Corneal thickness changes associated with daily and extended contact lens wear

Evaluation of Corneal Densitometry and Endothelial Layer in Soft Contact Lens Users

SIGNIFICANCE

Soft contact lens wear may lead to physiological changes in the cornea. Corneal densitometry is a new software tool that may detect these physiological changes by monitoring their effects on corneal transparency.

PURPOSE

This study aimed to assess corneal densitometry values and corneal endothelium in patients using soft contact lenses.

METHODS

The study included 36 eyes of 36 subjects with soft contact lens use (group 1) and 36 eyes of 36 healthy control subjects (group 2). Anterior segment parameters and corneal densitometry of all subjects were evaluated using Scheimpflug corneal topography (Pentacam; Oculus Inc., Wetzlar, Germany). A total corneal area of 12 mm was divided into four annular zones (0 to 2, 2 to 6, 6 to 10, and 10 to 12 mm) and into three regions as anterior, central, and posterior for densitometry examination. Cell density, cell distribution, and cell morphology (quadrangular, pentagonal, hexagonal, and other cell percentages) of the endothelial layer were examined in both groups using specular microscopy (Nidek CEM 530; Nidek Co., Gamagori, Aichi, Japan).

RESULTS

In group 1, the mean duration of contact lens wear was 4.45 ± 2.15 years. The anterior segment parameters were not significantly different between the two groups (P > .05). The anterior 0- to 2- and 2- to 6-mm corneal densitometry values were significantly higher in group 1 compared with group 2 (23.2 ± 4.7 for group 1 and 17.3 ± 5.6 for group 2 [P = .03]; 22.4 ± 6.4 for group 1 and 17.8 ± 1.4 for group 2 [P = .03], respectively). There were no significant differences between the groups for corneal densitometry values between 6 and 12 mm, or for cell density, cell distribution, and cell morphology (P > .05).

CONCLUSIONS

Although the cornea remains transparent in soft contact lens wearers, corneal densitometry may increase in the 0- to 6-mm annular zone of the anterior region.

Assessment of modifications in thickness, curvatures, and volume upon the cornea caused by disposable soft contact lens wear

PURPOSE

To assess the effect of different disposable soft contact lenses on several corneal parameters-thickness, anterior and posterior curvature, and volume-by means of a Scheimpflug imaging-based device (Pentacam HR). Diurnal variations of these parameters were taken into account.

METHODS

Twenty-one young, healthy subjects wore 4 different types of daily disposable soft contact lenses on 4 different days: Dailies AquaComfort Plus, SofLens, Dailies Total1, and Acuvue TruEye. The lenses had different material and water content. Pachymetry and curvature maps and corneal volume values were obtained using the Pentacam HR twice a day: one before putting the lens on and one after an 8-hour period of contact lens wear. Measurements were also taken without any contact lenses being worn.

RESULTS

Regarding corneal thickness, the lens with the most similar behavior to the naked eye scenario was the Dailies Total1, causing a thickening of 0.2 ± 0.1% in the central zone and 0.6 ± 0.2% in the periphery. All 4 lenses caused a slight but not significant flattening in the anterior corneal curvature, whereas the posterior corneal curvature only experienced a significant but small steepening with the SofLens. The use of these lenses increased corneal volume slightly.

CONCLUSIONS

Variations in corneal parameters seem to depend on the type of contact lens used (material, oxygen transmissibility, water content). However, the magnitude of the changes introduced by the use of soft contact lenses over the 8-hour period was small and probably not large enough to influence either visual acuity or comfort.

Assessment of corneal morphological changes induced by the use of daily disposable contact lenses

PURPOSE

To assess the effect of different disposable soft contact lenses upon corneal thickness, and upon anterior and posterior corneal curvatures using a dual-Scheimpflug imaging based device.

METHODS

Twenty-eight young, healthy subjects wore four different types of daily disposable soft contact lenses on four different days: Dailies Total1, Proclear 1 Day, Clariti 1-Day and 1-Day Acuvue Moist. The lenses had different material and water content. Pachymetry maps and keratometry values were obtained using the Galilei G4 twice a day: one before putting the lens on and one after an eight-hour period of contact-lens wear. Measurements were also recorded without any contact lenses being worn during a day.

RESULTS

Clariti 1-Day lens caused the greatest thickening in the central (8.9±2.8 μm; p<0.01) and in the peripheral cornea (10.1±4.6 μm; p<0.01), whereas Dailies Total1 was the lens that had the most similar behaviour to the non-contact lens scenario. All the lenses caused a slight flattening in the anterior corneal curvature, except Clariti 1-Day, which induced a very slight steepening. The four lenses caused a steepening of different magnitude in the posterior corneal curvature.

CONCLUSIONS

The magnitude of the changes introduced by the use of soft contact lenses over the eight-hour wearing period was rather small. Thus it is probable it will not influence the vision nor the comfort of the subject. Also, variations on corneal parameters seem to depend on the type of contact lens used.

Corneal stability after discontinued soft contact lens wear

PURPOSE

To determine the time needed to reach corneal stability after discontinued daily wear of soft contact lenses and to identify corneal topographic and pachymetric changes during this period.

METHODS

We prospectively studied the eyes of 21 consecutive soft contact lens wearers evaluated for keratorefractive surgery. Each eye was examined once immediately after lens removal, 3 and 7 days later, and weekly thereafter. At each visit, manifest refraction, keratometry, corneal topography (EyeSys Corneal Analysis system, EyeSys Laboratories, Houston, TX) and pachymetry were performed. Incidence of associated corneal warpage and changes in corneal curvature and thickness during corneal stability time were determined.

