Hi Michele,
I have been taking trihexyphenidyl for nearly a year now, on the advice of my new neuro. It was prescribed to combat rigidity in my joints.
I find it seems to have little effect, I notice no difference if I don't take it. I have not noticed any side effects.
Your brother's symptoms don't seem to be typical for Parkinson's disease. It took 10 years of PD before I had difficulty in walking, (I have had PD for 16 years.). Difficulty in walking takes years, not overnight. PD is generally a slow developing disease,
It looks more like your brother is undergoing a breakdown of sorts, and is extremely stressed. A sudden onset of severe tremors and shaking is not PD, and I feel sure it is not due to the trihexyphenidyl.
What is worrying him at present, has he undergone some recent trauma? If you can remove the thing that seems to be causing the stress, he should improve. Is he going through any emotional or financial worries at present?
Sorry I can't help topo much, but keep us posted.
Hre is lucky to have you to look after him.
Best wishes
Ron