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

As well as understanding that the practitioner is measuring radial edge thickness, it is equally important that the laboratory should know at what distance from the edge apex the measurement is being taken. Port (1987) described a modified-thickness gage that enabled this distance to be set. In normal clinical situations, it is believed that the nearest to the edge that it is reasonably possible to measure is 0.50 mm.

INSPECTION OF EDGE PROFILE

The shape of any contact lens edge can have a significant influence upon its comfort in wear. It has been suggested that edge quality may be assessed by rubbing the lens edge on the wrist or fingers. Such tests are capable of disclosing grossly unacceptable edge forms but the profile is not determined.

A simple and relatively quick method is to inspect the lens using the magnification of a hand loupe, stereomicroscope, or slit lamp. At high magnifications it is useful to support the lens on a holder. The edge should ideally have either a centrally or posteriorly displaced apex. The apex is visible as a thin white line when the edge is wellilluminated. The use of a smooth rolled edge has been shown to be crucial for good RGP comfort (La Hood 1988). In addition, it has been shown that the anterior edge profile is more important than the posterior profile in terms of comfort (Picciano & Andrasko 1989). Careful verification of the lens edge is important to ensure the patient receives the most comfortable lens possible.

Whilst it is possible to take a plaster cast of the lens edge and then inspect this on the slit-lamp microscope, this is beyond the bounds of what most practitioners would consider reasonably achievable at every verification. However, it is a useful way of checking that the laboratory is producing consistent edges, if it is performed from time to time. A small pool of dental stone is placed on top of an elastic band that has been stretched around a plastic surface. The lens is inserted at right angles to the rubber band. When the dental stone has set, the lens is removed. This needs to be done with care if the lens is to remain in one piece! The dental stone is then snapped about the scoring imparted by the elastic band.

The form of the lens edge can then be inspected as an impression in the dental stone. Given the risks of damaging the lens whilst removing it from the dental stone, this is a "gold standard" edge inspection technique best reserved for occasional use on specially ordered lenses.

SURFACE QUALITY

The quality of the lens surface, particularly the rear surface, is vital to the clinical success of orthokeratology. Imperfections such as scratches or deposits on the back surface or poorly finished fenestrations can be a cause of corneal staining. In orthokeratology, the lens back surface may be only 3-10 I-Lm from the underlying corneal surface in the center of the lens. There are three possible instruments that can be used to examine the surface - the slit lamp, the band magnifier, and the radiuscope. Since a magnification of 20x or more is essential to detect surface defects that have been caused by poor manufacture, the band magnifier is inadequate. The authors prefer to hold the cleaned lens gently at the slit lamp between thumb and first finger and rotate whilst looking for any pitting, scratches, lathe marks, signs of incomplete polishing, or burning and mottling. Surface burning or mottling results from excessive heat during manufacture and is likely to cause poor in vivo wetting.

There are some special features to note and record. These include:

•engravings

•laboratory codes

•fenestrations - number, position, size, and finish

•tint

•carrier design - assessed by edge measurement.

Engravings may include a dot or letter to indicate whether the lens is destined for a right or left eye. It is important that these are not too deeply sunk into the lens as they can give rise to accumulations of deposited material that cannot easily be removed. In the longer term these can cause ocular irritation. In addition deep engravings can weaken the lens, giving rise to splitting, perhaps after several months of use. Due to the ease with which lenses can be switched and the highly customized

nature of modern orthokeratology lens designs, the authors do recommend some means by which the patient can differentiate the right from the left lens. This may take the form of different tints for each eye. Certain laboratories prefer to mark the lens with a black dot from an indelible pen rather than permanently mark the lens. This is less than ideal as eventually the ink wears off and the issue of toxicity from the ink also arises.

Some laboratories engrave the lens with a code indicating the type of construction that the lens has. This might be vaguely useful if the patient goes to another practitioner who is unable to access the patient's complete lens specification, but, in the main, the clinical record should contain all the information required to duplicate the lens. It therefore seems a relatively pointless exercise to mark the final lens in such a way. However, the marking is very useful on a trial or inventory lens.

Fenestrations are often incorporated into orthokeratology lenses, particularly when night therapy is being used, as it is thought they enable adherent lenses to unbind more easily. A typical order is for three fenestrations, at 1200 intervals, just within the optic zone of the lens. Thus in verification, the practitioner establishes that the fenestrations are present, that there are the correct number, and that they are in the correct position. Ideally, the practitioner should specify the size of the fenestration, or at least be aware of what the laboratory usually produces. A figure of 0.2 mm is fairly typical. Even more important than the size of the fenestration is the quality of polish around it. It is most important to inspect each fenestration on the slit lamp to see that there are no sharp edges or splits around it. Generally, polishing fenestrations has to be done by hand and it is an area where laboratories with even the bestquality lathes may fall down. Some laboratories are now producing fenestrations using a laser.

