Contact Lens Related Corneal Hypoxia Notes

Superficial Punctate Keratitis

  • Eliminate other possible causes.
  • Normally asymptomatic.
  • Premature desquamation of superficial cells.
  • Microscopic gaps in corneal surface into which fluorescein lodges.
  • Potential gateway for corneal infection.

Superficial Punctate Keratitis Management

  • Remove CLs for 24 hours.
  • Reduce wearing times.
  • Increase oxygen transmission.

Epithelial Microcysts Presentation

  • Small numbers (<10) observed in non-CL wearers
  • Occur particularly in EW CLs (soft hydrogel or rigid)
  • Appear as small (1-15µm) translucent / grey inclusions
  • Visual acuity is generally unaffected
  • Essentially asymptomatic
  • Extensive response:
    • Mild discomfort
    • Lens intolerance
  • Grading based on number of microcysts visible

Epithelial Microcysts Aetiology

  • Effect of tissue metabolic stress
  • Chronic hypoxic changes in corneal epithelium
  • Potential lens induced trauma also implicated
  • Develop in the deeper layers of epithelium
  • Eventually emerge and break epithelial surface, leaving a pit that will stain with fluorescein

Epithelial Microcysts Management

  • Resolve over period of months if CL wear is totally discontinued
  • Reduce frequency of overnight wear
  • Change from extended wear to daily wear
  • Increase Dk/t
  • Change from soft hydrogel to rigid lenses
  • Change to SiHy soft lens

Corneal Oedema (Stroma)

  • Refers to an increase in fluid content of tissue
  • Normal response to lid closure during sleep
  • Early texts refer to central corneal clouding:
    • Associated with PMMA wear
    • Rarely seen
  • Modern response observed to be much more subtle:
    • Stromal Striae
    • Endothelial Folds

Central Corneal Clouding

  • Traditionally associated with steep fitting PMMA CLs and reduced tear exchange
  • Rarely seen in modern practice
  • View by sclerotic scatter
  • Discrete round area of central corneal clouding

Stromal Striae Presentation

  • More prevalent in hydrogel soft extended wear
  • Normally asymptomatic
  • Fine, wispy, white, vertical lines, posterior stroma
  • Evidence of oedema > 5%
  • Greyer, thicker, more numerous as oedema increases

Endothelial Folds Presentation

  • More frequent in hydrogel soft EW
  • Depressed grooves, raised ridges, general area of buckling
  • Oedema in excess of 8%
  • Increase in number with increasing oedema

Striae & Folds Management

  • Resolve soon after lens removal, except in severe cases
  • Cease lens wear until resolution
  • Reduce wearing times
  • Change from extended wear – daily wear
  • Increase Dk of lens material

Endothelial Blebs Presentation

  • Observed as small black spots on endothelium
  • Occur within 10-20 minutes of CL insertion in unadapted patients
  • Large variation in intensity of response
  • Peaks after 20-30 mins, subsides to low after 45-60 mins
  • Low level response may be observed throughout remainder of lens wearing period
  • Reduction in response over a period of weeks

Endothelial Blebs Aetiology

  • Due to stromal hypoxic acidosis and build-up of CO_2 in the cornea (hypercapnia)
  • Asymptomatic, little clinical significance

Endothelial Polymegethism Presentation & Aetiology

  • Increased variation in size and shape of endothelial cells
  • Polymorphism / Pleomorphism
  • Natural age-related change, occurs in all humans
  • Also result of corneal disease, trauma & UV
  • Contact lens related hypoxia essentially has the effect of accelerating such changes
  • Considered a chronic response to contact lens induced corneal hypoxia
  • Common in long term PMMA wear and soft hydrogel extended wear
  • Cornea with polymegethism are at increased risk of corneal decompensation following cataract surgery

Endothelial Polymegethism Aetiology & Management

  • Aetiology similar to endothelial blebs, but a chronic response
  • Hypoxic acidosis and hypercapnia implicated
  • Management:
    • Change lens type
    • Increase Dk
    • Reduce lens thickness
    • Reduce wearing times

Neovascularisation

  • Formation and extension of vascular capillaries into previously non-vascularised regions of the cornea
  • Historically, not infrequently observed in CL wearers, with thick low Dk hydrogel lenses
  • Introduction of thinner and higher Dk lenses has significantly reduced the problem
  • Some studies suggest SiHy lenses have eliminated it altogether

Neovascularisation Type

  • Superficial Neovascularisation
    • Most common form of CL related response
  • Deep Stromal Neovascularisation
    • Occasional CL cases reported
    • Typically, large vessel in mid stroma
  • Vascular Pannus
    • Extensive band of ingrowth in peripheral cornea
    • May occur in CL wear
    • Corneal Disease / Infection
    • Trachoma (400-500M cases worldwide)

Neovascularisation Aetiology

  • Number of theories, aetiology probably multi-factorial
  • Hypoxia
  • Peri–limbal oedema
  • Softening of corneal stroma
  • Inflammatory factors
  • Release of vasostimulatory agents following epithelial trauma

Neovascularisation Other Causes

  • Rigid Lens Wearer
  • Radial Keratotomy

Grading Neovascularisation

  • Normal: Vessels up to 0.2mm in from limbus (as observed in non-lens wearers)
  • Efron describes a scale of “normality” for differing types of lens wear and suggests various grades in excess of these “norms”
  • Often simply recorded as estimate in mm
  • Observation aided by use of red-free filter

Neovascularisation Prevention & Management

  • Reduce wearing times
  • Avoid trauma / toxicity
  • Discontinue hydrogel soft lens wear and refit with SiHy or GP
  • Increase Dk/t
  • Monitor for further vessel growth ‘Ghost vessels’

Summary

  • Hypoxic complications generally associated with hydrogel lenses and extended wear
  • Advent of SiHy materials has reduced incidence of problems
  • Hypoxic signs much more subtle, require careful examination and observation
  • When fitting, should be mindful of prevention
  • Management usually involves reduced WTs and increasing Dk/t