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Ocular Surface Characteristics of the Asian Eye
Meeting Synopsis
Academy 2010
pective Analysis of Risk Factors Associated With Contact Lens Induced Inflammatory Events During Continuous Wear
Feature Review
Adequate tear mixing under a soft contact lens may play an important role in minimizing certain > more
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Glossary Terms
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Term Definition
Adverse event

General definition: unwanted effect(s) (i.e. physical and/or psychological symptoms and signs) resulting from treatment; any undesirable effect or problem that is present during the period of treatment.

In contact lens practice, common categories:

  • Serious adverse reaction = symptomatic adverse reaction of sufficient clinical concern to warrant permanently discontinuing lens wear and that produces or has the potential to produce significant visual impairment.(eg MK)
  • Significant adverse event = mostly symptomatic adverse reaction of sufficient clinical concern to warrant temporary or permanent discontinuation of lens wear.(eg CLPU, CLARE, IK, SEAL, CLPC, Corneal erosion)
  • Non-significant adverse events = asymptomatic adverse event not of sufficient clinical concern; may not warrant discontinuation of the patient from lens wear.(eg AIK, AI)
Aftercare Regular examination of extended wear patients, consisting of at least vision assessment, history and ocular examination with and without the lenses in place using a biomicroscope slit-lamp; recommended schedule for the first appointments after successful lens fitting:

1. @ 24 hours
2. @ one week
3. @ one month
4. @ 3 months

Every three months thereafter, additional unscheduled appointments whenever necessary. Also see Emergency care.
Alcian blue A copper-staining dye that selectively stains mucus and connective tissue.
Anoxia Complete deprivation of oxygen, suffocation.
Asymptomatic infiltrate(s) Corneal condition including the presence of one or more focal infiltrates (clusters of inflammatory cells), less than 0.2mm in size. Asymptomatic infiltrates are commonly seen in the periphery but may be observed anywhere in the cornea. The infiltrates are limited to the anterior stroma and overlying epithelium is intact. Bilateral events are common. The conjunctiva is normal. No symptoms are present and therefore asymptomatic infiltrates are usually only observed at routine aftercare visits. The condition resolves rapidly with no sequelae. Asymptomatic infiltrates are found in lens wearers and non-lens wearers. There is little variation in the incidence with various modes of lens wear.
Biphasic Having two phases or pathways for transmission through the lens. In silicone hydrogel lenses, one phase allows transmission of oxygen and the second phase allows transmission of fluids and therefore ions, essential for good lens movement.
Contact lens induced acute red eye.
Sudden onset, corneal infiltrative event observed during EW of hydrogel lenses which is always found associated with sleep. Patient reports generally include waking from sleep with symptoms of irritation or pain, redness and watery eyes. Sometimes these symptoms are not noticed until soon after waking. The condition is usually observed unilaterally.
Contact lens induced papillary conjunctivitis.
Signs may range from mild hyperemia of the upper tarsal conjunctiva with a few, small papillae to severe hyperemia with large, raised papillae, which have a cobblestone appearance. CLPC may present as a localized or generalized response. Symptoms include itching and a stringy or ropy mucous discharge. Excessive lens movement or decentration and blurred vision may also occur. Lens material, design or fitting characteristics may need to be modified to prevent reoccurrence of this condition.
CLPU Contact lens induced peripheral ulcer.

