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Visual performance with artificial iris contact lenses
JournaloftheB.C.L.A.
Vol. 10. No. 2, pp. 10 - 15, 1987
@1987 by the British Contact Lens Assoc.
Printed in Great Britain
VISUAL PERFORMANCE WITH ARTIFICIAL IRIS CONTACT LENSES RICHARD V . ABADI, ERIC PAPAS
(Received 5 June 1987, in revisedform 20 July 1987) Abstract - - O n e of the functions of the iris is to control retinal irradiance. Whilst this is the case in most individuals, albinos, due to a lack of ocular pigment found in their iris, have an inadequate pigment screen. This study was designed to examine and quantify the scattering properties of the iris in four Tyrosinase negative oculocutaneous albinos by measuring contrast detection when a small discrete light source was directed at the inferior iris. Three commercially available artificial iris contact lenses were then evaluated to determine h o w well they screened the retina from the source. T h e results suggest that a high water content dyed H E M A contact tens is as effective at shielding the iris as an Iris print lens.
METHODS
Visual performance is primarily dependent on the quality of the retinal image (Charman 1983). Of the many ocular characteristics which can influence the imaging characteristics of the eye, the role of the iris diaphragm is not insignificant. Ideally, light should only pass through the pupillary aperture. For normally pigmented individuals the extraneous light is screened out by the pigment in the uveal tract and the retinal epithelium; these two sites being responsible for the control of retinal illuminance and the minimisation of internal light scatter. Clearly inefficient screening will give rise to a reduction in the retinal image contrast and a subsequent degradation o f visual performance. Such a state of affairs exists for the albino eye. Albinism is not a single entity but comprises a group of conditions (Witkop 1979; Witkop, Quevedo and Fitzpatrick 1983). Affected individuals exhibit varying degrees of hypomelanosis of the skin, hair and eyes (Taylor 1978; Abadi and Dickinson 1983; Kinnear, Jay and Witkop 1985). In the eye, pigment is deficient in the uveal tract and the retinal epithelium. One clinical manifestation of this lack of pigment is the existence of iris transil!umination and many albinos experience discomfort and disability even under quite modest light levels. This is particularly the case for the tyrosinase negative oculocutaneous albino (Ty neg OCA) who are purported to have no melanin whatsoever. Not surprisingly, clinicians have advocated shielding the albino retina from the unwanted excess radiation with either tinted spectacles or cosmetic contact lenses (Bier & Lowther 1977; Taylor 1978; Stone & Phillips 1981). Whilst there has tended to be miach anecdotal literature concerned with the value of such aids, particularly contact lenses (Bier 1981; Kemmetmfiller 1981), there has been little attempt to quantify their usefulness. In this paper we shall firstly examine the transmission and scattering properties of the iris tissue in a group of Ty neg OCA. Secondly the effects on visual performance of three different artificial iris contact lenses worn by the same group of albinos will be evaluated.
Subjects Individuals belonging to a sub-group of albinism that has no detectable melanin pigment in the eye were chosen for the experiments. The four observers were classified as Ty neg OCA because they lacked the enzyme tyrosinase which is responsible for melanin synthesis (Witkop 1979). Together with other tests (Abadi & Dickinson 1983) a biochemical assay confirmed our subject classification (King and Witkop 1979). In this way we were able to examine a homogeneous hypomelanotic subject group of adults ranging in age from 19-55. Normally pigmented non-albinos acted as control observers for all the experiments. Both the albinos and controls were refracted and wore their appropriate corrections for all psychophysical tasks. None of the albino observers had Snellen visual acuities greater than 6/24. All the normals had Snellen visual acuities of at least 6/6.
Measuring contrast thresholds Sinusoidally modulated gratings were generated on a Telequipment D61a oscilloscope. The average screen luminance was 5 cd m -2 (P31 Phosphor) and the display was masked by a circular aperture subtending 4 ° with the rectangular surround 40 ° x 28 ° of matched colour and luminance. Viewing distance was 114cm. Horizontally rather than vertically orientated gratings were used in order to minimise the degrading effects of the involuntary bilateral conjugate horizontal nystagmus exhibited by all the albino observers (Abadi and Sandikcioglu 1975; Dickinson and Abadi 1985). Two parameters, contrast and spatial frequency, of the test pattern were varied. Contrast was defined as Lmax - Lmin Lmax + Lmin where Lmax and Lmin are the maximum and minimum luminance. Spatial frequency refers to the number of cycles subtending one degree of visual angle. Subjects varied the grating contrast manually by adjusting a potentiometer. Their instruction was to increase the contrast of the pattern from below threshold until they could just detect the grating pattern (ascending method of adjustment). Before recording any measurements, subjects made a series of practice settings. Thresholds were taken as a mean of five settings
Richard V. Abadi, MSc, PhD, FBCO, Eric Papas, BSc, MBCO. UMIST, Department of Ophthalmic Optics, PO Box 88, Manchester M60 1QD, England.
