Sometimes used in research, a muscle biopsy is a small piece of muscle tissue that is surgically removed for diagnostic purposes. Usually a local anesthetic is injected at the site of the biopsy. The invasive procedure takes less than an hour and is performed in an outpatient setting. The muscle tissue is examined by a pathologist under the microscope to determine the nature of any abnormality, specifically whether it is a primary problem with the muscle, such as a muscular dystrophy or polymyositis, or a problem with the motor neuron causing a secondary muscle atrophy, as with poliomyelitis (see Pathology).
A muscle fiber that has lost its motor neuron innervation months earlier appears as “nuclear bags” in biopsy. One that has recently lost its innervation displays intermediate (muscle fiber) atrophy. Biopsies of post-polio muscle show large numbers of nuclear bags (some from the original illness) and isolated atrophic muscle fibers. These isolated fibers indicate that the terminal axons are degenerating, not the motor neuron. Degeneration of the motor neurons would be indicated by whole sections of the muscle undergoing atrophy at the same time (Trojan & Cashman, 1996).
Reference
Trojan, D.A., & Cashman, N.R. (1996). Current trends in post-poliomyelitis syndrome. New York, NY: Milestone Medical Communications.