A circular, well circumscribed, dense, yellowish-white, focal corneal infiltrate (0.2-2.0mm in diameter) located in the peripheral to mid-peripheral cornea. It is always located in the anterior stroma and has a complete loss of overlying epithelium. Symptoms can vary, but may include pain or soreness, irritation and watering. The condition is almost always unilateral, consisting of a single lesion. Prognosis for healing is good. Typically the CLPU resolves to a circular, anterior stromal scar which persists for more than 6 months after the event. Some patients may not present with an "active" CLPU and the only evidence of the event may be the presence of the resulting scar.
Compliance The willingness to comply with instructions regarding lens wear schedule, removals and care regimen. Compliance with instructions is essential to maximize success with EW.
Conjunctival staining Fluorescein, lissamine green or rose bengal staining of the bulbar conjunctiva. A normal finding at low grades in most contact lens wearers, but may also be observed as a result of dry-eye conditions and/or due to microtrauma from the lens edge. The integrity of the bulbar conjunctiva should be assessed carefully with fluorescein at each contact lens aftercare visit. Conjunctival staining of grade 1 or 2 is considered to be normal or acceptable.
Contact lens induced acute red eye see CLARE.
Contact lens induced papillary conjunctivitis see CLPC.
Contact lens induced peripheral ulcer see CLPU.
Continuous wear Wearing a contact lens constantly, safely and effectively for up to 30 days and nights.
Also see Extended wear.
Corneal distortion Alteration of the corneal shape due to chronic hypoxia or poor lens fit. Often seen in patients with significant corneal astigmatism fitted with spherical, low permeability rigid lenses and long-term PMMA wearers. Also referred to as corneal warpage syndrome. Observed by keratometry, retinoscopy and corneal topography.
Corneal edema see Corneal oedema.
Corneal neovascularization Vessel penetration at the limbus into the cornea beyond the translucent zone. The vessels may empty (ghost vessels) but the condition is irreversible and they will refill with the return of stress, usually of hypoxic nature. Localized neovascularization may appear subsequent to trauma or inflammatory events. When fitting a patient with silicone hydrogel lenses on an EW schedule, baseline neovascularization should not exceed 0.5mm.
Corneal oedema Also called corneal swelling. Fluid diffuses into the stroma and interrupts the regularity of the stromal fiber structure, causing an increase in thickness (swelling) and loss of transparency. Usually occurs as a consequence to hypoxia or anoxia. Clinical signs include microcysts, corneal striae and/or stromal folds.
Corneal scrape A sterile blade is commonly used to remove a small amount of material from the edge/bed of a corneal lesion and placed in various culture media to see if there is any growth of microbes. If any growth occurs, this signals infection of the lesion, or microbial keratitis (MK). Specific treatment can then be prescribed to prevent escalation of the infection.
Corneal staining
Occurs when fluorescein penetrates damaged cell membranes or when it fills gaps in the epithelial cell surface. Gaps are created when cells are damaged or displaced. There are a large number of reasons for corneal staining caused by lens wear including hypoxia, deposits, care products, lens fit, lens surface or edge irregularities, foreign bodies, and tear film disruption.

The integrity of the epithelium should be assessed carefully at each contact lens aftercare visit. Attention should be paid to the extent, depth and type of staining. Corneal staining of grade 2 or more in any area of the cornea is considered unacceptable.
Corneal swelling see Corneal oedema.
Corneal vascularization The normal existence of vascular capillaries at the limbus encroaching no more than 0.2 mm into the cornea from the limbus.
Also see Corneal neovascularization.
Corneal warpage see Corneal distortion.
CW see Continuous wear.
Daily disposables (DD) Single use lenses that are worn during the waking hours and then discarded.
Daily wear (DW) A lens wear schedule requiring daily or more frequent removal for cleaning and other purposes. Lenses are not worn during sleeping hours.
Debris Waste products located between the contact lens and cornea. Post-lens tear debris has traditionally been considered to have the appearance of amorphous cellular material. If substantial amounts of debris are trapped under a lens, staining will be present upon lens removal and fluorescein instillation. Post-lens tear debris is a fairly common observation in patients who wear soft lenses on an EW basis. It should not be confused with Mucin balls.
An acquired protein and/or lipid film or haze on the anterior lens surface.
Disposable lenses Defined by the FDA as contact lenses worn for 1 week of extended wear and those discarded daily (or after one-time use). Defined clinically as contact lenses worn for 1 week of extended wear and up to 2 weeks of daily wear.
Distortion see Corneal distortion.
Dk Oxygen permeability of a contact lens material. Dk usually depends on the water content of a hydrogel lens material and the silicone content of a silicone hydrogel lens material. Oxygen is more soluble in water than in PMMA, but far more soluble in Silicone rubber than in water. Also known as P (permeability).

P = Dk = diffusion (D) * oxygen solubility (k)

Also see high Dk and low Dk.
Dk/L see Dk/t
Dk/t Oxygen transmissibility of a contact lens material with consideration of the thickness of the lens.
Dk/t = Dk / average thickness (L) of the contact lens.

Disposable soft contact lens.Also see Disposable lenses.

