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Managing Age-related Astigmatism: Part Two

2 CPD in Australia | 1G in New Zealand | 1 October 2018



By David Stephensen

The options for correcting refractive astigmatism are increasing. Within your arsenal are spectacle lenses, soft contact lenses for astigmatism, rigid contact lenses for astigmatism and intraocular lenses. This article is the final of a two-part series addressing issues associated with correctly fitting soft and rigid contact lenses and IOLs with a focus on soft contact lenses. 

LEARNING OBJECTIVES

  1. Realise the increasing global prevalence of near sightedness, particularly in younger adults
  2. Recognise the need to increase eye testing of young adults and check for astigmatism at >0.5 dioptres and above
  3. Understand and be ready to educate patients about the latest CL technology for patient comfort and fit
  4. Understand why contact lenses with high wettability increase comfort for patients using computers over extended time.

From a clinical perspective, the prescribing rate of contact lenses that correct astigmatism in Australia reflects the population incidence of refractive astigmatism. In a 2017 survey of international prescribing habits, Australian optometrists used a contact lens design that corrected for astigmatism in 45 per cent of monofocal contact lens cases.1 The mean age of contact lens wearers in this survey was 37.2 years of age.1 A measure of the percentage of the Australian population in this age group suggests that 18.7 per cent exhibit refractive astigmatism of greater than 1.00DC.2 This population measure contrasts with the suggestion that in a contact lens wearing population, 45 per cent of patients exhibit refractive astigmatism that would benefit from contact lens correction.1

With a higher percentage of contact lenses being prescribed for astigmatism than the population incidence of astigmatism in Australia, several conclusions may be drawn. The first conclusion is that contact lens prescribers are over-prescribing for astigmatism. However, this is unlikely because over-prescribing for astigmatism creates residual astigmatism, and the resulting visual blur would be unpleasant and create a perceptible loss of visual function.3

The next conclusion that may be drawn is that Australian contact lens prescribers are confident in the various contact lenses for astigmatism that are available to them to prescribe. In Australia, 99 per cent of soft contact lenses prescribed are disposable contact lenses, meaning that Australian contact lens prescribers are confident in the disposable contact lenses available to them for the correction of astigmatism.1

This confidence is reflected in the conclusion of the literature review, The impact of rotational stabilization methods on toric soft contact lens performance.4 The review shows that the different methods of rotational stabilisation – prism ballast (upper eyelid pressure applied to a prismatic wedge that is thickest inferiorly), peri-ballast (similar to the prism ballast, except that there is no prism in the optic zone of the contact lens), thin zone (a thicker central portion aligns with the eyelid pressure) – all show an equivalent interblink rotational stability on the eye.4 In terms of rotational positioning on the eye, there was no appreciable difference between stabilisation methods for contact lenses for astigmatism. While there was rotation on the eye, the stability of this rotation is the key issue for prescribing purposes.4

The final conclusion is that the recipients of contact lenses for astigmatism in Australia are satisfied with their contact lenses. A study, A comparison of toric and spherical soft contact lenses on visual quality of life and ease of fitting in astigmatic patients, which compared contact lenses for astigmatism with spherical contact lenses, supports this view. Contact lenses for astigmatism provided higher quality of life scores for patients, compared to spherical contact lenses in low levels of astigmatism.5

This finding of an increased quality of life score, reflects the basic science that shows that contact lenses for astigmatism, provide superior visual acuity compared with spherical contact lenses, even in low levels of astigmatism. In 2007, Richdale et al demonstrated that the improvement in visual acuity with the use of contact lenses for astigmatism rather than spherical contact lenses was statistically significant for patients in each of a low astigmatism group (=<-1.00DC) and a moderate astigmatism group (-1.25 to -2.00DC).6 In 2005, Morgan et al demonstrated that aspherisation of soft spherical contact lenses in order to decrease aberrations is not sufficient to improve vision compared to contact lenses for astigmatism.