10
11
Visual Performance with Artificial Iris Lenses
for each chosen spatial frequency between 1.0 c/deg and 6.0 c/deg. A limited spatial frequency range was chosen because of the low visual acuity of our albino observers (see Methods: Subjects). Contrast sensitivity was defined as the reciprocal of threshold. Natural pupils were used by all observers. All experiments were carried out under monocular viewing conditions. The discrete light source A lmm 2 circular patch of incandescent light (500 lux) was directed onto the inferior iris midway between the pupil and the limbus at 6 o'clock. The inferior portion was chosen in order to minimise the possibility of the discrete source entering the pupil due to the horizontal nystagmus of the observers. During the experimental sessions the source was always incident on the iris tissue. For safety reasons a Pilkington I.R. absorbing heat filter was introduced between the source and the eye. The Contact Lenses Two soft dyed and one laminate contact lens were used: (a) The Igel Hi-Tint (HT) contact lens. This was a high water content (77 %) soft lens with a full thickness dyed artificial iris (centre thickness 0.2ram, diameter 12mm, colour dark). Its overall size was 14ram and the pupil was circular (3.5mm diameter). (b) The Igel Hi-Colour (HC) Contact Lens. The characteristics of this lens are the same as the Igel HT except that an opaque material was applied to the posterior surface of the artificial iris before the lens was dyed. (c) The Titmus Eurocon Iris Print Lens. This was a laminate lens (38% HEMA) with a centre thickness of 0.35mm, overall diameter 15mm and pupil 3.5mm. The contact lenses were fitted to give minimum movement and to allow the artificial pupil apertures to be well centred with the natural pupil. Spectral transmission characteristics of the iris portions of the contact lenses were determined with a Pye Unlearn SP8-100 dual beam spectrophotometer (Fig. 1). None of the lenses were powered. THE EXPERIMENTS Our primary investigative goal was to compare and quantify the screening effectiveness of three artificial iris contact lenses when a discrete light source was directed on to the iris portion. Contrast thresholds for the grating stimuli were found for the following four test conditions both without and with the glare source in operation. (a) with the natural iris (ie. no contact lens on the eye) (b) with the Hi-Tint contact lens (c) with the Hi--Coloor contact lens (d) with the Iris Print lens.
Figure 2 for one of conditions: and (d) Iris
RESULTS illustrates the contrast sensitivity functions the albino observers under the four test (a) n~.tural iris, (b) Hi-Tint, (c) Hi-Colour print. The open circle symbols (O) indicate
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Fig. 1 The percentage spectral transmission functions for the three contact lenses used in the study. Notice that the Iris print lens completely absorbs all incident radiation between 200 and 80Ohm whilst the Igel Hi-colour and Hi-tint allow some light to be ~ransmit~edo
the visual responses when no light source was directed at the iris. The effect of the 500 lux source on the contrast sensitivity function is shown by the closed circles (0).
R. V. Abadi& E. Papas
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Fig. 2
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The contrast sensitivity curves for a Ty neg oculocutaneous observer under the four test conditions: (a) natural iris, (b) Hi-tint, (c) Hi-colour and (d) Iris-print. (©) -- the visual responses when no light source was directed at the iris. ( 0 ) -- a 500 lux discrete source is directed at the inferior iris midway between the pupil and the limbus at 6 o'clock.
improved matters there appeared to be a hierarchy of gains with the observers performing best with the Hi-Tint lens and poorest with the Hi-Colour lens. An analysis of variance was carried out on the data and revealed statistically significant differences (P<0.001) between the sensitivity ratio function describing the natural iris state and two of the other functions (Igel Hi-Tint and Titmus Iris Print).
For the natural iris condition it is clear that the discrete source markedly reduced the contrast sensitivity at all spatial frequencies (Fig. 2(a)). The light screening effects of the three artificial iris contact lenses are described by the functions seen in Fig 2(b)-(d). All three lenses improved contrast detection. The pooled results for all four albinos are illustrated in Fig. 3. Each point represents the contrast sensitivity ratio (ie. the contrast sensitivity with the glare source divided by the contrast sensitivity without the glare source) for the four test conditions -- natural iris (D), Hi-Tint (O), Hi-Colour (O) and Iris print (11). A contrast sensitivity ratio of 1.0 therefore indicates that the presence of the glare source made no difference to the detection thresholds. Ratios below 1.0 signify that the glare source had a detrimental effect on contrast threshold. It is clear from the family of curves shown in Fig. 3 that the glare source had the greatest disturbing effect for the natural iris test condition. A general trend is also observable in that although all the three types of contact lenses
DISCUSSION The intention of this study was to examine the scattering properties of the albino iris and determine which of three types of commercially available soft artificial iris contact lenses would be able to offer satisfactory shielding and so improve retinal image quality. In the natural state (ie. no contact lens) none of the control observers exhibited any statistically significant reduction in contrast sensitivity when the discrete glare source was directed at their inferior iris. This was not the case for the four albinos. All experienced contrast sensitivity impairment. However with the contact lenses in place visual performances improved. On the assumption that nothing but the light shielding properties of the contact lenses need be considered, the expected ranking of the lenses should be -- Iris print, HiColour and then Hi-Tint, with the Iris print giving the best performance. However, the results of our experiments did not concur with this rank order but showed the Hi-Tint lens to be the most effective. Clearly
13 Visual Performance with Artificial Iris Lenses []
