Over the last 15 years plus of operating on mature cataracts, I have not uncommonly encountered an iris prolapse during the operation. Usually, this is near the beginning of the operation. There certainly can be many causes for this — a beginning choroidal hemorrhage ( rare ), to poor wound construction [ fairly common ], the fluid trapped behind the nucleus or iris, and other causes.
With the MSICS, if you make your entrance into the anterior chamber too peripheral ( prematurely ), then you might have an ongoing iris prolapse throughout the rest of the operation. Wound construction is important with the clear cornea or also corneoscleral tunnel incisions.
Sometimes just changing surgical positions ( starting over ) at a different limbal location is the best course although surgeons are sometimes reluctance to begin again in a different location. You can put in a suture to reduce the size of your wound during I and A which is often when the iris prolapses reoccur.
You can try rotating or rocking the nucleus in case fluid is trapped behind the cataract but in my hands that usually is not helpful. You can reduce the pressure on the globe from the lid speculum but usually, that is not the problem.
However, if I get an iris prolapse, the first thing I do is to do a one snip radial full-thickness iridotomy. Just make a hole in the peripheral iris. You are not removing any iris [ iridectomy ] but rather just making a hole ( iridotomy ). If you look carefully through the slit lamp you will often see a small gush of fluid from the posterior chamber through the newly created iris opening ( hole ). If you get the gush then often the iris will simply fall back and no longer prolapse. This does not always solve the problem but often it will and worth the attempt/effort. It is fairly safe.
Be careful to make the hole in the iris toward the base/periphery of the iris and not near the pupil. You want a full-thickness cut ( opening ). It is quite easy to cut closer to the pupil than you want.
I would encourage you to try this as often the one snip full-thickness iridotomy will solve the problem with the iris prolapse.
Peace,
Baxter