Nerve and muscle biopsy are usually done simultaneously.
Nerve biopsy can help differentiate axonal from demyelinating polyneuropathies when other tests are inconclusive. A nerve supplying the affected area should be chosen.
If polyneuropathy may be caused by vasculitis Overview of Vasculitis Vasculitis is inflammation of blood vessels, often with ischemia, necrosis, and organ inflammation. Vasculitis can affect any blood vessel—arteries, arterioles, veins, venules, or capillaries... read more 
, the sample should include skin to increase the chances of finding a characteristic vascular abnormality.
If the biopsy shows that nerve endings are lost, skin punch biopsy can help confirm small-fiber polyneuropathy.
Muscle biopsy can help confirm myopathies Congenital Myopathies Congenital myopathy is a term sometimes applied to hundreds of distinct neuromuscular disorders that may be present at birth, but it is usually reserved for a group of rare, inherited, primary... read more .