Generally there are two reasons why RGP lenses are tinted. The first is to aid location once the lens is out of the eye. The second is to reduce any photophobia associated with the use of the contact lens and perhaps impart some degree of ultraviolet protection. In night therapy these latter issues are an irrelevance, but it is still worth ordering a tint as it helps the patient enormously in locating the lens and ensuring that it is in

the case after removing it in the morning. For daytime wearers the issue of reducing photophobia and offering some ultraviolet protection is no stronger than for a conventional RGP wearer. In fact, with modern lens designs and high-Dk materials, photophobia is rarely a reported symptom.

When verifying the tint of a contact lens, the important thing is to ensure that the color is as specified and that the density of tint is in line with expectations based on the thicknesses concerned. In typical orthokeratology work, the final thickness will not depart greatly from the trial lens thickness and so the trial lens will, in most cases, serve as a good reference for the tint hue and saturation.

The form of the carrier requires careful skilled interpretation. Where a negative carrier or lenticulation has been requested (normally to raise the habitual riding position of a positive lens), measuring the edge and junctional thickness is the best technique. In the case of a negative carrier, the junctional thickness should be less than the edge thickness. For an orthokeratology lens with a negative carrier and a positively powered BVp, the junctional thickness will typically be the lowest thickness recorded on the lens. This is found by carefully moving the lens in steps across the thickness gage (withdrawing the probe each time to avoid scratching) until the lowest thickness is measured. An alternative is to cast the lens in dental stone, as described above, but with the concomitant risks of breaking or scratching the lens.

Where a positive carrier or lenticulation has been specified, usually in the case of a negative lens that rides high, then the junctional thickness will be greater than the edge thickness. In fact, the junctional area in a negative lens will be the thickest point on the lens surface. Thus in a similar manner to that described above, the lens is moved over the surface of the thickness gage until the thickest point is found. This should be greater than the edge substance, demonstrating that a positive lenticulation has been made.

However, in night therapy different carrier forms have little effect in altering habitual lens resting position. In fact, it is a common observation that a lens that rides high or low with the eyes open in the primary position will produce a

236 ORTHOKERATOLOGY

perfectly centered zone of flattening on the cornea overnight providing it is correctly fitted. This indicates that the centration of the lens with the eyes closed is correct.

TOLERANCES

The dimensional tolerances for rigid contact lenses are set out in Table 9.1, which quotes from British Standard BS7208: Part 1: 1992; ISO 8321-1: 1991 (E).

Unfortunately, although all of these tolerances may be adequate for conventional RGP lenses, they are inadequate for reverse geometry orthokeratology lenses in the case of the standards for the BOZR. As has already been mentioned, this parameter needs to be specified to an accuracy of

± 0.02 mm to ensure reliable outcomes in orthokeratology. This imposes a high level of understanding between laboratory and practitioner if they are to work together in a professional manner. In addition, both parties must ensure that they have properly calibrated instrumentation to determine the lens radius. The laboratory has to be using a computer numeric-controlled (CNC) lathe with a good-quality diamond-tipped tool and a regular servicing calibration protocol. Although the standards on radius have to be tight, it is the authors' experience that a good laboratory can rise to the challenge and produce excellent, accurate reverse geometry lenses to the practitioner's and the patient's satisfaction.

Table 9.1 Dimensional tolerances forrigid gas-

Back venexpower ft(lf/l ± 125 0 to ± 1~hooD .;1: O~ Back vertex pqwer from pyer ± 14.~5 0

Prescribed prism

LENS DELIVERY

Assuming that a satisfactory trial of orthokeratology has taken place and the patient's lenses are verified to meet the specification, then the insertion and inspection of the final lenses should not produce any surprises. The fitting pattern should be as the trial lens and, assuming the initial overrefraction was performed carefully, the vision should be good. As has already been stated, generally in night therapy the lenses are left planopowered to give the best transmissibility profile. Thus there will often be a small, and insignificant, overrefraction. Obviously daytime wearers require full and accurate correction with the lenses in place.

