Ami Halvorson, OD

PCLI—Portland, OR

From the EDITOR   Over the last three decades, cataract surgery has evolved into a highly successful treatment with excellent refractive outcomes. However, occasionally, patients experience over or under-correction, known as a refractive surprise. In this issue, my colleague Rick Burk discusses this phenomenon, how it can happen, and how it is managed.

By Rick Burk, OD   |  PCLI—TUALATIN, OR

Cataract surgery is the most performed surgery in the U.S., helping millions each year by restoring clear vision, correcting refractive errors, and addressing presbyopia. New advancements, like extended depth of focus (EDOF) IOLs, improve vision for distance and near tasks without heavy reliance on glasses. However, some patients experience a “refractive surprise” where the intended refractive target is not achieved. Join me as we discuss this phenomenon and explore treatment options.

Evolving Expectations

During residency training in the early 90s, my discussions with cataract patients were straightforward. If cataracts were present, the recommendation was often surgery, with the understanding glasses would still be needed post-operatively. A lucky few achieved clear distance vision without glasses, but they were the exception rather than the rule.

Today, we have enhanced measurement techniques and a much broader selection of IOLs, which can significantly reduce dependency on glasses. However, these advancements also raise patient expectations for visual outcomes. So, what do we do when the refractive outcome deviates significantly from the intended target? First, let’s explore some causes of refractive surprise.

The Mechanics of IOLs

It is important to understand what IOLs can and cannot do. Unlike spectacle lenses, which can be ordered with precise power increments (as small as 0.12 diopters), IOLs typically come in 0.5 diopter steps. This limitation can create situations where the necessary lens power falls between available options, making it difficult to achieve perfect vision. Consequently, some patients have residual refractive errors after surgery. With most IOLs, reaching a perfect plano is often more luck than skill.

Pre-operative counseling is crucial, and setting realistic expectations helps mitigate post-operative disappointment. While most patients achieve outcomes close to the intended target, significant residual errors can lead to dissatisfaction.

IOL Power Calculation

Calculating IOL powers uses three main parameters, which I have listed in order of importance.

1. Axial Length – Today’s biometers can measure axial length with remarkable accuracy, typically within ±0.02 to 0.04 mm. To put this into perspective, a 1.0 mm error in axial length will equate to a refractive surprise of about 3.0 diopters. While errors in measuring axial length are rare, they can occur—especially in cases of dense cataracts where optical biometers may fail to provide accurate readings. In these cases, we rely on ultrasonic biometry, which is not quite as precise.
2. Corneal Curvature – Corneal curvature can be measured accurately within ±0.25 to 0.50 diopters, assuming the corneal surface is healthy. Pre-operative optimization of the corneal surface is vital to minimize variability in refractive outcomes.
3. Estimated Lens Position (ELP) – This is the least controllable variable. Modern IOL calculation formulas assume that the new lens will be centered in the same position as the natural lens. However, if the IOL settles in a different position than predicted, it can result in unexpected myopic or hyperopic outcomes. While such positioning errors are generally small and infrequent, they can contribute to a refractive surprise.

The Influence of Previous LVC

Patients with a history of laser vision correction (LVC) face a higher likelihood of a refractive surprise with cataract surgery. This is due to the alterations in corneal curvature that make accurate IOL power calculations more challenging. Access to pre and post-LVC refractive data can aid in refining IOL power selections, thus minimizing the risk of surprise.

Managing a Refractive Surprise

When a refractive surprise occurs, what are the options for correction?

1. Non-surgical Options – Glasses or contact lenses can effectively address residual refractive errors, which is often the simplest solution.
2. IOL Exchange – If the discrepancy is significant, an IOL exchange may be warranted. This procedure typically carries a low risk if timed carefully. Before considering an exchange, it is advisable to wait until any post-operative corneal edema has resolved. Once refractively stable, the sooner the IOL exchange, the better. A good time frame is 2 to 5 weeks post-surgery. Toric exchanges are a little higher risk as the IOL can only be placed inside the capsular bag. If the capsular bag is compromised during the exchange, placing any new IOL may be complicated, and the refractive outcome may be less predictable.

An option for patients seeking greater flexibility in their refractive outcomes is the Light Adjustable Lens (LAL). This innovative IOL allows for three post-operative UV light adjustments to correct residual refractive errors within a limited range (approximately 3.0 diopters). While not infinitely adjustable, it offers a significant advantage for patients who experience (or want to avoid) unexpected refractive outcomes.

3.  Laser Vision Correction – This is another viable option, especially for smaller refractive surprises. It can typically be performed 2.5 to 3 months after cataract surgery when the refractive status has stabilized. An IOL exchange may yield better results for larger errors, particularly in cases of significant hyperopia, where an IOL often provides more stable outcomes than LVC.

Conclusion

Thankfully, refractive surprises after cataract surgery are not common. However, when they do occur, timely intervention can increase the chance of resolving the problem with minimal risk. If you have questions or concerns, please do not hesitate to contact any of our OD staff. By navigating these challenges with informed strategies, we can enhance cataract surgery patients’ overall experience and outcomes.

An IOL that settles more anterior can lead to a myopic refractive surprise.