Section ONE Preliminaries

begins to rupture leading to isolated islands of break-up. This is the stimulus for blinking and cycle is repeated.3

6.2 Dry eyes

The symptom of dry eyes is the commonest complaint of contact lens wearers. The marginal dry eye manifests only in the presence of a provocative stimulus such as a contact lens. The causes include:

•Aqueous deficiency

•Mucin deficiency

•Lipid deficiency

•Ocular Surface implications

A pathological dry eye can be defined as having symptoms in most normal situations without a provocative stimulus.

6.2.1 Aqueous deficiency

The aqueous originates from the lacrimal gland and provides tear volume. The composition includes:

Electrolytes which regulate the osmotic pressure and pH, such as sodium, potassium, magnesium, calcium and chloride to maintain epithelial hydration.

Proteins some of which have an antibacterial role (e.g. Lysozyme); an antiinflammatory function (eg Lactoferrin); an immunological response (e.g. IgG, IgA); and enzymes for aqueous transport and flushing away debris.

Vitamins C, B2 and B12.

The dynamics of the tear aqueous equate to a non-Newtonian fluid. In between blinks it has high viscosity, giving a stable film and good ocular surface coverage. During blinks it has low viscosity with the collapsing film providing lubrication between the eyelids and ocular surface. The aqueous thickness reduces by about 20% five seconds after a blink and by about 50% after 30 seconds.

6.2.2 Mucin content

Mucin content is related to corneal surface wettability. Mucin is produced by:

•Goblet cells of the conjunctiva.

•Glands of Moll and Krause.

•The endoplasmic reticulum and Golgi apparatus of the corneal epithelium cells. These produce vesicles of mucopolysaccharides and keratosulphates which are probably responsible for the production of microvilli to which the mucin layer adheres.

A reduction in cell activity due, for example, to hypoxia, results in the nonproduction of vesicles and microvilli and therefore affects the wettability of the ocular surface. In addition, decreased goblet cell density causes a mucin deficiency which can be affected by systemic mineral deficiency and altered secondary to contact lens induced papillary conjunctivitis (CLIPC).

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6.2.3 Lipid deficiency

Lipids are expressed from the meibomian glands spontaneously and on a full blink. Meibomian gland disease (posterior lid margin disease) is the main cause of a deficiency. Frothing within the tear prism indicates lipid contamination produced by meibomian gland dysfunction (MGD). Hyperkeratinisation of the lid margin due to disease causes reduced tears production and an accumulation of inflammatory products. Any local disease, such as blepharitis (anterior lid margin disease), affects lipid production. This is also true of skin complaints such as acne rosacea and systemic conditions such as high cholesterol. Excessive tear evaporation due to blocked meibomian glands gives rise to surface inflammation and negative neural feedback to the lacrimal gland which exacerbates the

problem.

6.2.4 Ocular surface conditions

•Conjunctival folds or LIPCOF (lid parallel conjunctival folds), loose bulbar conjunctival tissue adjacent to the lid margin, are implicated in the irregularity of the tear prism and cause tear film instability. (see   Section 6.3.6)

•Pinguecula, a common degenerative conjunctival anomaly, can destabilise the tear film because of its elevated nature.

•Tylosis, an uneven lid margin or lid irregularity, is conducive to an irregular tear prism.

•Lid retraction can cause or exacerbate dry eyes. It is especially a problem when staring at a VDU as the blink rate will be markedly reduced. Those patients who during fluorescein instillation or slit lamp examination automatically retract the upper lid when asked to look down are very likely to have tear film problems.

•Proptosis of the eyes causes increased exposure of the ocular tissue which encourages tear evaporation and alters the blink mechanism.

•Habitual incomplete blinking or eye closure is even more relevant when wearing contact lenses, especially with visual tasks that involve staring such as computer work, intensive reading or sustained driving.

•Disruption of the tear film and corneal innervation occurs following surgery (e.g. LASIK). This is also described as an evaporative condition and although typically lasting for about three months following refractive surgery can persist for much longer.

