welcome to HCM.com.
First of all, the chiari malformation
that was picked up on your MRI may be just an incidental finding. If it is not causing you severe symptoms, there maybe nothing to worry about. Most commonly this may present as some headache and dizziness.
According to me, there is no connection with your psoriatic arthritis
for which you should consult a dermatologist
who will start you on some tablets.
If however you do develop symptoms like weakness, blurred vision
or any other disturbing symptom, it is worth getting your Chiari malformation looked into by a neurologist
Wish you good health and take care.