Should surgeons recommend toric IOLs to patients with irregular astigmatism?
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Many patients can benefit from toric IOLs
It is not necessary in every case that patients with irregular astigmatism should be denied the benefits of new technology IOLs, or NTIOLs.
While corneas with RK incisions, diffuse ectasia, significant post-infectious scarring, distortion from healed lacerations or advanced rheumatologic limbal ectasia are usually poor candidates, three conditions common to corneal practice allow potentially gratifying outcomes with appropriate preparation and documented stability when implanting toric IOLs.
Epithelial (or anterior) basement membrane dystrophy is highly prevalent in the cataract age group as well as eyes with previous trauma, inherited dystrophy, unstable ocular surface disease, excess actinic exposure or a previous episode of thermal or chemical injury. When the inciting causes have been controlled, a lamellar keratectomy (usually with judicious topical intraoperative mitomycin C application) can markedly improve corneal topography. When stable for 2 to 6 months after keratectomy, these eyes can benefit from toric IOL implantation, always respecting validation of cylinder and axis by at least three concomitant topographic modalities.
Keratoconic, pellucid marginal degeneration and post-LASIK ectasia eyes can be stabilized with corneal collagen cross-linking. In the absence of apical scarring, stable biometry with multimodality validation 6 months after CXL, confirmed by manifest refraction, often encourages the surgical team to pursue toric NTIOL implantation. Reasonable topography with maximum keratometry below 55 D yields the best results. Appropriate informed consent discussions partially mitigating the endemic fixation upon perfect 20/20 vision with every cataract operation must align expectations with anticipated results. Post-cataract contact lens fitting is far simpler in eyes implanted with standard non-cylindrical IOLs, so the potential or track record for successful contact lens wear might guide the surgeon away from a toric recommendation.
Patients with a history of PK reveal an incredible variety of postoperative topographies. Although essentially always more irregular than their endothelial keratoplasty counterparts, these eyes often benefit from a lucid astigmatic manifest refraction, especially when confirming a readily identifiable axis and cylinder gleaned from multimodal topography. With documented biometric stability and reproducibility accompanied by long-term pachymetric and specular endothelial stability indicating favorable graft survival longevity, these patients are often delighted with toric NTIOLs. Again, respect the difficulties with contact lens fitting over a toric IOL. Premium IOLs offer life-changing glasses or contact lens freedom, but only for carefully treated, stable, validated, compliant, thoroughly documented and informed patients.
John D. Sheppard, MD, MMSc, is from Virginia Eye Consultants in Norfolk, Virginia.
Toric IOLs may do more harm than good
Irregular astigmatism can result from corneal surgery, corneal degenerations or trauma. Common causes include dry eye and dry eye “masquerades” such as conjunctivochalasis, Salzmann’s nodules and epithelial basement membrane dystrophy. Corneal degenerations include keratoconus, pellucid marginal degeneration and keratoglobus.
The use of toric IOLs to correct irregular astigmatism is still controversial, and a careful preoperative examination should be considered in these eyes. As a first step in my decision making, I look at corneal topography and tomography. If I see a relatively regular bow tie pattern within the visual axis, I may consider the toric IOL option, explaining to my patients that results are not likely to be perfect. But whenever I am unable to identify the axis of the astigmatism, I do not implant a toric IOL.
Biometry is still challenging in corneal ectasia and keratoconus. Typically, it overestimates the power of the cornea and underestimates the power of the IOL. In these eyes, corneal astigmatism may not be accurately defined because the calculation of corneal power is based on an assumed ratio of the anterior to posterior corneal power, which is not constant in keratoconic eyes. A wrong keratometry value will lead to errors in the effective lens position, usually resulting in a hyperopic shift.
There are a few new IOL formulas with keratoconus adjustment that use a modified corneal power derived from anterior corneal radii of curvature that better represents the true anterior-posterior ratio. However, patients overall will likely have to deal with residual refractive error and need for hard contact lenses postoperatively. And here new problems arise because RGP fitting after toric IOL implantation in keratoconus may become complex because the RGP nullifies the corneal astigmatism.
In addition, keratoconus is a progressive disease, and even in elderly patients and after corneal cross-linking, unpredictable changes may occur, leading to further worsening of vision.
In my opinion, toric IOLs can be considered in keratoconus and other forms of ectasia only in limited cases, when a regular bow tie pattern is identified within the visual axis, in patients who can be corrected with spectacles and mandatorily when the disease is stable.
In the near future, patients with irregular astigmatism may benefit from the introduction of the IC-8 small aperture lens (AcuFocus) in our armamentarium. The pinhole effect of this lens cancels the degradation of vision from up to 1.5 D astigmatism, regular or irregular.
Karolinne Maia Rocha, MD, PhD, is an OSN Technology Board Member.
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