BLOG: How to get multifocal results with monofocal IOLs
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After my dad got cataract surgery with monofocal IOLs aimed for plano, he wasn’t supposed to be able to read without glasses. And yet, to his utter delight, he could.
We strenuously warn all cataract patients who elect to receive a standard monofocal IOL with a plano target that they will be absolutely dependent on readers or bifocals for all near tasks. However, every clinician knows that a small percentage of these patients will happily inform them after the surgery, “Doc, I can read without glasses!” How does this happen, and can you predict before the surgery who will get this unexpected benefit?
There are five factors that subtly improve depth of field, and if your patient gets lucky with enough of them, they’ll be able to read without glasses. This ability is referred to as “pseudoaccommodation” — the ability to see near targets well despite a pseudophakic lens (Nanavaty et al.).
First, all optometrists know that an optical system’s depth of field depends on the aperture size, and, in fact, patients with pseudoaccommodation do have smaller pupils. The best chance for a large depth of field occurs with a pupil smaller than 2.6 mm (Lim et al.). Fortunately, a near target stimulates accommodative miosis, so a pupil just larger than this might constrict enough to get the effect.
The second factor is a slightly myopic outcome. The key is to get enough myopia to increase near vision without unduly compromising distance vision, and the amount of blur that can be acceptable will vary by patient. Many patients can tolerate up to 0.50 D of myopia without noticing a significant drop in vision.
The third factor involves the beneficial aspects of astigmatism and higher-order aberrations. Astigmatism and higher-order aberrations decrease acuity and contrast sensitivity but increase Sturm’s conoid and provide multiple focal points on the visual axis. Therefore, a small amount of residual astigmatism will increase the depth of field. No correlation has been found between reading speed and whether the patient has against-the-rule or with-the-rule astigmatism. A slightly higher amount of spherical aberration also gives better near vision. Indeed, this is the mechanism used by IOLs like Johnson & Johnson’s Tecnis Eyhance and Rayner’s RayOne EMV, which purposely incorporate spherical aberration to elongate the depth of field.
Fourth, shorter axial lengths — and thus typically more hyperopic eyes — more often give a longer depth of field, especially eyes shorter than 23 mm (Lim et al.). The mechanism for this is unclear. Some studies have suggested a small forward movement of the IOL during attempted accommodation but only in shorter eyes. This could be explained by a smaller capsular bag exerting greater force on the IOL than in longer, larger eyes (Nemeth et al.). However, this effect is not seen with all IOL types, and the correlation is weak.
A fifth and important factor is the patient’s motivation. Despite very little objective accommodation, such as movement of the IOL and pupil miosis, some patients will want to be able to read badly enough that they will push through some blur to achieve their goal. This is subjective accommodation, the patient’s actual ability to read at near. For some pseudophakes, subjective accommodation will far outstrip objective accommodation.
A monofocal patient who is lucky with their pupil size, corneal aberrations and axial length, and who is willing to accept some blur at distance could get a surprising amount of functional vision at both distance and near — especially if they want it badly enough.
References:
- Lim DH, et al. J Refract Surg. 2013;doi:10.3928/1081597X-20130129-06.
- Nanavaty MA, et al. J Cataract Refract Surg. 2023;doi:10.1097/j.jcrs.0000000000001302.
- Nemeth G, et al. J Cataract Refract Surg. 2013;doi:10.3928/1081597X-20130129-06.
For more information:
Oliver Kuhn-Wilken, OD, practices at Pacific Cataract and Laser Institute’s Tualatin Clinic in Oregon.
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