This study compared visual acuity with spectacle correction, with aspheric soft spherical contact lenses, and soft contact lenses for astigmatism. The soft contact lens for astigmatism corrected -0.75DC of refractive astigmatism, and subjects were selected for this study on the basis that they had a refractive astigmatism of -0.75DC to -1.00DC irrespective of the degree of the myopic spherical component of their refraction. The study by Morgan et al, demonstrated that across a range of pupil sizes (2mm, 4mm, 6mm) the soft contact lenses for astigmatism provided a statistically significant improvement in high contrast visual acuity when compared to the aspheric soft spherical contact lenses. The soft contact lenses for astigmatism demonstrated high contrast visual acuity that was not statistically different from that of spectacle correction. These studies suggest that spherical contact lenses do not have a capacity to mask even low levels of refractive astigmatism, and that patients do recognise and enjoy the benefits of astigmatism correction with soft contact lenses for astigmatism, irrespective of their degree of refractive astigmatism.7

Clinical Issues with Soft Contact Lenses for Astigmatism

Australian contact lens prescribers have embraced soft contact lenses for astigmatism, and patients are able to perceive high contrast visual acuity improvements demonstrated in terms of patient quality of life scores. It should then be reasonable to approach all soft contact lens fitting in terms of the correction of the total spherocylindrical refractive error, irrespective of the degree of astigmatism. However, there are various factors that the contact lens prescriber should be mindful of in order to improve their efficiency in prescribing soft contact lenses for astigmatism.

The Vertex Correction Trap

In order to prescribe a contact lens, the patient’s spectacle prescription needs to be corrected for the vertex distance between the posterior surface of the spectacle lens and the apex of the anterior surface of the cornea. Both of these locations represent the approximate plane of the back-vertex power of the spectacle lens and soft contact lens respectively.

For lower levels of both myopia and hypermetropia there is no effective change in the spectacle prescription when translated to the plane of the apex of the anterior cornea. The spectacle prescription can be used verbatim when selecting an initial trial contact lens.

As the degree of spherical ametropia increases for patients with myopic astigmatism, the required spherical and cylindrical components of the spectacle prescription are decreased in magnitude, when translated to the apex of the anterior cornea.

As the degree of spherical ametropia increases for patients with hyperopic astigmatism, the required spherical and cylindrical components of the spectacle prescription are increased in magnitude, when translated to the apex of the anterior cornea. This means that the initial trial soft contact lens for astigmatism cannot be selected by a verbatim replication of the spectacle prescription without inducing a spherocylindrical residual refractive error.

The other factor to consider when selecting the first trial contact lens for astigmatism, is that the spherical and cylindrical components of the spectacle prescription need to be vertex corrected independent of each other. This means that each cylinder power cannot be converted by a standard factor across the range of spherical ametropias encountered in clinical practice. It also means that the cylinder power selected for the first trial contact lens for astigmatism may be significantly different from the spectacle cylinder power as the degree of ametropia increases. Indeed, for some of the lower degrees of astigmatism in a spectacle prescription, the magnitude of astigmatism in a contact lens prescription may fall below the degree that necessitates correction with a contact lens for astigmatism.

The Accuracy Trap

Converting the spectacle prescription to an ocular prescription at the plane of the anterior corneal apex creates an issue of accuracy when considering disposable contact lenses for astigmatism. It is possible to accurately calculate the required cylindrical power for a contact lens for astigmatism, and in the case of a custom made soft contact lens, it is possible to manufacture the contact lenses to 0.01DC, in order to be accurate when providing vision correction.

In the case of disposable soft contact lenses for astigmatism, there is an accuracy trap. This arises because disposable soft contact lenses for astigmatism come in fixed step sizes, in terms of the magnitude of the astigmatism that they are able to compensate. In a daily disposable contact lens, such as the Biotrue Oneday for Astigmatism, the cylindrical power options are -0.75DC, -1.25DC, and -1.75DC, and these powers are typical of those available in other one day disposable soft contact lenses for astigmatism.7,8,9,10

Soft contact lenses for astigmatism that are reusable on either a 14 day or 30 day cycle do come in a wider range of cylindrical powers. The Ultra for Astigmatism that is available in the USA is available as -0.75DC, -1.25DC, -1.75DC, -2.25DC, and -2.75DC.11 The accuracy issue arises from the difference between the accurate conversion to the ocular surface, and the available powers of disposable soft contact lenses for astigmatism.

