Choosing the refractive target in cataract surgery
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It would seem logical that emmetropia is the ideal refractive state of the eye.
Emmetropia puts far away objects in perfect focus. Then, the natural accommodation of the eye allows shifting of the focus to near points, thereby giving a full range of sharp vision. As we age, presbyopia sets in, and our accommodative amplitude declines.
While cataract surgery has the ability to restore a clear visual axis and correct the refraction of the eye, we still do not have a commercially available IOL that provides a wide range of accommodation. If we could restore 3 D or 4 D of true accommodation, then achieving an emmetropic outcome after cataract surgery would be ideal. Depending on the patient’s needs and habits, we may often choose refractive targets that are not plano.
Plano is not always best
A recent patient presented to our clinic with cataracts and a large degree of axial myopia and corneal astigmatism (Figure 1). For decades, he wore contact lenses and then used reading glasses for near vision such as reading a book or using his cell phone. For the preoperative biometry, he removed the contact lenses for an extended period so that we could obtain accurate measurements.
Source: Uday Devgan, MD
Cataract surgery was successful in both eyes, and toric IOLs were implanted with a goal of plano (Figure 2). The patient was accepting of the need to wear reading glasses after surgery because this is what he was doing for many years. The toric IOL alignment was spot-on accurate, and the spherical power of the IOL was also ideal. After healing, his postop refraction was just about perfect plano in both eyes (Figure 3), but the patient was not happy with the outcome.
At first it seemed unreasonable for the patient not to be thrilled with the plano outcome and 20/20 distance vision without glasses or contact lenses. But after carefully listening to the patient, we understood his issues. He was undercorrecting his contact lenses by about 1 D and then just using +1.25 reading glasses for the near work. While his far distance vision was not clear, it did not bother him because he spent the majority of his day doing work on his computer with the monitor about 1 m from his face. Now, after the cataract surgery, he achieved perfect distance vision but required +1.00 glasses for his computer work and +2.25 reading glasses for near work. For this patient, a postop refractive target of –1 D may have been better than plano.
We have seen patients in our clinic who have reasonable vision for far and near, without glasses, despite being presbyopic. These patients often have a degree of astigmatism with a refraction such as –1.50 +1.50 × 90 (which is the same as 0.00 –1.50 × 180). This means that the refraction in one meridian is plano and then 90° away it is –1.50, allowing a wider depth of focus despite the presbyopia.
For this cataract patient who rarely wears glasses, we must be careful in choosing the IOL and the refractive target. If we choose a toric IOL and deliver emmetropia, we will improve the distance vision significantly, but we will also collapse the depth of field. Some patients are used to seeing the world with a degree of astigmatism, and this just feels right to them.
Specialty IOLs
With the use of diffractive bifocal and trifocal IOLs, a target of plano is usually best to achieve the desired wide range of vision without glasses. But choosing the ideal IOL power to hit this target is not easy, and even the most detail-oriented surgeons have about a 90% success rate. For a trifocal IOL, I would rather the patient end up +0.25 D instead of –0.5 D, and we will choose the IOL power with this in mind.
With some extended depth of focus IOLs, the opposite is true because a slight degree of postop myopia will extend the range of useful near vision. The same applies to the early-generation pseudoaccommodating IOLs, and targeting a small degree of monovision can help with the visual range.
What does the patient desire?
Patient desires are the key in choosing refractive targets with cataract surgery. We use a detailed questionnaire and then an interview during the consultation, deciding what would be best for each patient. While most patients want the great distance vision that comes with emmetropia, there is an advantage to a myopic target for those who wish to emphasize near vision.
A recent patient was highly myopic and desired a postoperative target of –2 D for both eyes using monofocal IOLs. This gave him great near and intermediate vision from 30 cm to about 1 m away without glasses, allowing him to perform his work all day. We also target residual myopia in one eye for patients who are used to a monovision arrangement. Even for those who have not tried it, doing about 1 D of myopia is tolerated well by most patients. For the rare patient who cannot adjust to this mini-monovision, the residual myopia can easily be treated with an excimer laser ablation to restore emmetropia.
Patients also get used to their lifelong refractive state. When aiming for a plano outcome, patients who have always been myopic will be happier at –0.5 D than at +0.5 D, and the opposite is true for lifelong hyperopes. Image size is also a factor because spectacles can give image magnification for hyperopic prescriptions, but this will be lost when the refractive power is internalized into the IOL optic.
For most of our patients, emmetropia is the ideal refractive outcome. But different patient desires and different IOLs mean that some patients will do better by choosing a target other than plano.
Surgical video of this case is available on CataractCoach.com.
- For more information:
- Uday Devgan, MD, is in private practice at Devgan Eye Surgery, Chief of Ophthalmology at Olive View UCLA Medical Center and Clinical Professor of Ophthalmology at the Jules Stein Eye Institute, UCLA School of Medicine. 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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