6.2.5 Marginal dry eyes

The symptoms of marginal dry eyes include:

•soreness

•tiredness

•grittiness

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•dryness

•burning

•itching

•photosensitivity

•foreign body sensation

•irritation

•discomfort

•lid heaviness

Symptoms due to decreased tear production or increased evaporation tend to become worse as the day progresses. The dry eye patient is more susceptible to damage of the corneal and conjunctival surfaces and combined with changes to a contact lens surface can experience reduced visual performance. Evaporation begins on waking and increases tear film osmolarity as the day goes on. Eye closure during sleep allows a watertight seal to form over the tear film and gives the ocular surface a chance to recover.

Marginal dry eyes are the result of:

•Environment – smoke, wind, heat.

•Medication – anti-histamines, beta blockers.

•Age – HRT, post menopausal effects on the lacrimal gland.

•Diet – greasy food, alcohol

•Contact lens wear

Age related, idiopathic aqueous deficiency is the most common cause of dry eyes. It is prevalent in older men and women as well during the menopause. Histological changes to the lacrimal gland are implicated and it is considered an auto-immune disease with inflammatory mediators. The tear protein components are altered and the tear film osmolarity is increased.

There is a strong association with contact lens wear because it encourages tear film instability by initiating or exacerbating aqueous, mucin and lipid abnormalities. Lenses appear to increase the rate of meibomian gland dysfunction which in turn disrupts the tear film stability. This causes a loss of both surface lubrication and lipid layer integrity which allows increased  aqueous evaporation. Rigid and to some extent soft lenses cause corneal hypoaesthesia due to prolonged wear which decreases the basal and reflex tear secretion.4

There is alteration to the pre-lens tear film (PLTF) over any contact lens surface. Soft lenses show a reduced tear film; the lipid, aqueous and mucus layers are thinner and also have diminished wetting properties compared with those of the cornea. The PLTF over a rigid lens is thin and less stable than that over a hydrogel lens. It often has no lipid layer at all and so the aqueous layer thins through evaporation. Silicone hydrogel materials are hybrid in nature, between hydrogel and rigid gas-permeables; a lipid layer is usually present which limits evaporation but the effectiveness of these materials in dry eye problems may be limited compared to biocompatible hydrogels such as Proclear. (CooperVision).5 Contact lenses are also associated with altered mucin production; an accumulation of surface deposits with inadequate wetting can increase tear disruption.

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6.3 Assessment of tears

Assessment of tears is an important part of the preliminary examination in  order to:

•Predict potential success.

•Eliminate likely failures.

•Discover marginally dry eyes which would be adversely affected by contact lenses.

•Decide on the most appropriate lens type.

There are several ways of assessing the tears but basically it is necessary to decide whether any problem is due tear quality or quantity or both. Each technique has its own limitations depending upon factors such as temperature and humidity and whether the method is invasive or non-invasive.

Patient questionnaire

Screening with the aid of a questionnaire prior to contact lens fitting can give valuable information in assessing a marginally dry-eyed patient. Questionnaires have been published by McMonnies6 and Schiffman.7

6.3.1 Assessment of the tear volume

Tear prism observation

The upper and lower tear prisms hold 90% of the tears, so that the height  and width give a reasonable assessment of tear volume. To estimate tear  prism height, reduce the width of the slit lamp beam and rotate it to the horizontal position. Its value can then be used as a measurement guide for the  height of the tear prism. It is important to ensure the eye is in the primary position as the apparent height of the tear prism can depend on the position of gaze. For precise measurement a graticule can be employed in the slit lamp eyepiece.

Irregularity of the tear prism and the appearance of the conjunctival tissues give an indication of lid margin disease.

•Slit lamp observation either with or without fluorescein gives an overall view of the entire prism.

•The normal tear prism surface is convex where it is in contact with the cornea and lids but is concave centrally.

