acute rigid gas permeable contact lens intolerance

7
Contact Lens and Anterior Eye (2001) 24, 161-167 © 2001 British Contact Lens Association www.nature,com/cIae Original Article ACUTE RIGID GAS PERMEABLE CONTACT LENS INTOLERANCE A. Jonathan Jackson *, Clive Wolsley¶, jill L. Briggs'~ and David G. Frazer$ (Received 17th April 2001; in revised form 27th July 2001) Abstract- Rigid gas permeable (RGP) and polymethylmethacrylate (PMMA) lens wearers occasionally report episodes of acute intolerance which is experienced upon lens insertion. In this paper, we report two cases of such intolerance in which the probable cause was contact lens'inversion. We also present the results of a study in which a custom-built calibrated strain gauge was used to measure the force in Newtons (N), required to invert RGP lenses [oxygen permeability, or Dk, values between 30 and 90x 10 ~1 (cm2/s) (mlOJ ml x mmHg)] and PMMA lenses of different spherical back vertex powers (+_3.00 D, +_ 9.00 D). Significantly, less force was required to invert minus powered lenses (17.5+_4.8 N) than plus powered lenses (31.7++_ Z4 N), irrespective of the material. PMMA lenses required more force to induce inversion than that required to invert RGP lenses. Lenses with a Dk of 90 required only two thirds of the force (20.0 + 5. 8 N) required to cause inversion compared to PMMA lenses (32.9 + 11.0 N). High powered PMMA lenses were found to be more likely to fracture on inversion than any other lenses tested. The force required to return negatively powered lenses to their original shape, once inverted, was less than 25% of that initially required to induce inversion. Plus powered lenses either reverted to their original form spontaneously, or required less than 3% of the original inversion force to do so. It was concluded that practitioners should consider inversion as a possible reason for otherwise unexplained, acute RGP contact lens intolerance experienced upon lens insertion. The reason why inversion has eluded so many, as a possible cause of intolerance, is likely to be because minimal force is required to return those lenses, which do not crack or fracture, to their original shape. Contact Lens and Anterior Eye (2001) 24, 161-167. KEY WORDS: Contact lens flexure, contact lens intolerance, inversion, RGP, PMMA Introduction R igid gas permeable (RGP) contact lens wearers frequently experience episodes of acute discom- fort, excessive lacrimation and temporary intolerance. In the majority of cases, the symptoms experienced are short-lived and resolve on contact lens removal. Irrita- tion can normally be attributed to tear debris, lashes in the anterior fornices or foreign bodies including dust and grit. '~ Acute intolerance, experienced on contact lens insertion, may also be indicative of a contact lens defect. The term 'lens defect' encompasses abrasions to the lens surface, full thickness cracks, edge defects and alterations to lens geometry often referred to as warpage (Figure 1A and B). Alternatively, symptoms may indicate that lens care solutions have been used inappropriately or that other chemical substances have contaminated the lens surface. In some cases, where symptoms persist after contact lens removal, discomfort may relate to an underlying ocular disease process and not to the contact lens directly. The insertion of a contact lens under these circumstances is, however, likely to exacerbate symptoms. We report on two *BSc, PhD, MCOptom ¶BSc, MSc ~BSc, MCOptom ~BSc, MB, BCh, BAO, FRCS, FRCOphth episodes of acute contact lens intolerance, discuss the probable cause and speculate about the contributory factor. Case Report 1 A 29-year-old male, bilateral keratoconic with a two year history of uncomplicated bilateral RGP contact lens wear presented with a two day history of acute intolerance to the right lens. The normal average wearing time was 14 h per day on 7 days per week and the user was known to be compliant with the recommended contact lens care regimen using proprie- tary cleaning, wetting and soaking solutions. The contact lens responsible for the symptoms, which persisted as long as the lens remained in situ, was of a tri-curve design manufactured in an Aquasil material with a blue tint to a specification of: (C3 7.60:7.50/ 8.30:8.40/10.25:9.20 BVP -4.00 D.) On examination, the right contact lens fitting pattern, observed after the instillation of fluorescein, was unusual, characterised by deep central pooling, a narrow band of mid-peripheral touch and excessive axial edge clearance around the full circumference of the lens. The lens centred on the apex of the cone, although, on blinking, it became excessively mobile, decentring into the inferior fornices. The fitting pattern of the left lens was as one would expect in keratoconus

