Contact lens assessment

5.2.1

The evidence base: The importance of the pre-fitting case history

Many of the issues covered in section 2.3 also apply during a contact lens examination, such as the importance of communication and putting the patient at ease. Trying contact lenses for the first time can be a very daunting process for some patients; a common worry is that the lenses will cause pain when they are placed on their eyes. Encourage them that, at worst, the sensation is similar to having an eyelash in their eyes and at best they are simply not aware the lens has gone in. Also, reassure the patient that any discomfort will have significantly reduced within a week of starting wear. Spending sufficient time to fully understanding a patient’s wants from contact lenses, determining what they know about them, explaining the issues, and managing their expectations, are important factors to limit contact lens drop outs. Make it clear from the start that a successful fit may require more than one appointment, particularly in the case of complex lenses—correcting astigmatism, presbyopia, controlling myopia, or more complicated lens designs. Finally remind the patient that regular aftercare is essential. It is often useful to make your answers to questions more of a tutorial embracing areas, such as lens types and designs, hygiene, and wearing times, thereby increasing information exchange. Suitability may be determined by clinical, social, or financial constraints. Motivation may depend on social, occupational, sports, refractive, visual, or psychological factors. In the case of myopia progression management, a careful consideration of balancing the needs of patient and that of the parents’ expectations is required.

5.2.2

Procedure for pre-fitting contact lens case history

• 1.

  Observe your patient; their ability to speak and articulate, intellectual capacity, emotional state, cleanliness, length of fingernails, use of eye make-up, size of fingers, roughness of skin, and dexterity.

• 2.

  Consider their age and gender. For example, older females are more likely to have poorer tear quality, requiring careful lens material selection and perhaps the use of ocular lubricants. For myopia progression management, the maturity of the wearer and their ability to wear lenses independently from their parents should be considered.

• 3.

  Ask the following questions:

  • (a)

    Why do you want to wear contact lenses? What has sparked their interest in contact lenses? Any previous history of contact lens wear should be investigated thoroughly to determine previous lens types worn or trialled, and reasons for discontinuing use. Do not be afraid of refitting a former lens wearer because many contact lens dropouts are owing to poor compliance, or associated with older lens designs and materials, rather than a lack of patient suitability.

  • (b)

    What would you like to wear the lenses for? This will range from complete replacement for spectacles to occasional social wear. Some may wish to sleep in lenses for convenience or practical reasons (e.g., travelling abroad, antisocial working hours). ^, Most people need lenses that allow for the occasional nap in lenses, for example, on the train home from work. Reportedly, over 60% of single-use soft lens wearers had napped or slept in their lenses.

  • (c)

    What do you know about lenses? This is an opportunity to explain the different types of lenses, their pros and cons. Include the cost of the different types of lenses and their maintenance. For those who wear their lenses four or more days a week, monthly, or fortnightly disposable lenses are more cost effective than daily disposable lenses. It is important that the patient understands from the start, the cost of the lenses, fitting and aftercare appointments, along with the importance of regular aftercare, maximising healthy and successful contact lens wear. Outline the risks involved in contact lens wear and that compliance with instructions is important to minimise them. Direct patients to a website that provides unbiased, generic information on contact lenses, such as the British Contact Lens Association , American Academy of Optometry , or the Cornea and Contact Lens Society of Australia or Contact Lens Update .

  • (d)

    How do you feel about inserting lenses and touching your eye? Patients may be motivated, but fearful of touching their eye! If the patient is nervous, demonstrate how they can gently touch the lower forniceal conjunctiva with their finger, while they look up and suggest they do this a few times prior to the fitting visit. This will give them confidence. Those who habitually wear cosmetics are not likely to be as nervous.

  • (e)

    You do understand that you will need to clean the lenses after each wearing episode, or is convenience a major factor? You will have to outline what is involved in lens care, even for those with a lens-wearing family member. Any reluctance is an indication for the use of single-use daily disposable lenses.

• 4.

