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Changes in aniseikonia of an axial anisometrope at various stages of orthokeratology lens wear
Contact Lens and Anterior Eye xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Contact Lens and Anterior Eye journal homepage: www.elsevier.com/locate/clae
Changes in aniseikonia of an axial anisometrope at various stages of orthokeratology lens wear Singson Chi Keong Kwana,b, Sieu K. Khuub, Pauline Kangb,* a b
Ng Teng Fong General Hospital, National University Health System, Singapore School of Optometry and Vision Science, University of New South Wales, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Orthokeratology Aniseikonia Anisometropia Myopia
Purpose: To report a case of reduced aniseikonia in a myopic axial anisometrope during orthokeratology (OK) lens wear. Case report: A 19-year-old female university student with myopic anisometropia presented for an OK lens fitting consultation. At baseline, perceptual interocular image size difference or aniseikonia of 1.45% was found, with a smaller image seen by the right eye compared to the left eye. The patient was fitted with a pair of OK lenses and interocular image size differences, subjective refraction and corneal topography were measured after 7, 14 and 47 days of overnight lens wear. Hyperopic shifts in central refraction and corresponding flattening of the central cornea was measured during OK treatment. Aniseikonia reduced after OK wear with the most significant change measured from baseline to day 7. Aniseikonia recorded after 7, 14 and 47 nights of lens wear was 0.05%, 0.35% and 0.85%, respectively. Although minimum differences in refractive error between eyes was reached after 47 days of OK, aniseikonia was greater than that measured after 7 and 14 days of OK. Conclusion: This case report demonstrates reduction in aniseikonia with OK lens wear in a myopic patient with axial anisometropia, although this effect was not sustained beyond 1 week of lens wear. As the impact of corneal curvature changes on aniseikonia is not well understood, future studies on the impact of OK on aniseikonia and associated asthenopia is required.
1. Introduction Aniseikonia refers to a difference in the optical image size seen by the two eyes [1]. Patients with aniseikonia may present with symptoms such as asthenopia, headaches [2] and vertical diplopia at near [3]. Perceptual image size differences between the two eyes of up to 5% can significantly reduce stereopsis [4,5] and binocular contrast sensitivity [6]. Aniseikonia occurs in most patients with anisometropia that is mainly axial in nature [4,5,7]. Previous studies investigating the development of anisometropia have reported that the magnitude of anisometropia increases with age during childhood and adolescence, until relative stability is reached at middle age (30–50 years). A marked increase in prevalence and has been found beyond 60 years of age [8]. Large amounts of anisometropia due to unilateral aphakia surgery can also induce aniseikonia [4,9]. The prevalence of anisometropia is approximately 3–4% when the criterion of interocular dioptric difference of 2D or above is used [10]. Although Knapp’s Law suggests axial anisometropia is best corrected with spectacles to minimise the aniseikonia, clinical studies show
⁎
contradicting results [7,11]. Therefore, contact lenses are widely used to correct for anisometropic refractive errors [12,13] due to the effectiveness in reducing aniseikonia compared to spectacles [7,11]. Although orthokeratology (OK) lenses also corrects refractive error at the corneal plane, its usage in correcting anisometropia and impact on aniseikonia have not been investigated. To our knowledge, no previous study has documented the effect of OK on interocular perceptual image size differences. This case report describes changes in aniseikonia of a myopic patient with axial anisometropia during the course of OK treatment. It offers insight into the feasibility of using OK as an alternative option for correcting anisometropes with symptomatic aniseikonia. 2. Case report A 19-year-old female university student was seen at our research clinic for an OK lens fitting consultation in March 2018. She spent approximately 2–3 hours per day on reading and computer use, and approximately 4 h of outdoor activities. She wore two different types of
Corresponding author. Present address: School of Optometry and Vision Science, University of New South Wales, 2052, Australia. E-mail address: [email protected] (P. Kang).
https://doi.org/10.1016/j.clae.2019.10.135 Received 16 January 2019; Received in revised form 4 September 2019; Accepted 20 October 2019 1367-0484/ © 2019 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: Singson Chi Keong Kwan, Sieu K. Khuu and Pauline Kang, Contact Lens and Anterior Eye, https://doi.org/10.1016/j.clae.2019.10.135
Contact Lens and Anterior Eye xxx (xxxx) xxx–xxx
S.C.K. Kwan, et al.
