The medication is a steroid and helps get rid of inflamation. It will make you feel better so finish it as prescribed. I don't know about your arm as I don't have any more detailed information. Get the test done. I feel sure your doctor will be able to diagnose you soon.
Hi!
If the arm pain was due to some neuropathy, then this does respond to prednisolone or steroids. If the neuropathy started post viral infection (flu, cough cold, fever etc), then in all probability this should subside and not recur. However, if the neuropathic pain is secondary to diabetes, hypothyroidism, Lyme’s or lupus, then these conditions need to be treated.
Arm pain can also be due to Scalene Myofascial Pain Syndrome, inflammation of veins or phlebitis, inflammation of lymph channels or lymphangitis, deep vein thrombosis or compartment syndrome. So a correct diagnosis will help in correct treatment and pain relief. Do discuss this with your doctor. The MRI may show up something. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.