•The approximate height of the prism in a normal tear film is 0.2–0.4 mm at the centre and 0.1–0.2 mm at the periphery.5

•The shape of the tear meniscus can be photographed in the zone of specular reflection as a bright central band bordered by dark non-reflective areas.

•When the lower tear prism is illuminated from above the reflection should show a ‘with’ movement in the convex zone and an ‘against’ movement in the concave zone.

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•Reduced height suggests reduced tear volume.

•Increased height could indicate poor drainage because of obstructed puncta or an excessive aqueous layer giving a watery tear film.

•The regularity of the prism along the length of the lid margins indicates   the potential of the tear film to wet the eye consistently. This reduces   with age.

•Increased curvature is a sign of reduced volume; reduced hydrostatic pressure encourages movement of fluid from both the pre-ocular tear film and the tear meniscus, inducing localised disruption.

•Irregularity is associated with conjunctival folds.

•Abnormality is indicated if there is an uneven border and a height of less than 0.1 mm combined with a sharply demarcated black line at the junction of the tear film.

Non-invasive observation of changes to the meniscus during blinking can be made with the TearscopePlus (see Section 6.3.4). In the central area a black central band can be seen bordered by bright bands where the tear prism is in contact with the lid and cornea. Any irregularity of the meniscus shape caused by degenerative lid changes or meibomian gland blockage can be seen as a distortion of the central black band. Any variation in height can also be observed along its length.

Phenol red thread

The test consists of a cotton, two-ply thread impregnated with phenol red (ZoneQuick – phenolsulphonphthalein). The thread is pH-sensitive and changes from yellow to red as it is moistened by the tears. The folded 3 mm portion of the thread is placed against the bulbar conjunctiva near the outer canthus. The eye looks up and in, to avoid contact with the cornea. The lower lid is released  to retain the folded part in place. The remainder of the strip projects at rightangles from the lid. The normal result gives about 21 mm moistened in 15 seconds, borderline if less than 16 mm and low if less than 11 mm.8 Measurement must be taken of the entire length actually wetted regardless of the position of the fold.

Schirmer test

This is a well established quantitative test of tear production which uses strips of Whatman No.1 filter paper with notched ends 5 mm wide. The notched end is folded twice to give better balance on the lower lid and is placed as for the Phenol Red Thread test. The result is recorded either as the length of paper moistened in 5 minutes (normal = 15 mm) or the time taken to wet a length of 10 mm (normal = 3 minutes). The time taken for the test is a potential problem. The use of a local anaesthetic improves tolerance for the test but as it has been implicated in reducing tear volume is not recommended. The Schirmer test is currently used less often except as a provocative test to indicate severe lacrimal disease with the absence of reflex tearing.9

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PRACTICAL ADVICE

•The invasive nature of the strip causes reflex tearing.

•The results do not necessarily represent the patient’s norm but do indicate the presence or absence of reflex blinking.

•The result is quantitative and can be recorded for future comparison.

6.3.2 Tear mixing

Under normal conditions, fluorescein blends immediately with the tear film. If there is poor mixing, the tear prism and any dense conjunctival staining may both present a dull orange hue. This effect is often unilateral and with time and blinks the colour changes to the more usual bright fluorescent green. On fluorescein instillation, the patient should initially be encouraged not to blink. If dull orange is seen, it indicates tear film dysfunction, either temporary or habitual. On blinking, fluorescein eventually mixes normally with the tears when the lipid layer has been penetrated. This phenomenon is often associated with dry eyes or sinus problems. The phenomenon of ‘quenching’ occurs at high concentrations where fluorescein absorbs some of its re-radiated light, a further indication that there is minimal dilution by the tears.9

6.3.3 Tear film stability

Invasive tear break-up time

The tear break-up time (BUT, or TBUT) is the time in seconds for the so-called break-up of the precorneal tear film in a non-blinking eye. In fact it is an observation of tear thinning. BUT is a convenient and simple test to perform but its invasive nature requires careful interpretation of the results.