2.3. Corneal topography
daily disposable soft contact lenses (Aisei disposable daily colour lens, Aisei Co. Ltd., Japan and SEED daily lens, SEED Co. Ltd., Japan) once a week, for approximately 9 h each wear. When spectacles were worn, she experienced visual discomfort which remained relatively constant during both distance and near viewing, and felt there was a “difference” between her eyes. From a young age, such symptoms were apparent with various pairs of spectacles and reduction of symptoms was experienced with soft contact lens wear. To assess suitability for OK treatment, refraction and corneal topography were assessed as detailed below. Binocular visual function was also measured before OK lens wear only; with subjective refraction corrected with lenses in a trial frame, cover test indicated orthophoria at distance and 4 exophoria at near, and stereoacuity of 40″ was measured using the Randot Stereo test. Using the RAF rule, near point of convergence was less than 5 cm and amplitude of accommodation was 11D for the right eye, left eye and both eyes. Horizontal visible iris diameter was 11 mm in both eyes and pupil size was 4.6 mm in the right eye and 4.3 mm in the left eye measured under dim room light by the Medmont E300 corneal topographer (Medmont; Melbourne, Australia). Anterior ocular health was assessed at baseline and all subsequent visits using the slit-lamp biomicroscope. With good ocular health, mild to moderate degrees of myopic refractive error and regular corneal topography, the patient was deemed suitable for OK and fitted with a pair of Paragon CRT lenses (Paragon Vision Sciences, USA) for myopia correction. CRT lenses were selected according to the software supplied by the manufacturer, and according to manufacturer recommended guidelines. Changes in refraction, best corrected distance visual acuity (BCVA), corneal topography and aniseikonia from baseline were measured after 7, 14 and 47 nights of OK lens wear. The patient did not experience any clinically significant adverse effects of lens wear during OK.
Corneal topography was captured using the Medmont E300 corneal topography (Medmont; Melbourne, Australia). Flat K, Steep K and mean K values were averaged from 3 maps at each visit. After OK, there was central corneal flattening indicated by reduction in Flat K, Steep K and mean K values and corresponding hyperopic shift in spherical equivalent subjective refraction (Table 1). Axial and tangential difference maps captured at the day 14 visit (Figs. 1 and 2) illustrate central corneal flattening indicative of a relatively well centered OK fit. Due to poor topographic scan quality in the right eye, corneal curvature changes at the Day 47 visit are not reported.
2.4. Aniseikonia Aniseikonia was measured using a customised MATLAB program (Mathworks, Version R2016b). With sphero-cylindrical lenses in a trial frame in place to correct the residual refractive error in both eyes, the patient was asked to view the targets displayed on a computer screen presented at 50 cm (Fig. 3) through a pair of red-green glasses, with the green lens on the right eye and the red lens on the left eye. This ensured dichoptic viewing of the stimulus, which comprised of two differently coloured (red or green) semicircles with a Weber contrast of 0.4. The background luminance was approximately 32 cd/m2. The red semicircle acted as the reference stimulus with a radius of 5.6 degrees while the size of the green semi-circle was modified based on adaptive staircase method. The patient was asked to respond in a temporal twointerval forced-choice procedure in identifying whether the red or green semi-circle appeared smaller by pressing one of the buttons in the computer keyboard. The staircase procedure initially presented a randomly sized green semi-circle, and its size was increased or decreased by changing the radius of the semicircle at an initial step size of 4%, and after the first and for subsequent reversals the step size was halved. After the fourth reversal the step size remained at 0.5%. The staircase lasted for 6 reversals and the average of the last four reversals provided an estimate of the matched size. The staircase was repeated for 5 measurements and these values were averaged to calculate the aniseikonia. In order to show if there were significant interocular image size at baseline and subsequent visits, one sample t-tests were performed. Interocular image size differences were statistically significant at baseline and day 47 (P < 0.05), but not at day 7 and day 14 (P > 0.05) (Fig. 4). Although there was a reduction in aniseikonia from baseline at all review visits, it reached statistical significance at the day 7 visit only (post hoc t-tests with adjusted p-value of 0.0325). The relationship between the aniseikonia and spherical equivalent refraction differences between the right and left eyes measured at various visits are shown in Fig. 5. Aniseikonia decreased with smaller interocular differences in refraction at baseline, day 7 and day 14 (Fig. 5). However, aniseikonia increased at day 47 when both eyes reached emmetropia ( ± 0.50 DS) and the refractive difference between
2.1. Subjective refraction and visual acuity Non-cycloplegic subjective refraction was measured using standard optometric techniques [14] and monocular distance LogMAR VA was measured using an electronic chart presented at 6 m (Test Chart Pro, Thomson Software Solutions, UK). Spherical equivalent subjective refraction [15] and VA measurements during OK lens wear are detailed in Table 1. The most significant change in refraction occurred during the first 7 days of treatment and emmetropia ( ± 0.50 DS) was reach by day 47. BCVA did not vary significantly during the entire lens wear period.
