Corneal Response to Extended Wear of CAB Contact Lenses in Aphakia

Corneal Response to Extended Wear of CAB Contact Lenses in Aphakia •

BARNARD A. KAPLAN, MD, CONNELL J. TRIMBER, MD

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Abstract: We studied 41 aphakic eyes in 30 patients who had been wearing CAB (cellulose acetate butyrate) contact lenses on an extended basis . Patients were 51 to 81 years old and averaged 14.3 months of wearing time. Visual acuity was very good. Pachometry measurements ranged from 0.42 mm to 0.66 mm and averaged 0.550 mm. This compares favorably with data in the literature on corneal thickness seen with routine use of PMMA or hydrogel lenses or with extended wear of hydrogel lenses. Corneal thickness was also compared with unoperated fellow eyes and eyes using CAB lenses on a daily basis only. [Key words: aphakia, CAB lenses, contact lenses .] Ophthalmology 87 :292-297, 1980

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The optical correction of aphakia is a problem that is iatrogenically induced approximately 500,000 times each year. Recent development of intraocular lens implantation techniques and extended wear contact lenses have increased the options available when attempting to solve this problem . Traditionally, aphakia has been corrected with the use of spectacle lenses; however, there are many well-known disadvantages associated with their use. Image size is increased by about 25% ; if there is residual vision in the other eye, there will be superimposed diplopia, and binocular vision becomes impossible. Spherical aberration with pincushion distortion occurs as From Georgetown University, Washington, DC. Presented at the Eighty-Fourth Annual Meeting of the Americ an Academy of Ophthalmology, San Franc isco, November 5-9, 1979. Reprint requests to Barnard A. Kaplan, MD, State Highway 70 at East Gate Drive, Cherry Hill, NJ 08034 .

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well as a ring scotoma with the "jack-in-thebox" phenomenon . These spectacles also require continual readjustment of position for best visual acuity. The use of intraocular lenses has become increasingly common. While this mode of correction eliminates the optical problems induced by spectacle lenses, it introduces a new set of complications to the cataract procedure and to the postoperative cour.se. These include pupillary block, flat or shallow anterior chamber with corneal touch and corneal edema, lens precipitates and membrane formation, decentration and dislocation, uveitis, hyphema, and endophthalmitis. Contact lenses are another alternative to the optical correction of aphakia. The optical problems of spectacle lenses are eliminated and there are no added operative or postoperative complications to the procedure itself. However, contact lenses have disadvantages, including the inability of many patients to insert or remove the lens, especially true in older patients

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with decreased manual dexterity, and patient intolerance to the presence of the lens in the eye. This study investigates another solution to the problem of aphakic correction. A group of patients wearing the CAB (cellulose acetate butyrate) lens for extended periods of time was evaluated. The extended wear of contact lenses eliminates the problem of insertion and removal while the CAB material minimizes the problem of patient intolerance because of its unique properties. Special attention was given to corneal thickness, because we felt it would provide information concerning the effect of this material on the cornea's physiologic status and would have implications concerning the longterm compatibility of this material with corneal tissues.

MATERIALS AND METHODS Cellulose acetate butyrate is a material that has recently been introduced to contact lens use. Developed in the 1930s by George Eastman for use in the photography industry, its advantages included enhanced solubility in organic solvents, water resistance, flexibility, moulding properties, and resistance to weathering. With the proper formulation, it can be made into a lens that can be fabricated by molding or lathecutting. However, the great advantage of this lens for ophthalmologic purposes lies in its superior physiologic tolerance over other types of lenses. Compared with the hydrophilic lenses, the CAB lens is reported to be much more permeable to oxygen and carbon dioxide. Its 25% greater thermal conductivity permits greater dissipation of heat and serves to lower the metabolic needs of the cornea. Compared with the polymethylmethacrylate lens, the CAB lens has a 38% lower surface tension and an 18.5% lower wetting angle, which make it more wettable and, therefore, more comfortable than a hard lens. 1 The visual qualities of the CAB lens are quite good and the method of fitting, although different in some respects, is easily adapted to the techniques used with hard lenses. The material comes in different grades. The lenses used in this study were thermo-compression molded lenses manufactured by Danker Labs, Inc. and are known as the MESO lenses. Patients in this study all had routine intracapsular cataract extracts performed by Dr. Trimber. They were fitted with CAB contact lenses approximately six weeks postoperatively. Initially these patients wore their lenses only during the day. Those who tolerated them well were then allowed to wear the lenses for

