Toric IOLs can be successful in some irregular corneal astigmatism cases
Patients with irregular corneal astigmatism pose additional challenges when it comes time for cataract surgery. The corneal changes may be responsible for a large part of their visual loss, and simple cataract surgery may not be enough to restore vision sufficiently.
Depending on the condition and extent of corneal irregularity, however, the astigmatism may be addressed at the time of cataract surgery, typically with a toric IOL.
All cataract patients should have a detailed slit lamp microscopic examination prior to surgery. Significant corneal changes, such as stromal scars, pterygia, Salzmann’s nodules, degenerative changes or dystrophies, are usually seen well in advance of surgical planning. However, some corneal changes that can induce irregular astigmatism are subtle and easy to miss with slit lamp examination alone. Irregular mires on keratometry or curvature values that are outside the normal range alert us to do further evaluation.
Image: Devgan U
Corneal topography and tomography often provide the best overall assessment of the corneal status and degree of astigmatism, irregularity and symmetry. These platforms can also help in determining the central corneal power to be used in IOL calculations.
If the patient has a corneal issue that can be treated before cataract surgery, that should be the priority. By maximizing the corneal surface, we are able to get more accurate measurements for IOL calculations and a better postoperative refractive outcome.
Conditions such as pterygia, Salzmann’s nodules, epithelial basement membrane dystrophy and even dry eye syndrome can induce a significant degree of corneal irregularity. For all of these conditions, treatment should be initiated well in advance of cataract surgery; patients can then be followed up with serial topography to determine stability. In many cases, the cornea can be returned to having regular astigmatism with good symmetry, which is more amenable to treatment at the time of cataract surgery.
In some patients, corneal astigmatism may be best addressed without surgery. Be aware that patients with mild to moderate corneal irregularities who were successfully treated with rigid contact lenses previously, such as some keratoconic patients, may wish to return to their lenses after cataract surgery. When performing cataract surgery in these patients, we want to allow them to easily return to their contact lenses without inducing further changes.
Sometimes corneal stromal scars have less effect on the vision than expected. Performing cataract surgery as a first step to achieve better vision is sometimes all that is needed. Remember that if the cornea is highly irregular or unstable, do not touch it during cataract surgery; these patients can have any corneal issues addressed after cataract surgery. In some cases, these patients may benefit from a combined corneal transplant and cataract surgery at the same sitting.
If the patient will not likely require a corneal transplant in the near future and is best served by cataract surgery alone, obtaining accurate IOL calculations is important for visual recovery. Care should be taken to use the lowest keratometric values in the central cornea to avoid a hyperopic refractive surprise.
Use of toric IOLs
Toric IOLs are powerful tools for addressing corneal astigmatism at the time of cataract surgery, with the ability to treat 1 D of astigmatism to a bit more than 4 D of astigmatism. They are machine-made to be perfectly symmetric and regular, and because of their inert nature, toric IOLs are stable and will not change over time. Although these IOLs work best in eyes that have regular, symmetric and stable astigmatism, they can also be used in eyes with some degree of irregular corneal astigmatism as long as a few guidelines are followed:
- Look closely at the central corneal topography. Some patients with corneal diseases such as forme fruste keratoconus or mild cases of pellucid marginal degeneration can have peripheral corneal irregularities but also a reasonable degree of symmetry and regularity of the astigmatism in the central cornea, which encompasses the visual axis. When looking at the corneal topography, examine the central 3 mm to 5 mm pupillary zone separately from the peripheral cornea. Patients with symmetric and regular astigmatism in this central zone will likely do well with a toric IOL.
- Select patients who have previously done well with glasses. Spectacles will only correct the hyperopic or myopic refractive error and the regular, symmetric refractive astigmatism. Any irregular astigmatism cannot be corrected by standard glasses. This means that patients who have recently achieved good vision with conventional glasses must have a fairly regular and symmetric degree of astigmatism in their visual axes.
- Avoid toric IOLs in patients who wish to return to rigid contact lenses. Rigid contact lenses are able to address corneal irregularities by acting as a new anterior refracting surface within the eye. This means that patients who wish to return to wearing these rigid contact lenses will be best served by non-toric IOLs.
If we implant a toric IOL in a patient who then goes back to a rigid contact lens, that results in a direct neutralizing of the corneal astigmatism and an immediate unmasking of the toric IOL effect. This patient will then need to wear glasses to correct the toric IOL on top of wearing their original rigid contact lenses. Alternatively, they will need to obtain a specialty set of rigid contacts that have an anterior toric surface to balance out the now-unmasked toric IOL. The bottom line is to avoid using toric IOLs in patients who wish to return to wearing rigid contact lenses.
- Choose toric IOLs over relaxing incisions in patients with irregular corneas. Although we often use limbal or corneal relaxing incisions to address regular, symmetric astigmatism in normal corneas, we should shy away from these procedures when the cornea is irregular. Incisional relaxing techniques can further destabilize a weak cornea and produce even more irregularities.
The treatment of irregular corneal astigmatism needs to be individualized to each cataract surgery patient. Fix any issues that can be addressed prior to elective cataract surgery. For highly irregular or unstable corneas, avoid cutting into the corneal tissue so as to not worsen their course. And for patients with milder peripheral corneal irregularities but good central symmetry, consider a toric IOL. By tailoring our treatment regimen to each patient, we can address irregular corneal astigmatism and perform successful cataract surgery to maximally restore vision.