Ординатура / Офтальмология / Английские материалы / Medical Contact Lens Practice_Millis_2005

as when driving at night, light enters through the focused optic zone and the unfocused periphery of the lens, causing flare and glare.

Glare, caused by light scatter, occurs when lenses are dirty or scratched, and during the adaptation period.

Flare may be noticed with a badly centering lens, and with bifocal lenses that depend on pupil size to provide good vision.

Photophobia

Photophobia is common in the adaptive period with rigid lenses. If occurring later, review the lens size and exclude corneal edema, dryness, three and nine o’clock staining and a dirty lens as possible causes.

Lens loss

A rigid lens that is too small, too flat or with too much edge lift is likely to be lost. Similarly, a very mobile lens that moves excessively with eye movement, may lift off the eye and be ejected. Physical impact, such as occurs in contact sports, may cause loss of a rigid lens. The fit of the lens should be stabilized by increasing the total diameter, reducing the edge lift if this is the cause, or steepening the BOZR.

Soft lenses are less likely to be lost, but the edge of a lens that is too loose may ruckle (Fig. 5.3) and the lens is ejected with the blink. A larger or tighter lens should be tried. Lenses that dry excessively

Figure 5.3 Flat fitting silicone hydrogel lens resulting in edge ruckle.

are lost more often because the shape alters and the lens fits less closely, but the risk can be reduced by the use of wetting drops. Many patients find that Vislube® eye drops (Chemedica) have a mor prolonged effect. Soft lenses may also be lost from the eye by rubbing. This may occur inadvertently during waking hours, but in extended wear i often the result of eye rubbing during sleep and the patient may be unaware of the loss.

Lenses may be lost from the eye completely. I they are found they should be inspected for dam age, rinsed with normal saline to remove dirt and hairs etc., and then cleaned and disinfected. If the are not found immediately soft lenses will need to be rehydrated before they can be cleaned and dis infected and should be handled very carefully until fully hydrated because they are easily dam aged. These procedures need to be explained t the patient and should be included in written instructions on lens care. Wearers of one-day lense are advised to discard the lens and replace it with a fresh one because they have no means of disin fecting the lens.

Lenses may be dislodged from the cornea, bu remain on the ocular surface, often in the uppe fornix. The patient should try to identify the len on the eye and manipulate it using the eyelid onto the cornea or into the palpebral apertur from where it can be removed in the normal way If a rigid lens cannot be removed it may be possi ble to wash it from the eye using an eye bath or an eggcup filled with sterile normal saline.

In all cases patients should be reassured tha lenses cannot be lost round the back of the eye and disappear into the brain – a not uncommon fear Nor will significant damage ensue if a lens is lef overnight on the conjunctiva. If the lens cannot b retrieved the patient should be given an urgen appointment at which a search can be made fo the lens by using the slit lamp and doubly evert ing the upper eyelid. If a rigid lens is still not visi ble a drop of local anesthetic should be instilled into the eye, the upper eyelid everted and th upper fornix swept using a sterile glass rod. A “chink” will identify an RGP lens and it can b manipulated into the palpebral aperture and removed. If there is difficulty in finding a soft len a drop of fluorescein applied to the eye will con centrate in the lens and render it visible. The dy

will leach out of the lens with successive changes of solution.

Ptosis

Ptosis associated with contact lens use is discussed in Chapter 3.

SIGNS OF COMPLICATIONS

Staining

Staining of the cornea is best demonstrated with fluorescein instilled into the conjunctival sac and viewed using the cobalt blue filter of the slit lamp. The view is enhanced if a yellow barrier filter is placed over the viewing system. Too much fluorescein can obscure the view and it is preferable to apply the dye to the eye using a sterile disposable applicator. Sterile disposable strips impregnated with fluorescein are available, but the amount of the dye is less easily controlled.

Fluorescein should be washed from the eye with sterile normal saline before reinserting soft lenses, and the patient should be warned that some remaining stain may discolor the lens, but that this will not be permanent and will be removed by the care system. Corneal staining is a common finding in soft lens wearers as well as with RGP lenses.1

Conjunctival staining is seen best using Rose Bengal and white light. Unfortunately it is virtually impossible to remove Rose Bengal from a soft lens and so such lenses should not be reinserted until all traces of stain have gone.