2.2. Axial length The IOLMaster (Zeiss, Germany) was used to measure axial length and an average of 5 measurements at baseline and after 47 days of OK treatment are presented in Table 1. A difference of almost 1 mm in axial length between eyes with similar K values was found indicating axial anisometropia [16].
Table 1 Spherical equivalent subjective refraction, best correct distance visual acuity (BCVA), axial length and keratometry results at various study visits. R represents the right eye while L represents the left eye. Visit
Eye
Spherical equivalent subjective refraction (D)
BCVA (LogMAR units)
Axial length (mm)
Flat K (D)
Steep K (D)
Mean K (D)
Change of mean K from baseline
Baseline visit
R L R L R L R L
−3.25 −0.625 −1.25 −0.125 −2.00 −0.25 −0.125 −0.375
−0.04 −0.04 0.02 0.00 −0.02 −0.04 −0.02 −0.1
24.16 23.15 – – – – 24.08 23.21
44.52 44.11 43.07 43.89 43.65 43.8 – 43.34
44.97 44.98 43.97 44.31 44.31 44.47 – 43.64
44.75 44.55 43.52 44.10 43.98 44.14 – 43.49
– – −1.23 −0.45 −0.77 −0.41 – −1.06
Day 7 Day 14 Day 47
2
Contact Lens and Anterior Eye xxx (xxxx) xxx–xxx
S.C.K. Kwan, et al.
Fig. 1. a) Axial difference maps of the right and b) left eye between baseline and day 14 visits.
aniseikonia [17]. At baseline with trial lenses based on subjective refraction, the patient reported in this case study had an aniseikonia of 1.45% with the left eye seeing images larger than the right eye and this reduced with OK. In agreement with previous studies, changes in aniseikonia measured in this case report indicates that Knapp’s Law was unlikely applied to our patient. Contact lenses are commonly used to correct refractive error changes in children with unilateral aphakia post cataract surgery [12,13]. Aniseikonia reduced substantially with the use of contact lenses compared to spectacles lens wear [7,11]. Both conventional contact lenses and OK correct refractive error at the corneal plane. Therefore, OK could potentially produce a similar effect as contact lenses in reducing or correcting aniseikonia. Our results showed that the aniseikonia decreased as the interocular refractive difference decreased during the first 7 days of treatment (Fig. 5). Interestingly, even though interocular refractive error difference was smallest at day 47, the aniseikonia was larger when measured at day 47 compared to day 7 and day 14. While contact lenses correct myopia with no or minimal change in corneal curvature, OK lens induces myopic corrective changes by flattening the central cornea. The impact of progressive corneal flattening evident during the first 14 nights of OK on the image size is unknown. It has been reported that a patient with corneal curvature changes in the right eye due to a dendritic ulcer presented symptoms of aniseikonia (right eye image perceived to be smaller after resolution of ulcer), and differences in interocular image size was also measured. Provided that there is some evidence of the effect of corneal curvature on aniseikonia, it is plausible that changes in corneal curvature during OK treatment may have an impact on perceptual image size differences. Further studies are needed to improve our understanding of how image size changes caused by the alteration of corneal
eyes was minimal as shown in Fig. 5. At day 47, the patient reported good quality of vision in both eyes, and that the asthenopia associated with spectacles wear was not present while undergoing OK correction in both eyes. 3. Discussion In this paper, we report the reduction of aniseikonia in patient undergoing OK treatment. In axial anisometropic myopes, 1 mm difference in axial length corresponds to approximately 3D refractive difference between eyes [16]. This matches the interocular differences in refractive error and axial length measured in this case study indicating that the patient is an axial anisometrope [16]. According to Knapp's Law, with corrective lens placed at the anterior focal point of the eye, the image size of the eye with axial ametropia is equal to an eye with emmetropia. However, studies have provided evidence that contact lenses which correct refractive error at the corneal plane may reduce aniseikonia to a greater extent than using spectacles lenses. A previous study showed that all the 7 subjects with axial anisometropia experienced aniseikonia of more than 10% when corrective lenses were placed at a vertex distance of 15 mm away from the cornea corresponding to the anterior focal plane of the eye [11]. The authors commented that the perceptual image size was associated with the number of retinal cells being stimulated rather than the absolute retinal image size projected on the retina. Another study also reported that in 16 out of 17 patients with axial anisometropia, the aniseikonia became larger when spectacle lenses were worn compared to contact lenses [7]. It has been suggested that non-optical components, such as neural processing rather than just the actual retinal image formed on the retina may also play a role in the resulting 3
Contact Lens and Anterior Eye xxx (xxxx) xxx–xxx
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Fig. 2. a) Tangential difference maps of the right and b) left eye between baseline and day 14 visits.