extended periods of time. Office visits were frequent at first and then became routine. Patients were then evaluated by Dr. Kaplan. Data were obtained with regard to the patients' ages, sex, duration of CAB lens wear, and lens cleaning regimen. Subjective information was obtained concerning the patients' evaluations of their visual acuity and comfort as well as frequency of lens decentration or loss. Physical examination of the eye was performed with attention to visual acuity, conjunctiva, and cornea. Pachometry of the cornea was then done using the attachment developed for the Haag-Streit "900" slit lamp and the method described by Mishima and Hedbys. 2 Patients who had routine intracapsular cataract extractions performed by Dr. Trimber but who wore aphakic spectacles were also evaluated to provide a comparison group with those who wore CAB lenses on an extended basis. Another group of aphakic patients wearing CAB lenses on a daily basis only were also seen.

RESULTS

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Thirty patients with 41 aphakic eyes were seen who were wearing CAB lenses successfully for extended periods of time. These patients ranged in age from 51 to 81 years, with an average age of 65.0 years. Their length of CAB lens wear ranged from 1 month to 36 months with an average wearing time of 14.3 months. Visual acuity in these eyes was 20/20 or better in 17, 20125 or 20/30 in 19, and 20/40 or 20/50 in 5. Corneal thickness was measured as 0.42 mm to 0.66 mm with an average thickness of 0.550 mm. Fourteen of the patients wearing CAB lenses on an extended basis had a fellow eye that had not undergone an operation and was not wearing any kind of contact lens. The eyes with the CAB lens in this group averaged a corneal thickness of 0.548 mm; the fellow eyes averaged 0.515 mm. Twenty-five patients with aphakic eyes had spectacle correction. This group ranged in age from 59 to 79 years, averaging 70.1 years old. They were seen from 2 to 64 months postoperatively with an average of 29.8 months since cataract extraction. Only 4 eyes saw 20/20 with a total of 17 eyes seeing 20/30 or better. These corneas measured from 0.49 mm to 0.57 mm in thickness with an average of 0.525 mm. Thirteen eyes had undergone routine intracapsular cataract extractions and were subsequently fit with CAB lenses. However, these patients chose to wear their lenses only during the day and not on an extended basis. They

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Table 1. Aphakic Correction versus Corneal Thickness Average Corneal Thickness

Optical Correction

Cataract Extraction

No. Eyes

Duration Postop

Average Age

CAB extended wear

yes

41

14.3 mo

65.0 yr

0.550 mm

None

no

14 fellow eyes

16.2 mo

65.8 yr

0.515 mm

Spectacles

yes

25

29.8 mo

70.1 yr

0.525 mm

CAB daily wear

yes

13

22.1 mo

66.8 yr

0.538 mm

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averaged 66.8 years of age and were seen 22.1 months postoperatively. Their eyes averaged a corneal thickness of 0.538 mm (Table 1). Patients were also examined with regard to duration of contact lens wear versus corneal thickness. Those wearing the lenses for longer periods of time appeared to have smaller increases in corneal thickness (Table 2). Most of the patients wearing CAB lenses on an extended basis removed them for cleaning only everyone to two months. Several claimed that their lenses had not been removed until the time of this study, when they were removed by an investigator. The most significant complaints about the lenses concerned the loss of a lens, which occurred in 16 patients, and decentering of the lens, which 22 patients reported. Lost lenses were rapidly replaced, and often the patients had "back-up" lenses available for use. Decentering of the lenses was occasional in most patients, and they quickly learned how to rectify the situation on their own. Some patients did note increased exudation about the lid margins, especially when waking in the morning, and several patients felt that a tinted lens would be easier to handle than the currently available colorless CAB lenses.

