1. Quantify ocular parameters to aid selection of the first trial lens.

2. Confirm the normality of the ocular tissues and to record for future reference any acceptable abnormality (such as a corneal scar resulting from a historical eye injury).

3. Discover issues that potentially preclude or limit contact lens wear, (and manage or refer if necessary), or issues that indicate the need for a particular type of contact lens.

4. Allow the recording of baseline data against which to judge possible contact lens induced changes.

1. A pre-fit case history to determine what the patient wants from contact lenses, what they know about them and to help determine whether they are suitable.

2. Measurements to help determine lens parameters: Horizontal visible iris diameter, pupil diameter (average and mesopic), palpebral aperture and lid position, corneal curvature and regularity and subjective refraction (unless a recent refraction has taken place).

3. Assessments to help determine suitability for lens wear: Anterior eye health and tear film quality. Examination of the posterior segment is only included in the pre-fit examination if any new symptoms or signs indicate that further investigation is warranted or if there has been a significant time period since the last assessment.

4. Choose the trial lenses: After the preliminary assessment, you need to summarise your findings and discuss how they influence the choice of lens type. There may be a clear indication for a particular type of lens, such as single-use lenses for a patient who only wants to wear lenses 2–3 times a week; or there may be a range of possible options and you should discuss the pros and cons of each lens type (including cost and impact of lens care use) with the patient, so that they can make an informed decision.

5. Post-trial assessment: Assess the performance of the trial lenses in terms of fit, compatibility with the eye/tear film and visual acuity

6. Teach the patient to handle and care for the lenses: Ensure the patient fully understands the dos and don’ts of lens wear and the importance of lens and lens case maintenance.

7. Final check of trial lenses: Allow the patient to trial the lenses for a few days. Undertake the first follow-up check up with the lenses in situ and if everything is satisfactory, order the final lenses and provide the patient with a copy of their contact lens specification. Further changes to the lens and an extended trial may be required before the fitting can be considered complete, particularly for some toric, multifocal or complex lens fits.

5.2.1 Procedure for pre-fitting contact lens case history

5.2.2 Patient records

Both positive and negative patient responses must be recorded. Remember that from a legal viewpoint, if the response was not recorded the question was not asked. Use standard abbreviations (Table 2.1) and avoid personal ones. Using the patient’s own words, recorded in quotation marks, can be useful. An example is given below.

5.2.3 Interpretation

Interpretation of the data collected relies on an understanding of why questions are asked, and a good knowledge of lens characteristics such as wettability and lubricity (smoothness).

Selecting the most suitable lens modality, replacement schedule and lens type depends on:

Interpretation of the case history example in section 5.2.2 suggests that the patient wants lenses for occasional use only, specifically for contact sports (rugby), and therefore a single use lens would be the best option if available in his prescription. This will avoid the issue of lenses sitting in solution for extended periods between wearing episodes. In addition, mud in the eye and lens loss are common in rugby and therefore single use lenses are preferred for hygiene reasons. Regular swimming is yet another indicator for single use lenses, and tight fitting goggles over the top will reduce the risk of complications from swimming in lenses. The patient will need to be advised not to wear their lenses when they are suffering from a cold sore or at least to take extra precautions in terms of hygiene before handling the lenses. This patient may need more time spent with them for insertion/removal training due to their large fingers and possible squeamishness.

5.2.4 Common errors

5.3.1 Comparison of tests

Palpebral aperture height, HVID and pupil size are most commonly measured using a contact lens rule (Figure 5.1), with direct measurement of PA height and by matching the semicircles to the visible iris or pupil. Semi-circular templates and mm rules are equally accurate in estimating pupil size and measurements can be made in average and dim office illumination, but are not possible in scotopic illumination.¹⁴

Pupillometers are an expensive alternative and can provide pupil measurements at very low light levels, but the accuracy at higher light levels is similar to simple rulers and they are generally restricted to refractive surgery clinics, where pupil diameter measurements under controlled scotopic conditions are important.¹⁴

5.3.2 Procedure for PA height, HVID and pupil size measurement

5.3.3 Adaptation for pupil size measurement

5.3.4 Interpretation

The average HVID value is 11.6 mm with a range of 10.2 mm to 13.0 mm.¹⁵ The rule-of-thumb for calculating corneal RGP total diameter is at least 2 mm smaller than HVID and for soft lenses is at least 2 mm greater than HVID. It is also useful to consider the corneal diameter when selecting the first soft trial lens as lens fit is most closely related to the sagittal height of the cornea, which is dependent on a number of factors including curvature and diameter. Larger corneas generally need a flatter base curve and smaller corneas, a steeper base curve.¹⁶

The maximum pupil diameter under low illumination is of interest when selecting an RGP trial lens. To minimise the risk of flare and halos at night, particularly in someone who drives for a living, the back optic zone diameter of the contact lens should be at least 1mm larger than the maximum pupil diameter, or even larger still if the lens is lid attached and sits superiorly.

