Corneal Swelling Response to Contact Lenses Worn Under Extended Wear Conditions

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1 Corneal Swelling Response to Contact Lenses Worn Under Extended Wear Conditions Drien A. Holden, George W. Merrz, and John J. McNolly Corneal thickness changes were monitored in ten subjects (1) over a 36-hour period without contact lens wear and () during 7 days continuous wear of three types of hydrogel contact lenses of different back vertex powers. Measured on awakening, the mean overnight central corneal edema without lenses was 3.0% ± 1.%. With the lenses used in this study, the mean overnight corneal swelling over the week ranged from 9.7% to 15.1%. Following sleep with lenses, the cornea showed evidence of gross stromal edema (striae and folds in the posterior cornea) that reduced or disappeared during the day depending on the level of edema finally reached. The mean contact lens-induced edema 1 hours after eye opening varied from 1.6% to 5.8% for the various lenses. Considerable individual variations were noticed in the corneal edema response under the conditions of sleep, sleep with lenses, and open eye lens wear. Lenses of higher minus power (greater peripheral lens thickness) but the same center thickness produced greater central corneal edema, indicating that with hydrogel lenses of the same material, average thickness over a central zone of the lens determines swelling response. The levels of edema recorded with the high minus power ( 9.00 diopters [D]) low (38.6%) and medium (45%) water content lenses, an average of 13.9% overnight and 5.5% during the day, were cause for concern as obvious signs of stromal edema (striae) persisted with these lenses throughout the study. No decrease in the level of overnight edema (adaptation) was observed during the week of lens wear for any lens. The mean corneal deswelling for the group of subjects over the 1 hours following sleep with these lenses was found to be both limited and reasonably consistent for all lens types (8.% ± 1.1%). Consequently the level of daytime corneal edema was determined primarily by the overnight swelling. We suggest that the desirable maximum overnight swelling with extended wear contact lenses is 8% as this level of edema allows the cornea to regain normal thickness during the day. Invest Ophthalmol Vis Sci 4:18-6, 1983 If corneal oxygen supply is sufficiently reduced, normal metabolic activity is adversely affected causing the cornea to swell. 1 Prolonged and severe edema leads to serious complications such as loss of corneal transparency and corneal neovascularization. 3 It is known that most conventional hydrogel lenses restrict the cornea's oxygen supply enough to cause corneal swelling. 4 " 6 However, the wearing of these lenses has generally been considered acceptable on a daily wear basis (lens removed overnight) provided the level of corneal swelling is no greater than the physiological swelling experienced by the cornea during overnight sleep about 4%. 7>8 From the Cornea and Contact Lens Research Unit, School of Optometry, University of New South Wales, Sydney, Australia. Supported by the Optometric Vision Research Foundation of Australia (Grant No ) and the National Health and Medical Research Council of Australia (Grant No ). Presented in part at the 7th National Contact Lens Research Symposium, August 1980, Chicago, Illinois. Submitted for publication October 30, Reprint requests: Brien A. Holden, Ph.D., Cornea and Contact Lens Research Unit, School of Optometry, P.O. Box 1 Kensington N.S.W., Australia 033. The wearing of contact lenses continuously for a period exceeding 4 hours ("extended wear") may impose continuous corneal hypoxia with no periodic restoration of normal atmospheric levels of oxygen to allow corneal recovery. However, wearing contact lenses for extended periods is considered feasible with hydrogel lenses that supply high levels of oxygen. With hydrogel lenses, tear pumping from outside the lens is minimal, 9 " 11 and oxygen is supplied by diffusion through the lens material, 1 the amount determined by lens water content and thickness. 13 In recent years both high water content and extremely thin hydrogel lenses have become available for consideration as extended wear lenses, and although there have been some reports of the levels of edema encountered under closed eye and extended wear conditions, 14 " 19 a detailed study of the variation in corneal thickness throughout the day with extended wear of lenses has not yet been reported. The aim of this study, therefore, was to measure both the "normal" diurnal variation in corneal thickness (no lens wear) and the variation during 7 days continuous wear of three types of hydrogel lenses /83/000/18/$ 1.5 Association for Research in Vision and Ophthalmology 18

