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The Measurement of Corneal Epithelial Thickness in Response to Hypoxia Using Optical Coherence Tomography.

Jianhua Wang, Desmond Fonn, Trefford Simpson, and Lyndon Jones
American Journal of Ophthalmology 2002: Vol 133, No. 3

INTRODUCTION

The corneal stroma has so far been reasonably predictable in terms of thickness changes in response to hypoxia. The epithelium's response however has not been consistent between studies (both short and long-term) and does not appear to be as predictable. Recent advances in equipment and the introduction of high Dk lens materials, which allow differentiation between hypoxia and lens effects, have led to a resurgence of interest in this area.

HYPOXIC EFFECTS ON STROMAL THICKNESS

Short term studies have firmly established the 5-15% swelling response of the corneal stroma to hypoxia that is evidenced clinically by striae when greater than 5% and the additional presence of folds, when greater than 8% 1.

The classic Gothenburg study by Holden and colleagues showed thinning of the stroma with long term (62 29 months) wear of low Dk hydrogel lenses worn on an extended wear (EW) schedule 1. Recent studies have shown a decrease in posterior keratocyte density in long term EW of low Dk hydrogels that is most likely responsible for this reduced thickness 2, 3.

UNDISPUTED EFFECTS OF HYPOXIA ON THE EPITHELIUM

Associated with hypoxia, the epithelium exhibits:

• a significant increase in microcysts 4,
  
• abnormal metabolism that results in an acid shift in pH due to an accumulation in lactic acid and carbon dioxide 5,
  
• a reduction in cell synthesis due to enzyme shifts 6,
  
• reduced epithelial cell adhesion 7,
  
• increased binding of bacteria to cells 8,
  
• reduced apoptosis (or programmed cell death) 9
  
• an increase in cell size 10.

HYPOXIC EFFECTS ON EPITHELIAL THICKNESS: LONG TERM STUDIES

The Gothenburg study showed not only a decrease in stromal thickness, but also a decrease in epithelial thickness (5.6% compared to the control eye) with long term (62 29 months) wear of low Dk hydrogel lenses on an EW schedule. However, this difference decreased after ceasing lens wear and after 2 days was not significant, exhibiting recovery of the epithelium 1.

A recent long-term clinical study (over 10 years of low Dk hydrogel daily lens wear) has not, however, found evidence of epithelial thinning 11. Possible reasons for this difference may be less hypoxia with DW compared to EW of low Dk soft lenses, longer term survival in low Dk hydrogel lens wear of the "fittest" corneas or possibly a longer term adaptive effect.

HYPOXIC EFFECTS ON EPITHELIAL THICKNESS: SHORT TERM STUDIES

Most of the short-term studies have shown no change in epithelial thickness 12-15, although there have been several studies on excised rabbit corneas that have shown an increase in epithelial thickness with hypoxia 16-18.

THIS STUDY: WHY AND HOW

The purpose of the study was to determine if there was an associated increase in epithelial thickness with corneal edema induced by a contact lens worn during eye closure. Twenty subjects wore thick (0.34-0.41mm) HEMA 38% water content lenses (average Dk/t of 2.1 x 10-9 on one eye which was patched for 3 hours. The contralateral eye of each subject, which did not wear a contact lens and was not patched, acted as the control.

Optical Coherence Tomography (OCT) setup (Humphrey Instruments, Zeiss-Humphrey, San Leandro, CA)

The method used to measure corneal tissue thickness was optical coherence tomography (OCT) which uses low coherence interferometry to produce high resolution cross-sectional maps of the anatomical structure. OCT is similar to ultrasound B-mode imaging, using light instead of acoustic waves. This is a new optical imaging technique that was originally designed for measuring different layers of the retina and has recently been used in several studies involving measurement of the cornea and the epithelium. Good reproducibility of OCT measurement of the thickness of the cornea and epithelium has been previously reported by this group 19.

