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Managing Age-Related Astigmatism

2M CPD in Ausrtalia | 0.75G in New Zealand | 8 December 2018

By David Stephensen

In the course of our daily professional lives our patients will often ask the question, “What is astigmatism?”. In response, there is a brief period of silence as we run through, in our minds, which of our stock responses will be appropriate to this patient. This is part one of a two part article.

LEARNING OBJECTIVES

1. Understand the need to check for astigmatism at >0.5D and above.
2. Recognise the need to keep up to date with technology in order to offer the latest comfort and fit to patients.
3. Realise that increased computer use requires contact lenses with high levels of wettability over a longer period of time.
4. Recognise that more than one dioptre of astigmatism is clinically significant and new designs available in toric lenses can ensure visual clarity.

Some practitioners will describe astigmatism in terms of the shape of the eyeball, or more specifically the cornea. Holding a model eye in our hands we look at our patients and earnestly attempt to explain the toroidal surfaces found in astigmatic optics via various allusions. References to the difference between dessert spoons and soup spoons, rugby balls and soccer balls, eggs and ping pong balls are then offered to our patients in our best professional tone of voice. This explanation is followed by another period of silence, slightly longer this time, while the patient attempts to work out how having their eyeball being the same shape as a rugby ball equates to their visual experience of blur. In the meantime, the practitioner sits hopeful, in anticipation that the explanation has been sufficient.

In my practice I will admit that I have largely abandoned these shape-based allusions in favour of telling patients that astigmatism is simply a big word we use that may confuse people, and that it is simply part of their spectacle or contact lens prescription. If I am pressed, I then move bravely on to explain that it is a special type of blur. This is followed by me pointing to a wall in the consulting room and waving my hands in a direction parallel to the wall while I explain that a standard blur goes up and down and across the wall, much like an out of focus projector. I then start moving my hands in direction perpendicular to the wall and tell my patient that astigmatism is a three-dimensional blur that also blurs into and out of the wall. I then tell them that is why their blur is so annoying, in a hopeful attempt to reconcile my game show hand waving with their personal visual experience.

In truth, whichever technique is used to explain astigmatism to our patients is little more than a lie-to-children explanation. A lie-to-children being a simplified explanation presented as a foundation for understanding, with the expectation that later learning will reveal that the simplification is not in fact true.1

HISTORY OF ASTIGMATISM

The concept of astigmatism is held to arise from the work of Sir Isaac Newton, who described the optical effects and graphical methods of constructing the foci of an astigmatic system. The first practical application of astigmatism towards measuring the refractive error of an eye was presented to the Royal Society by Thomas Young in a Bakerian lecture in 1800. Young presented a measurement of his own astigmatism, a prescription equivalent to -4.00/-1.75x90. It is interesting to note that Young measured the astigmatism of his eye while it was immersed in water. He found that the astigmatism measure did not alter, and was also then the first person to measure his own internal ocular astigmatism.2

The first description of the application of a spherocylindrical corrective lens to compensate for ocular astigmatism was presented by Sir George Airy to the Cambridge Philosophical Society in 1825. In this presentation Airy describes the methodology used to determine the refraction of his own left eye and the production of a spherocylindrical lens to correct the optical defect. Airy also attributes the origin of the term ‘astigmatism’ to Dr. William Whewell of Trinity College Cambridge in 1846.2,3

The production and dispensing of spherocylindrical spectacle lenses to correct ocular astigmatism in Australia is first recorded in South Australia in 1880 when Charles Sawtell advertised the availability of astigmatic spectacles. However, such spherocylindrical spectacle lenses had been available in Europe and the United States of America from shortly after Airy’s lecture of 1825. This included trifocal lenses to correct astigmatism as first described by J. I. Hawkins in 1826, who also included notes on how to work out one’s own astigmatic correction. The production of such complex multifocal spectacles that corrected astigmatism demonstrated, in the early stages of ophthalmic optics correction of astigmatism, an initial burst of enthusiasm.2,4 This initial enthusiasm does not seem to be consistent through the 19th century. In clinical optometry practice, the practice of correcting astigmatism was still not globally conducted even by the mid-20th century. An anecdote presented by Thomas Grosvenor describes the attitude of one of his colleagues in 1946 as “Cylinders are for cars; I fit my patients with spherical lenses”.5

NATURE AND CLASSIFICATION OF ASTIGMATISM

Astigmatism is created by the presence of two different radii of curvature on an optical surface. If these two different radii of curvature are orthogonal, or 90 degrees apart, the optical surface is regarded as having regular astigmatism. If the two principal curvatures are other than 90 degrees apart, the optical surface is regarded as having irregular astigmatism. Light entering into an optical system from a direction that is oblique to the optical axis creates a form of astigmatism known as oblique astigmatism. For the purposes of this article most of the discussion will consider the nature of regular astigmatism and paraxial optics.2