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Fig. 3 Illustrates the pooled data for the four Ty neg oculocutaneous albino observers. The contrast sensitivity ratio contrast sensitivity with glare source contrast sensitivity without glare source is plotted against spatialfrequency for the four test conditions. A contrast sensitivity ratio below 1.0 signifies that the glare source had a detrimental effect on the contrast sensitivity. other factors need to be considered. The contrast sensitivity function In spite of the fact that we constantly direct our attention toward countless spatially complex suprathreshold stimuli surrounding us, visual scientists more often than not examine visual performance at threshold (Overington 1976). This approach has, nevertheless, proved most fruitful and has brought about a greater understanding of the fundamental mechanisms of perception (Kelly & Burbeck 1984). Over the last twenty years or so an increasing number of experiments have made use of the contrast sensitivity function as a measure of how we respond to low contrast targets of varying size (spatial frequency) (Campbell and Green 1965, Abadi 1974, Arden 1978, Woodhouse 1987). Measuring the contrast thresholds of periodic stimuli (eg. sinusoidal gratings) has greatly extended the traditional description of visual performance. Whereas
(c/deg)
the Snellen chart gives a standard measure of resolution at a fixed contrast, the contrast sensitivity function offers the opportunity to explore the relationship between contrast detection and a number of important variables such as spatial frequency, orientation and stimulus complexity. Use of the contrast sensitivity function has also increased in contact lens research (Applegate and Massof 1975, Woo & Hess 1979, Abadi 1980, Bemstein and Brodrick 1981, Guillon, Lydon and Wilson 1983, Grey 1986). In the present investigation we wanted to establish whether retinal image quality was adversely affected by a discrete light source directed on to the iris of albinos. Collecting visual responses at or just above threshold offered a sensitive method for investigating the quality of visual performance. Although we have described changes in the absolute contrast sensitivity function (Fig. 2) we felt a far better index would be a description of relative measures of visual performance in the form of contrast sensitivity ratios (Fig. 3). Making reliable psychophysical threshold judgements can be a difficult task for naive observers. To minimise the experimental time we arranged that the threshold would be found by the method of adjustment. This required the observer to continuously stare at the screen whilst the contrast was increased until the gratings were detected. All our albinos exhibited an involuntary bilateral, conjugate, congenital nystagmus. The oscillation was in
14
R.V. Abadi & E. Papas
the horizontal plane and waveform characteristics such as the amplitude, frequency and waveform shape were different in all four albinos. Past studies have shown that the constancy of the oscillations are often greatly affected by attention, stress and fatigue (Abadi and Dickinson 1987) which in turn differentially affect visual performance. During the experiments we attempted to minimise these effects by presenting the different spatial frequencies randomly, aligning the grating pattern parallel to the plane of the nystagmus and by always making the measurements with the discrete glare source after the no glare run in order to avoid changes in retinal adaptation. The characteristics of the contact lens The present study used lenses made from H E M A material with water contents of either 77% (Igel lens) or 38% (Titmus lens). All observers adapted to the lenses for at least 15 minutes before starting the experiment and no single experiment lasted more than 45 minutes. Even though the time taken for each experimental run was short and the minimum period of time between runs with the different lenses was always greater than 2 hours, the distinct oxygen transmissibility of each of the lenses may well have contributed to the intra contact lens differences in performance. Indeed, a recent review by Goldberg and Egan (1984) emphasised the need to exercise caution when the results of independent contact lens related psychophysical studies are compared. In further support, Grey (1986) reported that the contrast sensitivity when wearing soft lenses was influenced by their water content such that low water content soft lenses gave a greater reduction in contrast sensitivity than high water content lenses. The Igel and Titmus lenses used in our study were not only of different water content but of different thicknesses. Since corneal swelling has shown to be directly related to both these lens characteristics it is not unlikely that the performance of the Titmus iris print lens may have been adversely affected by its thickness and water content. Consequently any oedema produced would be disadvantageous in that it should increase the likelihood of the cornea introducing additional scatter of the test glare source used in our experiments. Finally, it is also possible that the optical qualities of each of the lenses may have contributed to the results. A direct method of describing the image performance of any optical system such as a contact lens, is to measure its modulation transfer function (Charman 1983). Previous studies (Charman 1979, Charman and Walsh 1986) suggested that so long as a lens remains centred there should be no significant degradation of the image quality. With this in mind we ensured that, for all three lenses used in our study, the natural pupil and the artificial iris apertures were aligned and that the involuntary nystagmus did not introduce any significant increase in the lens mobility. Furthermore, the spatial frequencies below 6.0 c/deg for which we examined contrast detection are minimally influenced by the degrading
effects of optical blur (Charman 1983). The Physiology of the Eye In the previous section it was pointed out that there was a need to consider the induced physiological changes in corneal function such as oedema. None of the observers exhibited gross corneal changes although on occasions some observers experienced short-term blur after removal of the iris print lenses. This may have been due to intersubject variability in the minimum levels of atmospheric oxygen required by the cornea (Holden, Sweeney, Sanderson 1984; Holden and Mertz 1984). When the eyeball of a Ty neg albino is transilluminated the iris becomes translucent. Whilst all our observers exhibited this feature there was a noticeable difference in its extent. One may therefore expect some relationship between the level of iris translucency and the contrast threshold when the discrete glare source was directed on to the iris. Unfortunately quantification of the iris hypopigmentation is fraught with uncertainties (Wirtschafter, Denslow and Shine 1973). However we felt that the level of the iris translucency was a reasonable index