Ectropionization see Lid eversion.
Edge buckling see Edge lift.
Edge lift
General definition in lens geometry: The distance between an extension of the BCOR (back central optic radius) and the absolute edge of the lens; when measured parallel to the optical axis, axial edge lift (AEL); when measured along the radius, radial edge lift (REL).
The term is also used when the base curve of a lens is too flat with the consequence that the edge of the lens is lifting excessively so that lens alignment with the cornea is interrupted. Often the lens will ride on the lower lid margin causing varying degrees of discomfort. Edge lift does not reduce or settle with silicone hydrogel lenses and the patient should not be fitted with lenses with this appearance. If possible a steeper basecurve should be trial fitted and assessed. Alternative expressions are lens fluting or lens buckling.
Emergency care The EW patient should have access to the practitioner or a representative of the practitioner at all times in case of an emergency such as CLPU or CLARE. 24-hour access is considered as mandatory for undertaking EW practice.
Endothelial polymegethism Increase in size of corneal endothelial cells due to structural damage of surrounding endothelial cells. The ratio of the smallest to largest cells increases from 1:5 to 1:20. The mosaic pattern of the endothelium becomes rather irregular with cells of significantly differing sizes when viewed under specular reflection. This irreversible condition is either age related (and therefore physiological) or caused by long-term hypoxia (and therefore pathological).
EW see Extended wear.
Extended wear Originally referred to wear of contact lenses for up to six nights and seven days without lens removal. The term EW is now commonly used for continuous wear (CW).
Fluorescein Vital dye used to assess the integrity of cornea and conjunctiva. Fluorescein stains the tear film and depending on the tear film layer thickness, more or less green fluorescence will be observed. A cobalt blue filter in the illumination system of the slit lamp biomicroscope is necessary to excite the dye to a green fluorescence. A yellow filter in or in front of the viewing system of the slit lamp biomicroscope will enhance the visibility of any fluorescent staining.
Fluorescein staining see Fluorescein, Corneal staining, Conjunctival staining and Staining
High molecular weight fluorescein dye used to assess static and dynamic lens fit of hydrogel lenses. A cobalt blue excitement filter and a yellow enhancement filter are necessary in the illumination and observation systems respectively. For optimal visibility, the dye should be instilled on the back surface of the contact lens before insertion of the lens. Minimal dosage is of importance because higher amounts of the dye cause stinging and burning which results in excessive tearing and therefore a false appearance of the lens fit will be observed.
Fluting see Edge lift.
Gas plasma surfacing A technique employed in silicone hydrogel contact lens manufacture to modify the lens surfaces in order to improve the surface wettability and resistance to deposits. The resulting surface treatment is an integral part of the lens and cannot be easily "stripped" away from the base material.
Ghost vessels see Corneal neovascularization.
GPC Giant papillary conjunctivitis. See CLPC.
High Dk Term for contact lenses or contact lens materials with Dk/t > 85 x 10-9 (cm x ml O2)/(s x ml x mmHg).
Also see Dk, Dk/t, Oxygen demand and Oxygen supply.
Hypoxia Reduced oxygenation of the corneal tissue causing a variety of problems. The severity of the consequences depends on the level and duration of hypoxia. Common signs of hypoxia are corneal vascularization, corneal oedema and microcysts. Any signs of hypoxia should be immediately addressed. In some cases temporary or permanent discontinuation of lens wear is necessary to prevent further damage. High Dk contact lenses should be fitted to avoid hypoxic stress or to improve oxygen supply.
