Patient anatomy, healing response greatly affect visual outcomes
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We perform a beautiful cataract surgery with the expectation that the patient will heal normally and achieve outstanding vision.
While that happens the vast majority of the time, there are occasional patients in whom the visual results are not as expected. We all know to temper expectations so that patients have a realistic concept of what is possible with modern-day IOL surgery. But sometimes, despite our best efforts, we have patients who are disappointed with the results of surgery despite a technically perfect procedure without complications. We must realize and explain that patient anatomy and healing ability greatly affect the visual outcome.
Source: Uday Devgan, MD
Anatomic considerations
When we take a photo with a camera, we know that the lens and focus are important, but equally critical is the aperture size, or F-stop in camera terms, which determines depth of field. For our patients, that is the pupil size, and it varies greatly among the population. While the pupil size does tend to decrease with age, there can still be considerable variation among patients. A larger pupil will allow more light to enter the eye, but it also lessens the depth of focus within the eye and the depth of field of the patient’s view of the environment.
When considering refractive targeting for lenses with central focusing elements, such as extended depth of focus IOLs, the pupil size can help determine where to aim. For a patient with a larger 4.5-mm pupil, only 25% of the incoming light will go through the central zone of the IOL, whereas in a patient with a smaller 2.5-mm pupil, fully 80% of the incoming light will be through the central focusing element (Figure 1). This means that for the patient with larger pupils, perhaps we should aim for a slight myopic outcome such as –0.25 D or even –0.5 D, whereas for the patient with smaller pupils, aiming for plano to even +0.25 D may give better visual results.
Also keep in mind pupil size when using newer design monofocal IOLs with a central 1 mm increase in curvature. This is an area of 0.8 mm2, so it has minimal impact for a 4.5-mm pupil (5% of incoming light), but for a smaller 2.5-mm pupil, it can have a bit more effect (16% of incoming light). Corneal aberrations such as astigmatism, spherical aberration and other higher-order changes can also affect the depth of focus by trading some image quality for a larger range. These measurements vary among patients and can significantly affect postoperative visual results.
Unpredictable healing
If we have 100 people and give them all the exact same cut on the arm, of course there will be variability in healing, with some patients healing beautifully and without a scar while others may form a keloid or have delayed wound closure. This is to be expected, and the same thing happens in the eye with cataract surgery. There is certainly a wide distribution of patient healing after cataract surgery, and we can only do a limited amount of modulation of that healing with our topical steroid and NSAID eye drops. Systemic factors affect healing, and we see this particularly in patients with diabetes in whom poor blood glucose control results in poor postoperative healing.
Patients can also have other ocular conditions that affect healing and may increase the likelihood of postoperative complications. Patients with epiretinal membranes and macular pucker are far more likely to develop cystoid macular edema and blurry vision even after a perfectly performed cataract surgery. Fortunately, this edema tends to resolve with time and medications, but the cataract surgeon may be unjustly blamed for the delayed recovery of vision.
Immediately after cataract surgery, patients tend to be impressed with their newfound vision, but in some cases, differences in healing can change the focus of the eyes over the next few weeks. We put the IOL in the capsular bag in a particular position with regards to centration, and with toric IOLs, rotation as well. Because we are replacing the 4- to 5-mm thick human cataractous lens with a thin IOL, the patient’s healing response can change the effective lens position. As the IOL moves more anterior, there is a shift toward myopia, and if it moves more posterior, the shift is more hyperopic. Again, despite a perfect surgical procedure, the patient healing can change the perceived results of surgery.
Patients are often surprised to learn that the pupil is not in the center of the cornea or iris, but rather it is typically somewhat nasally shifted. When we implant a trifocal IOL, we carefully center the diffractive rings of the lens with the patient pupil (Figure 2), and we can use the Purkinje images to help center an extended depth of focus IOL as well (Figure 3). But even with great centration, we need to keep in mind the patient’s variability in anatomy with regards to angle alpha and angle kappa. And when we perfectly center these IOLs, the patient’s healing response can cause them to decenter as the capsular bag contracts and fibroses.
Performing a beautiful cataract surgery is important to give patients the best vision possible, but it is not the only factor because patient healing response and ocular anatomy greatly affect the visual outcomes as well. The next time a patient feels like the results of her uneventful surgery are due to the surgical procedure, we should take time to gently remind her that it is probably her unusual anatomy and unpredictable healing response.
Video related to this column can be found at https://cataractcoach.com/2022/07/07/1522-pupil-size-affects-edof-iol-vision/.
- For more information:
- Uday Devgan, MD, is in private practice at Devgan Eye Surgery and partner at Specialty Surgical Center in Beverly Hills. He can be reached at 11600 Wilshire Blvd. #200, Los Angeles, CA 90025; email: devgan@gmail.com; website: www.CataractCoach.com.
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