Ординатура / Офтальмология / Английские материалы / Medical Contact Lens Practice_Millis_2005

production may be the result of a reduced reflex secretion, which has been associated with the reduced corneal sensitivity found in older people,6 and may cause steepening of the lens, so a tight fit should always be avoided.

Tonge et al.7 showed that in young people under 30 years of age the tear film stability is much greater than previously thought when a noninvasive technique is used to measure tear break-up time (TBUT). In contrast they found a very significant fall in TBUT in people over 30 years of age.

There is a progressive reduction in the concentration of lactoferrin, which correlates with that of tear lysozyme. Tear flow begins to reduce over the age of 40 years, but levels of lysozyme and lactoferrin fall more slowly until the age of 70 years, so maintaining their concentration levels in the tears.4

There is a less active immune system in the eyes of older people, and it has been shown that tear IgA, the main tear immunoglobulin, gradually decreases with age. In contrast IgG, which is found in very low concentrations before 60 years of age, gradually increases thereafter.4

Tomlinson and Giesbrecht8 investigated the tear evaporation rate and found no change with increasing age, which may be because reduced drainage is a compensatory mechanism.

In females the menopause may be associated with reduced tear secretion and symptoms that may be relieved by hormone replacement therapy. Meibomian gland function has shown a positive correlation with hormone levels.

Other changes

Changes in the cornea, pupil size, intraocular pressure, refractive state and spectral transmission also occur with age.9 These affect visual acuity, contrast sensitivity, stereoacuity, color vision and visual fields.10 Jaffe et al.11 found the upper half of the visual field to be more affected than the lower, and this may be due to senile ptosis. Visual acuity is affected by optical aberrations in the eye, retinal illumination and a defective retinal or nervous system.12

The recovery rate from corneal edema was found to be more rapid in younger than older eyes,13 which makes contact lens wear more risky. Fitting high Dk lenses (see Table 2.5) may improve

Figure 4.1 Abrasion caused by difficulty in handling the lens.

the situation and these patients should be kep under close observation.

In addition to the ocular changes, many olde patients have reduced digital sensation, which can cause problems with lens insertion and remova (Fig. 4.1) and may increase the risk of lens dam age. If a lens breaks at the junction of the optic with the periphery the junction thickness may b increased.

CONTACT LENS CORRECTION

The major problem in the older age group is pres byopia. Some have worn lenses for years and d not want to start wearing spectacles. Others hav only just found the need for a correction for clos work and find them awkward and cosmetically unsatisfactory.

The simplest method of correcting presbyopi in the contact lens wearer is by a reading addition in spectacles worn over contact lenses, and thi is an acceptable solution in many cases. There are however, increasing numbers of lens wearers who find that having to use spectacles for shopping is inconvenient. Others do not wish to wear spec tacles, while others feel that if they are wearing spectacles for close work, then they may as wel wear them for all distances and abandon contac lenses. For these patients there are several possibl solutions including monovision, and bifocal o multifocal lenses. Modified monovision is a tech nique in which one eye is fitted with a distance len and the other with a bifocal (or multifocal) lens.

Lenses are available for conventional wear, or in planned replacement programs with new lenses every 3 months, monthly, 2-weekly or even daily. Bifocal and multifocal lenses are available as RGP or soft lenses, and as simultaneous vision lenses or translating (alternating) bifocals.

Greatest success is experienced with previous lens wearers. For patients who are new to lens wear it may be preferable to agree to fit distance lenses in the first instance and consider a presbyopic correction later. This is easiest, and most costeffective, using soft, planned replacement, or disposable lenses. Emmetropes usually do less well than ametropes because they are used to good uncorrected vision.

Monovision

Monovision is the simplest and most successful technique for the presbyope. It is successful in 70% of cases. The patient is fitted with a distance lens on one eye and a reading lens on the other. It is usual to correct the nondominant eye for near vision and the other eye for distance vision. However, Schor et al.14 have suggested that sensory dominance may be important, and that the sighting dominant eye may not be the sensory dominant eye so, in some cases, correction of the sighting dominant eye for near vision may be more successful.