The average palpebral aperture height is around 9.75 mm, commonly ranging from 9.0–10.5 mm. The size of the PA influences the RGP lens fit. A particularly large PA will result in an interpalpebral RGP fitting. A smaller PA means that a larger proportion of the superior cornea is covered by the lid and lid attached fitting is likely. The position of the lower lid has an effect on the success of translating multifocal RGP designs. Such lenses require interaction between the lower lid and lens in down gaze, in order to displace the lens upwards so that the near portion lies over the pupil. With a lower lid that is 1 mm higher or lower than the inferior limbus, translating designs are less likely to work. Such designs also require reasonable tension of the lower lid to facilitate translation.

The angle of the lids, both in the stationary position and with a blink, should also be noted as this can influence the rotation of toric soft and front surface toric RGP lenses. An eye with oblique lid alignment or an unusual lid movement on blink is more likely to suffer from rotational instability if fitted with a soft toric lens.¹³

5.3.5 Most common errors

5.4.1 Comparison of tests

The traditional method to assess corneal topography was using a keratometer. This is an instrument that projects a symmetrical image onto the corneal apex, using it as a reflective surface and applying the optical principles of a convex mirror to estimate the radius of curvature at two assumed perpendicular principal meridians. When more information is needed an instrument called a corneal topographer or videokeratographer or videokeratoscope is used. To collect more data points from the corneal surface these instruments project a Placido image (series of concentric rings) onto the cornea (Figure 5.2) and the reflected image is captured by a high resolution CCD camera and the data are processed by a computer. In addition to providing more detailed information, the area covered by these projected rings is wider than that of a keratometer, up to 10mm compared to 3.0 to 3.5 mm. Topographers will also calculate nominal primary meridian values for the two principal meridians for RGP lens fitting, negating the need to use a keratometer as well as a topographer. The increased accuracy and information obtained with a corneal topographer and their lowering price tag mean the days for keratometers are numbered. Anterior segment optical coherence tomography (OCT; section 7.11) provides valuable information about the real sagittal height of the cornea which is more predictive of soft contact lens fits than either keratometry or topography measurements.¹⁷ Knowledge of the profile of the corneo-scleral junction is helpful when fitting gas permeable scleral and semi-scleral lenses, both of which are seeing a resurgence in use for both irregular and regular corneae. OCT is likely to become more widely used in contact lens practice in the future.

5.4.2 Procedure: corneal topography

While there are many different makes of corneal topographers, the following is a guide that is applicable to most, but you should also refer to the user guide for your specific instrument.

5.4.3 Procedure: Bausch and Lomb one-position keratometer

5.4.4 Procedure: two position variable doubling type keratometer

5.4.5 Recording

Topography results are usually printed and attached to the records (Figure 5.5). Many topographers display the corneal curvature data in a variety of ways:

5.4.6 Interpretation of corneal curvature measurements

Corneal topography: This has the advantage of showing the corneal height over a wide range of locations, making it easier to differentiate between symmetrical astigmatism and asymmetrical astigmatism. This can be easily observed by viewing the colour map. Astigmatism is represented on the map by the appearance of a bow-tie pattern. For symmetrical astigmatism this bow-tie pattern is seen as even bows on both sides (Figure 5.5). As the asymmetry increases, the disparity in the size of the bows increases. Increasing asymmetry in the pattern could be an indicator for correcting this astigmatism with soft toric lenses rather than RGP lenses, which may decentre due to the profile asymmetry. A pattern indicative of early keratoconus has an apex that is decentred inferior nasal (Figure 5.6). However, kerataconus can produce a range of different topographical patterns. Large changes in the degree of astigmatism within a short time can be indicative of keratoconus, lid neoplasms, pterygium, or a chalazion. Large changes in spectacle astigmatism without corneal astigmatic changes in the elderly are likely due to cortical cataract. Corneal curvature measurements can also be used to help indicate whether ametropia is refractive or axial. For example, a patient with increasing myopia but no change in corneal curvature probably has axial myopia. An anisometrope with different curvature readings probably has refractive anisometropia, while an anisometrope with similar curvature readings probably has axial anisometropia.

