How near vision can be compromised with a multifocal IOL

Patients with multifocal IOLs are implanted with an expectation that they would be spectacle or glass free for distance, intermediate and near. However, it is not uncommon for patients to land up with refractive surprise and may need glasses. Earlier in the article 'The Mystery of the Near with Presbyopia Correction IOL' (https://www.quickguide.org/post/problem-with-near-vision), I had written how a steep or flat cornea and a long or short axial length may impact the near vision of the patient. In this article, I will throw light on how a refractive surprise or wrong refraction may lead to a patient with an acceptable distance vision but a poor or suboptimal near vision.

Fig 1 - Depicting the distance focal point on the retina and the near focal point anterior to the retina quickguide.org

Before reading this short write up, please make sure that you have read and understood my article on 'Understanding Defocus Curve of Multifocal IOLs' (https://www.quickguide.org/post/defocus-curve). In that article, I described that a patient implanted with a bifocal diffractive IOL is expected to have the distance focal point on the retina, and the near before the retina (that is anterior to the retina, refer Fig 1) . In case of trifocal IOLs, there would be an additional focal point for the intermediate vision, and this will be between the distance and the near focal point.

In cases of hyperopic surprise, the distance focal point may shift behind the retina and the near focal point may come close to, or seat on the retina. How far the distance focal point will shift beyond the retina and thereby the near or intermediate (incase of trifocal IOL) will shift towards the retina depends on the level of hyperopic surprise.

As the clinician does the refraction, the patient may read the Snellen Chart at the distance fairly comfortably. Why? Because in this situation, the patient may utilize the near or intermediate focal point to read the distance chart at your clinic, but fail to read the near chart, as by then the near focal point shifts beyond the retina as the patient converges his eye to read (Fig 2- picture below).

Fig 2

As I said earlier, the near loss in reading will depend on the hyperopic surprise, that is the shift of the distance focal point beyond the retina. The higher the patient reads the distance Snellen Chart with the near or intermediate focal point, the lesser will be his near vision recorded when reading the chart. Thus a patient reading 6/6 the Snellen Chart at a distance of 6 meter, may experience less than N14 for the near reading. A patient utilizing the near focal point to read 6/9 may experience N12 for the near chart.

The question is how does the clinician realize that the patient is reading the distance chart with the help of the near or intermediate focal point. Locating the right focal point is important to determine this. Here are the steps:

First, determine, what is the patient reading unaided for the distance.

Second, if the patient reads 6/6 for the distance, dial in a plus glass of around +2.5 diopter. If the patient reads 6/9 dial in a +2.0 diopter or a +1.5 diopter for the patient reading 6/12.

Third, if the vision drops, you are confident that the patient has used the distance focal point to read the distance chart. The drop in near acuity may be because of other factors, decentration, pupil myosis post surgery, effective lens position, etc. The expected drop in patient's vision can be understood from the defocus curve of the particular IOL but in general are:

a) +2.5 diopter - less than .5 log mar

b) +2.0 diopter - less than .4 log mar

c) +1.5 diopter - less than .3 log mar

Fourth, if the patient's vision does not drop significantly with the +2.5 diopter add, you will understand that the patient has used the near focal point (or intermediate focal point) to read the distance chart.

As pointed earlier, the causes of drop in near vision may be multifactorial. After I have explained in an earlier article how near vision is affected by a steep cornea and a longer axial length or a deeper effective lens position, I have highlighted in this article how near vision can also be lost through a hyperopic outcome, masked by patient utilizing the intermediate or near focal point to see the far. Often, a wrong refraction, wherein the clinician fails to locate the right focus points is also a cause for a pseudo-near vision surprise. Thus, surgeons and clinicians should be careful to spend time doing a through refraction.