Eye Care - Contact Lens Types, Characterisation, and Care

Introduction to Contact Lenses

  • Reasons for Choosing Contact Lenses:

    • Visual Requirements: Eyeglasses may fail to provide satisfactory vision for certain individuals.
    • Cosmetic Preferences: Desire to change or maintain appearance without frames.
    • Activity-Specific Needs: Eyeglasses can be a significant hindrance during sports or other vigorous physical activities.

  • Benefits of Contact Lenses:

    • Visual Field: There is no obstruction of vision from eyeglass frames.
    • Peripheral Vision: Provides greater clarity in the periphery compared to glasses.
    • Environmental Stability: No fogging of the lenses caused by sudden temperature changes (e.g., moving from a cold to a warm room).
    • Mobility: Offers more freedom of motion during vigorous activities and sports.
    • Perspective: Objects are seen in their true sizes.
    • Psychological/Aesthetic: Perceived improvement in personal appearance.

Contraindications and Precautions for Contact Lens Usage

  • Occupational Hazards: Individuals exposed to the following may be advised against lenses:

    • Wind and glare.
    • Molten metals.
    • Irritants such as dust, particulate matter, and tobacco smoke.
    • Chemicals and chemical fumes.

  • Medical Conditions:

    • Ocular Conditions: Chronic conjunctivitis, blepharitis, and recurrent viral, bacterial, or fungal infections.
    • Blink Issues: Poor blink rate or incomplete blink habits.
    • Tear Film Issues: Insufficient tear production.
    • Systemic Conditions: Diabetes (specifically advised against extended-wear types), chronic common cold, or allergic conditions.
    • Hormonal/Life Stages: Pregnancy and the use of oral contraceptives.

Classification and Characteristics of Contact Lenses

  • Broad Categories: Lenses are classified based on chemical make-up and physical properties.

    • Hard Lenses: No longer in common use.
    • Rigid Gas-Permeable (RGP) Lenses.
    • Soft Lenses.

  • Comparison of RGP and Soft Lenses:

    • Material Rigidity:
      • RGP: −−−- - -
      • Soft: 00
    • Durability:
      • RGP: −−- -
      • Soft: +−+ -
    • Oxygen Transmission:
      • RGP: −−−- - -
      • Soft: +++ + to (+++)(+ + +)
    • Chemical Adsorption:
      • RGP: 00
      • Soft: −−−- - -
    • Visual Acuity (Optical Quality):
      • RGP: −−−- - -
      • Soft: −−- -
    • Correction of Astigmatism:
      • RGP: Yes.
      • Soft: Requires Toric lenses.
    • Spectacle Blur:
      • RGP: −−- -
      • Soft: 00
    • Convenience:
      • RGP: −−- -
      • Soft: −−−- - -
    • Comfort:
      • RGP: ++
      • Soft: −−−- - -
    • Adaptation Period:
      • RGP: Days.
      • Soft: Hours.
    • Extended Wear Availability:
      • RGP: Yes.
      • Soft: Yes.

Rigid Gas-Permeable (RGP) Lenses

  • Physical Properties:

    • Relatively inflexible.
    • Smaller in size compared to soft lenses.
    • Do not absorb water (water content is <10%< 10\%).
    • Retain their shape even when removed from the eyes.

  • Material and Oxygenation:

    • Made of flexible polymers with silicone incorporated to allow oxygen permeability.
    • Importance of Oxygen: Lack of oxygen transmission leads to Hypoxia, which can cause:
      • Red eyes.
      • Blurred vision.
      • Eye discomfort.
      • Increased risk of eye infections.

Soft Contact Lenses

  • Physical Properties:

    • Moderately to highly flexible.
    • Absorb a high percentage of water (>10%> 10\%
    • Conform to the shape of the supporting ocular structure.
    • Larger in size than RGP lenses: they cover the entire iris and part of the cornea.

  • Materials:

    • Hydrogel: Offers lower oxygen permeability.
    • Silicone Hydrogel: (e.g., Acuvue Oasys) Offers higher oxygen permeability, with some brands comparable to RGP lenses.
    • Note: Both are plastics that are hard when dry but absorb water to become soft and gel-like.

  • Usage and Replacement Schedules:

    • Disposable: Daily, Weekly, or Monthly.
    • Extended Wear: Allows overnight wear.
    • Toric: Specifically for astigmatism.
    • Conventional: Replaced every 6 months6 \, \text{months} or longer.
    • Average life span is 12 to 18 months12 \, \text{to} \, 18 \, \text{months}.

  • Advantages:

    • Easier to remove.
    • More comfortable than rigid lenses initially.
    • Photophobia is less likely to occur.
    • Unlikely to experience spectacle blur.
    • Less likely to trap dust, eyelashes, or foreign particles.
    • Less likely to become dislodged or fall out.

  • Disadvantages:

    • Excellent visual acuity is not achievable for all wearers.
    • Hydration levels may fluctuate.
    • The material tears easily.
    • Older hydrogel models have less oxygen permeability.
    • May contribute to dry eyes.
    • Can adsorb chemical compounds from topical ophthalmic products.

The Importance of Cleaning and Hydration

  • Cleanliness:

    • Normal tears contain hydrophobic components (proteins and oils) that adhere to lens surfaces.
    • Undisturbed residues harden into tenacious deposits and coatings.
    • Consequences of Deposits: Irritated eyelids, corneal epithelium issues, increased infection risk, decreased visual acuity, and reduced wear time. Deposits also act as a growth medium for bacteria.