Keratometer readings: The power of the anterior corneal surface (Fc) is estimated from the radii measurements (r) using the equation Fc = (n–1)/r. The refractive index (n) of the cornea is about 1.376, but most keratometers use a value for n of 1.3375. The lower value for n is intended to compensate for the negative power of the posterior corneal surface. It is assumed that the posterior surface reduces the overall corneal power by about 10% but this amount varies between individuals. This also assumes that the two surfaces have the same proportion of astigmatism. Other factors that lead to errors in keratometry readings include the assumption that the cornea is spherical (most are elliptical) and that the visual axis runs through the corneal apex, which it usually does not.

A small radius value means a steep corneal surface, which is more powerful and more myopic (or less hyperopic). Larger radii mean flatter surfaces, which are less powerful and more hyperopic (or less myopic). The anterior radii of curvature of the cornea are usually between 7.25 mm and 8.50 mm, with myopes having steeper (smaller) radii and hyperopes having flatter (larger) radii. Dioptric powers range between 46.50 D and 40.00 D, and the anticipated corneal astigmatism is usually less than 2.00 D.

5.4.7 Most common errors

5.5.1 Comparison of tests

To minimise error and reduce unnecessary chair time during the fitting of contact lenses, it is prudent for the patient to have undergone a recent eye examination prior to contact lens fitting, so they should attend with a copy of their spectacle prescription, which can be used as the starting point. If the visual acuity is similar in the two eyes and the patient is binocular, a binocular refraction is a quick way of confirming the prescription (section 4.12). Prescriptions greater than ±4.00 DS will require a correction for back vertex distance before selecting a trial contact lens power.

Wavefront aberrometry allows the higher-order optical errors of the eye to be measured in addition to sphere and cylinder. In eyes with corneal irregularity as a result of keratoconus, for example, higher-order aberrations have a much more significant impact on visual quality. Wavefront aberrometry is likely to play a more prominent role in custom contact lens fitting in the future as lenses capable of correcting some higher order aberrations become available for keratoconic patients.¹⁸

5.5.2 Determination of power for a correction greater than 4.00 DS

For spectacle refractions of >±4.00 D, a correction for back vertex distance (section 4.12.3) is required.

5.5.3 Determination of power for a soft contact lens

In the situation where a soft spherical lens is fitted to an eye with an astigmatic prescription, the power of the trial lens should be the mean spherical correction, i.e. sphere + half the cylinder. For example:

When fitting a spherical RGP lens to an eye with corneal astigmatism, the toric tear lens trapped between the lens and the eye neutralises the astigmatism, and therefore the trial lens power should be that of the spectacle sphere power alone, rather than the mean spherical error. In the above example, the trial lens power would be −2.00 DS.

1. To examine the health of the anterior portion of the eye and adnexa.

2. To look for issues that potentially preclude or limit contact lens wear, or indicate the need for a particular type of contact lens.

3. To allow the recording of baseline data against which to judge changes over time, contact lens induced or otherwise.

5.6.1 Procedure for preliminary slit-lamp and tear film assessment

For a detailed description of slit-lamp examination and tear film assessment, see sections 7.2 and 7.3. This section describes the procedures for assessing the tear film, which are generally undertaken as an additional component in the slit-lamp biomicroscopy examination. The tests shown to best predict contact lens induced dry eye in new wearers are symptoms, such as late in the day dryness. These symptoms can be reliably quantified using questionnaires.^20,21 The signs useful as predictors are non-invasive tear break up time and grading of lid parallel conjunctival folds.²²

5.6.2 Recording

It is useful to record and grade everything seen to provide baseline information. For example, grade 1 papillae on the lateral margins of the superior lid (online video 5.1) would not be considered abnormal but it is invaluable to note this prior to contact lens fitting. A number of grading scales are available but the most commonly used scales are from the Brien Holden Vision Institute (www.contactlensupdate.com/wp-content/uploads/2011/05/Grading_Scales_web.pdf) and the Efron grading scales, which are standardised images of common complications at different levels of severity.²³ Try to use just one grading scale, as this will improve your grading accuracy and repeatability over time. The detection of clinical differences can be improved by recording findings in 0.1 steps as these scales are generally quite coarse.²³

5.7.1 Soft lens application

Patients will be anxious about lenses being applied to their eyes for the first time and you need to try to put the patient at ease (section 2.1). Patients need to be comfortable and feel part of the process, this must involve an appropriate informed consent process, where explaining what you do at each stage and answering any questions they may have is key.

