Corneal Oxygen Requirements Summary

Corneal Oxygen Requirements

Learning Outcomes

• Describe corneal metabolic processes
• Define critical oxygen supply
• Discuss oxygen passage through CLs
• Describe the minimum oxygen requirements for daily and overnight CL wear

Corneal Function & Hydration

• Major refractive element of the visual system
• Maintains transparency to permit refracted light to be transmitted to the retina
• Stromal Organization, Hydration Control
• Increasing opacity = ↑ stromal hydration
• Regain of transparency = ↓ stromal dehydration

The Pump-Leak Mechanism

• Leak: Constant movement of water from the aqueous into the stroma through gaps between endothelial cells.
• Pump: Endothelial cells pump bicarbonate ions into the aqueous, creating an osmotic force that draws water out of the stroma.
• Metabolically controlled to maintain a constant level of stromal hydration.

Energy Requirements

• Cornea needs energy to maintain transparency and other functions
• Source of nutrient supply:
  • Glucose
  • Amino acids
  • Oxygen from Aqueous Humour

Sources of Corneal Oxygen

Open Eye

• Atmosphere
• Tear Film
• O_2 tension of 155mmHg

Closed Eye

• Limbus
• Aqueous
• Palpebral Conjunctiva
• O_2 tension of 55mmHg

Decreased Oxygen Availability

• Cornea swells by approximately 2-4\% overnight.
• Returns to baseline levels in less than 1 hour after opening.
• CL wear represents a barrier to available oxygen.
• Potential to inhibit metabolic activity and reduce corneal transparency.

Critical Oxygen Supply

How Much O_2 Does Cornea Need?

• Critical O2 level: Point to which O2 tension can be reduced before corneal “dysfunction” occurs.

Two Major Difficulties for Researchers

• Time over which "dysfunction" is measured.
• Criteria by which "dysfunction" is defined.

Research Concerns

‘Time’ Problem

• Most tissue can withstand a zero O_2 atmosphere for a number of minutes; the cornea can do so for hours.
• Over a period of days, the absence of O_2 would lead to permanent tissue degradation.

‘Criteria’ Problem

• What is the definition of “dysfunction” or “unacceptable corneal change”?
• Favored methodology is to record O_2 level at which measurable or observable changes to corneal function occur

Critical Oxygen Requirements - Various Authors

Critical O_2 Performance of CLs

Holden et al

• Without a CL in place, we need a minimum of 10% O_2 concentration in air to avoid corneal oedema.

Critical O_2 Performance of CLs

Daily Wear: Holden & Mertz (1984)

• Daily Wear: Zero Swelling
• Minimum Dk/t = 24.1 Barrer/cm

Critical O_2 Performance of CLs

Overnight Wear: Holden & Mertz (1984)

• Overnight Wear: 4% Maximum Swelling
• Minimum Dk/t = 87 Barrer/cm

Holden & Mertz Studies

• Only one data point for high Dk/t lens, very much an estimate
• Under normal physiological conditions of no lens wear, the cornea can eliminate up to 8% oedema during open eye conditions.
• Limit swelling to 8% overnight, then the cornea should be able to eliminate this during waking hours the following day.

Critical O_2 Performance of CLs

Overnight Wear: Holden & Mertz (1984)

• “Compromise Criteria”: Zero Residual Oedema Day 2
• Minimum Dk/t = 34.3 Barrer/cm

Limitations?

• Is Dk/t =87 Barrer/ cm safe for overnight wear?
• Studies represent the average response
• Normal distribution of responses
• Holden & Mertz criteria represent the absolute minimum values for Dk/t

Harvittt & Bonanno (1999)

• Research resulted in new benchmarks for critical oxygen supply for daily and overnight lens wear
• Daily Wear: Minimum Dk/t = 35 Barrer/cm
• Overnight Wear Minimum Dk/t = 125 Barrer/cm

Sweeney (2000)

• Re-evaluation of average overnight corneal swelling
• Corneal swelling with lens wear of 3.2\%%
• Minimum Dk/t = 125 Barrer/cm

Oxygen Flux

• Clinicians concerned with lens performance with respect to impact on corneal physiology.
• Oxygen Flux: Volume of oxygen which reaches a unit area of the corneal surface in unit time.
• Sometimes graphically represented as a percentage of the total available oxygen reaching the ocular surface

Oxygen Flux - Morgan & Brennan (2004)

• Graph showing Oxygen flux vs Dk for different lens thicknesses in open and closed eye states.

Oxygen Flux - Morgan & Brennan (2004)

• Table showing oxygen transmissibility and flux for a range of representative contact lenses.
• Units are \mu l cm^{-2}h^{-1}

Summary

• Modern SiHy lenses offer very high Dk/t.
• Recognition that does improve the appearance of eyes - fewer/more subtle hypoxic complications evident.
• Highlights the importance of slit lamp observational skills.
• Is hypoxia a thing of the past?
• Perhaps not all about Dk/t?
• How relevant is oxygen flux?

Further Reading

• Morgan P & Brennan N (2004) The Decay of Dk? Optician 6th Feb 2004, No 5937 Vol 227
• Holden B & Mertz G (1984) Critical oxygen levels to avoid corneal oedema for daily and extended wear contact lenses. IOVS 25: 161-1167
• Harvitt DM & Bonanno JA (1999) Re-evaluation of the oxygen diffusion model for predicting minimum contact lens Dk/t values needed to avoid corneal anoxia. Optom Vision Sci 76, 712-719
• Swarbrick NA et al (1998) The critical Dk/l to avoid oedema for daily wear RGP contact lenses. Clinical & Experimental Optom 81.2, 72-76