Hi Sparkles It seems that you have had a double load of bad in a short time. I am sorry to read about this. I hope that your cancer treatment results remain positive. About the chemo effects on Chiari, that is a good discussion.
I am sure that you have received information about chemotherapy and how it affects your brain processes. A lady I know often comments on her “chemo brain” when she is having difficulties remembering or processing conversations. Not talked about much by neurosurgeons nor neurologists, but more so by neuro-psychologists is that Chiari also affects brain processes. Of course, a neuro-psychologist would focus on the cognitive part of brain processing and give a shrug at vision or balance problems. Cognition is what they assess after all.
The anatomical abnormality of Chiari can affect the brain processes (even far away from the actual hernia site!) in a variety of different ways, which is why Chiari folk do not share all symptoms. But there are groupings. Brain processes include balance, vision, hearing, divided attention, reading comprehension and more. I have had it described that Chiari folk already have a type of brain injury and brain injured folk are more susceptible to further insult like chemotherapy, whiplash, and medications to name a few.
Perhaps your neurosurgeon will be aware of how Chiari affects how the brain works and be able to discuss those side-effects with you. Unfortunately, he will probably not have any solutions for you other than surgery if you are a medical candidate. Please know that there is information on this site about treating Chiari imposed balance and dizziness by retraining the brain’s processing they do work very well post surgery though you would probably have to do them regularly to achieve the same effect.
Good luck with your appointment. Stay calm.