Fig. 3. The semi-circle targets used in the aniseikonia test. Fig. 4. Aniseikonia with trial lens corrections after 7, 14 and 47 days of overnight OK treatment. Positive values indicate that the perceptual image size seen by the left eye was larger than of the right eye. Error bars represent standard error of the mean. Asterisk represents statistically significance difference in the image size between two eyes in baseline and day 47.
shape during OK lens wear. A limitation of the study is missing topographic data of the right eye at day 47. As the corneal topography was captured during eccentric viewing, the topographic data was not included in the analysis. Although there was no significant change from baseline in aniseikonia, the patient did not experience asthenopia associated with spectacle wear while undergoing OK correction. Other possible causes of asthenopia with spectacle lens wear include induced anisophoria in which the heterophoria varies at different gazes due to the changes of prismatic effect of spectacles lenses [18,19]. Other factors relating to spectacles such as tilting angle and face form which may create image distortion and dissimilarity between eyes, may also be responsible for the visual symptoms the patient experienced with spectacle lens wear, but not during contact lens wear.
4. Conclusions This case report demonstrates reduction in aniseikonia with OK lens wear in a myopic patient with axial anisometropia, although this effect was not statistically significant beyond 1 week of lens wear. As the impact of corneal curvature changes on aniseikonia is not well understood, future studies on the impact of OK on aniseikonia and associated asthenopia is required.
4
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[2] P.W. Kramer, V. Lubkin, M. Pavlica, R. Covin, Symptomatic aniseikonia in unilateral and bilateral pseudophakia. A projection space eikonometer study, Binocul Vis Strabismus Q 14 (1999) 183–190. [3] P. Hrynchak, Prescribing spectacles: reasons for failure of spectacle lens acceptance, Ophthalmic Physiol Opt 26 (2006) 111–115. [4] O. Katsumi, Y. Miyanaga, T. Hirose, H. Okuno, I. Asaoka, Binocular function in unilateral aphakia: correlation with aniseikonia and stereoacuity, Ophthalmology 95 (1988) 1088–1093. [5] J.R. Jiménez, A. Ponce, L. Jiménez Del Barco, J.A. Díaz, F. Pérez-Ocon, Impact of induced aniseikonia on stereopsis with random-dot stereogram, Optom Vis Sci 79 (2002) 121–125. [6] J.R. Jiménez, A. Ponce, R. González Anera, Induced aniseikonia diminishes binocular contrast sensitivity and binocular summation, Optom Vis Sci 81 (2004) 559–562. [7] B. Winn, R.G. Ackerley, C.A. Brown, F.K. Murray, J. Paris, M.F.S. John, Reduced aniseikonia in axial anisometropia with contact lens correction, Ophthalmic Physiol Opt 8 (1988) 341–344. [8] S.J. Vincent, M.J. Collins, S.A. Read, L.G. Carney, Myopic anisometropia: ocular characteristics and aetiological considerations, Clin Exp Optom (2014). [9] R.P. Rutstein, R.J. Fullard, A. WilsonJ, A. Gordon, Aniseikonia induced by cataract surgery and its effect on binocular vision, Optom Vis Sci 92 (2015) 201–207. [10] B.T. Barrett, A. Bradley, T.R. Candy, The relationship between anisometropia and amblyopia, Prog Retin Eye Res (2013) 120–158. [11] A. Bradley, J. Rabin, R.D. Freeman, Nonoptical determinants of aniseikonia, Invest Ophthalmol Vis Sci 24 (1983) 507–512. [12] A. Chia, K. Johnson, F. Martin, Use of contact lenses to correct aphakia in children, Clin Exp Ophthalmol 30 (2002) 252–255. [13] B. Russell, L. DuBois, M. Lynn, M.A. Ward, S.R. Lambert, The Infant Aphakia Treatment study contact lens experience to age 5 years, Eye Contact Lens 43 (2017) 352–357. [14] M. Rosenfield, Subjective refraction, in: M. Rosenfield, N. Logan (Eds.), Optom. Sci. Tech. Clin. Manag. 2nd ed., Butterworth-Heinemann, Edinburgh, 2009, pp. 209–228. [15] L.L. Thibos, W. Wheeler, D. Horner, Power vectors: an application of Fourier analysis to the description and statistical analysis of refractive error, Optom Vis Sci 74 (1997) 367–375. [16] A.H. Weiss, Unilateral high myopia: optical components, associated factors, and visual outcomes, Br J Ophthalmol 87 (2003) 1025–1031. [17] M. Scheiman, B. Wick, Clinical management of binocular vision: heterophoric, accommodative, and eye movement disorders, Lippincott Williams & Wilkins, Philadelphia, 2014. [18] A. Remole, Anisophoria and aniseikonia. Part II. The management of optical anisophoria, Optom Vis Sci 66 (1989) 736–746. [19] A. Remole, Anisophoria and aniseikonia. Part I. The relation between optical anisophoria and aniseikonia, Optom Vis Sci 66 (1989) 659–670.