Both myopes and hypermetropes, particularly if they have significant errors, do well with this type of correction, as do those with alternating strabismus and good vision in both eyes, because they do not suffer fusion problems. Patients who have poor corrected visual acuity in one eye should not be fitted with monovision, and those who experience motion sickness often do less well.

Monovision is useful for those who need to use near vision when looking up. Some myopes may be able to achieve monovision by ceasing to wear a lens on one eye if they have 2.0 D to 3.0 D of myopia in the eye selected for near vision. This lens wear modality is not pupil dependent and may be more successful with patients who have small pupils, who have difficulty with other types of presbyopic correction.

The monovision technique is simple. Two single vision lenses are fitted, one for near and one for

distance vision. Monovision is the least expensive of the options but has some disadvantages:

●distance vision in the eye corrected for near vision is reduced and the higher add needed by the older patient results in greater image blur – for the patient to accept the blurred image there must be interocular suppression, but this may be achieved, even with the higher adds

●there is loss of clear intermediate vision, which may be improved by undercorrecting the near add, while maintaining acceptable near vision for the patient’s needs, or by overplussing the distance correction without significantly reducing distance vision

●stereopsis is reduced, on average, by 100 s of arc.15

During the adaptation period, which may last up to 8 weeks,16 patients may experience hazy vision with occasional dizziness. Schor et al.14 showed that suppression of blur may be variable, and because the suppression in one eye corresponds to an area of clear image in the other, difficulties may occur with binocularity, which can cause problems with driving.

Many patients report a subjective improvement in vision during adaptation, but do not appear to have any significant improvement in visual acuities, near stereopsis or blur suppression.17 In some cases adaptation is made difficult by the sudden onset of anisometropia created by this technique, but usually resolves in a few days.18 A few myopic patients may have become undercorrected and have inadvertently acquired a low reading add, which may help their adjustment when fitting a presbyopic correction.

To maximize near and distance vision, and reduce blur, even small cylinders should be corrected if they can be shown to make a subjective improvement.19

The patient’s initial reaction to monovision is an indication of the likely success and the time that may be required for adaptation, which may vary from a few days to several weeks. It is helpful to ask the patient to assess visual comfort with the lenses in an outdoor environment; if this is not possible focusing on the far distance through a window may demonstrate any problems.

Patients who fail to adapt to monovision show greater levels of ghosting for near and distance vision, lose more stereoacuity, and tend to be older. They also have less good near and distance visual acuity and are less motivated than successful wearers.20

There are concerns about driving wearing monovision lenses. Schor et al.14 found there was considerable difficulty in suppressing the image from the blurred eye and a starburst effect occurred when car headlights were viewed at night. Patients should be advised not to drive until they have fully adapted to their monovision lenses. A pair of spectacles that will correct the near vision eye for distance can be prescribed.

Some patients are happy to buy three lenses – a distance lens for each eye and a reading lens, and to use these as appropriate. This is easier for RGP lens wearers than for those with soft lenses because, in the short term, they do not need to be stored wet. The advent of daily disposable lenses has made this modality easier for the soft lens wearer.

Often lens wearers are more comfortable if the reading add is reduced to a level that will still allow normal activities, such as shopping, without the aid of glasses, and accept the need for additional reading spectacles for fine work or print.

Bifocal contact lenses

Bifocal contact lenses are most likely to be fitted successfully in patients who fulfill the criteria listed in Table 4.2.

Translating (alternating) bifocals

Translating (alternating) bifocal lenses have two areas of correction, usually with a central or superior zone for distance vision and a peripheral or inferior zone for near vision, and one image is

Table 4.2 Patients for whom bifocal fitting is most likely to be successful

Successful single vision wearers

Those needing near and distance correction

Those who accept some compromise in vision

Those who are highly motivated

viewed at a time. The three types of design are a follows:

●the lower segment has a straight top extending across the whole of the lens

●a fused or solid crescent or D-shaped segment

●an annular concentric design.