•A drop of fluorescein is instilled into the eye.

•The slit lamp with blue light is used to observe the patient after a few blinks have mixed the fluorescein completely into the tear film.

•The patient stares while a wide blue beam is focused onto the cornea.

•The time is recorded for the first break in the tear film, shown as a dark blue patch against the otherwise green background of fluorescein.

•A normal result gives a BUT of 10 seconds or greater.

•The blue patches seen may be circular, streaked or uneven in shape.

PRACTICAL ADVICE

•If 15 seconds elapse with no break in the tear film, ask the patient to relax since the tear film is well within normal limits and the test can become uncomfortable after about 20 seconds.

•The position of the first break is often significant, especially if it corresponds to an area of staining or occurs at the lower limbus, since it suggests potential problems of desiccation.

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•A supplementary test, possibly of even greater value, is to observe the relaxed patient blinking normally to determine whether the tear film breaks up between blinks or remains unbroken until the next blink.

•The use of fluorescein destabilizes the tear film and can give erroneous results.

•The temperature and humidity of the consulting room can affect the result.

•Only a large variation in results (5–10 seconds) is significant.

Non-invasive tear break-up time

Non-invasive break-up time (NIBUT) methods measure the stability of the tear film without a staining agent. Most only show tear thinning rather than a true break in the tear film.

•One position keratometers with circular mires (e.g. Bausch & Lomb). The time is recorded when distortion is first seen in any part of the mire pattern. The three mires cover only a very limited amount of the central 3.00 mm of corneal surface, making this a rather inaccurate method.

•Hir-Cal grid. An extension of the above technique using a circular grid set within the Bausch & Lomb keratometer. Observation is still made only of the central corneal cap so that the first signs of break-up may be missed if they occur in the periphery.

•Loveridge grid. A modification of the Klein keratoscope with a fine grid pattern and +22.00 D observation lens. A convenient hand-held method with inbuilt stopwatch.

•TearscopePlus. Uses a cold cathode ring light source. Observation of the reflected light will show a true NIBUT, the norm being about 40 seconds.

6.3.4 Tear film analysis

Slit lamp techniques

•Specular reflection. Observes the bright zone of specular reflection with the slit lamp. Allows debris in the tears to show up as dark spots which move on blinking.

•Narrow-field specular reflection. The first Purkinje image, seen with reduced slit lamp illumination, appears either plain or enhanced with coloured fringes. It gives a relative estimate of lipid layer thickness and can indicate contamination of the lipid layer. Coloured fringes alone are difficult to assess, but when found together with irregularity of the tear prism, they strongly suggest a poor tear film.

Keeler Tearscope-Plus

The originalTearscope was devised to make use of specular observation of the tear film over the entire cornea. It was redesigned as the Tearscope-Plus with a range of inserts for more extensive tear film and ocular surface imaging. These include:

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•Concentric rings for Placido disc type observation.

•A coarse grid for hand-held observation of the rings. Coarser grids are also used to visualise localised corneal surface damage.

•A fine grid for tear film observation and NIBUT using the slit lamp.

•Blue and yellow filters for fluorescein observation.

•Dark field observation of contact lenses is also possible with the Tearscope-Plus.

The illumination source of the Tearscope-Plus is a double concentric, cold cathode light. The design positions the light away from the corneal surface eliminating heat transfer and avoiding tear evaporation.

Other observations can be made for the pre-ocular tear film (POTF):

•Lipid layer contamination and classification.

•Lipid layer spread during blinking.

•Tear prism classification for regularity and height.

•Tear prism continuity in the corneal and conjunctival area.

•Measurement of NIBUT with or without a grid.

•Meibomian gland manipulation and observation of lipid secretion spreading over the tear surface.