DISCUSSION CORNEAL THICKNESS AND CONTACT LENS WEAR

Measurement of corneal thickness was attempted as far back as 1723; however, only in Table 2. Duration of Extended CAB Lens Wear versus Corneal Thickness 0-6 6-11 12 - 23 more than

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months months months 23 months

0.561 0.577 0.541 0.523

mm mm mm mm

(9 patients) (12 patients) (7 patients) (13 patients)

the last 20 years has it been achieved with any reasonable accuracy. The pachometer developed to attach to the Haag-Streit "900" slit lamp is the most commonly used instrument today. I t measures the thickness of the corneal optical section when the slit-lamp beam passes through the tissue perpendicularly to its surface. In the original studies with this instrument, Mishima and Hedbys found the corneal thickness to be 0.518 ± 0.02 mm. An electronic corneal pachometer has been more recently developed. 3 While it is useful for measuring areas other than the central cornea and is easier to operate, it is no more accurate than the standard instrument. Variations in corneal thickness outside the normal range have many etiologies. Dystrophies or trauma will result in increased corneal thickness. A keratoconus or ulcers may cause corneal thinning. Alterations in oxygen consumption and changes in metabolism will also affect corneal thickness. Corneal thickness also reflects the state of hydration of the corneal tissue as there is a linear relationship between corneal thickness and hydration. The precorneal tear film and aqueous fluid are normally hypertonic with respect to the cornea. Contact lenses appear to disturb the baseline state of the cornea because of alterations in corneal metabolism and changes in the tonicity of the tear layer on the corneal surface. EI-Hage et al 4 found that hard and flexible lenses resulted in significant increases in central corneal thickness after one and three hours of wear. A significant increase in the central corneal thickness of the contralateral eye during hard contact lens wear was also found. Increases in thickness were less with soft lenses than hard lenses in all cases. Their study could not determine the exact cause of the increase in corneal thickness but suggested that an "osmolarity" effect, a decrease in glycogen content

of the cornea, or a decrease in "trans-corneal potential" may playa role. Farris et al 5 measured changes in the corneal thickness of successful hard contact lens wearers and found the following changes: increased thickness: 0.024 mm after 1 week 0.016 mm after 2-3 weeks 0.021 mm after 6- 8 weeks

These were measured after eight hours of wear. They felt this 4% increase in corneal thickness was due either to increased tearing that diluted the osmolarity of the tears or to decreased oxygen uptake. Mandell et al 6 also found an increase in corneal thickness of about 3% when a patient was wearing a hard contact lens in the contralateral eye. However, it was noted that this increase in thickness disappeared after a patient wore a lens for several weeks . They felt that this change was caused by the lower tonicity of tears induced by wearing the lens in the contralateral eye, and they observed that the corneal thickness returned to normal after the patient adjusted to contact lens wear and the tonicity of tears returned to normal. An increase of 4% or more was thought to be due to other causes. When contact lenses were made to fit tightly, patients reported burning, tearing, and photophobia; evidence of edema was found on biomicroscopy and corneal thickness was increased by more than 4% . Mandell thought that these findings were due to interference with oxygen supply, but no mechanism for corneal swelling itself was postulated. Millodot7 noted an average increase in corneal thickness of 6.9% after eight hours of wear in patients who were well-adapted to the use of hard contact lenses. He too felt that this was due to oxygen deprivation because these patients were successful lens wearers with normal tear production. He also found decreases in corneal sensitivity of 94% centrally and 116% peripherally, suggesting that this may be an even more sensitive measure of corneal changes due to contact lenses. CarneyS found an increase of 8.4% in the corneal thickness of six subjects wearing hydrophilic lenses. This increase was similar to PMMA lenses but differed in that it was more evenly distributed across the surface of the cornea while hard lenses caused an increase in thickness centrally. Leibowitz 9 demonstrated in vitro that increased oxygen permeability of the soft lens does not mean that all oxygen reaching the cornea comes through the lens. As with hard lenses, freshly oxygenated tears circulate beneath the lens with every blink. The hydrophilic lens tends to buckle with each blink creating negative pressure that draws in oxygenated

tears. Rather than oxygen going through the soft lens, it may be that this type of lens may just be more efficient in circulating tears. CORNEAL THICKNESS AFTER CATARACT EXTRACTION