  • Hydration:

    • A rigid lens typically absorbs 1 to 3%1 \, \text{to} \, 3 \% moisture by weight.
    • Lenses dehydrate when exposed to air and rehydrate upon contact with lacrimal fluid or soaking solutions.
    • Inserting a dehydrated lens causes significant discomfort.
    • Dryness during storage makes accumulated deposits much harder to remove.

Lens Care Products: Cleaning and Rinsing

  • Cleaning Solutions:

    • Contain non-ionic or amphoteric surfactants to emulsify oils and solubilize debris.
    • Weak alkaline solutions help dissolve proteins and lipids (though high pH can cause lens decomposition).
    • Usually contain preservatives.

  • Multipurpose Solutions (MPS):

    • An all-in-one system for cleaning, rinsing, disinfecting, and storing.
    • Components include surfactants, disinfectants (e.g., biguanide, polyquad), and lubricants.

  • Surface-active Cleaners:

    • Contain non-ionic detergents, wetting agents, chelating agents, buffers, and preservatives.
    • Effective for lipids; less effective for proteins.

  • Enzymatic Cleaners:

    • Used weekly to remove protein deposits.
    • Function by hydrolyzing polypeptide bonds of proteins.
    • Failure to use these may necessitate professional cleaning or lens replacement.

  • Rinsing Lenses:

    • Can use MPS or Saline.
    • Purpose: Remove loosened contaminants, remove residual cleaners, and rehydrate lenses.
    • Saline Note: Saline is a basic solution used for rinsing and storage, but it is NEVER to be used for disinfection.

Disinfection Methods

  • Definition: Disinfection is the inactivating/killing of microorganisms, whereas cleaning is the removal of deposits and debris.

  • FDA Recommendation: Disinfect soft lenses before every reinsertion.

  • Chemical Disinfection:

    • Lenses are stored for 4 to 6 hours4 \, \text{to} \, 6 \, \text{hours} in antimicrobial solutions.
    • Lenses can be worn immediately after a mild rinse.
    • Sensitivity reactions can occur depending on the preservative concentration.

  • Hydrogen Peroxide Disinfection:

    • Eliminates bacteria, yeasts, fungi, spores, and viruses by damaging cell membranes and creating free radicals.
    • Lenses are placed in 3%3\% hydrogen peroxide in a special case.
    • Neutralization: The peroxide must be neutralized to oxygen and water using a catalase tablet or catalytic disc.
    • Benefits: Preservative-free after neutralization; ideal for those with allergies to MPS preservatives.
    • Safety Warning: High risk of pain, stinging, burning, and corneal damage if an un-neutralized lens is inserted. Accidental exposure requires immediate removal and flushing with sterile saline.

  • Thermal Disinfection: Uses heat to kill organisms (less common now).

  • Subsonic Agitation: A mechanical cleaning/disinfection method.

Self-Care Procedures for RGP Lenses

  • Cleaning Steps:

    1. Place lens in the palm.
    2. Apply recommended cleaner (some contain silica gel to mechanically break adhesive bonds; these should be discarded 90 days90 \, \text{days} after opening).
    3. Rub in a circular motion with the index finger for 20 to 30 seconds20 \, \text{to} \, 30 \, \text{seconds}.
    4. Rinse thoroughly.

  • Conditioning (Soaking/Wetting) Solutions:

    • Enhances wettability.
    • Forms a cushioning tear layer for comfort.
    • Disinfects and stabilizes the lens on the fingertip for insertion.
    • Requires a minimum soak time of 4 hours4 \, \text{hours}.
    • Discard 90 days90 \, \text{days} after opening.

Maintenance and General Counselling

  • Rewetting Solutions:

    • Intended for use while lenses are in the eye.
    • Used when lenses dehydrate due to wind or high temperatures.
    • Contain low-concentration surfactants for cleaning and polymers for lubrication.

  • Lens Storage Case Care:

    • Clean thoroughly on a routine basis.
    • Air-dry the case between uses.
    • Scrub the case weekly.
    • Replace the case at least every 3 months3 \, \text{months}.

  • Essential Counselling Tips:

    • Do not wash or rinse lenses with tap water or saliva (risk of Acanthamoeba or infection).
    • Avoid swimming with contact lenses.
    • Remove lenses immediately if eyes become painful.
    • Always wash and dry hands before handling lenses.
    • Do not sleep with contact lenses (unless specifically prescribed for extended wear).

Questions & Discussion

  • True/False Concepts Check:
    • False: RGP lenses absorb around 30%30\% water. (Correction: RGP water absorption is <10%< 10\%, usually 1 to 3%1 \, \text{to} \, 3\%
    • False: Cleaning solutions should be discarded 30 days30 \, \text{days} after opening. (Correction: For many RGP products, it is 90 days90 \, \text{days}, though always check the label).
    • False: Conditioning solutions mechanically break adhesive bonds. (Correction: This is the role of specific cleaning solutions with silica gel).
    • False: Enzymatic cleaners are used daily. (Correction: They are typically used weekly).
    • False: Saline can be used as a disinfecting solution. (Correction: Saline is for rinsing/storage only; it has no disinfecting properties).