Only the lipid layer is visible by specular reflection. As it thickens, a pattern of flowing lipids appears. An amorphous pattern (i.e. devoid of detail) is seen with increased thickness. The ideal picture appears without coloured patterns. These are produced by interference and relate to abnormal clumps and irregularity in the thickness of the lipid layer. If the aqueous layer is observed, it will be seen as a dark grey surface where the lipid layer has broken (Table 6.1).

Similar observations can be made on contact lenses for the pre-lens tear film (PLTF). For soft lens wearers, the most common use of the Tearscope-Plus is the early detection of dry-eyed patients. This in turn can be used to suggest the optimum frequency of replacement for disposable lenses. With dry eyes, the lipid layer may be absent and the thin aqueous layer will display coloured fringes with surface contamination and a reduced NIBUT.

With rigid lenes the lipid layer rapidly disappears. The aqueous layer consequently thins, showing a coloured fringe distribution over the lens surface. The NIBUT recorded when the tear film is seen breaking up is typically about 5 seconds.

The non-invasive drying-up time (NIDUT) is recorded when the coloured fringes have all disappeared, on average at about 20 seconds. At this point, the surface is devoid of tears.

6.3.5 Staining agents

Fluorescein

Fluorescein staining in the horizontal or inferior sectors of the cornea is an indication of tear film dehydration. Staining of the conjunctiva may also be seen at the nasal and temporal positions and in the region of any elevated tissue. When undertaking slit lamp investigation it is important to scan the conjuncti-

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val tissue first because such exposure staining may become less obvious after several blinks and reflex lacrimation. This is also an opportunity to see if   there is poor tear mixing, another indication of dry eye. Precise control for the volume of fluorescein instilled can be maintained by using the Korb Dry Eye Test (DET).10

Table 6.1 

Lissamine green

Lissamine green is used for staining dead tissue as the current alternative to Rose Bengal because it causes much less stinging. It takes time, however, to penetrate tissues so there is a waiting period of 4 minutes after instillation before observation can be made. It is available in the form of strips which are wetted by saline and used to evaluate the cornea and conjunctiva. A red filter (Wratten 25) enhances the view of any stained area.

Rose Bengal

Rose Bengal has been used to assess the severity of any staining and help decide on the future management of the patient. Its disadvantage is stinging because of the acidic pH which cannot be buffered since the dye would become less effective. Discomfort can be reduced if a cotton wool bud is used for instillation. Any significant uptake of Rose Bengal prior to fitting suggests that it is unwise to proceed with contact lenses. The dye is viewed with white light and in the

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examination routine it follows fluorescein observation. Staining in an area which is already taking up fluorescein indicates chronic dehydration with numerous dead cells.

6.3.6 Lid parallel conjunctival folds (LIPCOF)

Conjunctival folds that border the posterior lid margin are associated with a significantly increased likelihood of dry eye. These can be graded according to their number and height relative to the tear prism as the patient looks in the primary direction (Table 6.2).

Table 6.2  LIPCOF

6.3.7 Upper lid margin staining (ULMS)

ULMS or Lid Wiper Epitheliopathy is caused by the brushing of the cornea and conjunctiva with the lid. This results in a ‘windscreen wiper’ type movement with resultant upper lid margin staining seen with fluorescein or rose bengal. The result of decreased lubrication is increased friction which causes irritation and tissue damage.

6.3.8 Osmolarity measurement

The most sensitive and specific test for dry eye is now considered to be the osmolarity measurement of nanolitre tear samples collected from the inferior tear prism. Change in the electrolyte composition of the tear film is a main cause of the breakdown of the ocular surface interface. If there is an increased concentration of solutes (osmolarity) in the tears this leads to desiccation which in turn activates the signals that precipitate an inflammatory cascade.11 A method that can be used in clinical practice is the TearLab which allows tear sampling  and biomarker analysis. Tears are collected by passive capillary action using a tear collection pen which contains a disposable microchip with gold electrodes. The tip is brought into contact with the tear prism and collects a tear sample. The pen is docked onto the TearLab reader which displays the osmolarity reading.12

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