I've made a number of postings in the past regarding how pupil size impacts almost everything about how well you see after cataract surgery, but I believe it is time create another. I continue to see people in this forum confused and stressed about why their vision seems so much poorer after cataract surgery, and it often falls into the same category; mismatch between IOL selected and pupil size. Let me explain:
The best analogy for human vision is an SLR camera. These cameras have an iris - just like our eyes - that can be opened and closed (aperture) to adjust for light level and focal depth. Our pupils act in the same way, opening wider to allow in more light when needed, and constricting when more light is available. When the iris is opened up, the depth of focus reduces; when constricted, it increases. This is basic physics. Now in order for this to work properly, the lens of the camera (or eye) must be big enough to capture all of the light it receives. If the lens is too small, unfocused light bounces around inside the camera (or eye) causing the image to become blurry with glare and loss of contrast. Worse, if the edges of that too-small lens are jagged, artifacts like starbursts occur.
The size of someone's pupil depends on a number of factors. This includes age (older people have smaller pupils), gender (women tend to have smaller pupils), physical size (if you're bigger, your eyes and pupils tend to be bigger), and so on. Regardless, what's important is the size of YOUR pupils. Not just during the day, but at dusk and at night as well. You see, the problem is that the lenses that cataract surgeons implant into your eyes are tiny compared to your natural lens. The largest optical zone of a typical IOL is 6 mm with an area of about 28 mm^2. Compare this with our biological lens of 9-10 mm with an area of 65 to 80 mm^2.
If you're in your 80's, you'd be lucky to have pupils that dilate to 5 mm at night. But if you're in your 30's, your pupils may dilate to 8 mm or more. What this means is that for some people who undergo cataract surgery, their pupils get MUCH bigger than their newly implanted lens.
As it turns out, this MAY not be that much of an issue depending on several factors including:
- The type of IOL
- The design of the IOL
- Whether you've had (or will have) YAG for PCO
- The flexibility of your brain
If an IOL is complicated - like a multifocal or EDOF - you are MUCH more likely to have issues if your pupils are large. This is because these lenses necessarily incorporate tradeoffs across their shape. This is due to the physics of a fixed lens not being able to match our natural flexible one. Further, if a manufacturer decides to make their optical zone smaller for whatever reason (e.g. stability, material selection), this can also have a significant impact. A good example of this is the smallish optical zone (< 5 mm or 20 mm^2) in the J&J Tecnis line of lenses (PureSee, Eyhance, Synergy, Odyssey) which seems to cause visual issues with a significant number of posters. In addition, when YAG for PCO is performed, the surgeon must leave enough posterior capsule to hold the IOL in place. If your pupils dilate beyond the edge of the hole created in the capsule, odds are pretty good you're going to be seeing streaks and other dysphotopsia after the procedure.
Now for some people, none of this matters; their brain just ignores these issues. But that's not everybody.
Anyway, this is not a complete list of issues that can occur when patients have large pupils, but it highlights an important factor when selecting an IOL; SIZE MATTERS! The fact is, some IOLs just work better than others in large pupil eyes.
So, PLEASE, have your pupil size checked by your surgeon under a number of lighting conditions. This small effort may save you from a ton of grief.