2 No. EXTENDED WEAR CONTACT LENS CORNEAL SWELLING / Holden er ol. 19 Materials and Methods Table 1. Summary of subjects' ocular parameters Subjects Mean S.D. Range Ten young adult subjects (8 men, women), mean age 19.5 ± 0.8 years, participated in the study. Ocular parameters for the group are summarized in Table 1. All subjects were unadapted to contact lens wear and free of ocular disease. Sph. refr. error (DS) Cyl. refr. error (DC) Horiz. K-reading (DK) Corneal toricity (DK) to to to Against rule to 1.00 with rule Lenses The subjects wore three types of hydrogel contact lenses: very thin low water content lenses (Bausch & Lomb Soflens 04 series); thin medium water content lenses (Soft Lens Inc. Hydrocurve II ); and thicker high water content lenses (Cooper Vision Permalens ). Nominal parameters of the lenses used are recorded in Table. Procedure Since corneal hydration is linearly related to corneal thickness, 0 corneal swelling response was determined by measuring corneal thickness with an electronic digital pachometer that has been described previously. 1 Fifteen readings of central corneal thickness were taken on each measurement occasion. The average standard deviation for a set of 15 readings was ±4 urn (0.8%). To investigate the diurnal variation in corneal thickness without lenses, one eye of each subject was monitored over a 36-hour period at 8,9, 10 am, noon,, 4, and 8 pm on the first day, and immediately on awakening, (usually 7 am) and 1, 5, and 1" hours later on the second day. The lens wearing part of the study was conducted in two phases separated by a three-month period of no lens wear. In each phase the subjects wore lenses in both eyes continuously for seven days and nights. During phase one five subjects (Group A) wore a D very thin low water content lens (Soflens 04 series) on one eye and a D lens of the same type on the other eye. The other five subjects (Group B) wore D and D Soflens 04 series lenses. During phase, Group A subjects wore 1.5D and D high water content lenses (Permalens) and Group B subjects wore 1.5D and 9.00D thin medium water content lenses (Hydrocurve II). For the lens wearing experiments, baseline pachometry measurements were taken immediately after lens insertion on the morning of the first day of wear. Subsequent measurements with lenses in place were taken on day 1 after 1,, 4, 6, 8, and 1 hours of lens wear. On days -7 measurements were taken at the moment of eye opening following 8 hours of sleep and at 1,5, and 1 hours after eye opening. Pachometry measurements were also taken prior to lens wear and following lens removal. Corneal swelling was calculated as the percentage differences in corneal thickness compared with the baseline measurements taken either without lenses or immediately after lens insertion. For example, if the thickness of the cornea was mm immediately after lens insertion and mm after 1 hrs of lens wear, then the percentage corneal swelling for that period of wear would be ( )/0.545 X 100 = 5.50%. The biomicroscopic appearance of the contact lenses and the anterior eye were assessed in the morning (within 5 min of eye opening), at noon, and in the evening. Striae and folds in Descemets' membrane and the posterior cornea were each rated on a 0 (absent) to 4 (severe) scale and the ratings for striae and folds combined to provide a measure (0 to 8) of the overall change in corneal appearance due to edema. The subjects were also asked to report any noticeable change in their vision. Table. Lens information and nominal parameters Manufacturer Lens name Water content Diameter Center thickness Back vertex power Bausch & Lomb, Inc. Rochester, NY Soflens 04 series 38.6% 14.5 mm 35 ixm 35 M m 35 /im D D D D D D D Cooper Labs, Inc. San Jose, CA Permalens 71.0% 13.5 mm 00 Mm 170 nm Soft Lenses, Inc. San Diego, CA Hydrocurve II 45.0% 13.5 mm 50 Mm 40 nm

