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, 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.