Infection Growth of microbes in or on one or more organs of the body. Usually accompanied by inflammation. Corneal infection, see Microbial Keratitis.
White or grey material in the normally transparent cornea, usually composed of inflammatory leukocytes and/or invading microorganisms. May be associated with symptoms.
See also Asymptomatic infiltrate(s)
Infiltrative Keratitis (IK) Inflammation of the cornea characterised by focal and/or diffuse sub-epithelial infiltrates with or without epithelial involvement; some cases may slightly stain with fluorescein. Infiltrates may occur anywhere in the cornea, but typically in the limbal area. Patients with IK report mild to moderate irritation (foreign body discomfort), mild hyperaemia and occasionally mild discharge. Lens wear should be discontinued until full resolution occurs. The patient should be monitored carefully over the first 24hours, but in many cases no medication is required. In the long-term there may be a small residual scar, depending upon the aetiology and depth of penetration of the infiltrate.
Inflammation Non specific host response to tissue injury characterised by swelling, pain, redness and heat. Tissue injury maybe due to infection, toxins released by bacteria or mechanical trauma.
Lens adherence Immobility of a lens on the eye - often observed immediately on eye opening after sleep with silicone hydrogel lenses. Normal movement patterns are gradually established after eye opening so that binding is usually not observed at an aftercare visit. Binding might occur when the aqueous layer of the post-lens tear film is decreased over night, leaving the tear film viscous and adhesive.
Lens adherence is also observed in poor fitting RGPs.
Lens dehydration  
Evaporation of the pre-lens tear film, consequently causing the lens to lose moisture to the environment. More common with higher water content and ionic lenses. May be related to dryness/discomfort symptoms reported by the patient. Pervaporation of the post-lens tear film may result, which has been shown to be more dramatic in high water content (>50%), thin soft contact lenses. A possible sign may include corneal staining in the lower third of the cornea. May be reduced with silicone hydrogel materials due to their high Dk, low water content and unique surface treatments.
Lens fit Optimal lens fit includes 0.2-0.5mm primary gaze movement with good corneal coverage in all directions of gaze. Slightly loose fit (45-50%) on push-up test and no edge lift. Lens settling is uncommon and therefore initial impression is important.
Also see Lens tightness.
Lens fluting see Edge lift.
Lens movement Due to the higher rigidity of the new silicone-based materials, most lenses exhibit approximately up to 0.5mm of movement in primary gaze compared with approximately 0.2mm for DSCL.
Lens tightness Lens tightness can be examined with the push-up test. Using a continuous scale from 0 to 100 per cent, lens tightness of 50% indicates optimal lens behavior at push-up, 100% would indicate the lens is too tight (almost impossible to dislodge) and 0% would be a lens which is too loose (slides from the cornea without lid support).
Lens tightness of slightly less than 50% (rather looser than tighter) is seen as desirable for silicone hydrogel contact lenses.
Lens wear modality see Lens wear schedule.
Lens wear schedule The lens wear schedule determines how many days and nights in a row the silicone hydrogel lenses should be worn without removal. This has to be carefully developed, for each patient individually, usually during the initial aftercare visits. The range may be from DW to flexible wear up to 30N EW. The actual wearing time can be shorter than the lens wear schedule prescribes. It might be necessary to advise patients to remove the lenses from time to time to clean them and rinse them if they are likely to develop deposits. The lenses should be discarded 30 days after opening the blister pack, regardless of whether they have been worn every day.

