who, despite clear verbal and written advice about the complete reversibility of orthokeratology, still ask when they can stop wearing the lenses. Having written advice is important for a variety of reasons, but it is particularly useful to remind patients that they have been informed about what they may regard as a disadvantage of the technique. A patient

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information pamphlet and consent form are shown in Figures 5.19 and 5.20.

There are certainly patients seeking a nonexistent "holy grail" treatment, who want perfect vision with minimum effort and expense. They must receive clear and detailed advice about the pros and cons of current orthokeratology techniques. Some of the extravagant claims made for orthokeratology and expounded on the internet and elsewhere make it doubly important that practitioners retain their integrity and advise the patient properly.

Patients may be motivated to undergo orthokeratology for a variety of reasons. They may have difficulties with existing contact lens modalities due to 3 and 9 o'clock staining and foreign-body problems with rigid lenses or corneal desiccation and lens awareness with hydrophilic lenses. They may wish to benefit from the considerable evidence for some degree of myopia control with rigid lenses (Stone 1976, Perrigin et al 1990). This is particularly the case for the young patient. As will be detailed later, night therapy is particularly suited to the child patient, since parents can directly supervise lens wear and there is no risk of lost lenses. There may be an occupational need for an improvement in unaided VA or a reduction in myopia below a certain level, as discussed above.

There are also a considerable number of patients who feel that, by remodeling their cornea and being free of optical aids for all or most of their waking hours, they are doing something positive about their visual defect. Providing they appreciate that orthokeratology is not a "cure," then this is a good motivating factor and should not be discouraged. Those practitioners who are not myopic may not always appreciate the degree of "disability" which myopes feel. It is this that drives their interest in refractive surgery as well as the desire to be "normal."

Additionally, patients with a history of poor compliance in previous contact lens wear are unsuitable for orthokeratology, particularly night therapy. The inability to care for lenses properly and attend for aftercare exposes the patient' and practitioner to risk. At the very least, such a patient should be forced to earn the right to

overnight orthokeratology by demonstrating a good response to daytime wear and showing that they will attend for follow-up and care for lenses correctly. If there is any doubt in the practitioner's mind, then it is better to err on the side of caution and not fit the patient.

Finally there are those patients who exhibit a total inability to remain still during topography. The results will always show large variations, and a high standard deviation in repeatability. This is particularly true for children when a small Placido instrument is being used. The invasion of personal space by the instrument makes them anxious and mobile, resulting in inaccurate topography data (Chui & Cho 2002). The poorquality topographic data with low reliability mean that the procedure will be fraught with difficulties and this should be seen as a relative contraindication.

DISCUSSION OF MODE OF WEAR, DAY OR

For most patients, the prospect of being free of all optical aids during waking hours makes overnight wear, or night therapy, very attractive. A practitioner who has never fitted RGP lenses for extended wear may be reluctant to offer the overnight-wear modality. However, all clinical experience to date indicates that extended wear of high-Ok RGP lenses produces far less corneal compromise than current-generation hydrophilic extended lens wear. High-Ok RGP materials are sufficiently permeable to meet the criteria set by Holden et al (1984) for safe extended wear, viz. an equivalent oxygen percentage (EOP) of 10%. The incidence of microbial keratitis has been shown to be half that of extended-wear soft contact lenses (Benjamin 1992).

It should be remembered that normal overnight corneal swelling is of the order of 3-4% (Mandell & Fatt 1965, Mertz 1980). In addition, subjectively graded limbal hyperemia, an index of corneal hypoxia, is increased in soft extended wear (Holden et al 1986). The recovery from the modest levels of edema after RGP overnight wear is also more rapid than for soft lens wear. This of course has been measured with lenses in situ -