for ranking the scattering effects of the iris. In addition, the results of our experiments suggested that the effects of the glare source directed on to the iris gave rise to a similar degree of inter-subject variability. Theoretically the scattering properties of the iris tissue are related to the density and distribution of the pigment cells and the fibre size and the arrangement of the stromal collagen. In 1978 a histological study of a Ty neg O C A human eye reported the complete absence of any iris stromal melanocytes or pigmental epithelial cells (Fulton, Albert and Craft 1978). This would conceivably argue for some degree of uniformity in the scattering properties of the Ty neg OCA. However a recent electron microscopy report (McCartney, Spalton and Bull 1985) described the presence of type IV mature melanosomes (normal pigment cells) in an iris biopsy specimen from a Ty neg oculocutaneous albino. The presence of such may well explain some of the variability seen in both iris transillumination and the contrast thresholds for our hypomelanotic group of observers. CLINICAL ISSUES History has shown that many adult albinos who suffer from the unwanted effects of light are quite happy to wear spectacle mounted tints. Whilst such a solution is satisfactory in an adult it may well not be a practical proposition for infants. Contact lenses, and in particular the scleral lens, have been advocated by practitioners in the past. Recently the soft artificial iris print lens has become a viable alternative. Factors which will influence the choice and ultimately the acceptance of the contact lenses include cost, availability and comfort. Interestingly, previous research has indicated that the albino population has high incidences of considerable degrees of astigmatism (Taylor 1978, Dickinson and Abadi 1984). It is predominantly with-the-rule and corneal in origin. Whilst optimal visual correction should
Visual Performance with Artificial Iris Lenses
theoretically be more likely achieved by practitioners using toric rather than spherical design lenses, this may not always be borne out in practice. Acceptable centration should be achievable with either lens design. In conclusion, this study has demonstrated that if the major concern of the practitioner is to minimise light transmission and scatter through the natural iris of a Ty neg OCA and there appears to be no specific advantage of the iris print lens, then high water content dyed H E M A lenses would seem to fit the bill admirably. ACKNOWLEDGEMENTS We would like to thank Igel and Titmus Eurocon for their kind donation of all the lenses used in this study. Ellen, John, Eve and Ralph are also thanked for their most helpful comments and advice, as is Janice for typing the manuscript. REFERENCES Abadi, RV (1974) Visual analysis with gratings. BrJPhysiol Optics29, 49-56. Abadi, RV (1980) Visual performance with contact lenses and congenital idiopathic nystagmus. Br J Physiol Optics 33, 3, 32-37. Abadi, RV & Dickinson, CM (1983} Monochromatic fundus photography of human albinos. Arch Ophthalmo1101, 1706-1711. Abadi, RV & Dickinson, CM (1987) Waveform characteristics in congenital nystagmus. Doc Ophthalmo164, 153-167. Applegate, RA & Massof, RW (1975) Changes in contrast sensitivity function induced by contact lens wear. A m J Optom & Physiol Opt 52, 840-846. Arden, GB (1978) The importance of measuring contrast sensitivity in cases of visual disturbance, Brit J Ophthalmo162, 198-209. Bernstein, IH & Brodrick, J (1981) Contrast sensitivities through spectacles and soft contact lenses. A m J Optom & Physiol Opt 58, 309-313. Bier, N (1981) Albinism. International Contact Lens Clinic, Sept/ October 10-15. Bier, N & Lowther, GE (1977) Contact lens correction. Butterworth Press. Campbell, FW & Green, DG (1965) Optical and retinal factors affecting visual resolution. J Physiol Lond 181, 576-593. Charman, WN (1979) Effect of refractive error in visual tests with sinusoidal gratings. Brit J Physiol Optics 33, 2, 10-19.
15
Charman, WN (1983) The retinal image in the human eye. In: Progress in retinal research (2), 1-50, Pergamon Press. Eds. N Osbourne and G Chader. Charman, WN & Walsh, G (1986) Retinal image quality with different designs of bifocal contact lens. Trans BCLA Conference 13-19. Dickinson, CM & Abadi, RV (1984) Corneal topography of humans with congenital nystagmus. OphthalPhysiol Opt4, 3-13. Dickinson, CM & Abadi, RV (1985) The influence of nystagmoid oscillation on contrast sensitivity in normal observers. Vision Res 25, 1089-1096. Goldberg, SM & Egan, DJ (1984) The effects of contact lens wear on contrast sensitivity. Can J Optom 46, 179-184. Grey, CP (1986) Changes in contrast sensitivity when wearing low, medium and high water content soft contact lenses. J Brit Contact Lens Assoc 9, 21-25. Guillon, M, Lydon, DPM & Wilson, C (1983) Variations in contrast sensitivity function with spectacles and contact lenses. JBrit Contact Lens Assoc 6, 120-124. Holden, BA, Sweeney, DF & Sanderson, G (1984) The minimum precorneal oxygen tension to avoid corneal oedema. Invest Ophthalmo125, 476-480. Holden, BA & Mertz, GW (1984) Critical oxygen levels to avoid corneal oedema for daily and extended wear contact lenses. Invest Ophthalmol25, 1161-1167. Kelly, DH & Burbeck, CA (1984) Critical problems in spatial vision. CRC Crit Rev Biomed Engng 10, 125-177. Kemmetmiiller, H (1981) The optical aid in albinism. The ContactLens JournallO, 5, 2-9. King, R & Witkop, CJ Jr (1976) Hairbulb tyrosinase activity in oculocutaneous albinism. Nature 263, 69-71. Kinnear, PE, Jay, B & Witkop, CJ (1985) Albinism. Surv Ophthalmol 30, 75-101. McCartney, ACE, Spalton, DJ & Bull, TB (1985) Type IV melanosomes of the human albino iris. Brit J Ophthalmol69, 537541. Overington, (1976) Vision and Acquisition. Pentech Press, London. Stone, J & Phillips, AJ (1981) Contact Lenses, Butterworth Press. Taylor, WOG (1978) Visual disabilities of oculocutaeous albinism and their alleviation. Trans Ophthal Soc UK 98, 423-445. Wirtshafter, JD, Denslow, GT & Shine, MD (1973) Quantification of iris translucency in albinism. Arch Ophthalmol90, 274-277. Witkop, CJ (1979) Depigmentations of the general and oral tissues and genetic foundations. Ala J Med Sci 16, 331-343. Witkop, CJ Jr, Quevedo, WC Jr, Fitzpatrick, TB (1983) Albinism and other disorders of pigment metabolism. In Stanbury, JB, Wyngaarden, JB, Fredrickson, DS (eds). The Metabolic Basis of Inherited Disease, pp301-346.5th Edition McGraw Hilt. Woo, GCS & Hess, RF (1979) Contrast sensitivity function and soft lenses. Int Contact Lens Clinic 6, 171-176. Woodhouse, JM (1987) Contrast sensitivity measurement. The Optician January 9th, 19-26.
