choosing refractive targets

Plano is not always the goal

It would seem logical that emmetropia is the ideal refractive state of the eye. It 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 or 4 diopters 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 computer work or using his cell phone. For the pre-operative biometry, he removed the contact lenses for an extended period so that we could obtain accurate measurements.

Cataract surgery was successfully performed 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 from the surgery, his post-op refraction was just about perfect plano in both eyes (figure 3), but the patient was not happy with the outcome.

At first it seems 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 under-correcting his contact lenses by about 1 diopter 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 since he spent the majority of his day doing work on his computer with the monitor about 1 meter from his face. Now, after the cataract surgery, he achieved perfect distance vision but requires +1.00 glasses for his computer work and +2.25 reading glasses for near work. For this patient, a post-op refractive target of -1.00 may have been better than plano.

Astigmatism can increase depth of focus

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 like –1.50 +1.50 x 90 (which is the same as 0.00 –1.50 x 180). This means that the refraction in one meridian is plano and then 90 degrees 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 very 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 may require different refractive targets

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 instead of -0.50 and we will choose the IOL power with this in mind.

With some extended depth of field IOLs, the opposite is true since a slight degree of post-op myopia will extend the range of useful near vision. The same applies to the early generation pseudo-accommodating 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 post-operative target of -2.00 for both eyes using monofocal IOLs. This gave him great near and intermediate vision from 30 cm to about 1 meter 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 mono-vision arrangement. Even for those who have not tried it, doing about 1 diopter 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.

Remember that 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 -0.50 than at +0.50 and the opposite can be true for lifelong hyperopes. Image size is also a factor since 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 cataract surgery patients, emmetropia is the ideal refractive outcome. But different patient desires and different IOLs means that some patients will do better by choosing a target other than plano.