  Medical and ocular history. If the patient has recently undergone an eye examination in your practice, ask open questions to verify that nothing has changed since. For those who attend for a contact lens fit with a spectacle prescription from elsewhere, a full history routine is required with a modified examination to take account of previous or potential contact lens wear. Questions should cover:

  • (a)

    General health, including whether the patient suffers from cold sores that may periodically have an impact on contact lens handling. The medical history may reveal contraindications to contact lens wear, the need for a particular type of lens, or more regular aftercare check-ups (e.g., diabetes). Ask about smoking because it is known to increase the risk of contact lens inflammatory events and is therefore a contraindication to extended wear lenses.

  • (b)

    Use of systemic medication particularly long-term treatment, such as steroids, beta-blockers, psychotropic agents (antidepressants), and regular use of over-the-counter pain medication. The main way in which medication can affect contact lens wear is through changes to the tear film and is summarised in the TFOS DEWS II report by Stapleton et al.

  • (c)

    Ocular history covers whether the patient has had previous ocular treatment or surgery or contact lens problems in the past. A history of an ocular abnormality requires you to look for the manifestations of the disorder that may have an impact on suitability for contact lenses or direct you towards a particular type of lens (e.g., RGP for an irregular cornea associated with a previous corneal injury). Previous surgery may dictate the lens type to be used (e.g., RGP lenses post corneal refractive surgery). Are they susceptible to hordeolums or chalazion that might be associated with recurrent blepharitis? Is the patient’s refraction stable? Are they interested in a better control of their myopia progression? With these last two, the patient should be advised that frequent changes to their contact lenses may be needed, with financial implications depending on the type of lens.

  • (d)

    Family history information determines if there are any hereditary ocular and/or medical conditions that may be relevant. A strong family history of myopia can indicate the need to consider lens designs that limit the progression of the patient’s myopia.

  • (e)

    Information regarding the patient’s occupation and hobbies is very useful, particularly when the patient is presbyopic because you need to ensure the lens chosen will give the patient good vision for the required working distances. Patients who spend a lot of time looking at a display screen are more likely to suffer from dryness associated with a reduced blink rate and tear film instability. Ask specifically about water sports because the patient may require the use of full scleral lenses or additional, single-use lenses if they swim regularly, and should be advised regarding the use of goggles over the top. Contact sports also require careful lens selection with single-use soft lenses most commonly providing the best option or no daytime lens wear as is provided by orthokeratology lenses.

  • (f)

    Environmental factors include regular exposure to a smoky atmosphere; an environment that is dusty, contains fumes, is of low humidity (associated with heating or air conditioning), or is unhygienic in some other way.

5.2.3

Recording

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

5.2.4

Interpreting the results

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:

• The patient’s requirements, including lifestyle (i.e., daily vs. extended wear, daily disposable vs. regular replacement)

• The patient’s ocular characteristics (i.e., lens design, small vs. large, spherical vs. toric)

• The financial position of the patient (i.e., soft vs. RGPs, daily disposable vs. regular replacement)

• The lenses you have available in your clinic (i.e., 3–4 types of single-use lenses covering a range of materials and prices is useful)

• Purpose of lens correction (i.e., distance vision only vs. presbyopia, control of myopia; consider specific designs)

Interpretation of the case history as shown in the example of a patient record above 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 their 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 owing to their large fingers and possible squeamishness.

5.2.5

Common errors

• 1.

  Not gaining a full understanding of what the patient wants from contact lenses.

• 2.

  Not recording all information obtained from the patient.

• 3.

  Assuming the same information is still current from the last eye examination.

• 4.

  Not applying all the information gathered to the selection of trial lens.

• 5.

  Agreeing to undertake a contact lens fit when the patient does not have a valid spectacle prescription (within recall date)

5.3.1

The evidence base: Corneal diameter, pupil and lid aperture measurement

PA height, HVID, and pupil size can be measured using a corneal topographer ( Fig. 5.1 ), or a contact lens rule ( Fig. 5.2 ). Many topographers allow a measurement of ‘white-to-white’ or HVID based on an anterior photograph. The measurement can simply be read off the screen, along with the photopic and mesopic pupil diameters, so it is not described further. Pupil measurements under low illumination can be made easily with a topographer, but are difficult with a contact lens (CL) rule.

Pupillometers, an expensive alternative, 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 critical.

5.3.2

Contact lens rule measurement procedure

• 1.

  Ask the patient to remove any spectacles.

• 2.

  Ask the patient to look directly at your dominant eye or an object held just below your eye.

• 3.