5.7.2 Problem solving

5.7.3 Removal procedure

5.7.4 Most common errors

1. Lens replacement frequency: The majority of soft lenses are disposable and replaced either daily, two-weekly or monthly. Deciding the replacement frequency starts the process of restricting your choice of lens material.

2. Lens material: Due to their increased oxygen transmissibility, silicone hydrogel lenses are now the preferred option. When a silicone hydrogel lens material is not selected, this is usually because a parameter needed to fit that patient is not available. It may be that the best option is a daily disposable lens and a parameter is only available in a hydrogel material.

3. Lens power: This will be based on the patient’s refraction, remember to convert the spectacle refraction to the ocular refraction when the back vertex distance change impacts the refractive result (>±4.00 D; section 5.5.2).

4. Base curve: While traditionally the lens base curve selection is driven by the patient’s keratometry results, most soft lenses are available only with one or two base curves. For soft lens fitting, keratometry is therefore only really useful to establish normality as the lens with the flattest base curve is trialled first. You should change to the steeper base curve only if lens fit was not acceptable or if the patient complains of lens awareness.

5. Total diameter: The lens edge must always rest on the conjunctiva well clear of the limbus. Lens edges that align with the limbus will result in complications, reduced wearing times and discomfort. This parameter is theoretically determined by measuring the horizontal visible iris diameter (HVID) although lenses are rarely available in more than one diameter. So if the lens you have selected is too small, you are likely to have to select another lens brand. Lenses that are too large may simply be too hard to remove.

5.9.1 Initial brief assessment of lens fit

Having selected the best lens and placed them on the patient’s eyes, the following procedure should be followed:

At this point the lens has probably not settled sufficiently to make an accurate assessment and it is often preferable to send the patient out of the consulting room, let them try the lens and see the world. If your risk assessment has determined that the patient is wearing a well-centred lens with acceptable acuity, say 6/9 (20/30) or better, then send the patient out for a walk, or if the vision or fit is questionable let them have a seat in the waiting room. The settling period should be for at least 10 minutes but may be longer if it fits better with your appointment schedule and the patient.²⁴

5.9.2 Detailed assessment of lens fit

See online videos 5.2-5.8. As part of the detailed assessment you will need to carefully measure the vision and assess how the lens is fitting. As both of these are interlinked, the order will be determined on how the patient is doing.

5.9.3 The ideal lens fit

In the primary position the lens should be well centred with an even lip of lens extending beyond the limbus. The centration is graded with reference to the centre of the pupil (or the HVID). A grade of +3 (poor) is not acceptable and is thus only likely to be seen during an unsuccessful initial lens trial. A natural blink should induce movement that is observable but that does not move the lens edge, such that it encroaches on the limbus. If a lens moves excessively where the edge abuts the limbus, a tighter lens will be needed. If no movement is observable but the lens moves easily during the push up test then the fit is acceptable. When a lens does not move during the push up test then it is considered too tight and needs to be refitted with a looser fitting lens.

5.9.4 The extended trial

If the lens is well centred, shows good movement and the patient is comfortable, you have a lens that can be used for an extended trial. This would be over a couple of days, so would require the patient to have been taught how to handle and care for the lens. The length of the extended trial should fit in with your appointment schedule as well as be convenient for the patient and therefore varies from 3 days to 2 weeks. When the patient returns, treat the visit as the first aftercare visit. As the lens will have demonstrated good vision and good fitting before being dispensed, the likely issues will relate to adaptation, lens handling and care.

5.9.5 Solving lens fitting problems

5.9.6 Over-refraction, VA and ocular motor balance

5.9.7 Post-fit health check

See online videos 5.9-5.11. Having removed the trial lenses, the final act before writing the lens order is to check the eyes for unwanted signs of contact lens wear. This involves a slit-lamp biomicroscope examination of the cornea and conjunctiva, looking for fluorescein and lissamine green staining (section 7.3). The findings should be recorded in a diagram and graded using the CCLRU or other grading scale, and compared with the preliminary assessment prior to lens insertion. Very occasionally the findings of the post-fit check indicate that the lens fitting process must begin all over again: circumlimbal staining of the conjunctiva indicates an interaction between the lens edge profile and the conjunctiva.²⁷ Lens edge designs vary significantly and swapping to a rounder edge profile can eliminate the staining. Alternatively, the high modulus of the lens may be the cause and a less stiff material can be trialled. A tight fitting lens can also cause such staining but this should have been identified during the fit assessment.