Upload: a-jonathan-jackson

Post on 17-Sep-2016

220 views

Category:

Documents


8 download

TRANSCRIPT

Page 1: Acute rigid gas permeable contact lens intolerance

Contact Lens and Anterior Eye (2001) 24, 161-167 © 2001 British Contact Lens Association

www.nature,com/cIae

Original Article ACUTE RIGID GAS PERMEABLE CONTACT LENS

INTOLERANCE

A. Jonathan Jackson *, Clive Wolsley¶, jill L. Briggs'~ and David G. Frazer$ (Received 17th April 2001; in revised form 27th July 2001)

Abs t r ac t - Rigid gas permeable (RGP) and polymethylmethacrylate (PMMA) lens wearers occasionally report episodes of acute intolerance which is experienced upon lens insertion. In this paper, we report two cases of such intolerance in which the probable cause was contact lens'inversion. We also present the results of a study in which a custom-built calibrated strain gauge was used to measure the force in Newtons (N), required to invert RGP lenses [oxygen permeability, or Dk, values between 30 and 90x 10 ~1 (cm2/s) ( m l O J ml x mmHg)] and PMMA lenses of different spherical back vertex powers (+_ 3.00 D, +_ 9.00 D). Significantly, less force was required to invert minus powered lenses (17.5+_4.8 N) than plus powered lenses (31.7++_ Z4 N), irrespective of the material. PMMA lenses required more force to induce inversion than that required to invert RGP lenses. Lenses with a Dk of 90 required only two thirds of the force (20.0 + 5. 8 N) required to cause inversion compared to PMMA lenses (32.9 + 11.0 N). High powered PMMA lenses were found to be more likely to fracture on inversion than any other lenses tested. The force required to return negatively powered lenses to their original shape, once inverted, was less than 25% of that initially required to induce inversion. Plus powered lenses either reverted to their original form spontaneously, or required less than 3% of the original inversion force to do so. It was concluded that practitioners should consider inversion as a possible reason for otherwise unexplained, acute RGP contact lens intolerance experienced upon lens insertion. The reason why inversion has eluded so many, as a possible cause of intolerance, is likely to be because minimal force is required to return those lenses, which do not crack or fracture, to their original shape. Contact Lens and Anterior Eye (2001) 24 , 161-167.

KEY WORDS: Contact lens flexure, contact lens intolerance, inversion, RGP, PMMA

Introduction

R igid gas permeable (RGP) contact lens wearers frequently experience episodes of acute discom-

fort, excessive lacrimation and temporary intolerance. In the majority of cases, the symptoms experienced are short-lived and resolve on contact lens removal. Irrita- tion can normally be attributed to tear debris, lashes in the anterior fornices or foreign bodies including dust and grit. '~ Acute intolerance, experienced on contact lens insertion, may also be indicative of a contact lens defect. The term 'lens defect' encompasses abrasions to the lens surface, full thickness cracks, edge defects and alterations to lens geometry often referred to as warpage (Figure 1A and B). Alternatively, symptoms may indicate that lens care solutions have been used inappropriately or that other chemical substances have contaminated the lens surface. In some cases, where symptoms persist after contact lens removal, discomfort may relate to an underlying ocular disease process and not to the contact lens directly. The insertion of a contact lens under these circumstances is, however, likely to exacerbate symptoms. We report on two

*BSc, PhD, MCOptom ¶BSc, MSc ~BSc, MCOptom ~BSc, MB, BCh, BAO, FRCS, FRCOphth

episodes of acute contact lens intolerance, discuss the probable cause and speculate about the contributory factor.

