Contact Lens I

TEAR FILM AND CORNEAL TOPOGRAPHY EVALUATION

 

  1. TEAR FILM EVALUATION
    1. Role of Tear Film
    1. Lipid Layer
      2.

      •thin anterior surface of tear film

      •secreted by the Meibomian glands and possibly glands of Zeiss and Moll

      •it functions to retard the evaporation of the tear film between blinks and promoted tear film stability
    2. Aqueous Layer
      3.

      •thick, central layer of the tear film

      •reflexly secreted by the lacrimal gland and basically by the glands of Wolfring and Krause

      •functions include the lubrication of the corneal surface, high optical and maintain neutral pH

    3. Mucin(Mucous) Layer

•thin layer located adjacent to the cornea

•it is formed by the goblet cells of the conjunctiva

•it is abundant in glycoproteins

•mucin has several important functions including:
    1. decreases surface tension of the tear film and minimizes dry spot formation
    2. minimizes hydrophobic effects contributed by the lipid layer and serves as the mechanism by which lipids and other metabolic wastes are removed from the tear film with the blink
    1. Blinking Action

•vital for a stable tear film

•if absent or improper, dry eye results

•during blink, the overlying aqueous tear film completely wets the mucin-coated epithelium

•betweeen blinks, the tear film thins via evaporation and exits into the fornices; thus the lipid layer becomes closer to the mucin layer. When proximity is such that the mucin layer is contaminated by the lipid layer, dry spots form as a result rupturing of the mucin layer and subsequent contact between the aqueous layer and the underlying epithelium. Therefore, the mucin layer is essential for maintaining the continuity of the tear film.
    1. Abnormalities/Deficiencies
    1. Aqueous (volume) Deficiency
      2.

      •associated with decrease in aqueous production of accessory and lacrimal gland

      •K sicca and Sjogren’s syndrome are associated with this deficiency

      •manage via artificial tears, punctal plugs
    2. Mucin Deficiency
      3.

      •decrease in mucin production form a decrease in conjunctival goblet cells; this results in an unstable tear film and low tear break-up-time

      •associated with an absence of Vitamin A

      •associated with ocular pemphigoid, Stevens-Johnson Syndrome

      •contact lenses are contraindicated
    3. Lipid Deficiency
      4.

      •a greasy tear film is present due to Meibomian gland infection resulting in dry spot formation

      •a close association exists between blepharitis, abnormal Meibomian gland function and dry eye

      •management via proper patient education, cotton swabs
    4. Lid Surfacing Abnormalities
      5.

      •if patient is a partial blinker or experiences incomplete closure, inferior punctate staining can result from improper blinking

      •drying also causes by entropion/ectropion
    5. Tear Base Abnormalities

•problems such as recurrent erosion, filamentary keratitis and persistent epithelial defects can result in dryness

•management via bandage lenses and lubricants
    1. Methods of Evaluation
    1. Biomicroscopic Evaluation: two methods
    1. Tear Meniscus

      2. •evaluation of the height and quality of lower tear prism; if not continuous, an aqueous deficiency is present

      •where the meniscus meets the cornea, a black line exists that represents localized thinning. This is observed with fluorescein and cobalt blue filter
    2. Interference Phenomena

•spectrum of colored interference patterns of lipid layer will be observed

•with the patient viewing superiorly, specular reflection in combination with high intensity illumination and high magnification should be used

•if the lipid layer is insufficient, no interference patterns will be observed
    1. Tear Break-Up-time (Tear B.U.T.)
      2.

      •most widely used test of lacrimal function

      •length of post-blink time for dry spots to develop as the mucin layer is contaminated by lipids

      •it decreases with age and with high ambient temperature

      •instill fluorescein and use a wide slit beam low magnification and cobalt blue filter

      •Avg. = 12.8 sec. With 15% < 10 sec.