Types of staining

Staining may be punctate, diffuse or confluent, and localized or generalized (Fig. 5.4). Disorders giving rise to corneal staining may cause pain, discomfort and redness, but may be asymptomatic.

Superficial punctate keratitis (SPK) consists of a localized diffuse epithelial disturbance, which appears as a gray clouding associated with discrete, punctate corneal staining.

Superficial punctate epithelial erosions (SPEE) are fine depressed areas of epithelial disruption seen when stained with fluorescein, and occur in exposure keratitis and toxic reactions.

Figure 5.4 Diffuse punctate stain.

Figure 5.5 Staining at three o’clock position caused by dryness in patient wearing RGP lens.

Causes of staining

Corneal desiccation

Three and nine o’clock staining Three and nine o’clock staining is characteristic of desiccation associated with RGP lens wear (Fig. 5.5). Areas of punctate stain occur in the horizontal meridia of the cornea at 3 and 9 o’clock. Dryness occurs as a result of a thick lens edge or too great an edge lift holding the eyelid away from the eye, so preventing effective resurfacing with tear film in these areas. Eventually stromal thinning occurs and dellen form (Fig. 5.6). Topical lubricants and improved blinking are useful. Itoi et al.2 found sodium hyaluronate eye drops preferable to other artificial tear solutions. Desiccation can be minimized by refitting the lens with a larger total diameter, providing an adequate tear reservoir can be maintained under the lens edge, and reducing lens edge thickness.3 In all cases

Figure 5.6 Dellen resulting from corneal desiccation associated with rigid lens wear.

it is important to avoid a low-riding lens. Three and nine o’clock staining in a wearer is a contraindication to extended wear use of RGP lenses.4

Inferior arcuate staining Inferior arcuate staining (smile stain) may occur with ultra-thin highwater content hydrogel lenses. These lenses are associated with a higher incidence of post-lens tear film depletion, have greater front surface dehydration and are more likely to adhere to the eye.5 A thicker, lower water content lens is required. Care should be taken when considering prescribing a thinner soft lens to increase oxygen transmission that inferior arcuate staining does not occur.

Environmental drying conditions Environmental drying conditions such as air conditioning, car heaters and aircraft cabins may cause a dry atmosphere resulting in SPK and steepening of the lens, making the fit tighter.6 Lagophthalmos due to failure to completely close the eye during sleep should be excluded as a non-contact lens related cause of inferior staining, and the eyelids should be examined to ensure complete closure.

Mechanical causes

Foreign body staining Staining caused by a foreign body is seen most commonly with RGP lenses that do not cover the entire cornea (Fig. 5.7) and are more prone to dust blowing beneath the lens. Lashes and mascara may be carried onto the eye by a soft contact lens during insertion, and the reduced tear exchange beneath a soft lens means that it is less likely to be flushed out than from under a rigid lens. In hot dusty climates soft lenses are more likely to be worn successfully than a small rigid lens.

Figure 5.7 Corneal abrasion due to foreign body behind a rigid lens.

Foreign bodies cause redness, pain and water ing, and characteristically cause a looped or zigza stain as the lens rotates on the eye. A carefu search of the eye should be made to locate the for eign body and to exclude the presence of mor than one. In all cases the upper eyelid should b everted and the lower fornix also examined. If foreign body is found it is sensible to seal it int the notes with transparent, sticky tape, so that permanent record is kept. Most foreign bodies ca be removed from the eye by irrigation or using sterile cotton wool bud. If the foreign body i embedded in the cornea it will need to be remove under topical anesthesia, using a sterile needle. I a significant defect remains in the epithelium antibiotic ointment and a pad should be applied.

Superior epithelial arcuate staining (SEAL) Punc tate corneal staining in an arcuate pattern unde the upper eyelid and adjacent to the limbus i most commonly seen with daily wear low-wate content soft lenses (Fig. 5.8). There is a horizonta lesion with heaped up epithelium associated with central split. It is caused by mechanical forces dur ing the blink. Discomfort occurs on insertion o removal of the lens, but is not present with th lens in situ. The lens should be refitted using a dif ferent, less rigid, soft lens material; it may b necessary to redesign the lens.