Fig. 5. Aniseikonia compared to interocular differences in spherical equivalent refraction difference at various follow up visits during OK lens wear. Positive spherical equivalent difference values indicate that the right eye was more myopic compared to the left eye.
Declaration of Competing Interest Pauline Kang is a scientific consultant for CooperVision, USA. Authors have no other conflicts of interests to disclose. Acknowledgements The lenses used in this study were kindly donated by Paragon USA. We also thank Bausch + Lomb (Australia) for providing contact lens solutions. References [1] W. Lancaster, Aniseikonia, Trans Am Ophthalmol Soc (1938) 227–234.
5
Contents lists available at ScienceDirect
Contact Lens and Anterior Eye journal homepage: www.elsevier.com/locate/clae
Changes in aniseikonia of an axial anisometrope at various stages of orthokeratology lens wear Singson Chi Keong Kwana,b, Sieu K. Khuub, Pauline Kangb,* a b
Ng Teng Fong General Hospital, National University Health System, Singapore School of Optometry and Vision Science, University of New South Wales, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Orthokeratology Aniseikonia Anisometropia Myopia
Purpose: To report a case of reduced aniseikonia in a myopic axial anisometrope during orthokeratology (OK) lens wear. Case report: A 19-year-old female university student with myopic anisometropia presented for an OK lens fitting consultation. At baseline, perceptual interocular image size difference or aniseikonia of 1.45% was found, with a smaller image seen by the right eye compared to the left eye. The patient was fitted with a pair of OK lenses and interocular image size differences, subjective refraction and corneal topography were measured after 7, 14 and 47 days of overnight lens wear. Hyperopic shifts in central refraction and corresponding flattening of the central cornea was measured during OK treatment. Aniseikonia reduced after OK wear with the most significant change measured from baseline to day 7. Aniseikonia recorded after 7, 14 and 47 nights of lens wear was 0.05%, 0.35% and 0.85%, respectively. Although minimum differences in refractive error between eyes was reached after 47 days of OK, aniseikonia was greater than that measured after 7 and 14 days of OK. Conclusion: This case report demonstrates reduction in aniseikonia with OK lens wear in a myopic patient with axial anisometropia, although this effect was not sustained beyond 1 week of lens wear. As the impact of corneal curvature changes on aniseikonia is not well understood, future studies on the impact of OK on aniseikonia and associated asthenopia is required.