Case Report 1 A 29-year-old male, bilateral keratoconic with a two year history of uncomplicated bilateral RGP contact lens wear presented with a two day history of acute intolerance to the right lens. The normal average wearing time was 14 h per day on 7 days per week and the user was known to be compliant with the recommended contact lens care regimen using proprie- tary cleaning, wetting and soaking solutions. The contact lens responsible for the symptoms, which persisted as long as the lens remained in situ, was of a tri-curve design manufactured in an Aquasil material with a blue tint to a specification of: (C3 7.60:7.50/ 8.30:8.40/10.25:9.20 BVP -4.00 D.)

On examination, the right contact lens fitting pattern, observed after the instillation of fluorescein, was unusual, characterised by deep central pooling, a narrow band of mid-peripheral touch and excessive axial edge clearance around the full circumference of the lens. The lens centred on the apex of the cone, although, on blinking, it became excessively mobile, decentring into the inferior fornices. The fitting pattern of the left lens was as one would expect in keratoconus

Page 2: Acute rigid gas permeable contact lens intolerance

Acute RGP lens intolerance AJ Jackson et a/

162

Figure 2. Geometric profile of rigid gas permeable contact lens in inverted (L) and normal (R) forms. Note the difference in edge profile of the two lenses (arrows).

Figure 1A and B. Rigid gas permeable contact lenses with edge (A) and central (B) defects which are associated with acute intolerance. Arrow 1 illustrates a small edge defect caused when the lens edge was inadvertently trapped between the lid of the lens case and the basket. Arrow 2 illustrates the position of a full thickness split in the centre of a negative lens whereas Arrow 3 identifies linear surface abrasions.

and was as recorded on previous visits. On removal of the contact lenses, both corneae exhibited findings which were characteristic of early keratoconus and, in both cases, the underlying epithelial surfaces were intact and showed no evidence of fluorescein staining. Visual inspection of the right contact lens revealed an abnormal surface geometry which, it was felt, might be indicative of inversion (Figure 2). Topographical map- ping of the front lens surface indicated that the apical radius of curvature had steepened considerably from 7.6 mm to 5.9 mm and that the periphery of the lens had flattened (Figure 3A and B). The geometrical appear- ance of the left lens appeared normal in all respects. The right contact lens was re-inverted and the surface

Figure 3A and B. Appearance of circular mire reflections as observed during topographical mapping of contact lens in normal (A) and inverted (B) profile. Note the irregular peripheral geometry of the inverted contact lens.

resumed its normal profile. Lens dimensions were checked and shown to be identical to those stated above. On re-insertion, the patient experienced symp-

Contact Lens and Anterior Eye

Page 3: Acute rigid gas permeable contact lens intolerance

Acute RGP lens intolerance AJ Jackson et a/ l_U

toms of mild discomfort although no lens defect was apparent. The fitting pattern was, however, essentially as one would have expected from the original lens. The patient was advised to discontinue use of this 'defective' lens and a replacement lens was ordered and subse- quently supplied. No further episodes of intolerance have been reported over the subsequent 12-month period.

Case Report 2 A 20-year-old anisometropic female, trainee teacher, with a 3 year history of uncomplicated unilateral RGP wear on her right eye, presented complaining of acute intolerance following contact lens insertion after over- night soaking in conventional solution. The patient's normal average wearing time was 10 h per day on 5 days per week. The patient was known to be compliant with the recommended lens care regimen using proprietary cleaning, wetting and soaking solutions. The contact lens was manufactured to a tri-curve design (C3 7.70 : 7.60/8.40 : 8.50/10.50 : 9.30 BVP - 6.00 D) from Fluoroperm Dk30 material. On insertion, the contact lens fitting pattern, observed after the instilla- tion of fluorescein, was characterised by a 5 mm diameter zone of deep central pooling with a small central bubble (Figure 4). Peripheral edge clearance was excessive beyond a narrow band (360 degrees) of mid-peripheral touch. The lens was excessively mobile and decentred into the inferior fornices on blinking. The corneal surface observed after contact lens removal was intact and there was no evidence of epithelial staining. Visual inspection of the contact lens after removal revealed abnormal lens surface geometry, similar to that illustrated in the previous case report. Manipulation allowed the contact lens to be re-inverted and, although post-re-insertion dimensions demonstrated a return to the specified values, the patient was unable to wear the contact lens comfortably. Following this episode the patient requested a transfer to soft contact lens wear for daily use. The patient continues to wear a soft lens

successfully, without any discomfort, since the original episode of acute intolerance of the rigid lens.