Astigmatic optical systems form two focal lines rather than a point focus when analysed using Gaussian optics. It is the nature of this focal pattern that gives rise to the term astigmatism – combining the Greek ‘a-‘ meaning without and the Greek ‘stigmatos’ meaning a spot.6 The interval between the two focal lines is known as the Interval of Sturm, and the cross section of this refracted light is generally elliptical apart from a point that is dioptrically midway between the two image planes. In this region the cross-section of blur is circular and known as the circle of least confusion.2

Astigmatism is often then classified into a notation, in which surfaces with their maximum curvature near to the horizontal are notated as with the rule, and surfaces with their maximum curvature near to the vertical are notated as against the rule. This classification, based on the orientation of the primary optical power meridians, may then be expanded to describe the nature of the ametropia being corrected. This gives rise to simple astigmatism in which, for hypermetropia the first focal line is at the retina, and for myopia the second foal line is at the retina. As well, compound astigmatism in hypermetropia means that the retina is situated in front of the first focal line, and in myopia the retina is situated behind the second focal line. Finally, with mixed astigmatism, the retina lies between the two focal lines.2

ASTIGMATISM AND THE EYE

In the human eye, the primary contributor to an astigmatic refractive error is the anterior surface of the cornea. However, measurement of the anterior corneal radii does not achieve an accurate prediction of the total ocular astigmatism. From the early period of the use of keratometry, formulae were created to relate the corneal astigmatism to the total ocular astigmatism. The best known of these is Javal’s rule in which the internal ocular astigmatism is assigned a value of 0.5 dioptres. In practice these formulae were often lacking, and total ocular astigmatism was held to be better assessed through a process of subjective refraction.7

The difference between the anterior corneal astigmatism and the total ocular astigmatism may be accounted for by the astigmatism of the posterior cornea, the crystalline lens, and a contribution from other optical sources. These additional optical sources include tilting of the crystalline lens and a misalignment between optical and visual axes.8

In contact lens practice, it is useful to assess the curvature of the cornea using a keratometer to assess for the appropriate fitting shape of the posterior surface of a contact lens suitable to the eye. The direct measurement from the keratometer uses the reflected image of the anterior corneal surface. The resulting measurement provides a radius of curvature along a specific meridian. Measuring orthogonal meridians provides a geometric representation of the anterior corneal surface in both of the primary meridians. These radii may then be converted to meridional power measurements. If the refractive power of the anterior cornea alone is desired, then the actual refractive index of the cornea must be employed, and a typical value for the corneal refractive index is 1.376. However in general, the anterior corneal radii are used to make a prediction of the total corneal power and astigmatism. This is performed by assigning a notional relationship between the anterior cornea radii and the posterior corneal radii. In such a relationship the posterior corneal radii are reasonably described as being 81 per cent of the anterior corneal radii. From this relationship an adjustment can be made to the actual corneal refractive index to form the keratometric refractive index. Typically, the keratometric refractive index used is 1.3375. Table 1 shows the difference in power between the measured anterior corneal astigmatic power and total corneal astigmatic power for an example case. It should be noted that keratometric power calculations account for posterior corneal astigmatism in the majority of cases without further modification.7,9

At birth the eye has a significant prevalence of high astigmatism, with an average of six diopters of corneal astigmatism. Astigmatism in a neonatal population shows some correlation with birth weight and prematurity; in that low birth weight and premature newborns show higher degrees of corneal astigmatism.8

As the infant develops there is a reduction in corneal astigmatism, or in other terms, a tendency towards emmetropisation in terms of astigmatic power of the eye. There is also a shift from against the rule astigmatism in early life to with the rule astigmatism in children older than four years.8

In adult populations, the incidence of astigmatism is common, but the degree of astigmatism is mild. In an Australian population the magnitude of refractive astigmatism has been measured to a mean value of 0.44D, and this remains stable up until 50 years of age. Approximately 80 per cent of the Australian population has a refractive astigmatism between 0.25D and 1.00D, but only 19 per cent of the Australian population has astigmatism greater than 1.00D.10

After 50 years of age there is a gradual increase in the prevalence and magnitude of refractive astigmatism in the Australian population, and by 70 years of age 94 per cent of the population has refractive astigmatism between 0.25D and 1.00D, and the proportion of the population with refractive astigmatism greater than 1.00D increases to 37 per cent.10