Translating bifocals are usually rigid lenses, and although soft lenses have been designed, they ar generally not successful because the lens does no move sufficiently to translate satisfactorily.

Annular (concentric) bifocals also translate and are easier and less expensive to make and rotation is not a problem. On downward gaze the periph eral ring for near vision is raised over the pupil The Avision® lens is a concentric bifocal with th distance correction in the center – center distanc (CD). Concentric bifocals are usually lighter in weight than the equivalent segmented lens.

Translating lenses rely on head and eyelid posi tion and need to be fitted with the reading seg ment line just below the pupil in the primary position in normal illumination, and the lowe eyelid margin supporting the lens. RGP bifocal need tight eyelids that lie just above the lower lim bus at six o’clock. On looking down the reading segment is pushed up (translates) over the pupi by the lower eyelid. Loose lower eyelid are a contraindication to lenses that need t translate.

Translating lenses require precise fitting, which depends on the incorporation of a base-down prism or truncation to stabilize the position of th lens. Alteration of prism power or the angle o truncation changes the rotation of the lens. Som patients find the prism ballasting and truncation uncomfortable. The clinician should aim to fit th segment on the high side because it can be low ered by truncation, but a new lens will have to b ordered if a segment is too low.

Translating bifocals provide good vision in both eyes for near and distance vision, but are no good if the patient requires clear near vision abov the midline, as in VDU use. Better intermediat vision has been obtained in some patients with the RGP Tangent Streak® trifocal lens, which has 1 mm middle segment across the width of the lens This should ride into the inferior pupil margin on primary gaze in normal illumination. It require

very precise translation and pupil position to ensure proper focusing.21

It is important to use trial lenses when fitting translating bifocals because they require precise orientation and translation to succeed and the dynamic fit of the lens is crucial.

Translating bifocal lenses have been used successfully to treat patients with accommodative esotropia and a high accommodative convergence/ accommodation (AC/A) ratio.22

These lenses are less pupil dependent than simultaneous vision lenses, but even with translating lenses small pupils are better than large pupils. Flare may occur, particularly when driving at night and when the pupil dilates in poor illumination, and may extend into the reading zone. The transition line may also cause flare at night.

A tight fit must be avoided because the lens will not translate. To achieve a less tight fit the peripheral curve or the back optic zone radius may be made flatter or the lens made smaller. The position of a low segment may be improved by:

●increasing the back curve

●increasing the segment height

●reducing the prism by 0.5 D.

If a segment is too high, reduce the segment height or increase the prism, or flatten the base curve to loosen the lens and increase the truncation.23 Altering the segment size or the prism will require a new lens.

The Lifestyle® GP lens (Lifestyle Co. Inc.) combines translation with simultaneous vision. It does not need either a prism ballast or lower eyelid interaction. Centration is not essential and it is best as a slightly high-riding lens or with eyelid attachment. It is a CD progressive add lens that provides good distance and intermediate vision and translates on downward gaze to the reading add. The back surface provides up to 1.5 D of near add, but greater amounts can be obtained by additional curves on the front surface of the lens so it will cope with up to 3.0 D add. Accurate centration is more important in the older presbyope to obtain adequate near vision.

Simultaneous vision bifocals

Simultaneous vision bifocal lenses are designed so that distance and near images are coincident on

the retina. They are easier to fit, and the vision is more predictable because they do not need to be stabilized, nor do they need to translate. They are useful for patients with loose eyelids. The optical zones may be concentric and corrected for either CD or center near (CN), or they may be aspheric with continuous power variations between distance and near vision.

Simultaneous vision bifocal lenses produce two or more images from a single object and patients select which they will view. There is loss of contrast sensitivity, which is worse in low illumination and for near vision.

Shapiro and Bredeson24 found that new lens wearers were almost as successful as previous wearers with simultaneous vision bifocal lenses, and patients with higher errors were as successful as those needing little or no distance correction.