Tight lens fit A tight RGP lens may indent an sometimes abrade the cornea and cause arcuat staining. Immobile, tight lenses may be asymptom atic or even described as “more comfortable” tha a previously fitted lens because the lack o

Figure 5.8 Superior arcuate epithelial lesion found with thicker and higher water content lenses.

movement may reduce eyelid sensation. The lens should be removed and refitted. A soft lens may cause indentation of the conjunctiva and pooling of fluorescein, which can mimic staining. In the elderly this should be differentiated from folds of lax conjunctiva that overlap the lens edge, but do not cause redness or the blanching of blood vessels, which occur with a tight fit.

Trapped air bubbles Dimple veiling (Fig. 5.9) is the result of a bubble of air trapped beneath an RGP lens, usually during insertion. The large bubble breaks into myriads of small bubbles, which each indent the cornea and into which fluorescein will pool. They can be differentiated from true staining by irrigating the eye with normal saline (all the stain will be washed from the indentations) and by the rapid recovery of the cornea if the lens is removed.

Mucin balls A similar pattern of pooled fluorescein may be seen in the corneal indentations seen in the presence of mucin balls, which are associated with silicone hydrogel lenses (Fig. 5.10).

Toxic and allergic staining All preservatives used in eyedrops and contact lens solutions are potentially toxic, depending on the concentration and exposure time. In normal therapeutic circumstances eyedrops are used for limited periods of time and remain in contact with the eye only for short periods. When used in contact lens solutions the period of wear and consequent solution use is likely to extend over years and the solution may bind to the lens causing irritation, or concentrate in the lens and be released in uncontrolled amounts.

Figure 5.9 Bubbles beneath rigid lens causing small indentations in corneal epithelium. There is no true staining.

(A)

(B)

Figure 5.10 A&B: Mucin balls. Post-lens debris beneath silicone hydrogel lens.

Toxic staining may be due to insertion of the lens without rinsing off the surfactant cleaner, or failing to neutralize hydrogen peroxide. It may also occur if the coating of the tablet used to

Figure 5.11 Toxic keratopathy resulting in superficial epithelial punctate staining.

neutralize peroxide is inadequately rinsed off the lens. Toxic reactions can result from chemical solutions being accidentally splashed in the eye. They may cause localized or diffuse SPK or an area of frank epithelial loss on the cornea and conjunctiva (Fig. 5.11).

Toxic reactions can also result from the instillation of nonprescribed medication, and care should be taken to enquire about such a possibility.

Corneal abrasions These usually form linear staining areas and are often related to difficulties with insertion or removal, or are caused by long or false fingernails or garden plants (Fig. 5.12). The eye may be red, watering and painful, but if the lesion is small and superficial there may be little discomfort. If symptoms are severe a mydriatic and a broad-spectrum antibiotic should be applied. If the abrasion is large, a patch may be necessary, but pressure patches should be avoided.

It has been suggested7 that damage to a soft contact lens may occur as an initial event, producing particles and deposits at the lens–cornea interface. These can cause a corneal abrasion and the onset of other complications.

Lens-related problems

Lens damage and defects

It is more difficult to create a good edge on RGP lenses than on PMMA lenses. Careful examination by holding the lens and using slit-lamp magnification may elicit a finely ridged edge that is invisible to the naked eye or to lower magnification. The

Figure 5.12 Corneal abrasion caused by plant.

lens should also be inspected for lathe markings Lens breakage may occur either in the eye, usually as a result of rubbing the eye, or when handling the lens. Often a piece may be missing from th carrier or the break may occur at the junction o the carrier with the optic, particularly with high plus lenses and in the elderly and diabetics wh may have reduced sensation in their fingers. It i possible to order a lens with a slightly thicke junction or one made of less breakable material Central defects are less common than with sof lenses, but sharp junctions may cause staining The larger the defect the more severe the cornea damage, but it is surprising how often patients ar unaware of lens defects.

Lenses that break on the eye cause a great dea of concern in the wearer and the eye should b examined and any residual pieces of lens removed The cornea should be stained to ensure no signifi cant injury occurred.

RGP lenses stored in solution that has been allowed to evaporate may be coated in dried solu tion that is very abrasive. The edges of all such lenses should be checked before inserting the lens in the eye. The dried solution can be removed by rehy drating it in sterile normal saline, or cooled boiled water, and gently rubbing off the softened solution.

