6. Extended Wear Flashcards
What is the definition of Daily Wear (DW) contact lenses?
CL worn only through the day and removed at night before sleep
What is the definition of Extended Wear (EW) contact lenses?
CL worn continuously, both through the day and while sleeping, for a period of up to 6 consecutive nights or seven days (not removed in between).
What is the definition of Continuous Wear (CW) contact lenses?
CLs worn continuously for up to 30 consecutive nights, no removed in between.
What is the global prescribing trend of extended wear CLs between 2000 and 2020?
EW prescription rates stayed under 15%. EW is not expected to become the mainstream modality, unless the already low risks of ocular complications can be reduced to be equivalent to that for DW.
Why is oxygen essential for the cornea? What is the consequence of lack of oxygen?
O2 is essential for normal epithelial aerobic metabolism.
If oxygen is scarce, the cornea will undergo anaerobic metabolism. Anaerobic metabolism produces lactic acid. The accumulation of lactic acid causes corneal acidosis and trigger corneal oedema.
What are the typical oxygen concentrations at open eye conditions vs closed eye conditions?
- Open eye conditions = 20.9% O2
- Closed eye conditions = 7% O2 = short-term hypoxia
What causes low corneal pH?
Accumulation of carbon dioxide
What are the 4 changes that occur in the eye when the eye is closed?
- Corneal oedema → normal cornea swells 3-4% overnight
- Temperature = increases ~2ºC
- Tear pH = decreases ∴ more acidic
- Tear osmolarity = decreases ∴ hypotonic shift
What does the Holden-Mertz Criteria indicate?
The Holden and Mertz Criteria indicates the critical oxygen levels (Dk/t) to avoid corneal oedema/ anoxia at the basal epithelium for daily and extended wear contact lenses.
According the Holden-Mertz Criteria, what is the Dk/t for daily wear and overnight wear?
- Daily wear - Dk/t >24 x 10^-9
- Overnight - Dk/t > 87 x 10^-9
Who revised the Holden-Mertz Criteria? What was revised?
Fonn and Bruce revised the Holden-Mertz Criteria:
* Extended wear critical Dk/t > 125
* This is the Dk/t is necessary to prevent stromal anoxia (H-M focussed on basal epithelium)
Why are the theoretical critical Dk/t determinants not entirely accurate? (3)
- Corneal swelling is non-uniform → more swelling peripheral > central
- Adapted wearers more resistant to corneal swelling than non-contact lens wearers → consider decreased metabolic rate that would lead to less healing capacity
- Significant variability between individuals → diff in corneal thickness & variations in O2 demand
What are the 5 Extended Wear design properties?
- Allow adequate O2 to cornea (appropriate Dk/t)
- Minimal deposit formation
- Minimal mechanical interaction btwn lens & ocular surfaces (cornea & eyelid)
- Provide tear-carried metabolites essential for normal epithelial growth & repair
- Enable cleaning of post-lens debris
What are the 6 indications for Extended Wear CLs?
- Convenience
- Aphakes (eldery & infants)
- Anisometropic infants → difficult to fit glasses
- Therapeutic → bullous keratopathy, dry eye, post-corneal sx, RCE
- Occupation → emergency work force, shift workers, military
- Pre-refractive sx
What are the 7 considerations for not recommending Extended Wear CLs?
- Strong hx of CL non-compliance
- Smoking
- Regular swimming
- Chronic blepharitis or MGD
- Severe dry eye disease
- Hx of previous corneal inflammatory events
- Delayed wound healing e.g. diabetes
- Immunocompromised