1. Introduction Aniseikonia refers to a difference in the optical image size seen by the two eyes [1]. Patients with aniseikonia may present with symptoms such as asthenopia, headaches [2] and vertical diplopia at near [3]. Perceptual image size differences between the two eyes of up to 5% can significantly reduce stereopsis [4,5] and binocular contrast sensitivity [6]. Aniseikonia occurs in most patients with anisometropia that is mainly axial in nature [4,5,7]. Previous studies investigating the development of anisometropia have reported that the magnitude of anisometropia increases with age during childhood and adolescence, until relative stability is reached at middle age (30–50 years). A marked increase in prevalence and has been found beyond 60 years of age [8]. Large amounts of anisometropia due to unilateral aphakia surgery can also induce aniseikonia [4,9]. The prevalence of anisometropia is approximately 3–4% when the criterion of interocular dioptric difference of 2D or above is used [10]. Although Knapp’s Law suggests axial anisometropia is best corrected with spectacles to minimise the aniseikonia, clinical studies show
⁎
contradicting results [7,11]. Therefore, contact lenses are widely used to correct for anisometropic refractive errors [12,13] due to the effectiveness in reducing aniseikonia compared to spectacles [7,11]. Although orthokeratology (OK) lenses also corrects refractive error at the corneal plane, its usage in correcting anisometropia and impact on aniseikonia have not been investigated. To our knowledge, no previous study has documented the effect of OK on interocular perceptual image size differences. This case report describes changes in aniseikonia of a myopic patient with axial anisometropia during the course of OK treatment. It offers insight into the feasibility of using OK as an alternative option for correcting anisometropes with symptomatic aniseikonia. 2. Case report A 19-year-old female university student was seen at our research clinic for an OK lens fitting consultation in March 2018. She spent approximately 2–3 hours per day on reading and computer use, and approximately 4 h of outdoor activities. She wore two different types of
Corresponding author. Present address: School of Optometry and Vision Science, University of New South Wales, 2052, Australia. E-mail address: [email protected] (P. Kang).
https://doi.org/10.1016/j.clae.2019.10.135 Received 16 January 2019; Received in revised form 4 September 2019; Accepted 20 October 2019 1367-0484/ © 2019 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: Singson Chi Keong Kwan, Sieu K. Khuu and Pauline Kang, Contact Lens and Anterior Eye, https://doi.org/10.1016/j.clae.2019.10.135
Contact Lens and Anterior Eye xxx (xxxx) xxx–xxx
S.C.K. Kwan, et al.
2.3. Corneal topography
daily disposable soft contact lenses (Aisei disposable daily colour lens, Aisei Co. Ltd., Japan and SEED daily lens, SEED Co. Ltd., Japan) once a week, for approximately 9 h each wear. When spectacles were worn, she experienced visual discomfort which remained relatively constant during both distance and near viewing, and felt there was a “difference” between her eyes. From a young age, such symptoms were apparent with various pairs of spectacles and reduction of symptoms was experienced with soft contact lens wear. To assess suitability for OK treatment, refraction and corneal topography were assessed as detailed below. Binocular visual function was also measured before OK lens wear only; with subjective refraction corrected with lenses in a trial frame, cover test indicated orthophoria at distance and 4 exophoria at near, and stereoacuity of 40″ was measured using the Randot Stereo test. Using the RAF rule, near point of convergence was less than 5 cm and amplitude of accommodation was 11D for the right eye, left eye and both eyes. Horizontal visible iris diameter was 11 mm in both eyes and pupil size was 4.6 mm in the right eye and 4.3 mm in the left eye measured under dim room light by the Medmont E300 corneal topographer (Medmont; Melbourne, Australia). Anterior ocular health was assessed at baseline and all subsequent visits using the slit-lamp biomicroscope. With good ocular health, mild to moderate degrees of myopic refractive error and regular corneal topography, the patient was deemed suitable for OK and fitted with a pair of Paragon CRT lenses (Paragon Vision Sciences, USA) for myopia correction. CRT lenses were selected according to the software supplied by the manufacturer, and according to manufacturer recommended guidelines. Changes in refraction, best corrected distance visual acuity (BCVA), corneal topography and aniseikonia from baseline were measured after 7, 14 and 47 nights of OK lens wear. The patient did not experience any clinically significant adverse effects of lens wear during OK.
Corneal topography was captured using the Medmont E300 corneal topography (Medmont; Melbourne, Australia). Flat K, Steep K and mean K values were averaged from 3 maps at each visit. After OK, there was central corneal flattening indicated by reduction in Flat K, Steep K and mean K values and corresponding hyperopic shift in spherical equivalent subjective refraction (Table 1). Axial and tangential difference maps captured at the day 14 visit (Figs. 1 and 2) illustrate central corneal flattening indicative of a relatively well centered OK fit. Due to poor topographic scan quality in the right eye, corneal curvature changes at the Day 47 visit are not reported.