Edge defects in soft lenses due to manufactur ing defects or the lens edge catching in the baske eyelid may be asymptomatic. Splits situated mor centrally in the lens may abrade the cornea as they open and close with the blink. Central splits can be the result of overaggressive cleaning, handling the lens when it is dry, or folding the lens in hal

and rubbing it between the finger and thumb to clean it.

Efron and Veys8 showed that defects, including manufacturing defects, can occur with disposable as well as conventional lenses.

If patients are comfortable and the defect is not causing damage the lens may be worn, as long as there is no redness or discomfort, until a new lens is obtained.

Contact lens spoilation

A wide range of techniques is available to examine lenses for spoilage,9 but in the clinical situation the lens may be examined on the eye with the slitlamp biomicroscope, which may show evidence of colored deposits, jelly bumps, or mucus deposition. When the lens surface is wet the deposits are difficult to see, but if viewed between blinks, as the lens dries, they become more visible.

Deposits on all types of lens are often best viewed off the eye with the unaided eye, or with a loupe ( 10) after being allowed to dry. Deposits may be organic, including proteins, lipids, mucin, carbohydrates, cosmetics and pollutants, or inorganic such as iron or calcium salts, or they may be mixed deposits.

Mechanism of deposit formation As soon as a new contact lens is worn, tear film constituents start to adhere to the surface. Tear constituents adsorbed to the lens surface are mainly proteins (particularly lysozyme) and mucin, a complex carbohydrate. The exact proportions vary with the polymer and water content of the lens. The proteins are denatured. This may be reversible in the initial stages, but eventually the denatured protein becomes irreversibly bound to the lens surface and the lens matrix, and forms a deposit. This is called the “primary zone” (Fig. 5.13).

Further adherence of protein and mucin to the primary zone causes the formation of a coating or pellicle. In the pellicle, proteins are reversibly denatured and loosely bound to the underlying, inactivated, primary zone. The pellicle gradually builds until it is much thicker than the primary zone and eventually covers the whole lens. Up to 50% of some lens surfaces may be covered in 30 minutes of wear.10 The formation of the pellicle

TRANSIENT TEAR FILM

equilibrium

PELLICLE denatured partially reversible capable of reactivation

PRIMARY ZONE denatured irreversible inactivated

HYDROGEL POLYMER MATRIX

Figure 5.13 Schematic cross-section of the matrix of the lens polymer, the primary zone (deposit), the secondary zone (pellicle, coating), and the tear film.11

Figure 5.14 Deposits on the surface of a contact lens.11

is thought to be beneficial because it forms a protective layer between the lens surface and the palpebral conjunctiva, but the formation of a complete pellicle depends on an adequate tear film and correct blinking.

On the anterior lens surface the coating shows many convolutions (Fig. 5.14), and these extend over the lens edge to coat the periphery of the posterior lens surface. Centrally the posterior surface has a minimal coating without any convolutions. With time the proteins of the pellicle deposit onto

the primary zone and become denatured and irreversibly fixed. Hart11 believes that, in some patients, hydrophobic groups in the protein are exposed and the pellicle becomes unstable and breaks under the influence of eyelid shearing and evaporation, so that tear constituents can adhere directly to the primary zone, lens surface and matrix. These abnormal deposits form the basis for further abnormal deposition.

Types of deposit Clinically protein deposits appear as thin, superficial film on which other deposits may form. They cause reduced vision because they result in light scatter, irritation, and may act as an antigen to cause giant papillary conjunctivitis (GPC).

Lipid deposits are often referred to as “jelly bumps”. They are smooth, localized and raised above the lens surface and are surrounded by smaller satellite bumps. They may coalesce and extend into the lens matrix. Once this has happened attempted removal leaves a cavity in the lens surface, which encourages further deposits and alters the flow of the tear film and may cause discomfort. Large deposits often contain calcium and may contain protein. Lipid deposits are frequently associated with meibomian gland disorders. They consist mainly of cholesterol and its esters and triglycerides.