3 0 Vol. 4 INVESTIGATIVE OPHTHALMOLOGY 6 VISUAL SCIENCE / February 1983 Sleep 3 Fig. 1. Corneal swelling vs time for ten unadapted subjects during a normal sleep/wake cycle (no lens wear) Day 1- -Day - Results Diurnal Variation in Corneal Thickness The diurnal variation in mean central corneal edema without lens wear is recorded in Figure 1. For the subjects in this study the overnight edema ranged from 1.4 to 5.1%, with a mean of 3.0 ± 1.%. Within 1 hour of eye opening on day (approximately 8 am) the cornea had returned to the thickness recorded at the start of day 1. Overnight Corneal Swelling with Lenses At eye opening, after sleep with lenses, all subjects showed obvious signs of moderate to high levels of stromal edema and reported blurring of vision. The biomicroscopic ratings of overnight corneal edema for the various lenses ranged from 3.3 to 7,0 (Table 3 and Fig. ). Figures 3-5 show percentage central corneal swelling as a function of time of lens wear for the Soflens 04 series, Permalens, and Hydrocurve II lenses, respectively. The mean overnight central corneal swelling for the various lenses ranged from 9.7 to 15.1%, with a mean of 1.0 ± 1.9% (Table 4). Higher minus power Soflens 04 lenses caused significantly greater corneal swelling than lower powered lenses (P < 0.01). The D lenses produced 10.3 ±0.6%, the D lenses 13.1 ±0.9%, and the D lenses 15.1 ± 0.8%. No such difference due to lens power was observed for the Hydrocurve II or Permalens lenses (Table 4). Table 3. Biomicroscopic rating of corneal edema group average for days -7 Morning Noon Evening Striae Folds Striae Folds Striae Folds Soflens.4 ± ± ± ±0.9.6 ± ± ± ± ± ± ± ± 0.4 Hydrocurve 3.6 ± ± ± ± ± ± ± ± ± ± ± ± 0.5 Permalens 1.9 ± ± ± ± ± ±0.3

4 No. EXTENDED WEAR CONTACT LENS CORNEAL SWELLING / Holden er ol. 1 No decrease in overnight corneal edema could be detected during the week of extended wear for any of the lenses (Table 4 and Figs. 3-5). Corneal Edema during the Day MORNING -o.ee IS -e.ee Table 5 records the group mean daytime edema levels with lenses. The average for all lenses on days -7 was 4.0 ± 1.3%. As can be seen from Table 5 and Figure 3, there was a gradual increase in mean daytime swelling for all the Soflens 04 lenses over the week of lens wear. This trend was statistically significant (P < 0.01). No such trend was observed for Permalens or Hydrocurve II lenses (see Figs. 4, 5). Comparison of the amounts of edema at eye opening and 1 hours later (Table 6) indicates that the deswelling of the cornea, averaged for all subjects, was reasonably consistent for all lenses at 8. ± 1.1%. Table 6 and Figures 3-5 also clearly demonstrate that the mean corneal swelling on day 1 after 1 hours of lens wear (before the first night of sleep with lenses) was significantly less (P < 0.01) for all lenses than the swelling 1 hours after eye opening measured on subsequent days, the mean difference being.7 ± 0.5%. o NOON -1.5 EVENNG PERHALENS ee %</' ' /,' ' -t.s -e.ee SOFLENS -a.ee -e.ee p%?%?%3 -I.S f '""I HYOftOCURVE ee Individual Subject Differences There was considerable individual variation in the swelling response to lenses. For example, for the D Soflens 04 lenses (Table 7), the overnight edema levels ranged from 7.3 to 14.9%, with some subjects (1-3) recording consistently higher and others (8-10) consistently lower edema (P < 0.05) than the mean for the group (10.6%). Fig.. Biomicroscopic rating of corneal edema at eye opening, noon and in the evening, averaged over days -7. Discussion The Level of Edema in Extended Wear Overnight wear of the contact lenses used in this study caused obvious edema, reduction in corneal transparency and blurring of vision. The levels of 0 Sleep SOFLENS 04 Fig. 3. Corneal swelling vs time for ten unadapted subjects wearing Bausch & Lomb Soflens 04 series contact lenses continuously for a period of one week. All subjects wore D lenses on one eye; half wore D lenses and half wore D lenses on the other eye Lena Removed hrl hrl hrl