  PA height. Hold the CL rule vertically and align the zero on the millimetre rule with the lower lid margin. Estimate the PA height by reading off the average position of the upper lid with the patient looking in the primary position of gaze.

• 4.

  HVID. Position the CL rule on the patient’s forehead so that the semicircles are facing downwards and the rule bisects the iris. Move the rule horizontally until the semicircle on the rule and the visible semicircle of the iris match to create a continuous circle. It is often easier to judge when the diameter is slightly too large or small and take the HVID value as the reading in between.

• 5.

  Pupil size. Ask the patient to fixate an unlit distant object. Ensure that you do not get in the way of this fixation.

  • (a)

    Maximum pupil size. Best measured with a topographer or pupillometer using the dynamic or low light/mesopic setting ( Figs. 5.3 and 5.4 ). Significant pupil decentration can also be quantified. If a topographer or pupillometer is not available, a Burton lamp on the blue light setting is useful for providing sufficient illumination to view the pupil with less pupil constriction, and the crystalline lens fluoresces under the UV light.

    
    

• 6.

  Minimum pupil size. Additional local lighting directed towards a near reading chart can be used to measure the minimal pupil size at near, which is of interest when considering lens options for a presbyopic patient.

5.3.3

Interpreting the results

The average HVID value is 11.75 mm with a range of 10.8 mm to 13.6mm. The rule of thumb for calculating lens diameters: corneal RGP, total diameter is at least 2 mm smaller than HVID; larger RGP lenses can be as large as the HVID; mini sclerals and soft lenses are at least 2 mm greater than HVID. It is also useful to consider the corneal diameter when selecting the first trial lenses for larger lens sizes (RGPs, mini sclerals and soft lenses), because these lens fits are most closely related to the sagittal height of the cornea, which in itself is dependent on a number of factors, including curvature and diameter. Larger corneas generally need a flatter base curve, and smaller corneas, and steeper base curve.

The maximal 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 1 mm larger than the maximal pupil diameter, or larger still for a lid-attached lens.

The average PA height is around 9.75 mm, commonly ranging from 9.0 mm to 10.5 mm. The size of the PA influences the fit for corneal RGP lenses. A particularly large PA will result in an interpalpebral fitting corneal RGP lens. A smaller PA means that a larger proportion of the superior cornea is covered by the lid so that lid attachment 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 because 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.

5.3.4

Most common errors

• 1.

  When using some corneal topographers, pupil diameters and PA measures may be influenced by the measuring process (i.e., not all devices offer mesopic measurements) and a bright light source results in pupil constriction or the patient may be forcing their PA to be wider for the measurement.

• 2.

  When using a CL rule:

  • (a)

    Poor alignment resulting in a parallax error.

  • (b)

    Failing to notice that the patient is narrowing their PA by squinting to see a blurred distant target.

  • (c)

    Positioning yourself between the patient and target so the patient focuses on you, resulting in pupil constriction.

5.4.1

The evidence base: When and how to assess corneal topography

You will measure corneal topography in the initial assessments and monitoring of contact lens, orthokeratology, and refractive surgery patients. Tear film assessment using Placido disc-based topographers is a useful adjunct. Initial assessments will screen for keratoconus, surgically induced irregularity, and other diseases that change corneal shape. Topography also provides baseline data for monitoring purposes and indicates appropriate initial fitting parameters for RGP contact lenses. It is minimally needed in the fitting of soft contact lenses because the fit of a soft lens is more closely related to the sag of the cornea rather than the curvature at its apex, plus the limited range of soft lens radii and diameters means that the use of topographers for the determination of soft lens fit is not necessary. The traditional method to assess corneal topography used to be a keratometer. This is an instrument that projects a symmetrical image onto the corneal apex, using it as a reflective surface and the application of the optical principles of a convex mirror to estimate the radius of curvature at two assumed perpendicular principal meridians. The keratometer is increasingly being replaced in clinical practice by the corneal topographer (or videokeratoscope). To collect more data points from the corneal surface, these instruments project a Placido image (series of concentric rings) onto the cornea ( Fig. 5.5 ) or utilise the Scheimpflug principle. The image is captured by a high-resolution CCD Charge-coupled device camera, and data are processed by a computer. In addition to providing more detailed information, the area covered by these projected rings in the case of the more common Placido disc-based topographers is wider than that of a keratometer, up to 10 mm compared with 3.0 to 3.5 mm. Topographers also calculate nominal primary meridian values for the two principal meridians for RGP lens fitting, negating the need to use a keratometer ( Fig. 5.6 ). Depending on the topographer, other assessments can be made, such as corneal thickness, non-invasive tear break-up time (NIBUT), effect of tear film on aberrations, interblink interval, and meibography (see videos 5.1 and 5.2 ). 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. Knowledge of the profile of the corneo-scleral junction is helpful when fitting gas permeable scleral and mini-scleral lenses, which are used for both irregular and regular corneae. OCT is likely to become more widely used in the future, particularly for the rapid assessment of apical clearance in scleral and mini-scleral lens fits ( Fig. 5.7 ).