Mucin forms opaque, white deposits on the lens. Deposits on RGP lenses are shown in Figure 5.15. Although the primary zone forms initially from proteins and mucin, further deposits are formed mainly from lipid. The greater the silicone content of the polymer, the more hydrophobic and lipophilic it is, so that the greater the silicone content of the lens, the greater the risk of deposit formation. Fluoropolymers are wettable, but more deposit resistant. Surfactants will remove superficial lipid deposits from RGP lenses, but not the

protein deposits on soft lenses.

Factors affecting deposit formation summarized in Table 5.2.

Ionicity Deposit formation varies with the type of lens material (see Table 2.4, p. 25). Positively charged lysozyme is found in greater quantities on negatively charged, ionic, lenses, and these lenses have a thicker pellicle than nonionic

Figure 5.15 Deposits on RGP lens.

Table 5.2 Factors affecting deposit formation

lenses. Group IV (high water content, ionic) lense deposit greatest amounts of protein and Grou I (low water, nonionic) deposit least. Protei deposits more readily on Group II (high water nonionic) lenses than on Group I lenses, but les readily than on Group IV lenses.12,13 Jones et al.1 found protein deposition was largely controlle by ionic charge, but that lipid was predominantl controlled by the N-vinyl pyrrolidone (NVP) con tent of the lens.

Water content High-water content lenses hav a larger pore size that encourages absorption o deposit into the lens matrix, whereas small-pore low-water content lenses (e.g. crofilcon A) tend t be deposit resistant.

Manufacturing technique Lathe-cut lenses ar more likely to have deposits than spuncast lenses This may be related to the smoother surface of th spuncast lenses and the greater opportunity fo manufacturing marks causing a less regular sur face on the lathe-cut lens. Deposits also occur i relation to engravings on the lens (Fig. 5.16).

Figure 5.16 Deposits occurring in engraving on soft contact lens.

Modality of wear Greater deposition is seen with extended-wear lenses and this tends to occur interpalpebrally, possibly due to the greater evaporation in this area and the less frequent cleaning of these lenses.

Surface deposits occur less often on frequent replacement lenses, but cannot be totally prevented because of their rapid formation.15 Jones et al.16 found that visible deposits increase in highwater content lenses with longer replacement times for both lipid and protein deposits, so monthly replacement is preferable.

Some lenses need to acquire a pellicle (coating) to improve their surface wetting and make them biocompatible. If lenses are to be worn for 1 day only this needs to occur rapidly and this might not be achievable with nonionic lenses. Nevertheless work by Guillon et al.17 does not indicate that this is a problem and 1-day lenses have been shown to have fewer deposits than lenses worn for 2 weeks.18

Blinking An inadequate and incomplete blink increases deposit formation in the horizontal interpalpebral area. Lagophthalmos may cause deposits to form on the inferior portion of an extendedwear lens. A band of central deposits may also be seen in the extended-wear lens wearer after surgery under general anesthesia, due to the manipulation of the eyelid by the anesthesiologist to assess the pupils. All extended-wear lens wearers should be advised to either remove the lens for surgery or inform the attending nursing and medical staff.

Care systems Heat is rarely used in current contact lens practice, but may be useful in countries

where chemical solutions are not readily available or in patients who suffer allergic reactions to care solutions. Even if the lens material is suitable for heat disinfection, and many are not, any protein remaining on the lens after cleaning becomes denatured and is irrevocably bound to the lens matrix, so that the lens life is shortened.

Compliance with care regimens is often poor, even with the one-step systems now available, and failure to clean and disinfect the lens increases deposit formation. Enzyme tablets may remove some types of deposit, but not others, depending on the enzymes they contain. A review of the nature of the deposit may result in a change in the enzyme prescribed, but even regular treatment with enzyme tablets does not remove all the deposit. The condition of RGP lenses can be improved if the lens is cleaned and polished, providing this process does not interfere with any surface coating.

Lens design An uncomfortable lens, particularly when due to a poor edge design, causes an increase in deposits. Toric lenses tend to deposit inferiorly where any prism ballast will locate and such lenses may cause an incomplete blink.

Environment Dry conditions with high temperatures cause increased deposits, and smoky atmospheres will discolor lenses. Patients should be advised to adjust the airflow in their cars to avoid drying their lenses, and in centrally heated, air-conditioned rooms, humidifiers are of help.

Medication Medication that causes dry eye is likely to increase deposit formation.