Taking the example of a -9.00/-1.75x180 spectacle prescription, this then becomes a -8.09/-1.39x180 ocular prescription, assuming a back vertex distance of 12.5mm. In terms of the cylinder component of the ocular prescription, the two available options in disposable soft contact lenses for astigmatism are -1.25 or -1.75. Choosing a -1.25DC option for the initial trial contact lens will leave a residual refractive astigmatism of -0.14x180. Choosing a -1.75DC option for the initial trial contact lens will leave a residual refractive astigmatism of -0.31x90. Both of these statements assume that the initial trial contact lens aligns correctly on the surface of the eye in a rotational sense. Additionally, the residual refractive astigmatism will induce a small spherical component of +0.07 in the first case, and +0.15 in the second, the latter being enough to require reconsideration of the sphere power if a 0.25 step size is available. This sequential cascade of considerations is a by-product of the polar nature of the spherocylindrical representation used in spectacle and contact lens prescriptions.

The pattern of the necessary adjustments is demonstrated in Table 1, which shows the decomposition of a range of spherocylindrical spectacle prescriptions into the spherical and ocular components. These are then vertex corrected at an assumed back vertex distance of 12.5mm. Based on the vertex corrected sphere and cylinder components, a net astigmatism value was calculated. From this, an appropriate soft disposable contact lens for astigmatism power is nominated as an initial trial contact lens for astigmatism. In this case, the initial trial contact lens was determined by selecting the trial contact lens with the smallest difference in Fourier Power vector magnitude, between the actual vertex corrected power and available trial contact lenses for astigmatism.13

Table 1 – Conversion of spectacle prescription to an ocular prescription at the corneal apex assuming a back-vertex distance of 12.5mm. The accurate correction and net corrected cylinder are shown in the central two columns. A suggested initial trial contact lens for astigmatism is shown in the right hand column, this is based on the minimal magnitude of the Fourier power vector between the corrected ocular refraction, and available typical disposable contact lenses for astigmatism.12

The Rotation Trap

Aside from errors associated with vertex correction there are refractive power errors that arise from rotation of contact lenses for astigmatism on the eye. Contact lenses for astigmatism generally exhibit a sound degree of rotational stability on the eye, as well as generally exhibiting a tendency to sit close to their designed orientation on the eye.4

These statements about general performance of contact lenses for astigmatism do not convey a fitting strategy that may be employed for situations in which a contact lens for astigmatism shows good rotational stability, but a rotational orientation that is not as per the specification on the packet. For example we may be fitting a patient who has a spectacle prescription of -5.00/-1.75x180 with a soft contact lens for astigmatism. From Table 1 we can see that the vertex corrected prescription at a 12.5mm back vertex distance is now -4.71/-1.52x180 and that we have selected an initial trial soft contact lens for astigmatism of -4.75/-1.75x180.

If this contact lens then displays a consistent and stable rotation of 15 degrees nasally on the eye, there will be an induced residual spherocylindrical refractive error that results from this rotation. As a rough rule of thumb, a rotation of 30 degrees relative to the desired orientation will induce a residual degree of astigmatism equal to that astigmatism that was intended to be corrected.14 In other words, ten per cent of the effect is lost per three degrees of contact lens misorientation.

Mathematically, this becomes slightly more complicated in that these rules of thumb are accurate only if the vertex corrected refractive astigmatism is exactly equal to the astigmatic power in the soft contact lens for astigmatism. In the case of our example trial soft contact lens for astigmatism above, where the vertex corrected astigmatism is equal to -1.75DC at a rotation of 15 degrees nasally, the induced residual refractive error would be +0.46/-0.92x142. If we take the exact case that is described above, the induced residual refractive error would be +0.60/-0.88x135.

It is notable that for the exact case, the spherical component of the residual astigmatism is exactly half of the astigmatic component. If everything is exactly matched, then one could also use this pattern as the basis of a rule of thumb, to identify a situation in which the correct soft contact lens for astigmatism is on the eye and simply misorientated. The problem with working on this basis is that the very small nature of residual astigmatism, coupled with the tendency to refract in 0.25D intervals, may render a confusing result. In this case, the exact error may be rendered as +0.50/-0.75x135 and this would not quite comply with the sphere to cylinder ratio rule of thumb; and this must be exact to be true. Consequently, reliance on this rule of thumb may be fraught for the contact lens prescriber.

Identification of orientation of a soft contact lens for astigmatism on the eye is also a component of the rotation trap. Soft disposable contact lenses for astigmatism are generally identified in their rotational stability, by means of an inscribed marking located with its orientation pointed towards the inferior periphery of the contact lens. This is commonly described as a six o’clock marking.8,9,10,11 By aligning a vertical thin beam of light through the use of a slit lamp with this marking, it is possible to determine that this marking is in its intended location at the six o’clock position.