5.4.2

Procedure with corneal topographers

Although many different makes of corneal topographers exist, the following is a guide that is applicable to most. You should also refer to the user manual for your specific device.

• 1.

  Explain the procedure to your patient: “I am going to measure the shape of the front of your eye, the cornea.” You may add: “. . . so that I will know the size of contact lens to fit” and so forth.

• 2.

  Ask the patient to remove their glasses or contact lenses. If a contact lens wearer keeps their lens in place, then you will measure the topography of the contact lens front surface rather than the cornea.

• 3.

  Most topographers hold patient information on their self-contained database, so complete these details before measurements begin.

• 4.

  The next prompt screen will have a selection of options: to review existing data or to collect data. Once you have selected collect new data the camera will turn on and the Placido rings will illuminate (see Fig. 5.5 ).

• 5.

  Ask the patient to place their chin on the chin rest once the rings have illuminated to avoid unnecessary photophobia; the patient will have already adjusted to the light level. Alignment is automatic for some devices, but if not, follow the on-screen instructions to align the camera. The fine focusing to align the alignment targets will allow you to capture the image, either manually (pressing the button) or automatically. Before the image is captured, ask the patient to blink a few times and then hold. Some instruments have a chin rest that encourages a slight head turn, which helps remove the nose shadow from the captured images.

• 6.

  Switch eyes: again this may be automatic or manual. Usually the instrument moves rather than the patient. Align and focus the targets and capture the image of the second eye.

• 7.

  The patient can now rest their head back while you confirm the captured images are acceptable and the instrument processes the data and generates the colour maps.

5.4.3

Procedure for one-position keratometer

• 1.

  Ideally, before the start of the clinic, focus the instrument eyepiece by observing a distant object (e.g., the room wall). First turn the eyepiece anticlockwise as far as it will go and then back clockwise until the black cross hair just comes into sharp focus.

• 2.

  Seat the patient comfortably at the keratometer and ask the patient to remove any spectacles or lenses. Dim the room lighting.

• 3.

  Explain the procedure to the patient as with the corneal topography procedure.

• 4.

  Adjust the height of the patient’s chair and the keratometer to a comfortable position. Ask the patient to place their chin on the chin rest and forehead against the headrest. Occlude the eye not under test by swinging the keratometer’s occluder into place. Adjust the chin rest so that the outer canthus aligns with the headrest marker.

• 5.

  Ask the patient to look at the reflection of their own eye in the centre of the keratometer and to open the eye wide after a full blink. If a high refractive error prevents the patient seeing their own eye, then ask the patient to look down the centre of the keratometer. Make vertical adjustments of the keratometer if the patient is unable to see into the centre, or shine a pentorch through the observation eyepiece on to the patient’s face and adjust the keratometer’s height until the light shines on the patient’s eye.

• 6.

  Align the keratometer so that the lower right mire image is centred on the crosshairs and lock it into place.

• 7.

  Adjust the focusing of the keratometer by turning the focusing knob until the mires are clear and the lower right mire is no longer doubled. Keeping one hand on the focusing knob, constantly adjust it to exclude doubling of the lower right mire.

• 8.