Typical lens wear schedules are:

6N EW: 6 nights in a row, then one night without lenses (lenses will be cleaned after removal and disinfected over night). This will be repeated over a period of a maximum of 4 weeks with one set of lenses.

14N EW: 14 nights in a row followed by one night without lenses (lenses will be cleaned after removal and disinfected over night). After a further 14 nights in a row, there is one night without lenses. The next day a new set of lenses will be inserted.

30N EW: 30 nights in a row, then one night without lenses before inserting a new pair of lenses the next day.

Flexible wear
: lenses are worn on DW basis most of the time, but can be worn over night for up to x days. Total wearing time should not exceed 30 days.

DW: lenses are only worn in waking hours, removed before sleep, cleaned and disinfected during sleep and discarded after maximum 30 days.
Lens wettability Subjective assessment made after observations with the slit lamp biomicroscope. Takes into consideration the pattern in which the tears break over the contact lens, the speed of break up, the stability of the tear film, the lipid layer appearance and the non-invasive break up time (NIBUT).
Lid assessment see Lid eversion.
Lid eversion To assess redness and roughness of the upper palpebral conjunctiva. At every aftercare visit lids need to be everted. Instillation of fluorescein can be helpful in the assessment of palpebral roughness. With lens wear, levels of both redness and roughness increase to slight or moderate levels. Depending on subjective symptoms this would be considered as acceptable. A localized or general CLPC response has been observed with silicone hydrogel lens wear.
Limbal vascularization see Corneal neovascularization.
Lissamine green A vital stain that has a similar action to rose bengal but does not sting on instillation.
Low Dk Term for contact lenses or contact lens materials with Dk/t between 18 and 25 x 10-9 (cm x ml O2)/(s x ml x mmHg).
Also see Dk, Dk/t, Oxygen demand and Oxygen supply.
Lubricating drops Unit dose or multi-dose comfort drops formulated for use with contact lenses. Patients should be advised to use lubricating drops often and liberally, especially before bedtime and upon awaking.
Microbial keratitis (MK) MK occurs as a result of infection of the cornea by replicating microbes (bacterial, viral, fungal or amoebae). In contact lens wear it is usually preceded by hypoxia and/or an epithelial break. Contact lens wearers (particularly EW) are more prone to develop MK. The most common microbe associated with contact lens related MK, is a bacteria called Pseudomonas aeruginosa. Symptoms include rapidly increasing pain, severe redness, intense epiphora & photophobia. Paracentral or central lesion(s) are observed with full thickness loss, raised edges and an irregular appearance. Anterior chamber flare may also be present. If the patient is wearing contact lenses, they must cease lens wear and immediate medical management should be sought or instituted.
Microcysts Small (15-50µm), irregular shaped inclusions usually found in the paracentral to mid-peripheral zones of the cornea. Observation is best with retro-illumination. Microcysts show reversed illumination due to a suspected higher refractive index than the surrounding tissue. A hypoxic mechanism has been proposed for microcyst development involving a reduced epithelial mitotic rate and a slower regeneration of the epithelium. Patients refitted from low Dk lenses to high Dk silicone hydrogel lenses may exhibit a temporary increase in microcysts. This "rebound" effect is similar to that reported when patients discontinue low Dk lens wear following a period of long term anoxia.
Mucin balls
Small discrete particles or plugs seen between contact lens and corneal surface, composed mainly of mucin, tear proteins and lipids. Mucin balls are round and vary in size and clarity. Smaller mucin balls are generally 10-20µm in diameter and typically transparent. Larger mucin balls, 20-50µm in diameter, tend to appear opalescent. These 'deposits' may be either scattered or clumped. They do not move as the lens moves, so they seem to be trapped against the corneal surface. Lens removal and/or subsequent blinking causes them to be dislodged leaving an indentation in the corneal surface which resolves rapidly.
Necrosis The death of cells or tissues.
Neovascularization see Corneal neovascularization.
Oxygen demand The oxygen demands of the cornea are very high and vary from individual to individual. Closed eye demands range from as low as 1µl/cm²/h to as high as 10µl/cm²/h. Demand increases with temperature. The epithelium is thought to consume up to 95% of the oxygen available to the cornea.
Oxygen permeability (P) see Dk.
Oxygen supply In order to reduce the level of overnight corneal oedema to a physiological level of 3.2% (the mean level of no lens overnight oedema), the critical lens Dk/t would need to be around 125 x 10-9 (cm x ml O2)/(s x ml x mmHg).
Oxygen transmissibility see Dk/t.
Patient history A careful history of the patient is necessary to determine eligibility prior to fitting the patient with EW silicone hydrogel contact lenses. Patients should provide detailed information regarding their past ocular and general health. Motivational factors and compliance with instructions need to be assessed and addressed when the patient history is recorded.

EW contra indications include:

- diabetes
- compromised immune system
- conditions that require steroid or other systemic medication
- severe seasonal allergies
- previous reactions to solutions or to hydrogel contact lenses.

Caution is advised with patients who have had previous episodes of CLARE or CLPU. The risk of recurrence needs to be taken in account in these cases.
Phase separation This is the term used to describe the situation where the interconnections between chemically similar molecules in a material are stronger than the adhesive connections between them and the different molecules.
Post-lens tear debris see Debris.
Post-lens tear film (PoLTF) The PoLTF is the tear layer between ocular surface and contact lens back surface. It is made up of a thin aqueous phase (1µm) and of a compressed mucin phase (30-40µm). The mucin phase is of unknown thickness but it is probably at least marginally thinner than the pre-ocular tear film. The maintenance of the integrity of the mucin layer is essential to safe contact lens wear. The maintenance of the aqueous phase is essential to controlling the viscosity of the PoLTF and facilitating the elimination of back surface debris. The closed eye wearing period with it's associated aqueous depletion and reduced lens movement produces the most challenging phase in EW. The period immediately upon eye opening is therefore the key recovery phase when PoLTF aqueous replenishment takes place.
Regarding silicone hydrogels, the following clinical practice is recommended:

Follow-up visits should take place in the morning and include the evaluation of the PoLTF and the assessment of lens movement.

Patients should be instructed to carry out self-evaluation of lens binding upon waking.

