This article is more than 5 years old. Information may no longer be current.
Preoperative evaluation important for IOL selection in keratoconus, post-keratoplasty and post-LASIK eyes
Regular corneal astigmatism and irregular corneal astigmatism play a role when choosing an IOL
Patient selection is crucial in achieving success with toric and multifocal IOLs. Astigmatism is the most important factor that contributes to poor visual outcomes in grafted, keratoconus and post-LASIK eyes. Today, while maintaining a clear cornea is important, success is measured by the improvement of vision.
Preoperative examination is important
Accurate preoperative diagnostics are essential for patient counseling and surgical planning. In the preoperative examination, evaluation of visual acuity with spectacle refraction is also important. Toric IOL implantation can be considered in patients with mild to moderate amounts of irregular astigmatism who can be satisfactorily corrected using glasses. However, if patients require rigid gas permeable contact lenses to correct astigmatism, toric IOL use is less appropriate. In keratoconus patients, progression should be stopped by such means as cross-linking or intracorneal rings.
Astigmatism is multifactorial
Aylin Kiliç
Astigmatism measurement is important for the performance of IOLs. Patients with regular bow tie astigmatism are most suitable for toric IOL implantation. Although toric IOLs are most effective in the correction of regular astigmatism, in patients with irregular corneal astigmatism, including keratoconus, post-keratoplasty and post-LASIK eyes, lenses of this type can be used. But patients with any degree of irregular astigmatism are not good candidates for multifocal IOLs or multifocal toric IOLs because of the image degradation caused by this technology. It is important to be aware that patients who have had keratoplasty, LASIK or radial keratotomy or who have forme fruste keratoconus frequently show many aberrations, with the cornea itself often becoming multifocal. Placing a multifocal IOL behind a multifocal cornea runs the risk of an additional loss of contrast with a significant reduction in visual quality.
Topography is a mandatory preoperative measurement because it provides useful information on the regularity of the corneal surface. To understand the severity of irregular astigmatism, corneal topography using Placido disc videokeratoscopy and Scheimpflug imaging can give precise details of the anterior and posterior corneal surfaces and corneal thickness. We use the Orbscan (Bausch + Lomb), Pentacam (Oculus) and IOLMaster (Carl Zeiss Meditec) to estimate the cylinder power choice, which is based on the amount of corneal astigmatism. Any discrepancy between the two, in terms of axis of greater than 10°, can be remeasured after dry eye treatment. Because surface irregularity due to dry eye is more common after graft surgery, LASIK or keratoconus and dry eye may change keratometric values significantly.
In addition, routine preoperative work-up may include an anterior corneal aberration profile for an elevation of third- and fourth-order aberrations to exclude coma and keratoconus.
Keratoconus eyes and IOL selection
In eyes with keratoconus, measured keratometry readings are not accurate due to corneal distortion caused by ectasia. The visual axis of eyes with advanced keratoconus may not pass through the steepest portion of the cornea, and effective lens positioning plays a crucial role in the final outcomes. In these cases, multifocal IOLs are contraindicated. However, toric IOL implantation for the correction of astigmatism can be considered.
We had five cases of toric IOL implantation for keratoconus. One year postoperatively, visual acuity in all eyes increased and mean astigmatic refraction improved from –4.7 D to –0.7 D. I believe that in carefully selected cases of nonprogressive keratoconus, toric IOLs may be an excellent solution.
IOL power calculation different after LASIK
Patients who have LASIK surgery to avoid the use of spectacles have less predictable outcomes after developing cataract or presbyopia than routine eyes because all standard formulas for IOL power calculation are based on keratometric values. A major factor in the miscalculation of IOL power in irregular corneas is the wrong practice of measuring only the anterior surface with keratometry or corneal topography. Due to the altered anterior/posterior corneal curvature relationships after excimer laser vision correction, the keratometer will overestimate corneal power in previously myopic eyes and underestimate corneal power in previously hyperopic eyes.
The use of hybrid refractive-diffractive multifocal IOLs after hyperopic and myopic LASIK can be effective, but evidence is limited. Studies suggest that multifocal IOLs with an aspheric profile provide better visual and optical quality than spherical designs. Myopic LASIK induces positive spherical aberration, while hyperopic laser correction induces negative values. A spherical multifocal IOL in eyes with hyperopic LASIK compensates the negative spherical aberrations.
Multifocal IOLs have undergone modifications over time to enhance distance and near vision. One possible presbyopia-correcting IOL is a new category of IOLs known as extended depth of focus IOL. The latest generation of presbyopia-correcting IOLs intends to reduce unwanted side effects with distance performance comparable to a monofocal IOL.
We recently treated a 62-year-old woman presenting with vision loss who had LASIK in both eyes 10 years earlier. She was scheduled for cataract surgery with an achromatic technology, echelette design IOL (Symfony Tecnis, Abbott Medical Optics). For IOL power calculation, the hyperopic Haigis formula was used to determine the spherical equivalent IOL power for emmetropia. One month postoperatively, uncorrected distance visual acuity had increased from 20/200 to 20/25 in the right eye and from 20/100 to 20/20 in the left eye. The patient gained seven lines of uncorrected near visual acuity, achieving a mean binocular uncorrected near visual acuity of 20/25. The patient was satisfied with her distance and near vision and did not report halos or glare in the short-term follow-up.
In summary, implantation of toric and multifocal IOLs may be useful in selected patients with regular corneal astigmatism after LASIK and keratoplasty. In cases with irregular corneal astigmatism, such as forme fruste keratoconus patients, multifocal IOL implantation leads to unpredictable and unsatisfactory outcomes. However, toric IOL implantation offers the opportunity to achieve spectacle independence for distance. Accurate preoperative examination and measurements are essential for surgical planning.
References:
Alfonso JF, et al. J Cataract Refract Surg. 2009;doi:10.1016/j.jcrs.2009.04.045.
Kohnen T, et al. Ophthalmology. 2005;doi:10.1016/j.ophtha.2005.05.004.
For more information:
Aylin Kiliç, MD, can be reached at Dunya Eye Hospital, Istanbul, Turkey; email: aylinkilicdr@gmail.com.
Disclosure: Kiliç reports no relevant financial disclosures.