Another finding that would indicate the need to refit would be signs of corneal (punctuate fluorescein staining) and conjunctival (lissamine green staining) desiccation, as the result of a lens material incompatible with a poor quality tear film. Don’t confuse solution-induced staining with a lens fitting staining appearance (Figure 5.9) as the management to resolve either is different, i.e. with the first, you need to change the lens care system; with the second, you need to change the lens material or design.²⁸ With careful history taking and a thorough ocular and tear assessment informing the trial lens choice, this should be avoided.

5.9.8 Most common errors

5.10.1 Trial lens selection

The majority of off-the-shelf toric lenses are only available in one base curve and diameter, therefore trial lens selection is simple but if the lens does not provide an adequate fit, a different brand must be trialled. These lenses are generally not available in every cylinder power and axis combination, with many brands offering between two and four different power options, for example 0.75 DC, 1.25 DC and 1.75 DC. The practitioner must select the nearest astigmatic power with the rule of thumb that it is better to under-correct than over-correct, as a smaller cylindrical correction gives less induced astigmatism if the lens does rotate, leading to more stable vision. Custom made toric lenses with any power, axis and dimensions are also available in a wide range of materials, but they are more expensive and therefore tend to be offered on a less frequent replacement schedule and never as a single use lens.

The trial lens power will require the spectacle prescription to be adjusted for back vertex distance (BVD) if one or both meridians have a power greater than 4.00 D. To do this, the power of each individual meridian is calculated, the BVD adjustment is applied to each meridian and the resulting powers are recombined (see section 5.5.2). The initial trial lens is selected in much the same way as a spherical trial lens with the frequency of replacement determined by the initial discussion of patient needs, the lens material determined by the history and preliminary examination (section 5.2), the diameter determined by the HVID measurement (section 5.3) but also the availability of a suitable cylinder power and axis. The method of stabilisation is also a factor and in patients with a similar prescription in each eye, it can be informative to trial a lens with a different stabilisation method in each eye.

5.10.2 Toric lens fit assessment

See online videos 5.12-5.14. The fit of a soft toric lens is assessed in the same way as a spherical soft lens (section 5.9). The only differences are that the lens should be left to settle for around 10–15 minutes before assessing and the orientation of the lens must also be examined.³⁰ Rotational stability is essential for good vision with toric lenses, and the lens must remain reasonably stable as the eye moves around to avoid inducing astigmatism and blurring vision. It can be worth examining the fit and orientation before over-refraction as there is no point in checking the power of a lens that has poor rotational stability. To check the orientation:

If the rotational stability of the lens is poor and varies with eye movements, a lens brand that uses a different stabilisation mechanism should be trialled instead. A lens that consistently rotates by the same amount in the same direction is perfectly acceptable as the axis of astigmatism can be adjusted using two possible methods:

Whether you use LARS or CAAS the result is the same. This allows a contact lens to be ordered with an axis of astigmatism that differs from that of the spectacle prescription, knowing that this lens design will rotate to position the astigmatic power correctly.

For example: Figure 5.10.

5.10.3 Over-refraction of toric soft lenses

If a lens is not orientated correctly, the astigmatism of the lens will combine with the uncorrected cylinder of the eye and the resultant astigmatism will have an axis part way between the two, but there is no point in trying to measure this. A different lens should be trialled that either has a different axis of astigmatism (if the orientation is wrong but consistently at the wrong axis), or has a different stabilisation mechanism (if the orientation varies significantly with blinking and eye movements).

5.10.4 Common errors in toric soft lens fitting

5.11.1 Comparison of correction options

Using distance contact lenses with reading spectacles (known as over-readers in this situation) is probably the most commonly used option and as long as the patient accepts wearing reading spectacles, it is very successful. Morgan et al. reported 63% of presbyopes wearing contact lenses were not wearing a multifocal contact lens or monovision.³¹ Wearers do not have to be refitted with new contact lenses, just provided with over-readers, in the form of reading spectacles or half eyes (both of these can be in the form of ready readers and thus at relatively little cost) or spectacle multifocals (i.e. with a plano distance portion). There is no additional visual compromise unlike other contact lens corrections for presbyopia and this is the obvious choice for patients who have a high visual demand, whether for distance or at near, e.g. a patient who drives at night for a living. However, some patients will not appreciate the ‘ageing’ appearance and/or inconvenience of wearing over-readers and will prefer other options. Monovision provides a correction for distance viewing in one eye (the dominant eye) and near viewing in the other and so they are simple to fit. Current contact lens wearers only need a change in lens power in one eye and no refitting into a new design or material is required and lens costs are the same as distance only contact lenses. Patients who wear toric lens designs are ideal for monovision because although there are some custom designed soft lenses that provide correction for both astigmatism and presbyopia, these are not available as disposable contact lenses. Monovision is successful so long as the patient can tolerate the induced blur, reduced stereopsis and possible retinal rivalry.³² As the reading addition increases, this blur increases and particularly when above +2.00 D may lead to symptoms especially when driving at night, where it is difficult to suppress the bright myopic blur circles seen by the non-dominant eye. Introducing modified monovision may solve these problems or refitting with a pair of distance contact lenses with over-readers. Monovision used to be the preferred option for correcting presbyopia mainly due to the poor optical designs and poor reproducibility of multifocal contact lenses, but significant improvements in design and manufacture of multifocal contact lenses has changed that preference.³¹