Concentric, simultaneous vision lenses These can be CN or CD:

●in low illumination CD lenses are not good for distance vision because the pupil dilates into the reading correction, while in high illumination, with a small pupil, the distance vision is good, but not the near vision

●with CN lenses the distance vision is better in low illumination, when the pupil is larger, while in bright light the pupil constricts and improves near vision, but the patient may find distance vision more difficult and may need tinted glasses for driving.

Aspheric lenses

Most modern simultaneous vision lenses are aspheric lenses. These provide an increased depth of focus at the retina and therefore an increased range of near vision. These lenses are characterized by their eccentricity values (e-value), which is an indication of the degree of peripheral flattening. The relation between the base curve and e-value determines the near add. Changes in power are generated by the aspheric design as it moves away from the geometric center of the lens.

Aspheric lenses are less affected by pupil size than other lens designs, but CN lenses give better distance vision with larger pupils. Aspheric lenses are less dependent on centration, but unfocused

images are more likely with lens movement. Because the area of any given power is smaller, precise centering is necessary, but may result in a steep fit. Increasing the lens diameter will stabilize the lens.

Aspheric designs are good for low adds and do not give rise to ghost images. In the author’s experience a higher reading add is often needed, compared to that in the spectacle prescription. As with most types of presbyopic correction good lighting is important.

CibaVision has produced a 1-day disposable CN aspheric lens that will cope with up to 3.0 DS of presbyopia.

Modified monovision

If patients are unhappy with one lens for near and the other lens for distance vision, they may prefer a bifocal lens on one eye and a single vision lens on the other to gain better distance vision. Loss of accommodation affects intermediate as well as near vision and this may be improved by overplussing the distance lens.

Better intermediate vision with bifocal lenses may be provided by prescribing slightly different adds to each eye.

Multizone lenses

The introduction of the Acuvue bifocal lens minimizes the effect of pupil size because the optic zone has five alternating distance and near rings. A large lens library with a wide range of adds lets the clinician make an immediate assessment and, as the

lenses are used on a 2-weekly basis, the patient ca take home a trial pair.

Other modalities

Lenses for the older patient have their specific prob lems. RGP lenses with hydrophilic coatings an the new Hybrid FS material may improve com fort, and lenses of special materials such as th Proclear® lens may be better in dry eyes.

Bifocal lenses should be fitted using the appro priate fitting sets for soft lenses. RGP lenses of th appropriate parameters should be ordered from the laboratory and used as trial lenses. If it i necessary to alter any parameter, new lenses ca be ordered and exchanged. It is preferable fo patients to try lenses under normal living condi tions for 3–5 days and then review the fit.

Although visual acuity for distance and nea vision should be checked and recorded with eac eye separately, it is also important to assess visua acuity with both eyes together because, in prac tice, this is the vision that matters. Patients shoul be warned repeatedly that bifocal vision is a com promise and advised to avoid comparing one ey with the other, particularly if wearing monovisio or modified monovision lenses. Older patient requiring higher adds have greater difficult because they are limited in the types of correctio available and their distance acuity is more affecte if they use monovision.

Lenses for presbyopia (Table 4.3) are not perfect solution but are a valuable addition to th clinician’s armamentarium.

1.Vale J. The effects of drugs on the tear film and contact lenses. Trans Br Contact Lens Assoc Scientific meetings. 1990;13:21–23.

2.Hill JC. Analysis of the senile changes in the palpebral fissure. Trans Ophthalmol Soc UK 1975;95:49–53.

3.Norn N. Expressibility of meibomian secretion. Relation to age, lipid, precorneal film, scales, foam, hair and pigmentation. Acta Ophthalmol (Copenh) 1987;65:137–142.

4.Seal DV. The effect of ageing and disease on tear constituents. Trans Ophthalmol Soc UK 1985; 104:355–362.

5.Hamano T, Sachiko M, Kotani S, et al. Tear volume in relation to contact lens wear and age. CLAO J 1990;16:57–61.