Prospective studies of corneal thickness after cataract surgery show that the cornea undergoes little change between preoperative and po s toperative measurements. Wood and Maumenee 10 looked at 34 eyes that were thought to have normal corneas and had undergone intracapsular cataract extractions using microsurgical techniques. Using the HaagStreit pachometer and the measurement technique of Mishima and Hedbys, they found that preoperative measurements were 0.552 mm. On the fourth or fifth postoperative day, they averaged 0.616 mm; measurements averaged only 0.556 mm five to eight months after surgery. They felt that there was no significant residual corneal swelling several months after cataract surgery in patients with normal corneas. Stark II followed 100 patients who had intracapsular cataract extractions with pre- and postoperative pachometry readings and found that there was a significant (0.12 mm) increase in corneal thickness on the first postoperative day, but that by five weeks after the procedure, the corneas had returned to preoperative levels. EXTENDED WEAR OF CONTACT LENSES

There has been a great deal of excitement and activity about extended wear of contact lenses in the last several years. The first report of extended wear lenses was by Welsh,12 who fit 200 aphakic patients with tiny, hard (PMMA) contact lenses. He found no adverse signs or symptoms in 80% of his cases. Problems included a compression syndrome in which the lens would lodge in one peripheral area of the cornea (5%), displacement of the lens during sleep (2%), excessive cornea staining (3%), and fear of using the lens while sleeping (10%). Welsh advised great caution when recommending use of these lenses while sleeping, but claimed that symptoms of eye irritation were reliable in alerting patients to discontinue use of these lenses on an extended basis. After experience had been gained with use of hydrophilic lenses in the therapy of corneal disease, Liebowitz l3 felt that these lenses could be worn for prolonged periods without ill effect. He fit one eye in each of 10 normal volunteers with soft contact lenses for 10 days and found no evidence of overwearing; however, there was some variability in visual acuity, edema, and burning in several subjects that was transient and self-limited. These patients had a 30% increase in corneal thickness on the first day of

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wear, and the cornea stabilized at this thickness for the duration of the study with no interference in visual acuity . Binder and Worthen 14 studied 20 normal volunteers who wore a soft lens on one eye for 12 weeks. Slit-lamp examination of these patients revealed no gross edema, mild limbal injection in the first few weeks that decreased slowly, and no corneal neovascularization, conjunctival injection, or follicular changes. Only a minimal change in visual acuity was found in some patients. Four patients had a change in corneal thickness of 0.04 mm or more compared to the fellow eye at two weeks and again at 12 weeks. All corneas returned to normal three weeks after the removal of the lenses. Using this same group of patients, the bacterial flora was compared in the eyes with lenses and the fellow eyes. Hydrophilic lenses did not appear to change the bacterial flora, and the use of antibiotic drops did not alter the flora. Gasset 15 studied continuous wear soft contact lenses in aphakic patients. Thirty-seven patients with 49 eyes were seen; 45 had a visual acuity of 20/40 or better. Side effects included epithelial edema, 12.2% , superficial vascularization, 24.4%, and conjunctivitis, 16.3%. Corneal thickness averaged 0.57 mm with a range of 0.53 mm to 0.60 mm (normal was 0.50 mm, range was 0.43 -0 .56 mm) . Edema and superficial vascularization were felt to result from poor or nonexistent bevels on the back of the lenses. Gasset thought that this mode of aphakic correction would be a viable alternative to intraocular lenses. In a study from Australia, Benson l6 fit 280 ametropes for periods up to 2~ years of continuous wear with the Sauflon (80% water content) hydrophilic lens. He claimed that the overwearing syndrome was nonexistent and that 84% of these patients were completely comfortable with this lens. However, visual acuity was variable in 48% with 2% having reduced visual acuity. Thirty percent of the patients had episodes of infection. Patient dissatisfaction centered on complaints of dryness, which Benson thought was from environmental conditions, and reduced visual acuity, which he felt was from the thickness ofthe soft lens itself. Studies of extended wear gas-permeable lenses have been done by Garcia l7 ,18 and Boyd}9 Garcia initially used the RX-56 lens , which is similar in its properties to the CAB lens. He first reported on 42 phakic and 26 aphakic patients wearing 112 lenses for daytime use only and found neither irritation nor corneal staining or edema detectable by sclerotic scatter after six hours of wear. Most patients found these lenses more comfortable and more adaptable than PMMA lenses. This experience suggested that the cornea could tolerate lenses