Vol. 10. No. 2, pp. 10 - 15, 1987
@1987 by the British Contact Lens Assoc.
Printed in Great Britain
VISUAL PERFORMANCE WITH ARTIFICIAL IRIS CONTACT LENSES RICHARD V . ABADI, ERIC PAPAS
(Received 5 June 1987, in revisedform 20 July 1987) Abstract - - O n e of the functions of the iris is to control retinal irradiance. Whilst this is the case in most individuals, albinos, due to a lack of ocular pigment found in their iris, have an inadequate pigment screen. This study was designed to examine and quantify the scattering properties of the iris in four Tyrosinase negative oculocutaneous albinos by measuring contrast detection when a small discrete light source was directed at the inferior iris. Three commercially available artificial iris contact lenses were then evaluated to determine h o w well they screened the retina from the source. T h e results suggest that a high water content dyed H E M A contact tens is as effective at shielding the iris as an Iris print lens.
METHODS
Visual performance is primarily dependent on the quality of the retinal image (Charman 1983). Of the many ocular characteristics which can influence the imaging characteristics of the eye, the role of the iris diaphragm is not insignificant. Ideally, light should only pass through the pupillary aperture. For normally pigmented individuals the extraneous light is screened out by the pigment in the uveal tract and the retinal epithelium; these two sites being responsible for the control of retinal illuminance and the minimisation of internal light scatter. Clearly inefficient screening will give rise to a reduction in the retinal image contrast and a subsequent degradation o f visual performance. Such a state of affairs exists for the albino eye. Albinism is not a single entity but comprises a group of conditions (Witkop 1979; Witkop, Quevedo and Fitzpatrick 1983). Affected individuals exhibit varying degrees of hypomelanosis of the skin, hair and eyes (Taylor 1978; Abadi and Dickinson 1983; Kinnear, Jay and Witkop 1985). In the eye, pigment is deficient in the uveal tract and the retinal epithelium. One clinical manifestation of this lack of pigment is the existence of iris transil!umination and many albinos experience discomfort and disability even under quite modest light levels. This is particularly the case for the tyrosinase negative oculocutaneous albino (Ty neg OCA) who are purported to have no melanin whatsoever. Not surprisingly, clinicians have advocated shielding the albino retina from the unwanted excess radiation with either tinted spectacles or cosmetic contact lenses (Bier & Lowther 1977; Taylor 1978; Stone & Phillips 1981). Whilst there has tended to be miach anecdotal literature concerned with the value of such aids, particularly contact lenses (Bier 1981; Kemmetmfiller 1981), there has been little attempt to quantify their usefulness. In this paper we shall firstly examine the transmission and scattering properties of the iris tissue in a group of Ty neg OCA. Secondly the effects on visual performance of three different artificial iris contact lenses worn by the same group of albinos will be evaluated.
Subjects Individuals belonging to a sub-group of albinism that has no detectable melanin pigment in the eye were chosen for the experiments. The four observers were classified as Ty neg OCA because they lacked the enzyme tyrosinase which is responsible for melanin synthesis (Witkop 1979). Together with other tests (Abadi & Dickinson 1983) a biochemical assay confirmed our subject classification (King and Witkop 1979). In this way we were able to examine a homogeneous hypomelanotic subject group of adults ranging in age from 19-55. Normally pigmented non-albinos acted as control observers for all the experiments. Both the albinos and controls were refracted and wore their appropriate corrections for all psychophysical tasks. None of the albino observers had Snellen visual acuities greater than 6/24. All the normals had Snellen visual acuities of at least 6/6.
Measuring contrast thresholds Sinusoidally modulated gratings were generated on a Telequipment D61a oscilloscope. The average screen luminance was 5 cd m -2 (P31 Phosphor) and the display was masked by a circular aperture subtending 4 ° with the rectangular surround 40 ° x 28 ° of matched colour and luminance. Viewing distance was 114cm. Horizontally rather than vertically orientated gratings were used in order to minimise the degrading effects of the involuntary bilateral conjugate horizontal nystagmus exhibited by all the albino observers (Abadi and Sandikcioglu 1975; Dickinson and Abadi 1985). Two parameters, contrast and spatial frequency, of the test pattern were varied. Contrast was defined as Lmax - Lmin Lmax + Lmin where Lmax and Lmin are the maximum and minimum luminance. Spatial frequency refers to the number of cycles subtending one degree of visual angle. Subjects varied the grating contrast manually by adjusting a potentiometer. Their instruction was to increase the contrast of the pattern from below threshold until they could just detect the grating pattern (ascending method of adjustment). Before recording any measurements, subjects made a series of practice settings. Thresholds were taken as a mean of five settings
Richard V. Abadi, MSc, PhD, FBCO, Eric Papas, BSc, MBCO. UMIST, Department of Ophthalmic Optics, PO Box 88, Manchester M60 1QD, England.
