Most of the causes have been ruled out, even scabies was a possibility but whether any blood test was done for scabies as well? Diagnosis of scabies is made by scraping the skin and viewing the material under a microscope to see the characteristic mite or eggs.
In that case, if scabies turns out positive then instead of permethrin other treatment can be started.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.