Because monovision has been successfully used for many years to correct presbyopia, a modified version has been developed that incorporates multifocal lenses, by correcting the dominant eye with a distance bias multifocal and the non dominant eye with the near bias multifocal or various combinations of single vision lenses and multifocal lens designs. The suggestion is that these multifocal lenses provide functionality for both distance and near and thus reduce the disparity between the eyes as well as the blur. You are likely to try different combinations to get the correct visual balance and so may consume more chair time.

Studies comparing multifocal lenses with monovision now report patient preference toward multifocals and away from monovision.33,34 The most commonly used multifocal lenses use a simultaneous lens design and are typically distance biased lenses, where the central portion of the lens contains the distance power and is surrounded by an increased relative positively powered region providing the near element. When a high reading add is required, near biased design lenses (the central portion contains the near power and is surrounded by an increased relative negatively powered distance vision region) will need to be used resulting in a marked reduction in distance correction and explains why a multifocal design has a reduction in performance for distance vision as the reading addition increases.³⁴

These optical portions can be formed by two distinct spherical zones but are usually aspheric and create a multifocal progression from the distance portion to the near portion. The majority of lens designs are the latter: aspheric and centre distance. Another multifocal approach uses multiple concentric rings of alternating distance and near powers and are available in both low, medium and high reading additions. Situ et al., reported on the success of this lens design when refitting existing monovision wearers, with 53% still wearing the multifocal contact lens after one year.³⁵ Multifocal contact lenses have high levels of subjective preference and are very successful for lower reading adds, but there is a greater compromise in vision as the reading addition increases.³⁶ As a consequence, it is likely that distance contact lenses and over-readers become the preference as the reading addition increases. Disadvantages include the increased cost compared to single vision lenses and the fact that the multifocal design that works for your patient may not be available in the material that works for that patient.

An additional method to correct presbyopia with contact lenses uses translating designs where the contact lens has a distance correcting area and a near correcting area. In the primary position of gaze the distance vision area aligns with the primary visual axis and on downward gaze the lens translates such that the near vision area aligns with the visual axis of the eye. Translating soft lenses are only available in one design, which is not available as a disposable contact lens.³⁷ This limited availability restricts their usefulness and possibly reflects the limited success achieved with them; they are not frequently fitted due to their poor translation (limited unpredictable movement), increased discomfort (lenses are thicker and usually prism ballasted) and are only available as custom designs. These lenses are complex to fit and beyond the scope of this book. However, translating designs lend themselves well to being manufactured in RGP materials.

5.11.2 Fitting contact lenses for presbyopia

When assessing the performance of multifocal contact lenses or monovision it is important to let the patient have an extended trial with the lens and let them take them away for a few days. The optimum trial length need not be longer than 2–3 days.³⁸ Papas and colleagues also concluded that the reliability of clinical measures to determine success was limited and subjective responses were more reliable; ask the patient what they think about their vision.³⁸ Do they think it is successful? The assessment of visual acuity is used to optimise the lens power and to act as a safety measure, i.e. the driving standard.³⁸

Fitting the lenses is otherwise the same as that described in sections 5.9 and 5.10. In addition, note that:

5.11.3 Solving problems

Remember that patients with presbyopic contact lenses can suffer with all the problems of standard lenses. For example, symptoms of fluctuating vision should lead you to reassess the patient’s tear film stability and the lens fit. In addition:

5.12.1 Comparison of lens types

All RGP lenses have posterior (back surface) curves that are classified as follows:

The cornea has a shape that is similar to an oblate ellipse; the curvature progressively flattening from the corneal apex out to its peripheral area at the limbus. RGP lens back surface designs try to mimic this shape change and do so generally using one of two approaches:

Spherical designs have the advantage that you can change any curve to change the fit of the lens. This allows you greater control in designing the lens, for example, changing the optical area of the lens, increasing or reducing the edge lift, width or height. It is not possible to change an aspheric design other than the BOZR or the lens total diameter. However, the differing zones of a spherical design will have visible demarcations and these may present problems for patients with larger pupil diameters as the edge of the optical zone may lay within the pupil area and result in symptoms of blur or glare. The junctions between the different zones may compromise corneal physiology and leave concentric rings or compressions where lens and cornea touch. These do not occur with aspheric lenses unless the lens itself is too small. These lenses also generally show a closer alignment fitting relationship between the back surface of the lens and the cornea, which some consider to be desirable, although too close alignment in fit may cause the lens to bind.

