How much with-the-rule astigmatism would you intentionally leave in patients under 40 undergoing cataract surgery, considering the expected age-related shift toward against-the-rule astigmatism?

IOL selection and expectations are hugely important in this conversation. Will leave less for a multifocal, and more with a standard IOL. A sharpshooter, I might correct all the cyl with a standard IOL, but with the expectation they will need touch-ups.

A good place to start is their topo and their Mrx. IF they have 3D cyl in their topo and only take 1D in their Mrx, there is a good chance they will eat 2D cyl and only need 1D correction. It's not an absolute, but establishes a ball park.

In general, I'll leave 1.5 D WTR for someone under 40. Maybe 1 in 10 needs any cyl post-op correction, probably less.

I wouldn't even consider a toric for anything under 2.5D, instead can offer to "reduce, but not eliminate" cyl with an LRI and leave a rea...

I think there are a couple ways to look at this, and part of it depends on what your/the patient's goals are (crispest vision at post-op month 1 vs. better uncorrected vision 30 years from now) and what lens you are implanting (monofocal vs multifocal, the latter being less likely to tolerate higher levels of astigmatism without degrading visual quality more significantly).

The expected age-related shift towards against-the-rule astigmatism has been reported to be as high as 0.25D per decade. But under-correcting by more than 0.75D, even with a monofocal lens, is likely to have a more negative effect on visual performance in the immediate post-operative setting, which for a patient paying for a toric IOL is likely the setting that they will...

I agree with Dr. @Dr. First Last. I think leaving a small amount of WTR astigmatism is prudent. A discussion with the patient on what to expect and the possibility of needing a future astigmatism correction as a possibility.