10
11
Visual Performance with Artificial Iris Lenses
for each chosen spatial frequency between 1.0 c/deg and 6.0 c/deg. A limited spatial frequency range was chosen because of the low visual acuity of our albino observers (see Methods: Subjects). Contrast sensitivity was defined as the reciprocal of threshold. Natural pupils were used by all observers. All experiments were carried out under monocular viewing conditions. The discrete light source A lmm 2 circular patch of incandescent light (500 lux) was directed onto the inferior iris midway between the pupil and the limbus at 6 o'clock. The inferior portion was chosen in order to minimise the possibility of the discrete source entering the pupil due to the horizontal nystagmus of the observers. During the experimental sessions the source was always incident on the iris tissue. For safety reasons a Pilkington I.R. absorbing heat filter was introduced between the source and the eye. The Contact Lenses Two soft dyed and one laminate contact lens were used: (a) The Igel Hi-Tint (HT) contact lens. This was a high water content (77 %) soft lens with a full thickness dyed artificial iris (centre thickness 0.2ram, diameter 12mm, colour dark). Its overall size was 14ram and the pupil was circular (3.5mm diameter). (b) The Igel Hi-Colour (HC) Contact Lens. The characteristics of this lens are the same as the Igel HT except that an opaque material was applied to the posterior surface of the artificial iris before the lens was dyed. (c) The Titmus Eurocon Iris Print Lens. This was a laminate lens (38% HEMA) with a centre thickness of 0.35mm, overall diameter 15mm and pupil 3.5mm. The contact lenses were fitted to give minimum movement and to allow the artificial pupil apertures to be well centred with the natural pupil. Spectral transmission characteristics of the iris portions of the contact lenses were determined with a Pye Unlearn SP8-100 dual beam spectrophotometer (Fig. 1). None of the lenses were powered. THE EXPERIMENTS Our primary investigative goal was to compare and quantify the screening effectiveness of three artificial iris contact lenses when a discrete light source was directed on to the iris portion. Contrast thresholds for the grating stimuli were found for the following four test conditions both without and with the glare source in operation. (a) with the natural iris (ie. no contact lens on the eye) (b) with the Hi-Tint contact lens (c) with the Hi--Coloor contact lens (d) with the Iris Print lens.
Figure 2 for one of conditions: and (d) Iris
RESULTS illustrates the contrast sensitivity functions the albino observers under the four test (a) n~.tural iris, (b) Hi-Tint, (c) Hi-Colour print. The open circle symbols (O) indicate
100
80
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60
40
20
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60
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Titmus
iris print
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40
20
0
I
200
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I
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/
600
I
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800
Wavelength (nm)
Fig. 1 The percentage spectral transmission functions for the three contact lenses used in the study. Notice that the Iris print lens completely absorbs all incident radiation between 200 and 80Ohm whilst the Igel Hi-colour and Hi-tint allow some light to be ~ransmit~edo
the visual responses when no light source was directed at the iris. The effect of the 500 lux source on the contrast sensitivity function is shown by the closed circles (0).
R. V. Abadi& E. Papas
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Fig. 2
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The contrast sensitivity curves for a Ty neg oculocutaneous observer under the four test conditions: (a) natural iris, (b) Hi-tint, (c) Hi-colour and (d) Iris-print. (©) -- the visual responses when no light source was directed at the iris. ( 0 ) -- a 500 lux discrete source is directed at the inferior iris midway between the pupil and the limbus at 6 o'clock.
improved matters there appeared to be a hierarchy of gains with the observers performing best with the Hi-Tint lens and poorest with the Hi-Colour lens. An analysis of variance was carried out on the data and revealed statistically significant differences (P<0.001) between the sensitivity ratio function describing the natural iris state and two of the other functions (Igel Hi-Tint and Titmus Iris Print).
For the natural iris condition it is clear that the discrete source markedly reduced the contrast sensitivity at all spatial frequencies (Fig. 2(a)). The light screening effects of the three artificial iris contact lenses are described by the functions seen in Fig 2(b)-(d). All three lenses improved contrast detection. The pooled results for all four albinos are illustrated in Fig. 3. Each point represents the contrast sensitivity ratio (ie. the contrast sensitivity with the glare source divided by the contrast sensitivity without the glare source) for the four test conditions -- natural iris (D), Hi-Tint (O), Hi-Colour (O) and Iris print (11). A contrast sensitivity ratio of 1.0 therefore indicates that the presence of the glare source made no difference to the detection thresholds. Ratios below 1.0 signify that the glare source had a detrimental effect on contrast threshold. It is clear from the family of curves shown in Fig. 3 that the glare source had the greatest disturbing effect for the natural iris test condition. A general trend is also observable in that although all the three types of contact lenses
DISCUSSION The intention of this study was to examine the scattering properties of the albino iris and determine which of three types of commercially available soft artificial iris contact lenses would be able to offer satisfactory shielding and so improve retinal image quality. In the natural state (ie. no contact lens) none of the control observers exhibited any statistically significant reduction in contrast sensitivity when the discrete glare source was directed at their inferior iris. This was not the case for the four albinos. All experienced contrast sensitivity impairment. However with the contact lenses in place visual performances improved. On the assumption that nothing but the light shielding properties of the contact lenses need be considered, the expected ranking of the lenses should be -- Iris print, HiColour and then Hi-Tint, with the Iris print giving the best performance. However, the results of our experiments did not concur with this rank order but showed the Hi-Tint lens to be the most effective. Clearly
13 Visual Performance with Artificial Iris Lenses []
Natural
Iris
