Hi Steve, Mononeuritis multiplex actually is a group of disorders, not a true distinct disease entity.
It can be associated with (but not limited to) systemic disorders such as diabetes, prediabetes, vasculitis, amyloidosis, direct tumor involvement, polyarteritis nodosa, rheumatoid arthritis, systemic lupus erythematosus, and paraneoplastic syndromes, also possibly may be associated to Lyme disease, Wegener's granulomatosis, Sjögren syndrome, cryoglobulinemia, hypereosinophilia, temporal arteritis, scleroderma, sarcoidosis, leprosy, acute viral hepatitis A, and acquired immunodeficiency syndrome (AIDS) & about one third remain idiopathic [ cause unknown ] because the undelying condition can't be found as yet.
Whilst it is now Asymmetric in nature it can become symmetric if not treated, in answer to what a skin punch biopsy will do, it will test the small fibre nerves for damage, these nerves are responsible for the feeling of temperture and pain.
If your neuroligist is not aggressive in finding the cause then i suggest you find a neuro or medical centre that specializes Peripheral Neuropathy [neuritis means the same thing as neuropathy]..
good luck anyway