Contact lens management of keratoconus in a patient with residual astigmatism resulting from implantation of a toric intraocular lens

Authors


Abstract

Toric intraocular lenses (IOLs) are increasingly being used at the time of cataract surgery to treat regular refractive astigmatism. This paper outlines the contact lens management of a patient with keratoconus, who had marked residual astigmatism post-operatively due to implantation of a toric IOL during cataract surgery. It is strongly recommended that toric IOLs should only be prescribed for patients who have mild forms of keratoconus, with only slight irregular astigmatism that has been stable for at least the previous 12 months and acceptable vision with spectacles.

The contact lens correction of residual astigmatism in a patient with keratoconus, who is required to wear rigid gas-permeable contact lenses due to the significant nature of their condition, will increase the complexity of the contact lens fitting. This paper outlines the contact lens management of a patient with keratoconus who had significant residual astigmatism post-operatively due to implantation of a toric intraocular lens (IOL) during cataract surgery.

Case Report

A 48-year-old female patient presented with the complaint of blurred vision in her right eye following cataract surgery nine months earlier. Her ocular history was notable for having been diagnosed with both myopia (when she was nine) and keratoconus (in her early 20s). Soft contact lenses had been worn on a full-time basis from the age of 18 to 22, at which time she was refitted with rigid gas-permeable contact lenses, as she was no longer able to achieve acceptable vision with either spectacles or soft contact lenses. The patient also had suffered a retinal detachment in her right eye in 1986 that was treated with cryotherapy and a scleral buckle, as well as retinal holes in her left eye that were treated with cryotherapy in 1992 and 1994.

Examination at presentation revealed moderate bilateral keratoconus, slightly more advanced in the right eye. This was confirmed by videokeratoscopy (Figures 1 and 2) with the E300 Corneal Topographer (Medmont Pty Ltd, Melbourne, Victoria, Australia) that showed sagging (oval) cones in both eyes. A posterior chamber intraocular lens was observed in the right eye, while the crystalline lens of the left eye was still relatively clear. No corneal scarring was noted, although the patient was observed to have a mild tear film dysfunction bilaterally. Dilated fundus examination revealed healthy optic nerves and maculae.

Figure 1.

Topographical map of the patient's right cornea

Figure 2.

Topographical map of the patient's left cornea

Further investigation revealed that the patient's cataract surgery on the right eye nine months earlier had involved an uneventful phacoemulsification followed by implantation of a posterior chamber toric IOL (Staar AA403TL +10.5/+3.5 D) into the capsular bag with the lens selection and orientation based on ocular biometry performed with the IOLMaster 500 (Carl Zeiss Meditec, Jena, Germany). Slitlamp examination estimated the IOL to be positioned within five degrees of the specified axis. Post-operatively the patient had noticed that her visual acuity with the right eye wearing a rigid gas-permeable contact lens was now blurred, despite the spherical power of the contact lens being modified to take into account the change in the patient's spherical equivalent brought about by the insertion of the IOL. Subsequent yttrium aluminum garnet (YAG) laser posterior capsulotomy about four months after the cataract surgery did not bring about any significant improvement in the visual acuity of this eye.

It was postulated that the blurred vision in the right eye was probably not due to the slight misalignment of the toric IOL but rather the result of residual astigmatism created by the implantation of a toric IOL in an eye, which had keratoconus (and therefore a significant degree of irregular corneal astigmatism). Note that the term ‘residual astigmatism’ can be defined in various ways, most commonly as the component of the ocular astigmatism that is not due to the cornea. For the purpose of this paper—and more appropriately in the context of rigid lens fitting—the authors will adopt the definition proposed by Lindsay[1] that residual astigmatism is the astigmatic component of a lens required to correct fully an eye wearing a spherical powered rigid contact lens with a spherical back optic zone radius.

The patient was then referred for contact lens fitting and assessment of her visual status. Due to the marked irregular corneal astigmatism associated with the keratoconus, spectacle refraction of R -1.50/-1.00 × 70, L -5.25/-1.75 × 115 only improved the visual acuity to R 6/12=, L 6/9.5. A diagnostic rigid contact lens fitting subsequently confirmed the right eye to have a significant degree of residual astigmatism, with a 2.25 D cylinder identified by manifest refraction over a rigid trial contact lens on this eye. A rigid contact lens fitting of the left eye revealed the presence of no residual astigmatism based on a sphero-cylindrical manifest refraction performed over the rigid trial lens.

Consequently, the patient was prescribed a pair of rigid gas-permeable contact lenses (right front surface toric design and left spherical) incorporating the following parameters:

  • R 6.95:6.00/7.75:6.80/8.85:7.60/11.15:8.2 -2.00/-2.25 × 174 prism 2.5Δ base down
  • L 7.20:6.20/7.90:7.00/8.90:7.80/10.80:8.6 -7.50

Both lenses were manufactured using the Boston XO material.

