You are correct that anticholernergic drugs have been associated with an increased risk of dementia. I believe though that people need to weigh the risk of dementia against the very rule and pervasive problems that someone with small fiber neuropathy potentially faces. Obviously, there is pain and sensory issues which can be harsh but there is also the very real danger of autonomic neuropathy.

Many people with small fiber neuropathy experience some level of autonomic neuropathy. These range from cardiovascular complaints ( palpitations / arrhythmias, POTS, more generalized higher beat, exercise intolerance, etc.) to gastrointestinal issues ( constipation, diarrhea, gastroparesis, GERD, etc.), to other more miscellaneous issues ( sweating problems, other organ issues). Let me be clear: autonomic neuropathy can kill you. Cardiac Autonomic Neuropathy kills people all the time. Whenever you hear people dying from "complications of diabetes", one of the big ones is cardiac autonomic neuropathy triggering an episode of sudden cardiac death. As for Enteric Neuropathy ( GI neuropathy), constipation and diarrhea are a pain but gastroparesis can really negatively impact health.

If the M1R antagonists ( pirenzepine, oxybutynin, etc.) can trigger small fiber nerve regeneration, regrowing these nerves would have the potential of halting the progression of the neuropathy and reversing these symptoms. On balance, I would take halting the progression and/or reversing the sensory and autonomic neuropathic symptoms of this disease for an increased risk of dementia. In addition, if you get to a good place with your neuropathy, there is no indication that you would need to keep the dosage the same which might ameliorate somewhat the risk of developing dementia. Just my two cents.