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Fig. 3 Illustrates the pooled data for the four Ty neg oculocutaneous albino observers. The contrast sensitivity ratio contrast sensitivity with glare source contrast sensitivity without glare source is plotted against spatialfrequency for the four test conditions. A contrast sensitivity ratio below 1.0 signifies that the glare source had a detrimental effect on the contrast sensitivity. other factors need to be considered. The contrast sensitivity function In spite of the fact that we constantly direct our attention toward countless spatially complex suprathreshold stimuli surrounding us, visual scientists more often than not examine visual performance at threshold (Overington 1976). This approach has, nevertheless, proved most fruitful and has brought about a greater understanding of the fundamental mechanisms of perception (Kelly & Burbeck 1984). Over the last twenty years or so an increasing number of experiments have made use of the contrast sensitivity function as a measure of how we respond to low contrast targets of varying size (spatial frequency) (Campbell and Green 1965, Abadi 1974, Arden 1978, Woodhouse 1987). Measuring the contrast thresholds of periodic stimuli (eg. sinusoidal gratings) has greatly extended the traditional description of visual performance. Whereas
(c/deg)
the Snellen chart gives a standard measure of resolution at a fixed contrast, the contrast sensitivity function offers the opportunity to explore the relationship between contrast detection and a number of important variables such as spatial frequency, orientation and stimulus complexity. Use of the contrast sensitivity function has also increased in contact lens research (Applegate and Massof 1975, Woo & Hess 1979, Abadi 1980, Bemstein and Brodrick 1981, Guillon, Lydon and Wilson 1983, Grey 1986). In the present investigation we wanted to establish whether retinal image quality was adversely affected by a discrete light source directed on to the iris of albinos. Collecting visual responses at or just above threshold offered a sensitive method for investigating the quality of visual performance. Although we have described changes in the absolute contrast sensitivity function (Fig. 2) we felt a far better index would be a description of relative measures of visual performance in the form of contrast sensitivity ratios (Fig. 3). Making reliable psychophysical threshold judgements can be a difficult task for naive observers. To minimise the experimental time we arranged that the threshold would be found by the method of adjustment. This required the observer to continuously stare at the screen whilst the contrast was increased until the gratings were detected. All our albinos exhibited an involuntary bilateral, conjugate, congenital nystagmus. The oscillation was in
14
R.V. Abadi & E. Papas
the horizontal plane and waveform characteristics such as the amplitude, frequency and waveform shape were different in all four albinos. Past studies have shown that the constancy of the oscillations are often greatly affected by attention, stress and fatigue (Abadi and Dickinson 1987) which in turn differentially affect visual performance. During the experiments we attempted to minimise these effects by presenting the different spatial frequencies randomly, aligning the grating pattern parallel to the plane of the nystagmus and by always making the measurements with the discrete glare source after the no glare run in order to avoid changes in retinal adaptation. The characteristics of the contact lens The present study used lenses made from H E M A material with water contents of either 77% (Igel lens) or 38% (Titmus lens). All observers adapted to the lenses for at least 15 minutes before starting the experiment and no single experiment lasted more than 45 minutes. Even though the time taken for each experimental run was short and the minimum period of time between runs with the different lenses was always greater than 2 hours, the distinct oxygen transmissibility of each of the lenses may well have contributed to the intra contact lens differences in performance. Indeed, a recent review by Goldberg and Egan (1984) emphasised the need to exercise caution when the results of independent contact lens related psychophysical studies are compared. In further support, Grey (1986) reported that the contrast sensitivity when wearing soft lenses was influenced by their water content such that low water content soft lenses gave a greater reduction in contrast sensitivity than high water content lenses. The Igel and Titmus lenses used in our study were not only of different water content but of different thicknesses. Since corneal swelling has shown to be directly related to both these lens characteristics it is not unlikely that the performance of the Titmus iris print lens may have been adversely affected by its thickness and water content. Consequently any oedema produced would be disadvantageous in that it should increase the likelihood of the cornea introducing additional scatter of the test glare source used in our experiments. Finally, it is also possible that the optical qualities of each of the lenses may have contributed to the results. A direct method of describing the image performance of any optical system such as a contact lens, is to measure its modulation transfer function (Charman 1983). Previous studies (Charman 1979, Charman and Walsh 1986) suggested that so long as a lens remains centred there should be no significant degradation of the image quality. With this in mind we ensured that, for all three lenses used in our study, the natural pupil and the artificial iris apertures were aligned and that the involuntary nystagmus did not introduce any significant increase in the lens mobility. Furthermore, the spatial frequencies below 6.0 c/deg for which we examined contrast detection are minimally influenced by the degrading