5 INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / February 1983 Vol. 4 PERMALENS Lena Removed Fig. 4. Corneal swelling vs time for five unadapted subjects wearing Cooper Permalens contact lenses continuously for a period of one week. All subjects wore a D lens on one eye and D lens on the other eye. H hh hh H hh hh hh edema induced overnight were higher (10-15%) than the cornea normally experiences during sleep without a contact lens (3-4%) or during daily wear of conventional hydrogel lenses (4-8%). 4 " 6 Clearly these contact lenses do not, under the reduced oxygen availability of closed lid conditions, " 4 transmit sufficient oxygen to avoid relatively high levels of corneal swelling. Furthermore, once the eyes are opened, the cornea cannot successfully eliminate the edema that develops overnight. In every subject and with all lens designs, the corneal swelling measured 1 hours after eye opening on days through 7 was greater (mean.7%) than the level measured on day 1 following 1 hours lens wear, ie, prior to the first night of sleeping with the lenses (see Figs. 3-5). This indicates that the cornea has a limited deswelling capability under conditions of extended wear of contact lenses, and, therefore, it is the level of edema induced during sleep that determines the level of daytime corneal edema in extended wear. In this study the mean deswelling of the cornea following eye opening with lenses in place was very consistent for the various lenses at 8. ± 1.1%. It follows, therefore, that lenses inducing greater than 8% overnight edema will produce daytime swelling in extended wear. It is also reasonable to expect that lenses that cause 8% or less overnight edema will not cause daytime corneal edema. Adaptation Although the subjects in this study had not previously worn contact lenses, no reduction in the level of overnight swelling (adaptation) was observed over the week of lens wear. However, for some lenses, changes were observed in the level of daytime corneal 0 Sleep HYDROCURVEI 15 o to io Lena Removed Fig. 5. Corneal swelling vs time for five unadapted subjects wearing Hydrocurve II contact lenses continuously for a period of one week. All subjects wore a D lens on one eye and a D lens on the other eye. hh HH hh H hh hh hh hh

6 No. EXTENDED WEAR CONTACT LENS CORNEAL SWELLING / Holden er ol. 3 Table 4. Overnight central corneal edema (%) daily average for all subjects Day: Mean for days -8 Soflens ± ± ±0.8 Hydrocurve II ± ± 0.7 Permalens ± ± 0.9 Average for all lenses (days -8): 1.0% ± 1.9%. swelling. In Figure 6, daytime corneal swelling is plotted for the three types of D lenses studied. Subjects wearing Soflens 04 lenses showed a significant increase in swelling over the week of lens wear, while subjects wearing Hydrocurve II showed a gradual decrease in swelling (P<0.0\). Permalens wearers showed a levelling out of corneal swelling after 4 days. While the origin of these differences in daytime edema patterns over the week is not clear, possible explanations include differences in lens movement and/or differences in the effects of extended wear on lens characteristics such as water content. It is not possible to predict from the results of this study whether continued wear of these lenses beyond one week would have resulted in further changes in the levels of either daytime or overnight edema. The Effect of Lens Peripheral Thickness The results of this study for Soflens 04 Series lenses show significantly greater corneal swelling for higher minus power lenses than for lower power lenses (see Fig. 3) despite the lenses having the same center thickness. Clearly the effective lens thickness is the average thickness over a central zone of the lens. These results are consistent with the view expressed by various authors 5 ' 6 that average oxygen transmissibility across the lens should be considered when predicting the physiologic effects of hydrogel lenses. No such difference in swelling response with lens power was observed with Permalens or Hydrocurve II lenses (see Figs. 4, 5) probably because the center thickness of these lenses is reduced in higher minus powers (see Table ). Of the extended wear lenses investigated in this study, the high water content lenses gave the least overnight corneal edema (the mean edema for both powers of Permalens was 10.3 ± 0.9%). It is clear from the results in Table 4, however, that lenses made from very thin (0.035 mm) low water content material can give an equivalent performance, but only in lower minus powers. Increased peripheral thickness in high minus power low water content lenses significantly reduces overall oxygen availability to the cornea inducing greater levels of central corneal edema. Predicting the Problem Patient Another important issue in extended wear of contact lenses is selecting the appropriate patient. It Table 5. Daytime central corneal edema (%) 1 hours after eye opening daily average for all subjects Day: Mean for days -7 Soflens 04 Hydrocurve II Permalens ± ± ± ± ± ± ± 0.4 Average for all lenses (days -8): 4.0% ± 1.3%.