6.Millodot M. The influence of age on the sensitivity of the cornea. Invest Ophthalmol Vis Sci 1977; 16:240–242.

7.Tonge SR, Hunsaker J, Holly FJ. Non-invasive assessment of tear film break-up time in a group of normal subjects – implications for contact lens wear. J Br Contact Lens Assoc Trans Ann Clin Conference 1991;14:201–205.

8.Tomlinson A, Giesbrecht C. The ageing tear film. J Br Contact Lens Assoc 1993;16:67–69.

9.Woods RL. The ageing eye and contact lenses – a review of ocular characteristics. J Br Contact Lens Assoc 1991;14:115–127.

10.Woods RL. The ageing eye and contact lenses – a review of visual performance. J Br Contact Lens Assoc 1992;15:31–43.

11.Jaffe GJ, Alvarado JA, Juster RP. Age-related changes of the visual field. Arch Ophthalmol 1986;104:1021–1025.

12.Weale RA. Senile changes in visual acuity. Trans Ophthalmol Soc UK 1975;95:36–41.

13.Siu AW, Herse PR. The effect of age on the edema response of the central and mid-peripheral cornea. 1993;71:57–61.

14.Schor C, Landsman L, Erickson P. Ocular dominance and the interocular suppression of blur in monovision. Am J Optom Physiol Optics 1987;64:723–730.

15.Papas EB. The presbyope and the contact lens: a fatal attraction. J Br Contact Lens Assoc Trans Scientific Meetings 1991;14:51–54.

16.Collins M, Bruce A, Thompson B. Adaptation to monovision. Int Contact Lens Clin 1994;21:218–223.

17.Collins MJ, Bruce AS. Factors influencing performance with monovision. J Br Contact Lens Assoc 1994;17:83–89.

18.Josephson JE. The monovision controversy. Trans Br Contact Lens Assoc Ann Clin Conf 1989;6:60–65.

19.Cox IG, Cornstock TL, Orsborn GN. Presbyopic fitting strategies: fitting the lens to the patient, not the patient to the lens. J Br Contact Lens Assoc 1992;15:149–150.

20.Back A. Factors influencing success and failure in monovision. Int Contact Lens Clin 1995;22:165–172.

21.Gussler JR, Litteral G, Van Meter WS. Clinical evaluation of the Tangent Streak trifocal contact lens. CLAO J 1991;17:160–163.

22.Rich LS, Glusman M. Tangent Streak RGP bifocal contact lenses in the treatment of accommodative esotropia with high AC/A ratio. CLAO J 1992; 18:56–58.

23.Van Meter WS, Gussler JR, Litteral G. Clinical evaluation of three bifocal contact lenses. CLAO J 1990;16:203–207.

24.Shapiro MB, Bredeson DC. A prospective evaluation of Unilens soft multifocal contact lenses in 100 patients. CLAO J 1994;20:189–191.

CHAPTER CONTENTS

Symptoms 43

Signs of of complications 48 Patient-related problems 57 Avoiding complications 58 References 59

It is very unlikely that a patient will wear lense without, at some time, experiencing one or mor of the complications of lens wear. Patients wil view those practitioners who are able to accu rately identify the problem and resolve the situa tion swiftly as successful. Patients complain of wide variety of symptoms and present with range of signs that may or may not be related t lens wear (Table 5.1).

SYMPTOMS

Blurred vision

Corneal edema

Corneal edema is the most serious cause of blurre vision because it is associated with hypoxia. In th past it was most often due to PMMA hard lenses which are not normally fitted now. However some patients may still be wearing them, and few see better with PMMA than RGP lenses.

Edema may also be seen with low-Dk gas permeable lenses (see Table 2.5) if the amount o oxygen transmitted fails to meet corneal needs The edema characteristically causes central cornea clouding, which is best seen with the lense removed and using the sclerotic scatter techniqu with the slit lamp. Blurred vision is particularl noticed when the lenses are removed and replace by spectacles. It is symptomatic of a badly fittin lens and resolves over a period of hours when th lens is removed. In some cases the edema may b

sufficient to cause the patient to complain of haloes and to reduce wearing time.