In the light of these case histories and evidence that acute episodes of RGP contact lens intolerance, experienced on lens insertion, may occur as a result of lens inversion during handling, the authors designed the following study to measure the ease with which RGP contact lenses could be inverted.

Materials and methods Four different lens materials, all regularly used within the regional contact lens clinic at the Royal Victoria Hospital, were selected for analysis (PMMA, Fluoro- penn Dk30, Fluoroperm Dk60 and Fluoroperm Dk90). Five lenses of each material and in four different spherical back vertex powers +9.00 D and _+3.00 D (n=80) were obtained from the clinic's suppliers. All lenses were manufactured to the conventional tri-curve design regularly used by the clinic. Contact lenses were verified using a focimeter, an optical spherometer, a V- gauge and a thickness gauge. In all cases, lens power was within + 0.25 DS of that ordered. Total diameter was within + 0.01 mm of that required (9.00 ram). Back optic zone radius was within +0.05 mm of that requested (7.80 ram). Lens centre thickness varied with back vertex power; for -9.00 D (0.15 mm to 0.19 mm), for -3 .00 D (0.17 mm to 0.21 ram) for +3.00 D (0.30 mm to 0.35 ram) and for +9.00 D (0.39 mm to 0.47 ram).

A custom built calibrated strain gauge (Figures 5 and 6) was used to measure the force required to invert each gas permeable contact lens. The nylon support peg and piston were designed in such a way as to mimic the curvatures on the tip of an index finger and of the palm of the hand of a lens wearer. The mean force required to induce lens inversion, and the standard deviation, are expressed in Newtons. Prior to experimentation, each lens was cleaned with Boston surfactant cleaner and

163

Figure 4. Fluorescein fitting pattern exhibited by inverted contact lenses worn by a myopic subject. Note the steep central pooling and excessive edge clearance.

Figure 5. Custom-made calibrated strain gauge, SG, connected to signal controlling amplifier and voltmeter, the force, F, is applied to the contact lens (CL) as shown.

Contact Lens and Anterior Eye

Page 4: Acute rigid gas permeable contact lens intolerance

Acute RGP lens intolerance AJ Jackson et al

164

Applied force 1 ~ / Nylon

piston

Contact lens

Point at which Appliedl ........................... ~ lens inverts force

Time Figure 6. Schematic drawing of the experimental set-up used to record the forces required to invert the contact lenses. A custom built 'piston' and calibrated strain gauge were used.

pre-soaked for 12 h in Boston Advance conditioning fluid. These are the solutions normally used by RGP lens wearers fitted in this regional clinic. Each lens was inverted and re-inverted repeatedly to establish normal values for both initial lens inversion and ease of re- inversion. A general linear ANOVA was used to analyze the results.

Results With the exception of highly powered positive RGP contact lenses (+9.00 D), virtually all lenses could be inverted with relative ease. Significantly less force was required to invert minus powered lenses (17.5 ± 4.8 N) than plus powered lenses (31.7±7.4N) (P=0.001) irrespective of material (Figure 7). There was a trend for the force required to invert high minus powered lenses to be greater than that required to invert low powered minus lenses, irrespective of lens material. However the actual forces were very similar; 16.6±5.6 N for -3.00 DS compared to 18.7_+4.5 N for -9.00 D. The same trend was found for positive lenses; 30.8+7.1 N for +3.00 D compared to 32.7_+9.2 for +9.00 D.

Lens material had a significant impact on the force required to invert a contact lens. In all cases PMMA lenses (BVP of + 3 and ± 9 D) required more force to induce inversion or fracture than was the case for other lens materials (Fluoroperm Dk30, Dk60, Dk90) (P< 0.001) (Figures 7and 8). Furthermore, Dk90 lenses of all powers required only two thirds of the force applied (20.0±5.8 N) to induce inversion than that required to invert PMMA lenses of equivalent power (32.9± 11.0 N), P<0.001).