Figure 1

If the six o’clock marking is visibly rotated, the thin vertical beam may be rotated on the slit lamp, such that the slit lamp beam aligns with the marking in its rotated position. Using a measurement scale on the slit lamp, the exact degree of rotation may then be assessed. One problem with this method is maintaining the slit lamp beam in alignment with the geometric centre of the contact lens, to ensure accuracy of the rotational measurement. A single marking may not provide sufficient information to be certain of the centration of the thin beam of light from the slit lamp.

Some manufacturers provide an additional reference marking at three o’clock or nine o’clock as a secondary reference point. A new marking innovation from Bausch and Lomb is worth mentioning. The system employed on the Biotrue Oneday for Astigmatism and the Ultra for Astigmatism provides both a secondary reference point, as well as an elegant system for identifying the axis orientation on a sample contact lens for astigmatism.8,11 The latter is of great utility if the contact lens specification is unknown, and the patient cannot recall the power numbers on the packaging.

In terms of measuring the degree of on-eye rotation, as well as maintaining the centration of the thin beam of light from the slit lamp, these secondary markings are of great utility. Once the contact lens prescriber has aligned the thin beam of light with the primary six o’clock marking in its rotated position and that position is recorded, verification of the centration can be made by keeping the eye and slit lamp in a stable position and rotating the thin beam. If the beam rotates and exactly aligns with the secondary marking then the centering is sound. If the rotated beam does not exactly align with the secondary marking, then the initial measurement of rotation of the soft contact lens for astigmatism was not made with reference to the centre of the contact lens. The contact lens prescriber should then reset their slit lamp such that both markings may be aligned with the thin beam of light through rotation alone.

Once the degree of rotation of the soft contact lens for astigmatism has been established, an optical calculation may be made to establish the next trial soft contact lens for astigmatism. For soft contact lens fitting for astigmatism, it is a wise course to accurately calculate out the effect of each residual spherocylindrical refractive error to predict the next trial contact lens.

Rigid Contact Lenses for Astigmatism

Rigid contact lenses for astigmatism differ from soft contact lenses for astigmatism in that the rigid contact lens is not the only corrective lens on the eye. Beneath a rigid contact lens is a tear lens that functions to provide a compensation for anterior surface corneal astigmatism. This means that if there are no further sources of refractive astigmatism posterior to the anterior corneal surface, the act of placing a rigid contact lens with a spherical back optic zone radius and spherical back vertex power on the eye will correct the refractive astigmatism of the eye, without the need to incorporate an astigmatic component into the contact lens.15

Where there are sources of refractive astigmatism posterior to the anterior cornea, which may arise from the posterior cornea or the crystalline lens, it becomes necessary to incorporate an astigmatic component into the rigid gas permeable contact lens. This may be achieved through either a toric front surface with an appropriate stabilisation method – typically a prism ballast method – or through a bitoric system, in which both the anterior and posterior surfaces of the contact lens carry a toric geometry.

Intraocular Lenses for Astigmatism

In the past decade the application of intraocular lenses (IOLs) for astigmatism during cataract surgery has been increasing, and approximately 69 per cent of patients requiring cataract surgery will present with astigmatism that is worth consideration of using such IOLs. The utility of IOLs for astigmatism, even for small amounts of astigmatism, is well established.16

The key difference between IOLs for astigmatism and contact lenses for astigmatism is that IOLs for astigmatism do not seek to correct refractive astigmatism. With the removal of the crystalline lens from the eye prior to implantation of the IOL, the only source of residual astigmatism in the eye is the cornea. Caution must be taken in not regarding any refractive astigmatism, in consideration of the IOL power, as there may be significant lenticular astigmatism in the cataract.

The concepts of accurate translation of optical power from the corneal plane to the plane of the IOL, and the identification of induced refractive astigmatism from rotational misalignment of the IOL in the eye, are similar in concept to those of soft contact lenses for astigmatism.

Conclusion

Reliable methods for the correction of refractive astigmatism beyond spectacle lenses are widely available and the use of soft contact lenses for astigmatism should be a routine part of the contact lens prescriber’s practice methods. The contact lens prescriber should be mindful of the nature of fitting issues that may give rise to residual refractive astigmatism, and implement solutions for these as part of their prescribing protocol.