Corneal edema may be localized beneath a decentered, immobile lens or more generalized with a tight fitting lens.

Refraction may be performed on removal of an RGP lens, but, providing there is no evidence of edema, it is best to ask patients to remove PMMA lenses for 2–3 days before refraction to obtain more reliable results. Even then patients should be warned that vision can vary with time. If the patient has no back-up spectacles and is unable to cease wearing the lenses because of very high myopia or keratoconus, they may find it possible to function with one lens only so that one eye can be refracted, and a further appointment can then be made to refract the other eye.

In all cases of hypoxia there is a need to increase the gas transmissibility (Dk/L) by:

●changing to an RGP lens if the patient is fitted with PMMA lenses

●changing an RGP lens for one made of a higher Dk material or a thinner lens.

In the author’s experience it is unwise to change a PMMA lens for an RGP lens with a very high Dk because the difference in the polymers may alter the wetting characteristics of the lens. Instead it is preferable to choose a material with a moderate Dk, and increase this at a later date if necessary.

If signs of hypoxia occur with a soft contact lens, refit the patient with a lens with a higher water content, a thinner lens or a silicone hydrogel lens,

or change to an RGP lens with a moderate to high Dk/L.

All cases should be kept under frequent, regular review until it’s clear that the problem has been solved.

Incorrect lenses

Incorrect lenses may be the wrong power or have an incorrect base curve. Blurred vision caused by incorrect lenses is often because patients:

●are wearing old lenses

●have simply transposed the lenses into the wrong eye

●are wearing a lens belonging to another member of the family.

Transposition can be avoided with RGP lenses if the right lens is marked with a dot or an “R”, or if the lenses supplied are different colors. All lens parameters should be checked and compared with previous prescriptions because the patient may not be wearing the expected lens.

Altered power can also arise with RGP lenses because an overenthusiastic cleaning technique has removed power from the front surface.

More rarely the lenses may have been manufactured outside the limits of tolerance or may have been wrongly labeled, or a plus lens has been received instead of a minus lens, or vice versa. These problems should be identified, in most cases, when the lenses are checked on receiving them from the laboratory, before the patient attends to collect the lenses.

Dirty lenses

Deposit formation is most common in soft lenses (Fig. 5.1), extended wear lenses, and in the hay fever season when the copious production of mucus causes blurring of the vision.

Greasing of RGP lenses is associated with deficient or altered tear film, lens scratches or eyelid margin disease. Eyelid hygiene should be encouraged and lens cleaning with an alcohol-based cleaner, such as Miraflow (CibaVision), often improves the condition.

Infiltration of soft lenses by fungal hyphae destroys the lens matrix and affects vision if it occurs centrally. They appear as brown, white or gray feathery deposits. They usually result from poor lens hygiene. The patient’s care regimen should be reassessed and its importance stressed, and the lenses should be replaced.

Lens damage

Central deep scratches (Fig. 5.2) in RGP lenses reduce visual acuity and cannot be removed, so a new lens must be ordered. Superficial scratches do not usually affect vision and can be removed by polishing if sent to the laboratory. Peripheral damage and chips do not affect vision and often cause surprisingly few symptoms and signs. Lenses with only small areas of damage may continue to be worn until a new lens is available, providing the patient remains asymptomatic.

Soft lenses may split centrally as a result of poor handling, and the effective power of the

lens is consequently altered. The lens should b replaced.

Lens fit

Lenses that center badly may cause blurred visio because the patient may see through the peri pheral carrier or the junction of the optic and car rier, which causes a prismatic effect. Warpe lenses may blur vision and distort the cornea, an a decentered lens causes intermittent blurring o vision.

RGP lenses that drop may be too small or to flat and may be improved by making the len larger or steeper. Altering the back optic zon radius (BOZR) alters the effective power of the len and allowance must be made for this. Droppin lenses are often thick, high-plus lenses, and thei position can be improved by ordering a larger len with a minus carrier to achieve eyelid attachment Lighter thinner lenses with a lower specific grav ity may aid centration.