High power PMMA lenses were more likely to fracture on inversion than any other lenses tested. In all five cases, these lenses fractured on the first attempt at inversion. Lenses which were pre-soaked

prior to experimentation were 44% less likely to fracture on inversion than lens which were allowed to 'dry out', irrespective of material. Positive powered lenses were twice as likely to fracture as negative powered lenses.

Lenses which did not fracture, could in all cases be re-inverted to their original shape with ease. Positive lenses either re-inverted themselves once the applied force was removed or required only 3% (0.82 + 1.69 N) of the initial inverting force to do so. Negative powered lenses required on average 24% (4.21+3.75 N) of the original inverting force to re-invert them. The force required to induce repeated inversions on the same lens was only marginally less than that required to induce the initial inversion in lenses of all powers and on all materials. (Figure 9).

The back optic zone radius, back vertex power, total diameter and centre thickness were not found to have changed significantly following re-inversion.

Discussion Inversion of RGP contact lenses has not, to the best of our knowledge, been documented previously. The closest references to lens inversion are found in a series of papers on RGP lens flexure published in the 1980s) ,4,6,9 Fatt (1986) commented on the inverse relationship between lens stiffness and gas permeability and indicated that 'flexibility goes up faster as lenses are made thinner than it does as siloxane and fluoropolymer are added to the lens material') Additional data obtained from tests on a number of lens materials including Paraperm, Fluoroperm, CAB and PMMA indicated that the minimum lens thickness required to prevent flexure with PMMA is 0.15 mm, whereas for low to medium Dk RGP lenses, it is in the region of 0.24 mm. High Dk RGP lenses need to have a central thickness of approximately 0.35 mm to prevent flexure. 4,6 The introduction of new

Contact Lens and Anterior Eye

Page 5: Acute rigid gas permeable contact lens intolerance

Acute RGP lens intolerance AJ Jackson et al

°1; ° . . - 4

~ s ~ , ~"

wl N ..1~. ~ " ~ " "

. . . . . . . . . . . . . . . . .

_ I0 i -~

r - -

-9 -3 3 9 Lens power/DS

I - - 4 " - P M M A . - . - I - - 3 0 D K - A - 6 0 D K - = - 9 0 D K ]

Figure 7. Force required (++ SD) to invert lenses of four different materials in four different powers clearly showing the effect of the rnaterial properties of the lens (total lenses used n=80). In the case of+9.00 D PMMA lenses these could not be inverted and the force identified was that required to crack or fracture lenses.

165

60

F o r c e /

N

50

40

30

20

lO

T

. . . . . . - - ,3.1.1.r~1~

PMMA 30 DK 60 DK 90 DK Lens material

[ , -gDS - = - - a D s - J - * ~ D S - - ~ - ~ g D S )

Figure 8. Force required (+_ SD) to invert lenses of four different materials in four different powers clearly showing the effect of the back vertex power of the lens (total lenses used n=80).

generation flexure-resistant materials in recent years has provided manufacturers with a facility whereby lens thickness can be reduced without risk of flexure or warpage. The authors hope to investigate a selection of

these in due course. The influence that the toroidal cornea has on the flexure of different hard and gas permeable lens materials has been investigated by Harris et al. and Stone et al. who identified lens central

Contact Lens and Anterior Eye

Page 6: Acute rigid gas permeable contact lens intolerance

Acute RGP lens intolerance AJ Jackson et al

166

F

o 35- r c e

/ 30- N

25-

20-

1 5 - - r i

1 2

.--e. • 30 Dk - - 9 - 60Dk - • - 90Dk ]

3

Number of inversions

F i g u r e 9 . Force required ( ± SD) for repeated inversion of three different material lenses of +_ 9.00 DS back vertex power.

thickness as being a crucial factor. 5,6,9 More recently, the impact of using steep-, as opposed to flat-fitting, lenses on astigmatic and keratoconic eyes and the effect lens flexure has on visual performance, have been investigated. 1,8 None of these papers mentions total lens inversion.