Other methods to measure the thickness of the cornea include:
-in vivo confocal microscopy through focussing (CMTF) (the confocal microscope uses the one objective lens to illuminate and focus a bright spot of light and is thereby able to produce very sharp images; intensity curves of cell layers are generated and thickness calculated from the z-axis positions; invasive), and;
-modified optical pachymetry (an optical system that uses two parallel glass plates to bisect the image of the cornea; measurement is done by aligning the two images; non invasive but can be difficult to judge with increased light scatter as in stromal edema).


THIS STUDY: FINDINGS

For the test eyes, there was a 13.8+/-2.3% increase in overall corneal thickness and a small (1.7+/-4.8%) but not statistically significant increase in epithelium thickness. However, there was slight thinning of the epithelium in the test eye on removal of the lens (up to 3.0+/-4.5%) over a 100 minute time period. In addition, the control eye showed a decrease in total corneal thickness after removal of the contact lens from the test eye, compared to baseline readings of the control eye.

There was good agreement between the measurements of the test and control eye and a low standard deviation compared to other methods of measurement of the corneal epithelium, indicating good reliability of the OCT for this type of measurement.

THIS STUDY: SIGNIFICANCE OF THE FINDINGS

The increase in total thickness of the cornea falls within the 5-15% expected range and the small but not statistically significant increase in epithelium thickness is in line with the majority of previous short-term studies. It has been previously suggested that the absence of epithelial swelling may be due to a compensatory regulatory mechanism contained within the living epithelium or that radial rather than axial swelling of the epithelium occurs 12.

Epithelial thinning during the deswelling period has been previously reported 15 but confirmation of this phenomenon in this study leads the authors to hypothesize that this may be due to an auto-regulation effect where the cornea continues to expel fluid or thins beyond its baseline thickness.

The authors speculate that the corneal thinning of the control eye may be a sympathetic response. Sympathetic corneal swelling of the control eye has been previously reported, first by Harris and Mandell with PMMA 20 and more recently by Fonn et al 21, comparing the response with both low and high Dk lens wear (the control eye swelled more when the low Dk, compared to the high Dk lens was worn on the contralateral eye). Harris and Mandell proposed that the sympathetic response was due to an osmolarity effect as a result of lacrimation.

THE STROMA IN HIGH DK EW AND CW

Clinically, striae and folds are rarely seen in high Dk EW and CW 22.

Recent long term studies have shown a decrease in stromal thickness and posterior stromal cell density with both long term wear of low and high Dk material lenses 2, 3. This suggests other factors other than hypoxia, such as the presence of a contact lens and lens wear schedules, may be exerting an influence on stromal thickness.


THE EPITHELIUM IN HIGH DK EW AND CW

Microcyst levels are comparable to no lens wear and remain consistently low over at least 18 months of high Dk CW for new lens wearers 23. Patients transferring from low Dk EW to high Dk CW, show a transitory increase in the first three months of high Dk lens wear and then a decrease to levels similar to new high Dk lens wearers and non lens wearers 23.

Decreased bacterial binding of epithelial cells in high Dk soft lens EW and CW is arguably the most positive finding relating to the corneal epithelium 24, Article Review-Is it Time to Give Extended Wear Another Chance.

Dwight Cavanagh's group have also reported a decrease in epithelial thickness over the first 3 months of EW and CW of high Dk soft lenses, with adaptive recovery of epithelial thickness in subjects followed up to 12 months of lens wear . However, these changes were not as marked as those seen with high Dk RGP CW or low Dk EW 24.

The same study found an increase in the size of the corneal epithelial cells, however Stapleton et al have found no increase in epithelial cell size after 3months of CW of high Dk soft lenses 25.

The effect of long-term contact lens wear on epithelial thickness appears to be driven by not only hypoxia but also lens effects, as occurs with the corneal stroma.

CONCLUSION

Results with high Dk lenses are certainly positive. Research continues into the effects of hypoxia and contact lens wear, particularly high Dk EW and CW, on the corneal epithelium and the implications of these changes. New instrumentation such as the OCT has contributed to the surge in interest in this area and it will be interesting to see the results of long-term clinical studies with these new methods of measurement.

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