2.4. Aniseikonia Aniseikonia was measured using a customised MATLAB program (Mathworks, Version R2016b). With sphero-cylindrical lenses in a trial frame in place to correct the residual refractive error in both eyes, the patient was asked to view the targets displayed on a computer screen presented at 50 cm (Fig. 3) through a pair of red-green glasses, with the green lens on the right eye and the red lens on the left eye. This ensured dichoptic viewing of the stimulus, which comprised of two differently coloured (red or green) semicircles with a Weber contrast of 0.4. The background luminance was approximately 32 cd/m2. The red semicircle acted as the reference stimulus with a radius of 5.6 degrees while the size of the green semi-circle was modified based on adaptive staircase method. The patient was asked to respond in a temporal twointerval forced-choice procedure in identifying whether the red or green semi-circle appeared smaller by pressing one of the buttons in the computer keyboard. The staircase procedure initially presented a randomly sized green semi-circle, and its size was increased or decreased by changing the radius of the semicircle at an initial step size of 4%, and after the first and for subsequent reversals the step size was halved. After the fourth reversal the step size remained at 0.5%. The staircase lasted for 6 reversals and the average of the last four reversals provided an estimate of the matched size. The staircase was repeated for 5 measurements and these values were averaged to calculate the aniseikonia. In order to show if there were significant interocular image size at baseline and subsequent visits, one sample t-tests were performed. Interocular image size differences were statistically significant at baseline and day 47 (P < 0.05), but not at day 7 and day 14 (P > 0.05) (Fig. 4). Although there was a reduction in aniseikonia from baseline at all review visits, it reached statistical significance at the day 7 visit only (post hoc t-tests with adjusted p-value of 0.0325). The relationship between the aniseikonia and spherical equivalent refraction differences between the right and left eyes measured at various visits are shown in Fig. 5. Aniseikonia decreased with smaller interocular differences in refraction at baseline, day 7 and day 14 (Fig. 5). However, aniseikonia increased at day 47 when both eyes reached emmetropia ( ± 0.50 DS) and the refractive difference between
2.1. Subjective refraction and visual acuity Non-cycloplegic subjective refraction was measured using standard optometric techniques [14] and monocular distance LogMAR VA was measured using an electronic chart presented at 6 m (Test Chart Pro, Thomson Software Solutions, UK). Spherical equivalent subjective refraction [15] and VA measurements during OK lens wear are detailed in Table 1. The most significant change in refraction occurred during the first 7 days of treatment and emmetropia ( ± 0.50 DS) was reach by day 47. BCVA did not vary significantly during the entire lens wear period.
2.2. Axial length The IOLMaster (Zeiss, Germany) was used to measure axial length and an average of 5 measurements at baseline and after 47 days of OK treatment are presented in Table 1. A difference of almost 1 mm in axial length between eyes with similar K values was found indicating axial anisometropia [16].
Table 1 Spherical equivalent subjective refraction, best correct distance visual acuity (BCVA), axial length and keratometry results at various study visits. R represents the right eye while L represents the left eye. Visit
Eye
Spherical equivalent subjective refraction (D)
BCVA (LogMAR units)
Axial length (mm)
Flat K (D)
Steep K (D)
Mean K (D)
Change of mean K from baseline
Baseline visit
R L R L R L R L
−3.25 −0.625 −1.25 −0.125 −2.00 −0.25 −0.125 −0.375
−0.04 −0.04 0.02 0.00 −0.02 −0.04 −0.02 −0.1
24.16 23.15 – – – – 24.08 23.21
44.52 44.11 43.07 43.89 43.65 43.8 – 43.34
44.97 44.98 43.97 44.31 44.31 44.47 – 43.64
44.75 44.55 43.52 44.10 43.98 44.14 – 43.49
– – −1.23 −0.45 −0.77 −0.41 – −1.06
Day 7 Day 14 Day 47
2
Contact Lens and Anterior Eye xxx (xxxx) xxx–xxx
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Fig. 1. a) Axial difference maps of the right and b) left eye between baseline and day 14 visits.