Temporal displacement can be corrected with toric peripheral curve or a steeper lens, or a large optic zone may be used to stabilize the lens. Tori peripheries may be needed in cases of against-the rule astigmatism when there is a plus cylinder a 180°.

Excessive lens movement causes blur with th blink or with eye movement, and stability ca often be achieved with a larger total diameter.

Soft lenses tend to decenter if they are too loos and may benefit from a larger or steeper lens

Some of the very thin soft lenses move excessively with the blink if they are inserted inside out.

Residual astigmatism

In patients with a spherical refraction, corneal astigmatism (which can be revealed by keratometry or topography) is normally neutralized by astigmatism of the crystalline lens. If a rigid contact lens is fitted it will neutralize the corneal astigmatism by means of the liquid lens formed beneath the contact lens by the tear film and so the intraocular astigmatism will be revealed. These patients should be fitted with a spherical soft lens.

Excess watering

Excess watering may be due to discomfort, photophobia, corneal and conjunctival disease, or blocked nasolacrimal ducts. It results in liquid accumulating in front of the contact lens. Watering is often present during the adaptation phase of contact lens wear, particularly with rigid lenses.

Burning sensation

If the burning sensation commences on insertion of the lens it may be due to a dirty or contaminated lens and often occurs when hands have not been washed before lens insertion.

Damaged lenses may cause a burning sensation, as may foreign bodies, trapped mucus or solution sensitivity. The lens may have been inserted using a cleaning solution instead of a wetting solution, although there is less risk with the many one-step solutions now available. Hydrogen peroxide that has not been neutralized causes severe burning and watering.

Patients may also complain of burning and stinging when inserting disposable lenses. These are stored in sterile saline that may not be the same pH as the patient’s eye. A rinse with unpreserved saline of a different pH will prevent burning.

Burning after several hours of wear of a soft lens may be due to a tight fit with reduced tear flow. The lens fit should be loosened or the lens should be changed for an RGP lens. This symptom also occurs with poor blinking.

Pain and discomfort

Severe pain is likely to be due to serious problems. Acanthamoeba keratitis should be considered when pain is out of proportion to the findings.

Contact lens associated red eye (CLARE) syndrome (see below) affects those wearing extended wear lenses with the 3 a.m. syndrome (which is due to dryness and poor blinking) and typically occurs with PMMA and low Dk lenses. It causes patients to wake in the night with severe pain.

Discomfort may occur in the adaptive period, particularly with rigid lenses. RGP lenses that are too small or too large may irritate the eyelids and cause lens awareness. Soft lenses that do not extend over the limbus cause discomfort with the blink as the lens moves over the limbus.

Lenses that have thick, poorly shaped or damaged edges, and lenses that are too mobile are uncomfortable, and many patients find that they are aware of lenses if they incorporate a prism ballast. Some laboratories design soft toric lenses with the prism ballast sited more centrally to achieve a thinner, more comfortable lens edge.

Discomfort due to lens damage is common. Soft lenses are more easily damaged than rigid lenses, and patients should be discouraged from rubbing soft lenses between the finger and thumb to clean them. They should be shown how to place lenses in the storage baskets to avoid trapping the lens edge. Soft lenses that have been allowed to dry must be handled with great care, rehydrated in the storage solution for several hours, and then examined carefully for damage before cleaning and re-disinfecting before insertion.

High-plus rigid lenses are liable to damage at the junction of the optic zone and the carrier. Patients with these lenses are often older patients, particularly elderly aphakes, and those with diabetes mellitus, who often have reduced digital sensation and have difficulty with lens handling. If damage occurs frequently it is advisable to thicken the junction, but not above 0.21 mm, and to review the material from which the lens is made.

Flare and glare

Flare occurs most often with rigid lenses with an optic zone that is too small. When the pupil dilates,