Non-viscous eye drops such as saline (preferably single dose eye drops) should be dispensed for use upon waking.

Also see Lubricating drops.
Pre-lens tear film (PrLTF) The PrLTF is the tear layer on top of the contact lens front surface. It is alternately in contact with the air and tarsal conjunctiva. It is made up of an outer lipid layer and a base aqueous layer. The aqueous layer is usually of reduced thickness (3-4µm) compared to the pre-ocular aqueous layer (7µm).
Push-up test

The push-up test is a two-step assessment of the lens dynamic performance to determine lens tightness:
1. The resistance to decentration is evaluated. For this the lens is gently dislodged by digital upward manipulation of the lower eyelid border against the lower edge of the contact lens.
2. When the pressure of the lower eyelid is removed type and speed of the recentration movement are judged.

Red eye

Increased volume of blood in the conjunctival vessels results in an increased appearance of redness. Red eye may be due to a number of different reasons.
Also see CLARE.

Removal frequency see Lens wear schedules.
Rose bengal An iodine derivative of fluorescein which binds to mucus and cellular components. It is useful in the diagnosis of dry eye, in which it is seen binding to degenerated cells on the corneal surface. This dye often stings on insertion.
Scrape See Corneal scrape
Superior epithelial arcuate lesion. Thin arcuate white lesion in the superior cornea; within 1-3mm of the superior limbus between 10 and 2 o'clock. SEALs can be up to 0.5mm wide and from 1 to 5 mm in length, sometimes with heaped edges. Diffuse infiltration underlying the lesion is possible. Unilateral or bilateral but usually asymptomatic. Occurring symptoms may include edge awareness or irritation and/or foreign body sensation.
Silicone hydrogel contact lens material There are now two different materials of silicone hydrogel lenses on the market: Balafilcon A (B&L PureVision) and Lotrafilcon A (CIBA Focus Night&Day):

Balafilcon A is based on a homogeneous co-polymer of a vinyl carbamate derivative of TRIS. TRIS is a monomer that has been successfully used in the preparation of RGP lens materials. The water content of Balafilcon A is 35% and the Dk/t is 110 x 10-9 (cm x ml O2)/(s x ml x mmHg) at -3.00D.

Lotrafilcon A is based on a fluoroether macromer co-polymerized with TRIS monomer and N,N-dimethyl acrylamide in the presence of a diluent. It is a fluoroether-based silicone hydrogel having a water content of 24% and a Dk/t of 175 x 10-9 (cm x ml O2)/(s x ml x mmHg) at -3.00D.
Sodium fluorescein see Fluorescein.
The appearance of tissue disruption and other patho-physiological changes in the front surface of the eye as revealed with the aid of one or more of a number of dyes, such as fluorescein, rose bengal or alcian blue. Also see Fluorescein, Corneal staining and Conjunctival staining. test
Superior epithelial arcuate lesion see SEAL.
Swab Sterile swabs (either cotton or calcium alginate) are used to sample microbes on the tissue. The swab is removed from its protective covering, slightly moistened with sterile saline and passed along the tissue surface and then placed in a sterile transport solution. On transport to the laboratory, the solution is cultured onto various media to encourage growth of any microbes. Common ocular sites for swabs include: upper bulbar conjunctiva, lower lids, and fornix.
Tears see Post-lens tear film and Pre-lens tear film.
Tear layers see Post-lens tear film and Pre-lens tear film.
Tear exchange Outflow and inflow of tears at the lens periphery, triggered by the movement of the lens upon blinking. Clinical guidelines recommend that back surface debris accumulated during sleep must be eliminated within two hours of waking.
Also see Post-lens tear film and Pre-lens tear film.
Ulcer Pathological loss of substance of the surface of the cornea, due to progressive erosion and necrosis of the tissue.
Also see CLPU and Microbial Keratitis.
Undesirable side effects see Adverse event.
Vacuoles Fluid filled vesicles (20-50µm). Almost circular, intra-epithelial inclusions that show unreversed illumination indicating a lower refractive index than the surrounding tissue.
Vascularization see Corneal vascularization.
Warpage see Corneal distortion.
Wearing schedule see Lens wear schedule.
Wearing time The number of consecutive days and nights that the patient is able to wear the lenses before either a scheduled or unscheduled removal.
Also see Lens wear schedule.
Wettability see Lens wettability.
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