The other characteristic of the change in astigmatism with age is an increase in the proportion of against the rule astigmatism. Under the age of 60, the majority of the Australian population exhibits with the rule astigmatism. After 60 years of age, the majority of the population shows against the rule refractive astigmatism, and there is a further significant increase in the proportion of against the rule refractive astigmatism after 70 years of age with 75 per cent of the population showing against the rule refractive astigmatism.10

Corneal astigmatism remains predominantly with the rule astigmatism until the age of 70 in the Australian population. This suggests that the increase in against the rule refractive astigmatism between 50 and 70 years of age arises from changes in the crystalline lens astigmatism in this age cohort. However, after 70 years of age there is a sharp rise in the prevalence of against the rule corneal astigmatism, and by 80 years of age, 75 per cent of the Australian population demonstrates against the rule corneal astigmatism. Interestingly, in the same age group there is a lower prevalence of against the rule refractive astigmatism.10

These findings suggest that in the Australian population there are lifelong changes in both corneal and crystalline lens astigmatism. This would then require gradual adaptive changes to the modality of refractive correction over an individual patient’s lifetime. Patients that have spent the earlier decades of their life wearing spherical contact lenses, may in later life require toric contact lenses in one or both eyes through natural development of astigmatism with age. As we will see later, this can necessitate the practitioner giving consideration to the contact lens design philosophy in order to maintain consistent visual performance for patients as their refractive needs evolve over time.

SOFT CONTACT LENSES AND ASTIGMATISM

The use of soft contact lenses to correct astigmatism is now a longstanding practice. The late Professor Brien Holden, writing in 1975, described that 70 per cent of new contact lenses being prescribed at that time were soft contact lenses, and that a significant proportion of new contact lens patients had refractive astigmatism significant enough to require specific correction in their soft contact lenses. He described the level of refractive astigmatism that would be significant enough to require correction in soft contact lenses as 0.75D, and that patients with lower degrees of refractive astigmatism would generally find their vision acceptable in soft contact lenses without a specific correction for astigmatism.11

Professor Holden described the percentage of patients at the University of New South Wales clinic that demonstrated refractive astigmatism greater than 1.00D as 25 per cent, which is a figure slightly higher, but roughly in line, with that of Sanfilippo and colleagues in their later study. Consequently, we may conclude that the need for correction of astigmatism in soft contact lens wearing patients has remained relatively stable over time.10,11

Additionally, Prof. Holden outlined, in very simple and elegant terms, the underlying principles that apply to soft toric contact lenses, and these have not altered in the intervening time – even in the face of immense changes in contact lens materials, technology and wearing modalities. These fundamental principles are that the contact lens should conform to the shape of the central cornea, that the contact lens should be chosen with appropriate surfaces to provide correct in vivo meridional powers, and that the contact lens should remain in the correct orientation throughout the wearing period.11

Conformity of the soft contact lens posterior surface to that of the cornea is, in our modern practices, rarely a decision that the practitioner needs to make. In Australia in 2017, the majority of soft contact lenses prescribed (99 per cent) were disposable contact lenses, according to a survey of contact lens prescribing patterns.12 Disposable contact lenses typically come in one pre-formed back optic zone radius that may be either spherical or aspheric. Some newer soft toric contact lenses, such as the Biotrue ONEday for Astigmatism (Bausch and Lomb, Rochester), also carry a toric posterior surface. The design concept of these lenses aims to achieve a more complimentary match of the contact lens geometry and the astigmatic curvature of the patient’s anterior corneal surface.13 Practitioner satisfaction levels with disposable contact lens fitting characteristics remains high, despite the inability to alter the posterior contact lens geometry. A survey of practitioners fitting the Biotrue Oneday (Bausch and Lomb, Rochester) contact lens recorded a 99 per cent satisfaction rate with lens centration characteristics.14 As a result, contact lens practitioners should feel confident in the ability of the posterior surface of modern disposable toric contact lenses to maintain an adequate fit on the ocular surface. The selection of appropriate in-vivo meridional contact lens powers is predominantly determined as a function of the patient’s subjective refraction once corrected for the back vertex distance used during the measurement. Prior to prescribing soft contact lenses to correct astigmatism, the practitioner should ensure that a spherocylindrical spectacle refraction has been obtained that maximises the patient’s visual acuity. If there are approximations or the refraction has not been constructed to maximise the patient’s visual acuity, then the predicted contact lens back vertex power will be unlikely to prove appropriate to satisfy the patient’s visual needs.

Figure 1.