aniseikonia [17]. At baseline with trial lenses based on subjective refraction, the patient reported in this case study had an aniseikonia of 1.45% with the left eye seeing images larger than the right eye and this reduced with OK. In agreement with previous studies, changes in aniseikonia measured in this case report indicates that Knapp’s Law was unlikely applied to our patient. Contact lenses are commonly used to correct refractive error changes in children with unilateral aphakia post cataract surgery [12,13]. Aniseikonia reduced substantially with the use of contact lenses compared to spectacles lens wear [7,11]. Both conventional contact lenses and OK correct refractive error at the corneal plane. Therefore, OK could potentially produce a similar effect as contact lenses in reducing or correcting aniseikonia. Our results showed that the aniseikonia decreased as the interocular refractive difference decreased during the first 7 days of treatment (Fig. 5). Interestingly, even though interocular refractive error difference was smallest at day 47, the aniseikonia was larger when measured at day 47 compared to day 7 and day 14. While contact lenses correct myopia with no or minimal change in corneal curvature, OK lens induces myopic corrective changes by flattening the central cornea. The impact of progressive corneal flattening evident during the first 14 nights of OK on the image size is unknown. It has been reported that a patient with corneal curvature changes in the right eye due to a dendritic ulcer presented symptoms of aniseikonia (right eye image perceived to be smaller after resolution of ulcer), and differences in interocular image size was also measured. Provided that there is some evidence of the effect of corneal curvature on aniseikonia, it is plausible that changes in corneal curvature during OK treatment may have an impact on perceptual image size differences. Further studies are needed to improve our understanding of how image size changes caused by the alteration of corneal
eyes was minimal as shown in Fig. 5. At day 47, the patient reported good quality of vision in both eyes, and that the asthenopia associated with spectacles wear was not present while undergoing OK correction in both eyes. 3. Discussion In this paper, we report the reduction of aniseikonia in patient undergoing OK treatment. In axial anisometropic myopes, 1 mm difference in axial length corresponds to approximately 3D refractive difference between eyes [16]. This matches the interocular differences in refractive error and axial length measured in this case study indicating that the patient is an axial anisometrope [16]. According to Knapp's Law, with corrective lens placed at the anterior focal point of the eye, the image size of the eye with axial ametropia is equal to an eye with emmetropia. However, studies have provided evidence that contact lenses which correct refractive error at the corneal plane may reduce aniseikonia to a greater extent than using spectacles lenses. A previous study showed that all the 7 subjects with axial anisometropia experienced aniseikonia of more than 10% when corrective lenses were placed at a vertex distance of 15 mm away from the cornea corresponding to the anterior focal plane of the eye [11]. The authors commented that the perceptual image size was associated with the number of retinal cells being stimulated rather than the absolute retinal image size projected on the retina. Another study also reported that in 16 out of 17 patients with axial anisometropia, the aniseikonia became larger when spectacle lenses were worn compared to contact lenses [7]. It has been suggested that non-optical components, such as neural processing rather than just the actual retinal image formed on the retina may also play a role in the resulting 3
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Fig. 2. a) Tangential difference maps of the right and b) left eye between baseline and day 14 visits.
Fig. 3. The semi-circle targets used in the aniseikonia test. Fig. 4. Aniseikonia with trial lens corrections after 7, 14 and 47 days of overnight OK treatment. Positive values indicate that the perceptual image size seen by the left eye was larger than of the right eye. Error bars represent standard error of the mean. Asterisk represents statistically significance difference in the image size between two eyes in baseline and day 47.
shape during OK lens wear. A limitation of the study is missing topographic data of the right eye at day 47. As the corneal topography was captured during eccentric viewing, the topographic data was not included in the analysis. Although there was no significant change from baseline in aniseikonia, the patient did not experience asthenopia associated with spectacle wear while undergoing OK correction. Other possible causes of asthenopia with spectacle lens wear include induced anisophoria in which the heterophoria varies at different gazes due to the changes of prismatic effect of spectacles lenses [18,19]. Other factors relating to spectacles such as tilting angle and face form which may create image distortion and dissimilarity between eyes, may also be responsible for the visual symptoms the patient experienced with spectacle lens wear, but not during contact lens wear.
4. Conclusions This case report demonstrates reduction in aniseikonia with OK lens wear in a myopic patient with axial anisometropia, although this effect was not statistically significant beyond 1 week of lens wear. As the impact of corneal curvature changes on aniseikonia is not well understood, future studies on the impact of OK on aniseikonia and associated asthenopia is required.
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Fig. 5. Aniseikonia compared to interocular differences in spherical equivalent refraction difference at various follow up visits during OK lens wear. Positive spherical equivalent difference values indicate that the right eye was more myopic compared to the left eye.
Declaration of Competing Interest Pauline Kang is a scientific consultant for CooperVision, USA. Authors have no other conflicts of interests to disclose. Acknowledgements The lenses used in this study were kindly donated by Paragon USA. We also thank Bausch + Lomb (Australia) for providing contact lens solutions. References [1] W. Lancaster, Aniseikonia, Trans Am Ophthalmol Soc (1938) 227–234.
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