Papillary conjunctivitis A characteristic feature of active GPC is the production of mucus, which coats the lens and reduces vision, and renders the lens more mobile so that it moves with the blink.

Sulphur Sulphur from the tear film has been found in association with deposits and has penetrated deep into the lens matrix from cosmetics, pollutants and solution components.19

Corneal exhaustion syndrome

Corneal exhaustion syndrome has been recorded in long-term wearers of contact lenses. There may be severe ocular discomfort, redness, reduced vision and photophobia with corneal edema. There is distortion of the endothelial mosaic and

severe polymegethism.20 The changes are believed to be the consequence of chronic hypoxia and acidosis. Lens wear should cease and the situation reviewed when the patient is asymptomatic. It may be possible, under careful, regular supervision, to recommence lens wear with a higher Dk lens or by changing to an RGP lens. This is a situation in which the new silicone hydrogels may prove useful if worn on a daily basis.

Giant papillary conjunctivitis

Giant papillary conjunctivitis is discussed in Chapter 7 (see p. 78).

Microbial keratitis and corneal infiltrates

Microbial keratitis and corneal infiltrates are discussed in Chapter 6 (see p. 61).

neovascularization occur. A search should be mad for new vessel spikes at each follow-up visit. Boyc and Carman21 describe a useful method of record ing blood vessel status and growth. Concentri circles, representing

●the corneoscleral junction

●2 mm within the junction

●4 mm pupillary zone.

These circles are intersected by lines perpendi cular to one another. The length and location o vessels can be recorded on the diagram. This can be annotated with details of vessel depth, cornea clouding and the etiology of the vascularization (Fig. 5.17).

Vascularization may be superficial or deep Deep vascularization may result in a lipid ker atopathy and diminished vision. It may rarely cause an intracorneal hemorrhage, which usually resolves spontaneously with no sequelae.

Contact lens-related vascularization

There is some confusion in the literature about contact lens-related vascularization due to differences in terminology and the difficulty in standardizing the point of measurement. Most recently vascularization has been defined as the presence of blood vessels in the normally avascular cornea and neovascularization as new blood vessel growth occurring in a cornea. The limbus is a transition zone where transparent cornea changes to opaque cornea with an overlying conjunctiva. Vessels should be measured from the point where the conjunctival overlay meets the clear cornea. Vessel ingrowth is unacceptable if greater than 2 mm.

Examination of the vessels at the limbus is best undertaken using the slit lamp, direct focal illumination and a green (red-free) filter. A graticule in one eyepiece makes measurement easy and sufficiently accurate for clinical purposes. Alternatively the height or width of the slit beam can be used and the measurements read from the scale. Empty, ghost vessels are best viewed by retroillumination.

Details of limbal vessels and any corneal penetration should be recorded at the initial visit to provide a baseline for comparison should vascularization or

Causes of vascularization

Vessel ingrowth and new vessel formation result from a number of factors including hypoxia hypercapnia, inflammation, edema, trauma and toxic reactions. Vascularization is much less com mon with RGP lenses than with soft lenses and i seen most commonly with extended-wear lenses Localized vessel ingrowth may occur as a conse quence of desiccation with RGP wear (see Fig. 5.5) With extended-wear lenses inflammation give rise to metabolites, which stimulate angiogenesis and hypoxia and lactic acid accumulation can cause macrophages to release angiogenic factors Lens wear itself may cause repeated mild trauma, which can result in the release of angio genic mediators. Care solutions, which ar absorbed into the lens and released into the eye may cause limbal dilatation, which may give ris to neovascularization.

Management of vascularization

Management of vascularization and neovascular ization (Table 5.3) depends on careful supervision of the patient. Most patients assume that all is wel if they are asymptomatic, and it is essentia that complications such as vascularization ar

discussed so that patients understand the need for regular checks.

Lens wear should cease until the vessels have regressed and emptied, and only ghost vessels are seen. Refitting with a higher water content, thinner hydrogel lens on a daily wear basis may prevent vessels recurring.

Some patients are happy to wear their lenses on a part-time basis and to use spectacles at other times.

RGP lenses will provide greater oxygen transmission than most soft lenses and additional tear exchange beneath the lens. If vessels recur during RGP wear a higher Dk material, a smaller lens or