for longer periods of time. Garcia then looked at 20 aphakic patients (48 to 79 years old) who wore RX-56 lenses on an extended basis. These patients were motivated by the greater freedom and flexibility of life-style this mode of correction offers and by the elimination of trauma and anxiety often experienced during insertion and removal of these lenses. During the 3 to 15 months these patients were followed , their visual acuity was better than with spectacle correction in all cases; there were no signs of corneal edema or vascularization and there was only occasional slight limbal stippling. No change in bacterial flora was observed. Recently, Garcia has reported on 111 CAB lenses fit in 80 aphakic patients (43 to 88 years old). In this group, there were no significant ocular infections, no episodes of corneal decompensation, and no evidence of vascularization. Eighty-two percent of these patients continued to wear the CAB lens on an extended basis. Boyd has also reported an 80% success rate with 135 aphakic eyes fit with CAB lenses for extended wear. More recently, the extended use of silicone rubber contact lenses has been reported. Silicone lenses are more gas-permeable than CAB lenses. Dahl and Brocks 20 studied 34 of 42 eyes that had been fit with these lenses. Duration of wear was from 2 to 11 months. Visual acuity was good with the silicone lens, but it only exceeded spectacle correction when there was an overcorrection of the contact lens with spectacles . Decentration occurred in all except five eyes. The greatest problem with this lens appeared to be deposition of mucus on the lenses, which caused visual blurring and occasional irritation.

SUMMARY This study has demonstrated that the extended wear of CAB lenses provides excellent visual results with minimal effects on corneal thickness. Corneal thickness measured in patients wearing CAB lenses on an extended basis was 0.550 mm; those wearing the CAB lenses on a daily basis averaged 0.538 mm. A normal population averages 0.518 mm. A previous study of extended wear soft lenses had an average corneal thickness of 0.570 mm. Though problems of lens loss and decentration do exist, patients wearing the lenses were extremely satisfied with their good visual acuity and comfort. Extended-wear contact lenses provide a reasonable alternative to aphakic correction with spectacles or intraocular lenses, and the CAB material appears to demonstrate long-term compatibility with corneal tissue.

REFERENCES 1. Feldman GL. Chemical and physical properties of cellulose acetate butyrate as related to contact lenses. Contact Lens 1977; 11 :25-31. 2. Mishima S, Hedbys BO. Measurement of corneal th ickness with the Haag-Streit pachometer. Arch Ophthalmol 1968; 80:710-13. 3. Binder PS, Kohler JA, Rorabaugh DA. Evaluation of an electronic pachometer . Invest Ophthalmol 1977; 16:855-58. 4. EI Hage S, et al. Evaluation of corneal thickness induced by hard and flexible contact lens wear. Am J Optom Physiol Opt 1974; 51 :24-32. 5. Farris RL, et al. Epithelial decompensation with corneal contact lens wear. Arch Ophthalmol 1971; 85:651-60. 6. Mandell RB, et al. Corneal swelling caused by contact lens wear. Arch Ophthalmol 1970; 83:3-9. 7. Millodot M. Effect of hard contact lenses on corneal sensitivity and thickness . Acta Ophthalmol 1975; 53 :576-84. 8. Carney LG . Hydrophilic lens effects on central and peripheral corneal thickness and corneal topography. Am J Optom Physiol Opt 1975; 52 :521-23. 9. Leibowitz HM, et al. Continuous wear of hydrophilic contact lenses. Arch Ophthalmol 1973; 89:306-10.