Both contact lenses demonstrated excellent centration and movement on the eye as well as good alignment with the corneae. Visual acuity with the contact lenses was R 6/7.5, L 6/6= with negligible residual ametropia evident on over-refraction. Most importantly, the patient was no longer aware of any blurred vision with her right eye.

The patient wore these lenses successfully for about six months, at which time her contact lens wear started to become more symptomatic due to a combination of her bilateral dry eye condition and also the thicker (toric) lens design she was now wearing on her right eye. As a result, the patient was refitted with a piggyback contact lens system in both eyes, with this arrangement comprising a disposable silicone hydrogel contact lens worn beneath a rigid gas-permeable contact lens. In addition, it was decided to incorporate the correction for the residual astigmatism of the right eye into the disposable silicone hydrogel lens. By doing this, the patient could now revert back to a spherical rigid gas-permeable lens design for the right eye, with the expectation that this lens would be more comfortable than the previous toric design due to the reduced thickness and weight.

The patient's new contact lens prescription was as follows:

  • R: 6.95:6.00/7.85:6.80/9.15:7.60/11.55:8.4 -3.25
  • with R: CooperVision (Fairport, NY, USA) Biofinity Toric: 8.7/14.5/-0.50/-2.25 × 170
  • L 7.20:6.40/8.00:7.20/9.30:8.00/11.60:8.8 -8.50
  • with L: CooperVision Biofinity Sphere: 8.6/14.0/-1.00

Both rigid gas-permeable lenses were now of spherical design and made of the Boston XO material.

Visual acuities with the piggyback contact lens system were R 6/7.5+, L 6/6=, once again with negligible over-refraction and the patient was very happy with the vision of both eyes. The fit of both the rigid gas-permeable and silicone hydrogel lenses was deemed to be satisfactory in both eyes and two years later, the patient is still happily wearing the piggyback contact lens system on both eyes with no concerns.

DISCUSSION

Toric IOLs are increasingly being used to compensate for regular refractive astigmatism at the time of cataract surgery.[2] The toric IOL is orientated at an axis to offset the combined pre-existing corneal and surgically induced astigmatism with the aim of minimising the post-operative manifest refractive astigmatism. Toric IOLs have been shown to achieve a 60 to 80 per cent reduction in refractive astigmatism compared with pre-operative keratometric astigmatism[3, 4] and demonstrate good long-term rotational stability.[3] The best result is achieved where there is accurate, reproducible measurement of the regular corneal astigmatism pre-operatively and when the corneal shape is likely to remain stable for the long-term.

Keratoconus represents a progressive non-inflammatory thinning and protrusion of the cornea usually manifesting itself in youth or adolescence.[5] Considerable visual impairment often results, even with spectacle correction, due to the development of a high degree of irregular astigmatism. If the keratoconus materially impacts on spectacle-corrected visual acuity, the condition is best managed by fitting with rigid gas-permeable contact lenses, as these provide a regular refracting surface over the cone.

Toric IOL implantation is an option for those patients with keratoconus who are required to undergo cataract surgery. There are reports of patients with keratoconus who have achieved a 70 to 75 per cent reduction in their refractive astigmatism after lensectomy and implantation of a toric IOL; however, these cases involved patients with moderate forms of keratoconus, who had relatively regular corneal astigmatism and a history of good visual acuity with spectacles.[6, 7] The authors strongly support the assertion that toric IOLs should only be prescribed for patients who have mild forms of keratoconus, with low levels of irregular astigmatism and acceptable vision with spectacles.[7, 8] Stability over a period of at least 12 months is also important.[7, 8]

For more advanced cases of keratoconus, implantation of a toric IOL after cataract surgery is more problematic, as the corneal astigmatism cannot be accurately defined due to its highly irregular nature. Related to this, clinical experience has shown that there is often a marked disparity between the measured anterior corneal surface astigmatism and the total refractive astigmatism in keratoconus. Devices used to measure the anterior corneal surface astigmatism (such as the IOL Master) make the assumption of regular corneal astigmatism. This is obviously not the case when the keratoconus is more advanced, meaning that the specifications of the toric IOL will be calculated using corneal measurements that are both misleading and spurious.

The implantation of a toric IOL into a patient with keratoconus and significant irregular astigmatism may also be inappropriate, if the patient is required to wear rigid contact lenses to achieve acceptable vision. In this scenario, the toric IOL power is derived from the irregular corneal astigmatism resulting from the keratoconus; however, once the patient resumes rigid gas-permeable contact lens wear after cataract surgery, this irregular astigmatism is virtually all but eliminated by the rigid lens, so that the patient ends up with a large degree of residual astigmatism due to the toric IOL.