The correction of a spherocylindrical spectacle refraction for back vertex power requires decomposition into the individual meridional refractive powers. For example, a spectacle prescription of -5.00/-3.75x90 must have the meridional powers of -5.00 and -8.75 corrected separately for the backvertex distance of the spectacles. Assuming a back vertex distance of 12mm, the vertex corrected prescription at the anterior corneal plane would then become -4.71/-3.18x90, with a reduction in the astigmatic component of 0.57DC. As a result of this compensation, the magnitude of astigmatic power required in the contact lens will be slightly higher than that in spectacles for patients with hypermetropic astigmatism, and the magnitude of astigmatic power required in the contact lens will be slightly lower in myopic astigmatism. For some higher degrees of myopia, an apparently significant degree of spectacle astigmatism may in fact prove not to require toric correction at the plane of the contact lens.

Ensuring the correct orientation of a soft toric contact lens on the eye during wear may be achieved in a variety of ways. In the past, a truncation was applied to the inferior portion of soft toric contact lenses to achieve stabilisation through the support of the lower eyelid against the truncation. In modern contact lens practice this methodology is rarely used. Other ways to stabilise the contact lens are prism ballasted contact lenses, in which a base down prism is used to stabilise the contact lens under the action of the blinking mechanism, and peri-ballast stabilisation, which uses thicker zones, peripheral to the optic zone of the contact lens. Peri-ballast stabilisation also uses the blink mechanism to stabilise the contact lens.11

The configuration of this additional contact lens thickness is one that must be considered against other visual goals in terms of the overall contact lens design, both in terms of shape and optical goals. In modern disposable contact lenses there is also a need to consider the design methodology across an entire platform when considering the design of a soft toric contact lens, as some patients may require a spherical contact lens in one eye and a toric contact lens in the other.

Taking the example of the Biotrue Oneday (Bausch and Lomb, Rochester) platform of contact lenses, the existing spherical Biotrue Oneday contact lens has a compensation for 0.18 micrometres of spherical aberration. This has a result of improving image quality for patients compared to a contact lens that does not correct this degree of spherical aberration.15 In contemplating the design of the new Biotrue Oneday for Astigmatism contact lens, a deliberate decision was made to include a peri-ballast stabilisation configuration so that there was no prism in the optic zone of the contact lens, as well as enabling a central optic zone that is not decentred as found in prism ballasted contact lenses. The result of this configuration is that the Biotrue Oneday for Astigmatism is able to convey the same degree of spherical aberration compensation as its spherical equivalent in the Biotrue platform.13 Consequently, if the optical design is not consistent between the spherical and astigmatic forms of a particular platform of contact lenses, a patient may experience an inconsistent visual experience.

 

      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 the 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. Lie-to-children, Wikipedia, 2018, accessed 5th June 2018, https://en.wikipedia.org/wiki/Lie-to-children
2. Bennett AG, Rabetts RB. Clinical Visual Optics, 2nd Ed, Butterworth-Heinemann, Oxford, 1989 pp 93-109
3. Airy’s Astigmatism, Aust J Optom, 1938, 21(10), 503 accessed 6/6/2018 via https://doi.org/10.1111/j.1444-0938.1938.tb01188.x
4. Cole BL. Did T.R. Proctor Bring Modern Optometry to New Zealand and Australia? An Investigation through Archival Newspapers, Hindsight, 2017, 48 (2), 34-51
5. Grosvenor T, How much do we know about astigmatism?, Clin Exp Optom, 2007, 90(1), 3-4
6. Astigmatism, Online Etymology Dictionary, accessed 6th June 2018, https://www.etymonline.com/word/astigmatism
7. Gutmark R, Guyton DL, Origins of the Keratometer and its Evolving Role in Ophthalmology, Surv Ophth, 2010, 55(5), 481-497
8. Read S, Collins MJ, Carney LG. A review of astigmatism and its possible genesis, Clin Exp Optom, 2007, 90(1), 5-19
9. Aitchison D, Smith G. Optics of the Human Eye, Butterworth Heinemann, Edinburgh, 2000
10. 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
11. Holden BA, The principles and practice of correcting astigmatism with soft contact lenses, Aust J Optom, 1975, 58, 279-299
12. Morgan P, Woods C, et al. International Contact Lens Prescribing in 2017, Contact Lens Spectrum, 2018, 33(January), 28-33
13. Bausch and Lomb, Data on File, 2016
14. Bausch and Lomb, Data on File, 2012
15. Bausch and Lomb, Data on File, 2013
16. Final Results from a 7-investigator, multi-site, randomized, crossover study of Biotrue ONEday for Astigmatism.
17. Bausch and Lomb, Data on File BFA-0036-BODfA Overaching Memo - LD165005A - FIT 2016

' After 50 years of age there is a gradual increase in the prevalence and magnitude of refractive astigmatism in the Australian population '