 

David Stephensen conducts a specialty contact lens practice with his primary practice located in the southside Brisbane suburb of Moorooka. He is a Fellow of the Cornea and Contact Lens Society of Australia, a Fellow of the British Contact Lens Association, a former President of the Cornea and Contact Lens Society of Australia and former Honorary Vice President of the Cornea and Contact Lens Society of Australia. Mr. Stephensen is the current Chair of the Case Report Section for the Fellowship programme.

Mr. Stephensen has worked in the field of toric intraocular lens optics, particularly in applying toric intraocular lenses to complex cases. He has interests in anterior segment eye conditions, toric intraocular lenses, contact lens correction for presbyopia, orthokeratology and in bespoke design of rigid gas permeable contact lenses for visual correction of abnormal corneas – particularly pellucid marginal degeneration, post graft, and keratoconus.

 

References
1. Morgan P, Woods C, et al. International Contact Lens Prescribing in 2017, Contact Lens Spectrum, 2018, 33(January), 28-33
2. Sanfilippo PG, Yazar S, Kearns L, Sherwin JC, Hewitt AW, Mackey DA. Distribution of astigmatism as a function of age in an Australian Population, Acta Ophthalmol. 2105, 93: e377-e385
3. Wolffsohn JS, Bhogal G, Shah S. Effect of Uncorrected Astigmatism on Vision, Presentation at World Optometry Congress, June 2010
4. Edrington T. A literature review: The impact of rotational stabilization methods on toric soft contact lens performance, Contact Lens & Anterior Eye 34 (2011) 104–110
5. Nichols J et al. A comparison of toric and spherical soft contact lenses on visual quality of life and ease of fitting in astigmatic patients, Contact Lens and Anterior Eye, 41S (2018) S42
6. Richdale K et al.  Visual Acuity with Spherical and Toric Soft Contact Lenses in Low- to Moderate-Astigmatic Eyes, Optometry and Vision Science, Vol. 84, No. 10, October 2007 pp 969-975
7. Morgan PB, Efron SE, Efron N, Hill EA. Inefficiacy of Aspheric Soft Contact Lenses for the Correction of Low Levels of Astigmatism, Optometry and Vision Science, Vol. 82, No. 9, September 2005 pp 823-828
8. Bausch and Lomb, Biotrue ONEday Contact Lenses, www.bausch.com/ecp/our-products/contact-lenses/myopia-hyperopia/biotrue-oneday-contact-lenses: Accessed 26/7/2018
9. Johnson and Johnson, Acuvue Oasys 1 Day for Astigmatism, www.jnjvisionpro.com/products/acuvue-oasys-1-day-astigmatism: Accessed 26/7/2018
10. Alcon, Dailies AquaComfort Plus Toric Contact Lens Parameters, www.myalcon.com/products/contact-lenses/dailies/aquacomfort-plus-toric-parameters.shtml: Accessed 26/7/2018
11. Bausch and Lomb, Ultra for Astigmatism, http://www.bausch.com/ecp/our-products/contact-lenses/astigmatism/ultra-for-astigmatism: Accessed 26/7/2018
12. Thibos LN, Wheeler W, Horner D. Power Vectors: An Application of Fourier Analysis to the Description and Statistical Analysis of Refractive Error, Optometry and Vision Science, Vol. 75, No. 6, June 1997 pp 367-375
13. Miller JM. Clinical Applications of Power Vectors, Optometry and Vision Science, Vol. 86, No. 6, June 2009 pp 599-602
14. Young G, McIlraith R, Hunt C. Clinical Evaluation of Factors Affecting Soft Toric Lens Orientation, Optometry and Vision Science, Vol. 86, No. 11, November 2009 pp E1259-E1266
15. = Douthwaite WA, Contact Lens Optics and Lens Design, 3rd Ed., Butterworth Heinemann, Edinburgh, 2006 pp 204-205
16. Statham M, Apel A, Stephensen D. Comparison of the AcrySof SA60 spherical intraocular lens and the AcrySof Toric SN60T3 intraocular lens outcomes in patients with low amounts of corneal astigmatism, Clin Exp Ophthal, 2009, 37, pp 775-779
This article was sponsored by Bausch and Lomb Australia BOD.0031.AU.18

' Australian contact lens prescribers are confident in the various contact lenses for astigmatism that are available to them to prescribe '