5.12.2 Observing how an RGP lens fits

The fitting relationship between the posterior surface of a contact lens and the front surface of the cornea can be observed by instilling fluorescein into the tear film and illuminating the eye with ultraviolet/blue saturated light from a Burton lamp or a slit-lamp using low magnification. Changing various aspects of the peripheral curves of the RGP lens can change the fitting pattern seen.

Some lens materials contain a UV inhibitor and so appear ‘black’ under UV light. This prevents observations of the fluorescing tears behind the lens and thus an assessment of the lens-to-cornea fitting relationship. When using materials that contain a UV inhibitor, observations can only be made with a slit-lamp, which provides a significant amount of blue light, but not a Burton lamp as it has a predominantly UV light source.

The basic goal when fitting an RGP lens is to ensure:

5.12.3 Simplified fitting approach

The starting point when fitting an RGP lens is to determine three parameters: the BOZR, Back Optical Zone Diameter (BOZD) and total diameter (TD). The following can be applied to either a standard spherical lens design or an aspheric design:

Once these parameters have been determined for both eyes, you have your first pair of trial lenses to assess. The following procedure is suggested:

5.12.4 Assessment of lens fit

See online videos 5.15-5.18. You are interested to know whether you have the desired fluorescein fitting pattern and how dynamic the fitting pattern is. Having instilled the fluorescein (section 7.3.2), assess the fit using a slit-lamp as follows (Figures 5.11 to 5.14).

5.12.5 Assessment of lens (and tear film) power

Once you have a lens that appears to have an acceptable fit for each eye, perform a spherical over-refraction and measure VA (sections 4.6 or 4.7). Alterations in the fit of an RGP lens can have very subtle changes to the patient’s vision and you may find two fits that you think would work but the patient reports a preferable visual performance. Where you find this, go with the lens of optimum fit and preferred vision, alternatively select the lens with the steep BOZR.

5.12.6 Correcting astigmatism with RGP lenses

Variations in corneal shape can be neutralised by the tear lens and a spherical RGP lens can mask corneal astigmatism. However, as the astigmatism increases, the fit of the lens becomes increasingly compromised resulting in decentration, instability or increased discomfort. To compensate for this, it is common practice to steepen the BOZR of the RGP lens using the following rule of thumb:

5.12.7 Ordering lenses

On completion of the trial fitting an RGP lens can be ordered for the patient, the details required for standard or aspheric designs are: BOZR, TD, BVP and lens design.

Most lens manufacturers will allow you to exchange lenses if you need to modify how it fits or adjust the lens power for a period of up to three months. RGP lens manufacturers understand the complexity of fitting RGP lenses and are very supportive and knowledgeable, especially about their lenses, and can help you to get it right. If something does not make sense, ask them and listen to their advice as it is in their interest you get it right.

5.12.8 Common problems

5.13.1 Lens care and maintenance

Other than single use lenses, all other contact lenses need a cleaning solution, a disinfecting solution and a condition solution and these requirements are usually combined as a multipurpose lens care system. As RGP lenses are not replaced as frequently as soft lenses, patients must be educated on the need of supplemental cleaning processes. With age, lenses become increasingly deposited (deposit removal may be needed on an ongoing basis) and scratched. You may consider introducing a planned replacement scheme whereby lenses are replaced on an annual basis.

5.13.2 Contact lens application and removal training

Spending the necessary time to teach a patient to handle their lenses correctly is an important part of the lens fitting procedure. It is an activity that is often delegated to support staff, which is fine as long as they are well trained and able to answer the patient’s questions. Contact lens handling should be taught in a quiet area with good quality diffuse lighting that is away from the communal areas of the practice, to avoid putting pressure on the patient to perform in front of others. The patient should not be allowed to take the lenses away until they can safely apply and remove them and they have demonstrated a full understanding of the importance of hygiene, the cleaning regime and how to minimise the risk of complications.⁴⁵ With all aspects of compliance, it is important to give patients reasons why certain steps should be included, to encourage them not to drop them from their regime.⁴⁶ Do not simply give them a list of instructions. Areas to cover include hand washing, lens inspection, lens application technique, lens removal technique, lens cleaning, case cleaning, lens and case replacement, recommended wearing times, minimising risks: contact with water, napping, illness and when to seek advice.