  Measure the principal meridian that is closest to the horizontal first. Rotate the instrument so that the plus signs are set ‘in step’ ( Fig. 5.8 b) and the minus signs are parallel. This ensures that the instrument is aligned precisely on a principal meridian. This is easier to judge when the mires are adjusted to be relatively close to the endpoint. Continue adjusting the focusing knob to ensure a single, clear plus sign. Note that you will need to adjust the keratometer’s position constantly to maintain image focus, so keratometric measurements are always a two-handed operation. Note the radius of curvature (or dioptric power) and orientation of this meridian.

  

• 9.

  Measure the second principal meridian, which is theoretically 90° to the primary meridian. Adjust the focusing knob to give the best focus for the minus signs and then adjust the vertical alignment wheel until the minus signs are superimposed (see Fig. 5.8 ). Note the radius of curvature (or dioptric power) and orientation of this meridian. On a toric cornea, the plus signs will be out of focus and not superimposed, but this does not matter as you have completed your measurement of the first principal meridian.

• 10.

  Repeat the measurements on the other eye.

5.4.4

Procedure for two position variable doubling type keratometer

• 1.

  Set up the patient and the instrument as described in steps 1 through 8 above.

• 2.

  Move the telescope forward by adjusting the focusing knob or joystick appropriately. You may need to make minor adjustments both horizontally and vertically to centre the mire images and achieve a view as depicted in Fig. 5.9 . If the blocks and staircase are in step (see Fig. 5.9 a), then the orientation of the instrument arc is aligned to one of the two principal meridians and you can now proceed to step 4.

  

• 3.

  If the mires you see are similar to those in Fig. 5.9 c where the blocks and staircase are out of step, then the angle of the instrument arc is not aligned along a principal meridian. Rotate slowly until the staircase and block mires are aligned as in Fig. 5.9 d. This is easier to judge when the mires are relatively close together.

• 4.

  Ask the patient to blink and then keep their eyes as wide open as possible. Turn the knurled knob until the staircase and block mires are just touching. You must simultaneously adjust the instrument position with your other hand to maintain focus of the mire images. If you turn the knob too much and the mires overlap, a yellow/white area of overlap will be seen. Adjust the position of the mires until they are just touching with no overlap. If the hair wire does not pass through the middle of the touching mires, make final horizontal and vertical adjustments to achieve this.

• 5.

  Record the angle of the arc from the degree scale of the instrument and the corneal curvature along this meridian from the millimetre scale.

• 6.

  Rotate through 90° and make adjustments as in steps 4 and 5 to achieve a picture similar to those in Figs. 5.9 b and 5.9 e. Record angle and curvature values for this second meridian.

5.4.5

Recording

Increasingly practice management software will link with your corneal topographer and transfer the results to the patient records. Many topographers display the corneal curvature data in a variety of ways:

• a.

  Absolute colour map. The colour codes on the map represent fixed values relating to specific radii of curvature. Typically, steep areas of the cornea are shown as red, average areas as yellow through to green, and flatter areas as blue. The colour scale covers the whole range of curvature values for the instrument’s normative dataset ( Fig. 5.10 ). Fixed values allow comparison of corneal curvatures between eyes or between visits, and give an idea of how steep or flat a patient’s cornea is in comparison to the population. The disadvantage is that the intervals are large, so the map may lack sensitivity to small differences in curvature.

  

• b.

  Relative or normalized colour map. The colour codes are distributed across the range of curvature values for that specific eye measurement. This reduces the step interval so that colour maps have increased sensitivity and provide more detail on shape variations for that individual cornea ( Fig. 5.11 ). Be cautious when comparing relative colour maps between eyes or between visits, because the same colour will not necessarily represent the same curvature value. A relative colour map may show an area of curvature as red, because it is the steepest part of that cornea, but the absolute colour map may show the same area as green or blue, because in comparison to the normative dataset, that area is relatively flat.

  

• c.

  Difference colour maps. These compare two measurements (absolute maps) and allow a visualisation of the difference, which is helpful to observe change over time (i.e., the impact of wearing orthokeratology lenses or the progression of keratoconus).

• d.

  Indices of symmetry. Different topographers have different names for indices that quantify the coefficient of variability of a cornea (e.g., Regularity Index or Keratoconus Indices). They quantify measures such as how symmetrical a cornea is, the curvature of the steepest point, and how quickly the curvature changes away from that steep point—all early indicators for the development of conditions such as keratoconus.

• e.