The stability of the keratoconus is also a relevant factor, as any progression of the keratoconus after implantation of a toric IOL will change the refractive status of the eye, with the likelihood that the toricity of the IOL would no longer be appropriate. More importantly, if progression is seen in a spectacle-wearing patient with keratoconus, such that rigid contact lens wear becomes necessary, we have the same problem outlined previously, where a large amount of residual astigmatism will now be observed with rigid gas-permeable contact lens wear due to the rigid lenses neutralising the (irregular) corneal astigmatism on which the specification of the toric IOL had previously been predicated.

Progression of keratoconus in a patient with a toric IOL becomes an even bigger problem if the keratoconus becomes so advanced that keratoplasty is required. In this situation, the posterior chamber toric IOL must be removed and replaced with a spherical IOL as the post-keratoplasty astigmatism cannot be predicted, in terms of either degree or axis. Replacement of the IOL can be problematic and carries surgical risks for the patient.

Thankfully, progression of keratoconus beyond the fourth decade of life—when cataract surgery is most often indicated—is uncommon and most patients over 40 with keratoconus tend to experience minimal changes in their corneal curvature.[5, 9] While stability of the keratoconus could not be documented for the patient in this case report, it was noted that the patient had not required refitting in either eye for three years prior to the cataract surgery on her right eye.

In routine contact lens practice, residual astigmatism will only very occasionally need to be corrected in cases where the patient is fitted well with a rigid gas-permeable contact lens using a spherical back optic zone. Therefore, such a lens requires a toroidal front surface and lens rotation must be avoided, otherwise visual disturbance will result. Consequently, rigid lenses with a spherical back optic zone and a toroidal front surface (front surface toric lenses) will generally incorporate some form of lens stabilisation, such as prism ballast or truncation. Prism ballast is the most commonly used method of lens stabilization—as in this case—with anywhere between one and three prism dioptres of base-down prism being incorporated into the lens.[1] Unfortunately, for patients with significant keratoconus, a toroidal back optic zone cannot be used to stabilise the lens, as it will not align well on the irregular (conical) cornea.[1]

Prism ballasting can often cause rigid lenses to sit inferiorly, resulting in patients experiencing symptoms of discomfort and flare. Hence, trying to correct for residual astigmatism by means of a front surface toric rigid gas-permeable lens in advanced cases of keratoconus can be quite problematic, especially if the patient has a sagging (or oval) cone. In this situation, an alternative solution is simply to incorporate the correction for the residual astigmatism into a spectacle prescription, which is worn over the contact lens.

Another possible option when a patient with keratoconus has residual astigmatism is to use a piggyback contact lens system. This system would consist of a toric soft contact lens (usually a disposable silicone hydrogel lens) to correct for the residual astigmatism worn underneath a rigid gas-permeable lens.[10] Piggyback contact lens systems are generally used in contact lens practice when fitting with just a rigid gas-permeable lens may lead to areas of significant corneal insult due to an underlying ocular surface disease and/or a highly irregular cornea. Placing a soft lens under the rigid gas-permeable lens helps to protect the cornea from any excessive lens bearing, thereby minimising the possibility of complications associated with contact lens wear and also greatly improving contact lens comfort.[11] There is also the additional advantage, as demonstrated here, of being able to incorporate the correction for residual astigmatism into the (toric) soft lens, so that the rigid gas-permeable lens can be spherical and therefore not require any of the lens stabilisation features that may make the lens less comfortable on the eye.

Concern is often expressed as to whether the oxygen transmissibility (Dk/t) through a piggyback contact lens system can reach the level required to avoid hypoxic stress to the cornea during daily wear.[12] Weissman and Ye[13] have calculated that under open-eye conditions, the oxygen supply to the cornea is sufficient when both rigid gas-permeable and soft lenses have been manufactured from materials that have oxygen permeabilities (Dk) greater than 60 Barrer. The piggyback contact lens system prescribed for the patient in this report incorporated a rigid lens manufactured from the Boston XO material (Dk of 100 Barrer) and the disposable soft Biofinity lens (Dk of 128 Barrer). The thicker, toric soft lens for the right eye would have a lower oxygen transmissibility than a spherical lens with a power equal to the spherical equivalent of the toric lens; however, given the very high Dk of the lens material, it would be expected that the piggyback system for this patient would still adequately meet the Holden-Mertz criterion for daily wear.[12]

CONCLUSION

This case highlights an important issue in toric IOL implantation at the time of cataract surgery in patients with keratoconus who wear, or are likely to wear, rigid gas-permeable contact lenses. In fact, the same issue needs to be considered in any patient undergoing cataract surgery, who has significant irregular corneal astigmatism. A toric IOL provides the potential benefit of a reduction in refractive astigmatism and the secondary benefits of less spectacle lens-induced image distortion (due to lower differential meridional magnification) and increased spectacle independence. Notwithstanding these potential benefits, consideration needs to be given pre-operatively to refractive difficulties that may result. As highlighted by this case, contact lens fitting may become complex and problematic, if significant residual astigmatism is present.

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