The lens application procedure for a patient is almost identical to that for the practitioner (section 5.7.1), but the patient should lean over a mirror or use a mirror stood on a clean surface; for their right eye, they should position the lens on the very tip of the right index finger and use their right middle finger to pull down the lower lid and the left hand to lift the upper lid and it can be easier to place the lens directly on the cornea rather than on the conjunctiva and sliding the lens.

5.13.3 Cleaning lenses

With the exception of single use lenses, all contact lenses require cleaning after every wearing period. The current recommendation is for lenses to be rubbed and rinsed with the cleaning fluids prior to storage, so as not to leave the storage solution with too much to do in terms of microbial load.⁴⁷ Specific instructions vary between brands and types of lens solution but are summarised below:

5.13.4 Case cleaning and replacement

This is just as important as good lens care but frequently overlooked.⁴⁸ The patient should be shown how to clean their case and the importance of removing the biofilm should be explained.

• Identify any difficulties the patient may have with their eyes and lenses.

• Ensure that the patient’s needs are met by the current contact lenses.

• Consider the lens parameters and make changes where indicated.

• Ensure that the lenses are not having an adverse effect on the eyes and manage where necessary.

• Check patient compliance with lens wear and care instructions and re-educate where required, to maximise healthy, successful lens wear and reduce the risk of future complications.

5.14.1 Patient discussion – aftercare history taking

5.14.2 Over-refraction, acuity and ocular motor balance

See section 5.9.6.

5.14.3 Assessment of lens and lens fit

See online videos 5.19-5.21. Assess the fit as described in sections 5.9 (soft lenses) and 5.12 (RGP lenses). An unacceptable fit requires you to trial a different lens or book the patient in as a matter of urgency for a refit.

Whilst the patient is on the slit-lamp biomicroscope, the integrity of the contact lens and the condition of the lens surface can be examined. Specular reflection off the pre-lens tear surface can be very informative (section 7.2.3) and using the placebo disc from a topographer is also useful to assess the global impact surface deposits are having on the tear film stability, by observing the disruptions in the quality of the placebo disc image as well as measuring the non-invasive break-up time (NIBUT; section 7.3.5). All lenses need to interact with the tear film and it is inevitable that the lens surface will become deposited; in fact some of the interactions and deposition are desirable as they help condition the lens and contribute to its wettability. Clinically observable lens surface deposition has been shown to occur within minutes of wear and significant amounts after just one day of wear.^51,52 The level, composition and appearance of deposition on the lens surface is influenced by the tear film, environmental variables such as air quality, lens care products used, cosmetic and skin care products and industrial environments (particles and vapours), the replacement frequency of the lens and the lens material.^53,54

5.14.4 Management of deposition

If a patient presents with lens surface deposition that is clinically significant, the following strategies should be considered:

5.14.5 Observation of lens removal

The point in the routine where the lenses need to be removed provides an opportunity for you to observe the patient’s contact lens handling skills. It is also useful to ask the patient to demonstrate their lens cleaning technique at this point. Check:

Deficiencies should lead to re-education (section 5.14.7).

5.14.6 Check ocular health

Following removal of the lenses, perform a slit-lamp biomicroscope examination of the anterior segment (section 7.2), including staining with fluorescein sodium and lissamine green, lid eversion (see online video 5.22) and tear assessment.

Topography or keratometry (section 5.4) should be undertaken to assess the corneal shape to look for changes, particularly in terms of regularity. Unlike the days of hard polymethylmethacrylate lenses and thick hydrogel soft lenses, corneal warpage is now rare but it occasionally occurs with RGP and tight fitting, high modulus silicone hydrogel lenses.⁵⁹

Examine the posterior segment of the eye (section 7.10) unless it has been undertaken during a recent eye examination and nothing to raise suspicion, such as unexplained symptoms or visual acuity loss, has occurred since. Be mindful of the possibility of pathology in contact lens patients. For example, do not making protracted changes to a patient’s contact lens specification over a period of weeks in response to a symptom like ‘vision not quite right’, without checking the whole eye for signs of pathology.

5.14.7 Summary of outcomes and re-education of patient

Finally, you need to create an action plan and discuss it with the patient. This summary must include the importance of regular aftercare visits and should cover the following:

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