Some patients may benefit from a small degree of residual hyperopia, myopia or astigmatism.
There is a pride that we feel as surgeons when our patients can read the 20/20 line without glasses after surgery. Achieving true emmetropia for the lower-order aberrations of sphere and cylinder puts the Snellen chart clearly in focus. For our younger patients undergoing keratorefractive surgery, having a perfect plano outcome is great because they still have a large amount of accommodation, providing a wide range of vision from far to near. But for cataract patients, plano may not be the best refractive outcome for every patient.
In the absence of a truly accommodating IOL with a wide amplitude of dioptric effect, there can be a benefit of targeting on the side of mild myopia with a monofocal IOL for patients undergoing cataract surgery. The small amount of residual myopia will increase the functional depth of field, giving more intermediate range vision. The Snellen chart is at 20 ft, or approximately 6 m, and it corresponds to the focal point of an eye with a refraction of –0.17 D. A true refractive state of plano (0.00) corresponds to a focal point of infinity because the focal point in meters equals one divided by the myopic refraction in diopters. Classically, the teaching is to avoid hyperopia at all costs because it will result in less depth of field and it is inferior to being slightly myopic. For most patients, I agree — but is that always the case?
There are a few patients in whom a small degree of residual hyperopia may be a reasonable choice. Patients with lifelong hyperopia tell a consistent story: no glasses in their youth due to the large degree of accommodation, then earlier than expected reading glasses around age 40 years due to loss of accommodative amplitude, and then finally full-time glasses for near and far by age 50 years or so. Patients with a large degree of hyperopia before cataract surgery, such as +3 D or more, may be used to it, and they may not appreciate being left significantly myopic. In these patients, it may be better to err on the side of slight hyperopia. Most of the time, these patients would rather be +0.25 D instead of –0.5 D after cataract surgery.
For patients who receive a multifocal IOL, particularly one with a stronger add power (+3 to +4 add power on the IOL), it is important to hit the postop target of plano. And in this case, it may be better to err on the side of slight hyperopia (+0.25 D) instead of slight myopia (–0.25 D). For these eyes, choosing the IOL power that lands just barely on the side of hyperopia will result in excellent distance vision and then appropriate near vision provided by the multifocal IOL. For patients who receive a multifocal IOL with a lower add power (+2 to +3 add power on the IOL), targeting the first IOL power that lands just barely myopic may be a better choice because that will give a little better near point.
Being a low myope has really been useful for me as I become increasingly presbyopic. And many patients who are lifelong myopes will be pleased to retain a small degree of myopia after cataract surgery with a monofocal IOL. A refraction of –0.25 D will give excellent distance vision and will be comfortable for these patients, whereas leaving them at +0.25 D will give them the feeling of overcorrected glasses. Even a refraction of –0.5 D will typically give a good distance vision of 20/25 or so during the day, while increasing the effective intermediate range of vision. Increasing the residual myopia to –1 D will shift the optical focal point to 1 m away, thereby improving near vision, but it will come at the expense of distance vision.
If a patient spends most of his time doing near work such as computer and reading, and he is amenable to wearing glasses for distance, targeting –1 D to –2 D would be a great option. Keep in mind that pupil size also plays an important role in the depth of field and that people with smaller pupils may tolerate more myopia while preserving their distance vision. For patients who do not drive, being somewhat myopic after cataract surgery may be far more useful than being plano.
Some specialized IOLs such as the FDA-approved pseudoaccommodating and extended depth of field models can increase the functional range of vision, and they would benefit from a slight degree of residual myopia such as –0.25 D to –0.5 D.
The classic teaching is that we should fully correct all refractive astigmatism in order to provide the best focus and image quality. But remember that a mild degree of residual astigmatism can increase the depth of field. We have all seen astigmatic patients who seem to get by without distance or near glasses despite being presbyopic. These patients are making use of the astigmatism to increase their functional visual range, even though it comes at the expense of somewhat decreased image quality at all distances. Patients may also get used to seeing the world with a certain type of astigmatism, such as with-the-rule or against-the-rule. For a patient with a large degree of astigmatism, such as 3 D or more, it may feel more natural for the patient to end up with a slight degree of residual astigmatism at the same axis instead of achieving perfect plano with no cylinder.
The take-home message is that the refractive targeting for cataract surgery should be individualized to the specific patient, his needs for his vision and his particular biometry. There is no such thing as “one size fits all” when it comes to refractive surgical planning.