10. Wood WJ , Maumenee AE . Corneal thickness after cataract surgery. Trans Acad Ophthalmol Otorhinolaryngol 1975; 79 :631-41 . 11. Stark WJ , et al. Two-year trial of intraocular lenses at the Wilmer Institute. Am J Ophthalmol 1977; 84:769. 12. Welsh RC. Continuous use of tiny hard corneal lenses for aphakia. Ann Ophthalmol 1973; 5:1003-04. 13. Leibowitz HM , et al. Continuous wear of hydrophilic contact lenses . Arch Ophthalmol 1973; 89 :306-10. 14 . Binder PS , Worthen D . Clinical e va luation of continuous-wear hydrophilic lenses. Am J Ophthalmol 1977; 83:549-53. 15. Gasset AR. Permanent wear of soft contact lenses in aphakin eyes . Am J OphthalmoI1977 ; 83 :115-21. 16. Benson C. Continuous use of contact lenses. Trans Ophthalmol Soc NZ 1976; 28:71-74. 17. Garcia GE. Continuous wear of gas-permeable contact lenses in aphakia. Contact and Intra-Ocular Lens Med J 1976; 2:29-34. · 18: G · arcia GE. Extended wear of CAB contact lenses in aphakia . Paper presented at the annual meeting of the American Academy of Ophthalmology and Otorhinolaryngology, October 22, 1978. 19. Boyd B. Long-term wear of contact lenses: a reality. Highlights Ophthalmol 1977; 5:1- 8. 20 . Dahl AA, Brocks ER . The use of continuou s-wear silicone contact lenses in the opti cal correction of aphakia. Am J Ophthalmol1978 ; 85454 - 61.

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Discussion by George E. Garcia, MD I compliment Doctors Kaplan and Trimber on their paper. The planning, execution, and analysis were all of high caliber. The use of the contralateral unoperated eye in 14 patients in the study group, 25 aphakes wearing spectacles, and 13 aphakes who had previously undergone routine intracapsular cataract extraction and were wearing CAB lenses on a daily basis provided the appropriate controls for the group under study, and were appropriately matched with respect to age and period of time elapsed since surgery. A 6% increase in pachymetry measurements with CAB in the extended wear mode, versus a 4% increase with daily wear CAB lenses, and the greater than \0% increase with soft lenses worn on an extended wear basis as reported by Gasset, suggests that this material is well tolerated under the circumstances and produces minimal alteration of corneal metabolism. The accuracy of pachymetry measurements varies considerably with different observers and the fact that all measurements were done by the same person is helpful in improving their accuracy. Even with a single observer it is possible for the measurements to vary over quite a broad range. Unfortunately the authors do not permit an analysis of their data in a manner which permits us to determine their degree of accuracy. Mishima and Hedbys reported a standard deviation of 0.02 mm with a mean of 0.518 mm when From the Department of Ophthalmology, Boston University School of Medicine.

performing pachymetry measurements by this method . A two standard deviation range would suggest a variation of just under 8% is possible on the basis of technique and instrument error. While the data presented support the efficacy of extended wear of CAB lenses from the point of view of changes in corneal thickness, and from a clinical standpoint this seems to be well established, further statistical analysis of their data would hopefully be even more conclusive. It would be useful to know whether the groups actually differed from one another on a statistical basis. Since the extended wear of contact lenses is highly desirable for some of our patients, this type of study is most important and it is hoped that further studies will be done with larger numbers of patients and following a longer period of extended wear. Additional investigations such as the study of endothelial cell population in patients wearing contact lenses following cataract surgery would be helpful in determining whether or not any significant alterations of endothelial cell morphology and/or cell population occur. In addition, biochemical studies of corneal epithelial metabolism, such as those performed by Thoft and others, to determine whether or not permanent changes in aerobic versus anaerobic metabolism occur with extended versus daily wear would be valuable. Clinical experience to date supports extended wear of CAB lenses as a safe approach to the management of aphakia but supportive laboratory evidence would be reassuring and is something we owe ourselves and our patients.

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