Charcot-Marie-Tooth disease (CMT) describes a group of inherited peripheral neuropathies, or disorders, that affect the nerves outside the brain and spinal cord. These peripheral nerves are involved in sending messages that trigger muscles to contract, and in transmitting sensory information from the body back to the brain. There are different types of CMT, caused by different mutations. These mutations either affect the nerve cells themselves or the myelin sheath, the insulating layer protecting them.

Several tests may be carried out to diagnose CMT. Common tests include nerve conduction studies or electromyography to check how well electrical signals are being transmitted by the nerves to the muscles, or genetic testing to check for mutations known to cause CMT.

A peripheral nerve biopsy may also be performed to confirm a diagnosis of CMT, as — in rare cases — the above tests can be inconclusive. As the procedure is invasive, its necessity should be decided on a case-by-case basis.

What is a nerve biopsy?

A nerve biopsy involves the surgical removal of a small piece of peripheral nerve tissue to be examined in a laboratory.

For the procedure, a small incision is made under local anesthetic, allowing the doctor to remove a piece of the nerve before closing the cut. This biopsy is generally carried out on the sensory sural nerve in the leg.

What can a nerve biopsy show?

Particular abnormalities in the peripheral nerves — and specific to disease type — can be evident in CMT patients and visible under a microscope in tissue removed by biopsy.

For example, CMT type 1 can cause repeated demyelination and remyelination, or the loss and reformation of the protective myelin sheath. This is associated with nerve fibers being surrounded by “onion bulb”-shaped clumps of Schwann cells, a cell involved in the production of the myelin sheath. Some CMT type 1 patients may also have “tomacula,” or a thickening in the myelin sheath. This thickening can also be present in CMT type 4 and CMT type 3.

Nerve biopsies in CMT type 2 patients show fewer nerve fibers and so-called Wallerian degradation, where the nerve fiber starts to break down after sustaining damage.

A nerve biopsy can also help rule out CMT. For example, if there is evidence of inflammation or the presence of particular immune cells and no other abnormalities typically associated with CMT, the doctor may decide that the patient does not have this disease.

Risks associated with a nerve biopsy

A nerve biopsy carries a very low risk of causing nerve damage. Such damage usually results in a small area being permanently numb, and potential discomfort or the risk of infection as the procedure site heals. There is also a risk of the patient having an allergic reaction to the anesthetic.

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