RESULTS

Of the 42 eyes examined, 26 corneas showed no significant change after the initial evaluation (stable group). The minimum stability time was 3 days in the remaining 16 eyes (unstable group), 7 of which had significant evidence of abnormal topography. Statistical analysis showed no significant differences between two groups regarding age, sex, duration of contact lens wear, and refractive and topographic measures. The mean central corneal thickness on the final visit was significantly different between two groups (P<0.05).

CONCLUSION

Contact lens induced corneal warpage may occur in a considerable proportion of soft contact lens wearers. A 2-week contact lens free period seems to be adequate for the cornea to stabilize; however, one cannot predict the minimum time needed for each individual patient. Repeating examinations to document corneal stability before refractive surgery may be a safer alternative.

The impact of silicone hydrogel materials on overnight corneal swelling

PURPOSE

To identify the corneal swelling response associated with silicone hydrogel contact lens wear.

METHODS

Twenty-five subjects were recruited to participate in this single-center, controlled, single-masked, clinical trial. The corneal swelling response was evaluated by central corneal pachymetry after overnight wear of balafilcon A, etafilcon A, lotrafilcon A, senofilcon A, and no lens wear. More specifically, after 8 hours of closed-eye wear of each material, corneal thickness was measured immediately on eye opening, in addition to the first 4 hours of open-eye wear. Paired t tests were used to compare the swelling response from each condition to that associated with senofilcon A, and given the numerous statistical comparisons, a P value of <or=0.01 was considered significant.

RESULTS

At eye opening, there was no difference in the swelling response when comparing balafilcon A (1.35%), lotrafilcon A (1.22%), or no lens wear (0.77%) to senofilcon A (1.40%), although the swelling response associated with etafilcon A was significantly greater (P<0.001) than that associated with senofilcon A. There were no differences between balafilcon A, lotrafilcon A, and no lens wear compared to senofilcon A during the open-eye deswelling period. However, 1 hour after eye opening, corneal swelling remained significantly greater with etafilcon A than with senofilcon A (1.76% vs. -0.51%, respectively, P=0.01), although no other time-specific comparisons differed.

CONCLUSIONS

Silicone hydrogel lenses, with high oxygen transmissibilities, are associated with very little corneal swelling after overnight wear of the lenses.

The effects of long-term contact lens wear on corneal thickness, curvature, and surface regularity

PURPOSE

To evaluate the effect of long-term contact lens wear on corneal thickness, curvature, and surface regularity.

DESIGN

A prospective, clinic-based, case-control study.

PARTICIPANTS

A total of 40 eyes of 20 normal subjects and 64 eyes of 35 subjects wearing contact lenses for more than 5 years were evaluated.

METHODS

The Orbscan Corneal Topography System was used to evaluate the entire corneal thickness and curvature, anterior curvature and the anterior and posterior elevation topographic patterns in normal eyes and subjects wearing contact lenses on a regular basis for more than 5 years. Indices of TMS-1 Corneal Topography System were used to determine corneal surface regularity in subjects wearing contact lenses and normal eyes. All topographic examinations were performed after contact lenses had been removed for at least 2 weeks.

MAIN OUTCOME MEASURES

The entire corneal thickness, curvature, surface regularity index (SRI), surface asymmetry index (SAI), potential visual acuity (PVA) and topographic patterns were compared between normal eyes and subjects wearing contact lenses for more than 5 years.

RESULTS

The 64 eyes of 35 subjects evaluated by the Orbscan instrument had an average of 13.45 +/- 6.42 years of contact lens wear. The mean corneal thickness in the center and in eight peripheral areas measured in contact lens wearing subjects was significantly reduced by about 30 to 50 microm compared to normal subjects (P < 0.001 for central and peripheral sites). No correlation was noted between central corneal thickness and degree of myopia in diopters (r = 0.15, 0.10 < P < 0.25). The corneal curvature, maximum keratometry (Max K) and minimum keratometry (Min K) readings, were significantly steeper in eyes wearing contact lenses than normal eyes (P < 0.01 for Max K and Min K measured by both instruments). No difference in the mean corneal astigmatism was noted between groups. The SRI and SAI, but not the PVA index, of the TMS-1 were significantly greater in contact lens wearers than in the control group (P < 0.01 for both SRI and SAI, P = 0.15 for PVA). The color-coded patterns of all curvature and elevation maps made with both instruments showed no significant difference between subjects wearing contact lenses and the control group. There was no significant difference between corneal curvature measurements taken with the Orbscan System or the TMS-1 System in both normal and contact lens groups.

CONCLUSIONS

Long-term contact lens wear appears to decrease the entire corneal thickness and increase the corneal curvature and surface irregularity.

Corneal pathophysiology with contact lens wear

Contact lens wear induces a wide spectrum of changes in the appearance and function of the cornea. The most salient effect of lens wear is the hypoxically induced reduction in the rate of metabolic activity of the corneal epithelium and its sequellae. Other important alterations to corneal health associated with contact lens wear may be caused by antigenic and toxic stimuli, mechanical forces, osmotic effects and carbon dioxide retention. Perhaps the most important task facing the contact lens clinician is to distinguish between an acceptable state of physiological modification and an anomalous or pathological state of hypofunction. In this article, we review the assortment of corneal changes primarily on the basis of the causative agents and time scale with reference to the physical and chemical processes leading to the observed signs or symptoms. This procedure allows a strong foundation for understanding the etiology and management principles for the variety of effects that contact lenses may have on the cornea.