Understanding Refractive Surprise with Multifocal IOLs

The Challenge of Multifocal IOLs

Patients with multifocal IOLs expect to be free from glasses for distance, intermediate, and near vision. However, refractive surprises can occur, leading some patients to still require glasses. In my earlier article, "The Mystery of the Near with Presbyopia Correction IOL" (link), I discussed how factors like corneal shape and axial length can affect near vision. In this article, I will explain how a refractive surprise or incorrect refraction can result in good distance vision but poor near vision.

Before diving into this topic, please read my article on "Understanding Defocus Curve of Multifocal IOLs" (link). In that piece, I explained that a patient with a bifocal diffractive IOL should have their distance focal point on the retina and the near focal point located anterior to the retina (see Fig 1). For trifocal IOLs, there is an additional focal point for intermediate vision, situated between the distance and near focal points.

Hyperopic Surprise Explained

In cases of hyperopic surprise, the distance focal point can shift behind the retina. Consequently, the near focal point may move closer to or even sit on the retina. The extent of this shift depends on the level of hyperopic surprise.

During refraction, a patient may comfortably read the Snellen Chart at a distance. Why is that? They might be using their near or intermediate focal point to read the distance chart in your clinic. However, they may struggle to read the near chart, as the near focal point has shifted beyond the retina when they converge their eyes to read (see Fig 2).

The loss of near vision due to hyperopic surprise can be significant. If a patient reads 6/6 on the distance Snellen Chart, they may experience less than N14 for near reading. Conversely, if a patient reads 6/9, they might see N12 for the near chart.

Identifying the Focal Points

How can clinicians determine if a patient is reading the distance chart using their near or intermediate focal point? Here are the steps to identify the correct focal point:

1. Assess Unaided Distance Vision: First, check what the patient can read unaided for distance.

   
   

2. Add Plus Glasses: If the patient reads 6/6, add a +2.5 diopter lens. For those reading 6/9, use a +2.0 or +1.5 diopter lens for patients reading 6/12.

   
   

3. Monitor Vision Drop: If the vision drops, it indicates that the patient used the distance focal point to read the distance chart. The drop in near acuity can also be due to other factors, such as decentration, pupil miosis post-surgery, or effective lens position. The expected drop in vision can be understood from the defocus curve of the specific IOL, but generally, it can be summarized as follows:

4. +2.5 diopter: less than .5 log mar

5. +2.0 diopter: less than .4 log mar

7. +1.5 diopter: less than .3 log mar




8. Evaluate Near Focal Point: If the patient's vision does not drop significantly with the +2.5 diopter addition, it suggests they used the near or intermediate focal point to read the distance chart.

   
   

Multifactorial Causes of Near Vision Loss

As mentioned earlier, the causes of near vision loss can be multifactorial. I previously explained how steep corneas and longer axial lengths can affect near vision. In this article, I have highlighted how hyperopic outcomes can also lead to near vision loss, often masked by patients utilizing their intermediate or near focal points for distance vision.

A wrong refraction, where the clinician fails to locate the correct focal points, can also lead to a pseudo-near vision surprise. Therefore, it is crucial for surgeons and clinicians to take the time to perform a thorough refraction.

In conclusion, understanding the dynamics of multifocal IOLs and the potential for refractive surprises is essential for improving patient outcomes. By carefully assessing focal points and considering various factors affecting vision, we can enhance our approach to patient care in cataract surgery.

For more insights and resources, visit quickguide.org.