      Important Considerations

      •do not use digital manipulation of lids

      •do not use benzalkonium chloride (BAK) preserved solutions

      •have patient make several blinks prior to test

      •no tonometry should be performed prior to test

      •repeat test several times
    2. Schirmer
      3.

      evaluates basal tear secretion and part of the reflex secretion

      •a strip of filter paper (5 x 30mm) is placed slightly temporal over the lower lid with upward gaze

      •use low room illumination

      •avg. = 22.3mm for normals and 7.6mm for dry eye patients

      •>10mm for 5 minutes or >6mm for 1 minute is normal

      •anesthetic use???

      •inconsistent, uncomfortable
    3. Phenol red Cotton Thread Test

•developed in Japan and used on several hundred thousand patients, it consists of a 70mm cotton thread soaked in phenol red dye

•t is performed similar to Schirmer

•avg. = 16.7mm; <9mm = dry eye

•benefits include:
    1. no anesthesia, minimal discomfort
    2. 15 second test time
    3. little reflex secretion
    4. more valid than Schirmer

•criticisms include:
    1. relatively low absorption capacity
    2. may only measure residual tears in cul-de-sac – not tear volume
    1. Rose Bengal

      2. •stains dead and degenerated epithelial cells

      •beneficial in diagnosis of keratoconjunctivitis sicca

      •typically, inferior staining is present
    2. Laboratory tests
    1. Lacrimal Protein Concentration
    1. lysozyme agar diffusion test
    2. lactoplate lactoferrin immunological test system
    1. Tear Film Osmolality – increase results from insufficient tear film turnover and evaporation

•these procedures are typically very time-consuming and expensive, but diagnostic of dry eye
    1. What Should we Use?
    1. Tear B.U.T. = very accurate
    2. Biomicroscope – tear prism and staining should be evaluated
    3. Schirmer – unless close to 0, don’t contraindicate patient; cotton thread better (upon availability)
    4. Fonti and Holly equation:

W = f1(B.U.T.) + f2(Schirmer)

If >25, contact lens wear is recommended
  1. CORNEAL TOPOGRAPHY EVALUATION
    1. Corneal Contour
    1. Corneal Cap (apical zone, apical cap) – the corneal cap is the center region of the cornea. If the corneal cap is defined as the area within which the corneal power does not decrease > 1D and therefore, equals about 4mm. Average curvature of the corneal cap = 7.9mm (42.75D)
    2. Corneal topographic studies show cornea is spherical centrally and aspheric in the periphery
    3. There is a gradual flattening of the cornea from the center to the periphery
    1. Evaluation
    1. Keratometer: Disadvantages include:
      2.

      •only central 3mm evaluated

      •assumes central 3mm to be spherical; as corneal cap is smaller in steep corneas, the K measurement is inaccurate. In addition, if the central cornea is aspherical, an inaccurate measurement will result. The magnitude of the error is related to the rate of peripheral corneal flattening

      •only portions of the cornea used for measurement are 2 small areas per meridian

      •experimenter error

      •inaccuracy with decentered corneal cap
    2. Peripheral Keratometry

•keratometry used for the peripheral cornea will make corneal cap appear abnormally large and corneal periphery abnormally steep

•Mandell disk – assumes the cornea has a true cap

Auto-keratometry

•Advantages include:
    1. horizontal area = 6.4mm (vertical = 2.6mm)
    2. can calculate curvature at corneal apex, location of apex and shape factor
    3. easy to use
    4. good consistency and reliability

•Disadvantages include:
    1. cost ($10,000)
    2. limited area

 

 

    1. Corneal Curvature Change with Contact Lens Wear
    1. Contour change will occur with rigid lens application; this may not be detected with keratometry
    2. Cornea typically becomes more spherical
    3. Keratometry change often does not correspond with refactive change
    1. Bottom Line
    1. Keratometry is most often used; if used, it is only a starting point
    2. It is important to consider individual corneal variability when fitting rigid lenses
    3. Biomicroscopic evaluation is mandatory