The issue of anesthetic use was raised in Chapter 6. Whilst it is extremely useful to enable a rapid appraisal as to the quality of a trial lens fitting, by virtue of the reduction in reflex lacrimation, it arguably serves less purpose at the time of the final lens issue. Particularly when night therapy is being employed, comfort is not a major issue, providing the lens fit and material are appropriate. Even in daytime wear, the large diameters used in orthokeratology designs mean that the lens comfort is usually superior to any rigid lens the patient may have worn previously. Even soft lens wearers are frequently surprised, and vocal, about the high level of initial comfort with their new lenses. Furthermore, since handling instructions will usually immediately follow the issue visit with the practitioner, the cornea needs to have full sensitivity for these to be carried out safely. Therefore the use of an anesthetic is not recommended at this time.

The typical protocol to follow at the issue visit is as follows:

•practitioner to wash hands

•remove any existing lenses and check corneal integrity

•clean patient's lenses, rinse with saline, and apply wetting solution

•insert lenses and ask patient to take up fixation with head supported and gaze lowered

•after a few minutes, measure the visual acuity and check for residual overrefraction

•check quality of fit using slit lamp and barrier filter

•if all is satisfactory, patient is instructed on insertion and removal and lens care

•first aftercare visit is scheduled.

The importance of demonstrating good hygiene in all aspects of contact lens practice cannot be overstated. Whilst the risks of developing infection with all RGP lenses are low, patients need to be set a good and clear example of procedures to follow when handling their lenses on all occasions. All practice staff need to be similarly instructed.

The patient's existing lenses may be rigid or soft. In either case, they should be removed, cleaned and rinsed, and then stored. If patients do not have their case with them, then the practitioner needs to have a supply of temporary cases to hand, so that patients can leave the practice with their original lenses safely stored away in an appropriate solution. On the other hand, patients may want to leave the practice wearing their original lenses, in which case they can be reinserted, after the handling session, from the temporary case. This will usually be the case for night therapy patients.

From a medicolegal point of view, it is important to establish the condition of the cornea and the rest of the anterior segment immediately prior to embarking on orthokeratology treatment. To this end the cornea should be examined on the slit lamp using fluorescein, together with a rapid examination of the anterior segment. Given that this will have been done in some depth at the initial visit, this is a purely confirmatory check and need take no longer than a few seconds. Occasionally, a significant (usually corneal) lesion may be found which precludes immediately embarking on night therapy. This is typically due to wearing a torn or damaged soft lens. In these circumstances, the practitioner will need to follow the patient until the cornea recovers and then orthokeratology can begin.

In some practices, the verification of new lenses is performed by skilled technicians. It is important that the practitioner is confident that the new lenses are scrupulously clean prior to placing them on the patient's eyes. It reinforces good hygiene if the lens fitter is seen to clean and rinse the lenses prior to insertion for the first time. The solutions employed are described later in this chapter, but in the event of a polymeric bead cleaner being the

practitioner's choice, it is most important that the lens is thoroughly rinsed prior to insertion and the appropriate wetting agent applied.

In all cases of RGP lens fitting, initial comfort is increased if, after insertion, the patient takes up an inferior gaze position. The patient is instructed to rest the head against the head support (set so that the head is upright, not tilted back). The patient looks down slightly and the lens is placed on to the cornea directly from the fitter's index finger, whilst the upper lid is retracted with the other hand. Patients are then instructed to keep the gaze in the same position and encouraged to look at an appropriately placed object (or their hands). The upper lid is then released and they are instructed to blink gently. It is wise to tell patients who are embarking on daytime wear that they must be careful not to develop any strange blinking habits and consciously to relax the facial muscles to avoid any anomalous mannerisms or appearances.

It is most important to tell patients that the comfort can only improve from now onwards. Over a period of minutes they are encouraged to raise their gaze progressively until they are looking straight ahead. At this time the acuity can be recorded, where appropriate. A quick spherical overrefraction, perhaps using the duochrome chart, is then made to establish that the lenses are correctly powered. This procedure can be particularly rapid in the case of night therapy, where the vision through the lens is obviously of much less importance.

The lens fitting can then be checked. Initially, the degree of movement can be assessed using white diffuse light. Clearly a nonmobile lens at this stage would be unacceptable. Following this, fluorescein is instilled and the fitting pattern evaluated, using cobalt blue light and a barrier filter, as described in Chapter 6. Where the issued lens is identical in specification to the trial lens, clearly there should be no significant difference to the fluorescein pattern. There will be many occasions where the lens fitter has slightly altered the lens specification following careful appraisal of the corneal response trial. This is the moment when the practitioner gets the first opportunity of seeing whether the changes made have improved the fitting pattern further. Often the changes are sufficiently subtle that it is only by inspecting

the topographic change map at future aftercare visits that any improvement becomes obvious. Sometimes, particularly in the early days of learning to work with an orthokeratology lens design, the fitting is actually worse following the changes made and the practitioner has to reschedule the lens issue appointment and reorder new lenses.

