PN can be a primary disorder.... like Charcot Marie Tooth which is a genetic disorder. The damage arises as a primary causative factor. Poisoning, toxins, vaccines, are other primary types. Often with primary causes, they can be permanent, or we haven't found a way to deal with them yet. Dorsal root damage, is also an example of primary causation. There is rarely improvement, and treatments don't exist to help. Only symptomatic treatment is typically offered. Finding the toxin and removing it may help. But some toxins like thallium (heavy metal) have no cure.
Drugs toxic to nerves fall into this category.

PN can be a secondary disorder...arising from another disease process. Diabetes, hypothyroidism, gammonopathies like MGUS (bone marrow disorders), and autoimmune diseases like vasculitis, lupus, etc.
In secondary causes, treating the primary disease may lead to
a remission or improvement in the neuropathy, and damage may be arrested. Treating the primary disease successfully may arrest progression or even allow for some healing.

I thought I would just clarify that to help the newbies out.