  Quality of vision indices. Most topographers assess the position of the pupil, the level of corneal irregularity within the pupil area; some quantify the likely impact of corneal shape on the patient’s vision. Often this is expressed in terms of the level of higher order aberrations relating to the anterior surface of the cornea, the principal refracting surface of the eye. This helps you decide how likely it is that a patient’s reduced vision is caused by corneal irregularity ( Fig. 5.12 ).

  

Both topography and traditional keratometry provide keratometry measurements for the principal meridians, usually over a similar area around the centre of the cornea. The results can be recorded with the radius of curvature of the most horizontal meridian first followed by the most vertical as follows:

R 7.75 @ 175/7.60 @ 85

L 7.70 @ 180/7.60 @ 90

The @ nomenclature can be replaced by ‘along’ or ‘al.’ The millimetres and degree sign ( ⁰ ) need not be used. With a keratometer, if the mires are distorted, this should also be recorded.

Alternatively, the results can be recorded in dioptres (equivalent power), in which case the amount of corneal astigmatism is usually calculated and recorded. It can be useful to consider the amount of corneal astigmatism in relation to the spectacle astigmatism, when deciding whether a spherical RGP lens can be used to correct astigmatism.

OD: 42.00 @ 175/43.75 @ 85, total corneal astigmatism −1.75 × 175

OS: 43.50 @ 180/44.25 @ 90, total corneal astigmatism −0.75 × 180

5.4.6

Interpreting the results

Corneal topography has the advantage of showing the corneal curvature 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 each side ( Fig. 5.13 ). As the asymmetry increases, the disparity in the size of the bows increases. Increasing asymmetry in the pattern could be an indicator for correcting the astigmatism with soft toric lenses or larger diameter RGP lenses, rather than corneal RGP lenses which may decentre owing to the profile asymmetry ( Fig. 5.14 ). An exaggerated asymmetrical bow-tie pattern is indicative of early keratoconus, particularly if the apex is decentred inferior nasal. However, keratoconus can produce a range of different topographical patterns ( Fig. 5.15 ). 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 caused by a 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, whereas 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 instruments 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 among 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.

Small radius values mean 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 generally 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

The evidence base: Preliminary tear film assessment

The tear film and the way it interacts with the lens surface is very important in successful contact lens wear. Contact lenses have been reported as one of the modifiable factors that lead to symptoms of dry eye. A large proportion of contact lens wearers cite discomfort and symptoms of dryness as reasons for reducing their wearing time or ceasing lens wear. ^, 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. The signs useful as predictors are NIBUT ( Fig. 5.17 ) and surface staining. A thorough assessment of the tear film and its impact on the ocular surface allows the practitioner to select a more suitable lens material, such as one with low water content (to limit dehydration) and high lubricity (to minimise friction). It also allows better management of the patient’s expectations.

5.5.2

Procedure for preliminary slit lamp and tear film assessment

For a detailed description of slit lamp examination and tear film assessment, see sections 7.1 and 7.2 . These sections describe the procedures for assessing the tear film, which are generally undertaken as an additional component in the slit lamp biomicroscopy and/or topography examination.

5.5.3

Recording

See recording sections for slit lamp biomicroscopy ( 7.1.4 ) and tear film ( 7.2.10 ). 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 ( video 5.3 ) 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 ( http://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. Try to stick to 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 because these scales are generally quite coarse.

5.6.1

Soft lens application

Patients may 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, which must involve an appropriate informed consent process, where explaining what you do at each stage and answering any questions they may have is key.

• 1.

  Wash your hands thoroughly as per the World Health Organization guidelines (WHO) ( http://www.who.int/gpsc/clean_hands_protection/en/ ): rubbing both sides of the hands, in between the fingers and the fingertips. Rinse thoroughly and dry with a lint-free towel.

• 2.

  Check the lens specification on the container and the expiry date.

• 3.

  Remove the lens from the container, place on your index finger (finger next to the thumb) tip and check whether the lens is inside out or the correct way round by:

  • (a)

    Checking the lens profile, which should be slightly bowl-shaped ( Fig. 5.18 a) rather than saucer-shaped ( Fig. 5.18 b).

    

  • (b)

    Gently nipping the lens should result in the edges curving inwards rather than outwards ( Fig. 5.18 c).