THE INSTRUCTION SESSION

This is a crucial time for any new contact lens wearer, particularly patients embarking on a somewhat novel and different clinical technique like orthokeratology. They are about to start wearing rigid contact lenses, putting them into and removing them from their eyes. It may be the first time that they have tried to use contact lenses. The care and completeness of the dispensing appointment set the stage for the style of care the patients will exert themselves when wearing lenses.

Sloppy demonstrations suggest that hygiene is unimportant, when everything should point to the need for a rigorous system. Contact lens wearers will often modify their care systems with time and mostly in the direction of less rather than more care.

New contact lens wearers assume some of the responsibilities for the long-term success of contact lens wearing. They need careful training and complete instructions for them to be confident of their own skills and abilities and to understand their ongoing role.

Patients expect:

•training and instruction in the successful use of their lenses and the care system

•advice on the typical normal occurrences to expect

•a source of contact to deal with problems, emergencies, and adverse responses. In the case of overnight orthokeratology, a 24-h pager number is advised

•advice on ongoing care and necessary appointments.

New wearers need:

•self-confidence boosting with instructions on lens insertion and removal

•instruction in the proper care and storage of the lenses, use of solutions, and case hygiene

•clear instructions on the need to follow carefully the care system regimens and advice against changing any part of the system without reference to the practitioner

•a wearing schedule, where daytime wear is being employed

•warning of adaptive symptoms and how to deal with them

•advice on what to do in an emergency and how to obtain professional assistance in cases of difficulty.

A great deal of information is imparted on this occasion and it is useful to have printed instructions which can be given to the patient for future reference. These instructions should cover handling, insertion, and removal of lenses and routine use of the care system. This is reassuring from the patients' point of view as it provides an available source of information and guidance. It is also a source of protection for practitioners should they forget to mention anything and is desirable from a legal standpoint. It is worthwhile having patients sign a part of this booklet to say that they have received and understood instructions in the care and handling of their orthokeratology lenses. Figure 9.6 shows the suggested general advice written information for patients and Figure 9.7 shows the written handling instructions that might be issued. Where daily disposable lenses are being used and where patients have no experience with soft lenses, they will need to be shown how to insert and remove these. Figure 9.8 shows written advice that may be given to augment the practical instruction session.

WEARING SCHEDULES

Adaptation to orthokeratology contact lenses worn during the day is usually necessary and the daily wearing time is therefore gradually increased. Wearing schedules can be variable and the schedule should provide for the fastest passage possible through to complete daily wear, provided that patients are physically and visually comfortable in the process. There is no advantage in slow adaptation if proper safeguards are met. Typically, wearers start with 4 h on the first day, with 2-3 h added daily. Whilst it is often the case that these periods can be split into two sessions at

ADVICE ON CARING FORVOUR LENSES

•Hands must be clean andfree ofcreams. Alkl traces ofsoap must berinsed off. Avoid contamination of the lenseswith perfumes, hair spray etc.

•Whilst handling the lenses alway work overa smooth soft surface (hand towelspread over a

table). This avoids scratching or losing a lenson the floor.

•If the lensis dropped, do notmove untilthe lensis located. Always lift a dropped lensby wetting a fingertip or suction holder. lansesmustnot be slid across the table surfaces,

•Never Wipe lenses with a handkerchief or other material. If wiping is necessary use onlythe softest of papertissues (lint-free)

•To avoid distorting the lenses never leave them nearheatand handlevery gently. Avoid holding the lenses by the edges.

•Fingernails should be cleaned and keptreasonably short to avoid scratching the eyes.

Hygiene and storage

Absolute cleanliness is essential in the handling and storage of contact lenses. In orderto prevent dirt or harmful organisms beingtransferred to the eye.

The following rulesshould be observed:

• The lenses should be stored in a disinfecting solution intended for contact lens use.Solutions intended specifically for RGP lenses improve lenswettability and comfort.

•The case should be emptied andfresh solution should be usedeachday. The lensesshould be completely immersed in solution.

•The lenses should notbe stored dryas this mayaffect thecurvature of the lensandthewettability of its surface.

•The lenses maynormally be inserted directfrom the storage solution. This helpsthe surface

wetmore easily andforms a protective cushion in theevent of the lensbeing inserted tooquickly. If sensitive to the storage solution this can be rinsed off with salinesolution and a change of solution discussed with the practitioner.

•Remove lenses, clean withthe recommended cleaner and rinse off with saline, prior to placing the lenses in the casewiththe soaking solution.

