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The prognosis for neuropathies really depends on the diagnosis, as some can improve with time and treatment, while others do not. So it is a bit premature to know in your situation if this is something that can improve.
A spinal tap can be done fairly quickly, and generally less than 5 minutes.
There are risks of the needle hitting a nerve in the lower back, as well as the general risks of infection and bleeding, but these risks are generally minimal.