7 4 INVESTIGATIVE OPHTHALMOLOGY 6 VISUAL SCIENCE / Februory 1983 Vol. 4 Table 6. Central corneal swelling (%) and deswelling (%) group mean data for all subjects for day 1 and days -7 Lens BVP Day 1 Edema after 1 hr lens wear Overnight edema Days -7 1 hr after eye opening Deswelling over 1 hr Difference between edema measured on day and day 1 after 1 hr lens wear Soflens ± ±..3 ± 1.8 ± ± ± ± ± ± ± ± ± Hydrocurve II Permalens 1.6 ±.3.4 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Mean S.D. 1.3 ± ± ± ± ±0.5 would be of considerable benefit if predictive tests could be developed to detect the edema-prone patient. The ten subjects used in this study showed considerable individual variation in their responses to the same lens (see Table 7). We correlated daytime percentage corneal swelling with baseline corneal thickness (in micrometers) for these subjects. The correlation (r = 0.71) was significant (two-tailed Pearson r, P < 0.0) indicating that with extended wear, thicker corneas show greater edema. Thicker corneas also showed a greater percentage overnight corneal edema, but the correlation (r = 0.50) was not significant. The amount of deswelling following eye opening did not correlate with corneal thickness. Though confirmation of these findings is required with a larger sample, it would appear that overnight wear of lenses produced slightly greater edema in thicker corneas and because of the limited corneal deswelling, thicker corneas show greater residual edema during the day. From the high levels of overnight edema recorded in this study, it is clear that the oxygen availability beneath these lenses under closed eye conditions is very low. It may be that the thinner corneas derive some metabolic relief from oxygen diffusing in from the aqueous humor during the closed eye phase of extended wear. Acceptable Levels of Edema with Extended Wear of Contact Lenses The purpose of this study was to quantify the levels of edema encountered during extended wear of var- Table 7. Mean central edema (%) for the individual subjects with 1.5 Soflens 04 Subject Baseline corneal thickness (um) Day 1 After 1 hr lens wear Overnight Days -7 1 hr after eye opening r\ 'IT Difference (deswelling) ± 13.5 ± 1.4 ± ; 10.8 ± 10.4 ± 9.7 ± 9.3 ± 9.0 ± 8. ± 7.3 ± ± ± ± ± ± ± ± ± ± ± ±. 9.9 ± ± ± ±. 9.8 ±.0 8. ± ± ± ± 1. Mean S.D ± ± ± ± 1.4

8 No. EXTENDED WEAR CONTACT LENS CORNEAL SWELLING / Holden er ol. 5 Fig. 6. Daytime corneal swelling vs time for three types of D hydrogel contact lenses worn continuously for a period of one week. g 10 8 Z 6 < UJ Z CC o *" * -1.SD SOFLENS 04 Q -1.SD HYDROCURVE II " -I.SD PERMALENS LENS REMOVED 4- '-Day 5- - TIME (DAYS) 7- ~Day 8^- Dayl Recovery ious hydrogel lenses. Clearly, overnight wear of these lenses caused significant corneal edema, reduction in corneal transparency, and some deterioration of vision. The question is how much edema can the cornea tolerate without undesirable changes in corneal structure and function being induced? The normal edema cycle for the cornea is 3-4% overnight, reducing to 0% 1 hour after eye opening. Three to 4% might, therefore, be considered an acceptable limit for daytime edema for daily lens wear. However, the results of this study indicate that if, under extended wear conditions, the daytime edema is 4%, the cornea would have to contend with a cycle of 1% overnight edema and 4% during the day. Very little information exists to judge whether this level of edema, if sustained over a long period of time, would induce unacceptable changes in corneal structure or function. Certainly the levels of edema encountered with the high minus power low and medium water content lenses (15.1% overnight and 5.8% during the day with the D Soflens 04; and 1.8% overnight and 5.1% during the day with the 9.00 Hydrocurve II lens) are cause for concern as these lenses caused striae that persisted throughout the day for all subjects (Fig. ). From experience with thick low water content hydrogels causing similar levels of edema (5-8%) during daily wear, we would expect these high minus power lenses eventually to cause changes in refractive error, corneal shape, 67 " 9 contrast sensitivity, 9 " 30 and limbal vascularization. 31 The long-term effect of lower levels of edema, eg, 10% overnight and -3% during the day, are not known. Therefore, we suggest that until long-term studies on the effect of such an edema cycle are available, 8% should be considered the desirable limit for overnight edema during extended wear as this level would allow the cornea to recover to normal thickness soon after eye opening. Key words: corneal edema, extended wear, diurnal variation, hydrogel lenses, corneal deswelling. Acknowledgments The authors thank Kenneth A. Poise, Debbie Sweeney, and Irving Fatt for their helpful comments and Grant Sayer, Donna LaHood, Steve Payor, and Donald Martin for their assistance in the preparation of the manuscript. References 1. Smelser GK and Ozanics V: Importance of atmospheric oxygen for maintenance of the optical properties of the human cornea. Science 115:140, Maurice DM: The structure and transparency of the cornea. J Physiol 136:63, Cogan DG: Corneal vascularization. Invest Ophthalmol 1:53, Poise KA, Sarver MD, and Harris MG: Effects of Soflens parameters on corneal edema. Int Contact Lens Clin 3():35, Mertz GW: Electronic pachometry and contact lens physiological response. J Br Contact Lens Assoc (1):5, Bradley W and Schoessler JP: Corneal response to thick and thin hydrophilic lenses. Am J Optom Physiol Opt 56:414, 1979.

