I live north of Baltimore, and go to a neuro at Franklin Square. Initially I saw the head of neurology there, but he's semi-retired now. I do like the one I see, but with all tests normal, she has no ideas either. And to answer your question, it's not at all unusual for them not to have a clue what the cause is.

I've had MRIs on my brain, neck, thoracic spine and lumbar spine, as well as a spinal tap, nerve conduction study, EEG, and multiple rounds of blood work. The brain MRI and lumbar MRIs were repeated this year as it had been four years since I'd had them. The only abnormalities are a non-specific signalling anomaly in my brain that hasn't changed in four years, and evidence that my leg nerves are being impacted by degeneration in my lumbar region. The latter is not a cause of the neuropathy.