Lens manufacturers are thus faced with a dilemma concerning the choice of lens thickness, lens dimen- sions, the elastic properties of the lens material and the net effect on oxygen permeability. Interestingly, Lin et aIF in a study on the flexure characteristics of RGP lenses with Dk values ranging from 15 to 150, state that practitioners should not be overly con- cerned about flexure when choosing high Dk materials and that material selection should be made principally on the basis of permeability. As is often the case, the answer is in obtaining a compromise based on clinical findings, one of which should include patient contact lens handling skills and overall dexterity. The forces that we have measured in this experiment are relatively small and are likely to be less than the forces used during conventional manual lens cleaning. We postulate that the mechanical action of lens cleaning may, however, be one mechanism for lens inversion, particularly if the lens is inadvertently placed convex side upon the palm of the hand while undertaking a rub and rinse procedure. The authors have been able to induce inversion of RGP lenses, using this approach, fairly easily. A second possible mechanism for lens inversion concerns the use of basket type lens cases with a raised dome-like base on which to place lenses, convex side up. If a lens is placed in the basket the wrong way up, the mechanical action of closing the basket and thus forcing the lens down onto the raised dome, is enough to invert the lens. We have also been able to induce lens inversion using this technique.

Conclusion These cases illustrate the possibility that improper lens handling and cleaning of a RGP lens can lead to its inversion. Since collection of these data, an additional patient presented at the clinic and in this case the patient was consciously aware of having inverted the lens whilst using a barrel lens case. Upon realisation of what had happened, the patient re-inverted the lens and, upon re-insertion, continued to wear the lens without discomfort or subsequent complications. These cases highlight the need for practitioners to consider every possible cause of intolerance to RGP lens wear and to investigate the seemingly impossible.

Acknowledgements The authors would wish to thank Mr David Cantor of Cantor Nissel Limited for supplying all contact lenses used in this study. They would also wish to acknowl- edge the secretarial assistance of Miss E. Eltiman.

Address for Correspondence Dr. A.J. Jackson, Department of Ophthalmology, The Royal Victoria Hospital, Grosvenor Road, Belfast BT12 6BA.

REFERENCES

l Corzine, J.C. and Klein, S.A. Factors Determining Rigid Contact Lens Flexure. Optom. Vis. Sci., 74, 639-645 (1997).

2 Dougal, J. Abrasions Secondary to Contact Lens Wear. In Tomlinson, A., (Ed), Complications of Contact Lens Wear, St Louis, Mosby, pp. 123-156 (1992).

a Fatt, I. Performance of Gas Permeable Hard Lenses on the Eye. Trans. Br. Contact Lens Assoc. Annual Clinical Congress, 32, 32- 37 (1986).

4 Fatt, I. Flexure of Hard Contact Lenses - An Old Problem and a New Twist. Contax, September, 12-16 (1987).

5 Harris, M.G. and Chu, C.S. The Effect of Contact Lens Thickness and Corneal Toricity on Flexure and Residual Astigmatism. Am. J. Optom. Physiol. Opt., 49, 304-307 (1972).

Contact Lens and Anterior Eye

Page 7: Acute rigid gas permeable contact lens intolerance

Acute RGP lens intolerance . ~ !_!U AJ Jackson et al

6 Harris, G.M., Gale, B., Gansel, K. and Slette, C. Flexure and 8 Residual Astigmatism with Paraperm 02 and Boston II Lenses on Toric Corneas. Am. J. Optom. Physiol. Opt., 64, 269-273 (1987).

7 Lin, M.C. and Snyder, C.S. Flexure and Residual Astigmatism with 9 RGP Lenses of Low, Medium and High Oxygen Permeability. Int. Contact Lens Clin., 26, 5 - 9 (1999).

Sorbara, L., Chong, T. and Fonn, D. Visual Acuity, Lens Flexure and Residual Astigmatism of Keratoconic Eyes as a Function of Back Optic Zone Radius of Rigid Lenses. Contact Lens and Anterior Eye, 23, 48-52 (2000). Stone, J. and Collins, C. Flexure of Gas Permeable Lenses on Toroidal Corneas. Optician, 188 (4951), 8 -10 (1984).

167

Contact Lens and Anterior Eye