•If working overa handbasinbe sureto put the plug in.

• Never lickthe lenses as harmful organisms can be transferred from themouth to the eye.

•Examine the lenses regularly for scratches and chipsand to ensure each lens is beingworn in the correcteye.

Figure 9.6 Suggested general advice issued to patients.

the beginning and end of the day, in orthokeratology the practitioner must consider how patients are going to correct their sight with the lenses out.

Where patients only have modest degrees of myopia «2 D), it may be that the period between the two wearing sessions will be spent at work and that they will function perfectly well with uncorrected myopia, replacing the lenses for the journey home. In the instance where the patient has a higher degree of myopia (>2 D) it may be necessary to give patients daily disposable lenses to use in the interval between wearing sessions. The decision on what power to use is similar to that with night therapy. Patients seem perfectly able to decide if they need to increase or decrease the daily disposable power as the days pass and indeed seem to enjoy performing a simple visual test each day to see if they are fully corrected. They will typically choose an object requiring good acuity to visualize, such as the clock on a domestic video recorder.

With night therapy no adaptation is required and patients will wear the lens for their entire sleeping period. This is very convenient for patients, but once again they will need to use daily disposables in the first week or so whilst their myopia is reducing.

Schedules should always be given with the proviso that daily wearing time can be held at the same level for some days or built up more slowly if there are problems. It should be stressed that the lens wear should not be continued if there is persistent discomfort. In these circumstances the practitioner should be contacted.

Where the handling and care instructions are given by a technician, it is worth giving a checklist to work from so that he or she can be certain always to give patients all the instructions they need. A sample list is shown in Figure 9.9.

Night-therapy orthokeratology patients should be given some preservative-free artificial tears

240 ORTHOKERATOLOGY

ADVICE ON CARING FOR YOUR LENSES

Insertion of RGP lenses

Figure 9.7 Suggested advice issued to patients on lens handling.

to instil on awakening. These help the lens to unbind and move. If the patient attempts to remove a bound lens, epithelial damage can occur and this is prevented by using the drops. Patients should be shown how to loosen up a bound lens after instilling the drop. This involves gently indenting the sclera immediately below the lens, allowing the tears to enter, and enabling the lens to move again. Patients can look in a

mirror to confirm that the lens is moving before removing it from the eye.

SCHEDULING AFTERCARE VISITS

When starting out in orthokeratology practice, practitioners are advised to schedule visits more frequently than later, so that they can appreciate the typical time course and learn the nuances of

ADVICE ON CARING FORYOUR LENSES

Figure 9.8 Suggested

refining the fit to give optimal results. A typical visit schedule is shown in Table 9.2. The superscript a denotes visits that the more experienced

practitioner may later only perform if there is an indication, such as when a change to the lens fit has been made at a previous visit.

be in place, so that the practitioner can see if the lenses have spontaneously unbound and that there is no significant corneal stain.

The investigations that should be carried out at this and all subsequent aftercare visits are summarized in Table 9.3.

The investigations listed in Table 9.3 will now be considered in more detail.

SYMPTOMS AND HISTORY

Patient education is an important part of orthokeratology. In order to gain valuable information from the patient the practitioner must teach the patient what to look out for and when to look for it.

Any symptoms reported after wear of the trial lens should be reduced with the final dispensed lenses. This should be the case for two reasons. Firstly, a week will have passed, allowing adaptation to occur and secondly, the fitting may have been further improved between the trial lens and the final lens, producing even better results. To elicit symptoms the practitioner should avoid "closed" questions like "Is everything alright?" A better starting point is: "Tell me how you feel about the vision when you remove the lenses," followed by: "Are you aware of the lenses at any

time?" and "How do your eyes look just before you take the lenses out?" Additionally the question: "When did you last sleep in the lenses?" should be asked in case the patient has missed a night's wear. In this case there is little point in examining the patient's cornea as clearly it is unlikely to show any adverse signs. Later on, more closed questions can be used like "What power of disposable lens are you using now?" and "Does the vision vary much during the day?"

Practitioners routinely using night therapy frequently remark how few symptoms patients report to them. Only mild irritation on rising, a definite absence of redness, and a slight increase in dried mucus at the inner canthi are typical observations reported by patients. Additionally, daytime wearers report typical mild symptoms associated with RGP lens wear: foreign-body sensation from time to time, mild 3 and 9 o'clock stain {never seen in nighttime wearers}, and occasional lens greasing.

It follows that any extra symptoms or more severe symptoms should be treated seriously by practitioners. The avoidance of any significant complications of contact lens wear should be our primary concern when examining patients at aftercare visits.