  • (c)

    Using the crease test, which involves placing the lens in the palm of your hand along the main crease, cupping the hand slightly so that the lens is partially folded and looking to see if the edges roll inwards (correct) or outwards (incorrect). There is an alternate inversion test where by the lens is pinched between thumb and index finger (see videos 5.4 and 5.5 ).

  • (d)

    Some lenses have inversion indicator engravings. Make sure you know from which side of the lens the engraving should be correctly viewed.

  • (e)

    If incorrect, simply pick up the lens and turn it over, repositioning it on your fingertip.

• 4.

  Check for debris or defects. Any debris should be rinsed off with saline or multipurpose solution. Dispose of defective lenses.

• 5.

  Make sure you have your three key fingers reasonably dry; the index finger where the lens is resting, the middle finger of the same hand, and the index finger (or thumb) of the other hand. These two ‘extra’ fingers will hold the eyelids apart during the application of the lens. If the lens or finger is too wet, there will be too much contact between the two and it will not be easy to apply the lens. Too little contact, usually as a result of lens dehydration, may result in the lens falling off the finger during application.

• 6.

  Stand slightly to the side of the patient on the side where you are going to apply the lens first. This may necessitate rotating the patient’s chair away from you. The following instructions assume that the practitioner will apply the right lens first using their right hand.

• 7.

  Ask the patient to rest their head against the headrest of the chair, turning their head slightly away from you for the right eye application to make the process easier.

• 8.

  Explain that you are going to gently hold the lids apart and place the lens on the eye. Make it clear that once the lens is on, the patient will hardly feel it.

• 9.

  Ask the patient to look down and use the thumb (or index finger) of your left hand to lift their superior eyelid from just behind the lashes. Hold it firmly against the brow bone; do not push into the orbital cavity.

• 10.

  Ask the patient to look straight ahead and use the middle finger of your right hand to pull the lower lid down from just behind the lashes.

• 11.

  Gently place the lens on the eye in one of the following ways :

  • (a)

    Directly on the cornea whilst the patient looks straight ahead

  • (b)

    On the inferior conjunctiva whilst the patient looks up

  • (c)

    On the temporal conjunctiva whilst the patient looks nasally

  • The preferred procedure is usually determined by practitioner preference influenced by the patient’s compliance.

• 12.

  The lens will often have a bubble of air underneath it when first applied, particularly if the lens has been placed on the conjunctiva, which is flatter than the cornea. Keep your finger in position initially and ensure that the lens has adhered to the surface of the eye rather than sticking to your finger. A very gentle massaging motion will help this. Slowly withdraw your finger, asking the patient to gradually look straight ahead to centre the lens.

• 13.

  Release the lower lid then ask the patient to look down before slowly releasing the top lid to cover the lens.

• 14.

  The patient should now be encouraged to make slow, gentle blinking movements whilst looking down.

• 15.

  Repeat for the left eye, standing on the other side of the patient and using your left hand to apply the lens.

• 16.

  These instructions assume that the practitioner is able to use both their dominant and non-dominant hand for lens application and will therefore use their right hand for the right eye, and left hand for the left eye. If this is not the case, the practitioner will have to use their dominant hand for one eye and reach over the patient, taking care not to touch the patient’s nose, in order to apply the lens to the other eye.

The different insertion techniques are shown in the online videos (see videos 5.6 and 5.7 ).

5.6.2

Problem solving

• 1.

  If lens application produces a sensation of grittiness or discomfort, ask the patient to look up, displace the lens down and/or temporal with your index finger and allow it to recentre. If the grittiness persists, remove the lens, check for debris and defects, rinse and reapply or replace as appropriate (see video 5.8 ).

• 2.

  If lens application results in a stinging sensation and watering of the eye, remove the lens, rinse with saline, and reapply. Profuse watering on lens application can be indicative of something on the lens, such as soap from handwashing, or a difference in tonicity or pH of the patient’s tears and the solution on the lens. Significant lacrimation tends to give an apparently tight-fitting lens on assessment; the hypertonic tears cause the lens to adhere to the surface of the eye, especially lenses of ionic material.

• 3.

  If a patient is very motivated to wear contact lenses, but is very unsure about someone else placing a lens on their eye, teach them to apply and remove the lenses themselves during the fitting appointment.