9 6 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Februory 1983 Vol Mandell RB and Fatt I: Thinning of the human cornea on awakening. Nature 08:9, Mertz GW: Overnight swelling of the living human cornea. J Am Optom Assoc 51:11, Fatt I and Lin D: Oxygen tension under a soft or hard, gaspermeable contact lens in the presence of tear pumping. Am J Optom Physiol Opt 53:104, Hayshi T: Mechanics of contact lens motion. Doctoral Dissertation, University of California at Berkeley, Poise KA: Tear flow under hydrogel contact lenses. Invest Ophthalmol Vis Sci 18:409, Fatt I and St Helen R: Oxygen tension under an oxygen-permeable contact lens. Am J Optom Physiol Opt 48:545, Refojo MF and Leong FL: Water-dissolved-oxygen permeability coefficients of hydrogel contact lenses and boundary layer effects. J Membr Sci 4:415, Mandell RB and Poise KA: Contact lenses worn during sleep and rest periods. J Am Optom Assoc 41:937, Harris MG, Sanders TL, and Zisman F: Napping while wearing hydrogel contact lens. Int Contact Lens Clin (1):84, Sarver MD and Staroba JE: Corneal edema with contact lenses under closed-eye conditions. Am J Optom Physiol Opt 55:739, Sarver MD, Baggett DA, Harris MG, and Louie K: Corneal edema with hydrogel lenses and eye closure: effect of oxygen transmissibility Am J Optom Physiol Opt 58:386, Zantos SG and Holden BA: Ocular changes associated with continuous wear of contact lenses. Aust J Optom 61:418, Hirji NK and Larke JR: Corneal thickness in extended wear of soft contact lenses. Br J Ophthalmol 63:74, Hedbys BO and Mishima S: The thickness-hydration relationship of the cornea. Exp Eye Res 5:1, Holden BA, Poise KA, Fonn D, and Mertz GW: Effects of cataract surgery on corneal function. Invest Ophthalmol Vis Sci :343, Fatt I, Freeman RD, and Lin D: Oxygen tension distributions in the cornea: a re-examination. Exp Eye Res 18:357, Efron N and Carney LG: Oxygen levels beneath the closed eyelid. Invest Ophthalmol Vis Sci 18:93, Roscoe WR and Hill RM: Corneal oxygen demands: a comparison of the open- and closed-eye environments. Am J Optom Physiol Opt 57:67, Fatt I: The definition of thickness for a lens. Am J Optom Physiol Opt 56:34, Wilson G: Hydrogel lens power and oxygen transmissibility. Am J Optom Physiol Opt 56:430, Hill JF: Diurnal variations in the corneal curvature after wearingflexible (HEMA) contact lens. Am J Optom Physiol Opt 51:56, Mandell RB: Corneal edema from hard and hydrogel contact lenses. Contacto 0:5, Grosvenor T: Changes in corneal curvature and subjective refraction of soft contact lens wearers. Am J Optom Physiol Opt 5:405, Hess RF and Garner LF: The effect of corneal edema on visual function. Invest Ophthalmol Vis Sci 16:5, Tomlinson A and Haas DD: Changes in corneal thickness and circumcorneal vascularization with contact lens wear. Int Contact Lens Clin 7(1):45, 1980.