effects of optical blur (Charman 1983). The Physiology of the Eye In the previous section it was pointed out that there was a need to consider the induced physiological changes in corneal function such as oedema. None of the observers exhibited gross corneal changes although on occasions some observers experienced short-term blur after removal of the iris print lenses. This may have been due to intersubject variability in the minimum levels of atmospheric oxygen required by the cornea (Holden, Sweeney, Sanderson 1984; Holden and Mertz 1984). When the eyeball of a Ty neg albino is transilluminated the iris becomes translucent. Whilst all our observers exhibited this feature there was a noticeable difference in its extent. One may therefore expect some relationship between the level of iris translucency and the contrast threshold when the discrete glare source was directed on to the iris. Unfortunately quantification of the iris hypopigmentation is fraught with uncertainties (Wirtschafter, Denslow and Shine 1973). However we felt that the level of the iris translucency was a reasonable index for ranking the scattering effects of the iris. In addition, the results of our experiments suggested that the effects of the glare source directed on to the iris gave rise to a similar degree of inter-subject variability. Theoretically the scattering properties of the iris tissue are related to the density and distribution of the pigment cells and the fibre size and the arrangement of the stromal collagen. In 1978 a histological study of a Ty neg O C A human eye reported the complete absence of any iris stromal melanocytes or pigmental epithelial cells (Fulton, Albert and Craft 1978). This would conceivably argue for some degree of uniformity in the scattering properties of the Ty neg OCA. However a recent electron microscopy report (McCartney, Spalton and Bull 1985) described the presence of type IV mature melanosomes (normal pigment cells) in an iris biopsy specimen from a Ty neg oculocutaneous albino. The presence of such may well explain some of the variability seen in both iris transillumination and the contrast thresholds for our hypomelanotic group of observers. CLINICAL ISSUES History has shown that many adult albinos who suffer from the unwanted effects of light are quite happy to wear spectacle mounted tints. Whilst such a solution is satisfactory in an adult it may well not be a practical proposition for infants. Contact lenses, and in particular the scleral lens, have been advocated by practitioners in the past. Recently the soft artificial iris print lens has become a viable alternative. Factors which will influence the choice and ultimately the acceptance of the contact lenses include cost, availability and comfort. Interestingly, previous research has indicated that the albino population has high incidences of considerable degrees of astigmatism (Taylor 1978, Dickinson and Abadi 1984). It is predominantly with-the-rule and corneal in origin. Whilst optimal visual correction should
Visual Performance with Artificial Iris Lenses
theoretically be more likely achieved by practitioners using toric rather than spherical design lenses, this may not always be borne out in practice. Acceptable centration should be achievable with either lens design. In conclusion, this study has demonstrated that if the major concern of the practitioner is to minimise light transmission and scatter through the natural iris of a Ty neg OCA and there appears to be no specific advantage of the iris print lens, then high water content dyed H E M A lenses would seem to fit the bill admirably. ACKNOWLEDGEMENTS We would like to thank Igel and Titmus Eurocon for their kind donation of all the lenses used in this study. Ellen, John, Eve and Ralph are also thanked for their most helpful comments and advice, as is Janice for typing the manuscript. REFERENCES Abadi, RV (1974) Visual analysis with gratings. BrJPhysiol Optics29, 49-56. Abadi, RV (1980) Visual performance with contact lenses and congenital idiopathic nystagmus. Br J Physiol Optics 33, 3, 32-37. Abadi, RV & Dickinson, CM (1983} Monochromatic fundus photography of human albinos. Arch Ophthalmo1101, 1706-1711. Abadi, RV & Dickinson, CM (1987) Waveform characteristics in congenital nystagmus. Doc Ophthalmo164, 153-167. Applegate, RA & Massof, RW (1975) Changes in contrast sensitivity function induced by contact lens wear. A m J Optom & Physiol Opt 52, 840-846. Arden, GB (1978) The importance of measuring contrast sensitivity in cases of visual disturbance, Brit J Ophthalmo162, 198-209. Bernstein, IH & Brodrick, J (1981) Contrast sensitivities through spectacles and soft contact lenses. A m J Optom & Physiol Opt 58, 309-313. Bier, N (1981) Albinism. International Contact Lens Clinic, Sept/ October 10-15. Bier, N & Lowther, GE (1977) Contact lens correction. Butterworth Press. Campbell, FW & Green, DG (1965) Optical and retinal factors affecting visual resolution. J Physiol Lond 181, 576-593. Charman, WN (1979) Effect of refractive error in visual tests with sinusoidal gratings. Brit J Physiol Optics 33, 2, 10-19.
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Charman, WN (1983) The retinal image in the human eye. In: Progress in retinal research (2), 1-50, Pergamon Press. Eds. N Osbourne and G Chader. Charman, WN & Walsh, G (1986) Retinal image quality with different designs of bifocal contact lens. Trans BCLA Conference 13-19. Dickinson, CM & Abadi, RV (1984) Corneal topography of humans with congenital nystagmus. OphthalPhysiol Opt4, 3-13. Dickinson, CM & Abadi, RV (1985) The influence of nystagmoid oscillation on contrast sensitivity in normal observers. Vision Res 25, 1089-1096. Goldberg, SM & Egan, DJ (1984) The effects of contact lens wear on contrast sensitivity. Can J Optom 46, 179-184. Grey, CP (1986) Changes in contrast sensitivity when wearing low, medium and high water content soft contact lenses. J Brit Contact Lens Assoc 9, 21-25. Guillon, M, Lydon, DPM & Wilson, C (1983) Variations in contrast sensitivity function with spectacles and contact lenses. JBrit Contact Lens Assoc 6, 120-124. Holden, BA, Sweeney, DF & Sanderson, G (1984) The minimum precorneal oxygen tension to avoid corneal oedema. Invest Ophthalmo125, 476-480. Holden, BA & Mertz, GW (1984) Critical oxygen levels to avoid corneal oedema for daily and extended wear contact lenses. Invest Ophthalmol25, 1161-1167. Kelly, DH & Burbeck, CA (1984) Critical problems in spatial vision. CRC Crit Rev Biomed Engng 10, 125-177. Kemmetmiiller, H (1981) The optical aid in albinism. The ContactLens JournallO, 5, 2-9. King, R & Witkop, CJ Jr (1976) Hairbulb tyrosinase activity in oculocutaneous albinism. Nature 263, 69-71. Kinnear, PE, Jay, B & Witkop, CJ (1985) Albinism. Surv Ophthalmol 30, 75-101. McCartney, ACE, Spalton, DJ & Bull, TB (1985) Type IV melanosomes of the human albino iris. Brit J Ophthalmol69, 537541. Overington, (1976) Vision and Acquisition. Pentech Press, London. Stone, J & Phillips, AJ (1981) Contact Lenses, Butterworth Press. Taylor, WOG (1978) Visual disabilities of oculocutaeous albinism and their alleviation. Trans Ophthal Soc UK 98, 423-445. Wirtshafter, JD, Denslow, GT & Shine, MD (1973) Quantification of iris translucency in albinism. Arch Ophthalmol90, 274-277. Witkop, CJ (1979) Depigmentations of the general and oral tissues and genetic foundations. Ala J Med Sci 16, 331-343. Witkop, CJ Jr, Quevedo, WC Jr, Fitzpatrick, TB (1983) Albinism and other disorders of pigment metabolism. In Stanbury, JB, Wyngaarden, JB, Fredrickson, DS (eds). The Metabolic Basis of Inherited Disease, pp301-346.5th Edition McGraw Hilt. Woo, GCS & Hess, RF (1979) Contrast sensitivity function and soft lenses. Int Contact Lens Clinic 6, 171-176. Woodhouse, JM (1987) Contrast sensitivity measurement. The Optician January 9th, 19-26.