Nerve Biopsy

Rationale

Patients may have unexplained pain, weakness, or numbness in their extremities from a variety of diseases that damage nerves. Some are treatable and some are not. Electrical testing is used to help confirm this general diagnosis, which is called neuropathy. However, a specific diagnosis is required before treatment. When a medical history, clinical exam, laboratory tests, and imaging do provide the specific diagnosis, then a nerve biopsy may be requested. The results help guide further treatment. It must be kept in mind that even when a piece of your nerve is sent to the laboratory, there is still a chance that the diagnosis is not certain, or that your illness is not treatable.

Types of Nerve Biopsy

In general, the most affected nerve should be biopsied. The most common type of biopsy is a sensory nerve biopsy, where about one inch of a sensory nerve in the leg is removed for analysis. In removing this segment, the biopsied nerve no longer functions. This causes a patch of numbness on the top or side of the foot. Although this area of numbness usually resolves in 6-12 months, it nevertheless may be permanent. Fortunately, even when permanent, most patients do not notice it on a daily basis. The two most common sensory nerves to biopsy are the superficial peroneal nerve on the shin, and the sural nerve above the outer ankle. These nerves do not control muscle function where they are biopsied, and therefore, there is basically no risk of paralysis. Some diseases predominantly affect motor nerves. A selective motor nerve biopsy can be performed on a branch to the gracilis muscle along the inner thigh. Because there is another motor branch to this muscle, noticeable weakness does not occur. When neuropathy affects major motor/sensory nerves (e.g., sciatic nerve), then a fascicular biopsy of these nerves can also be done. During a fascicular biopsy, the nerve is exposed and carefully divided into its many fascicles. Each fascicle is tested electrically during surgery to determine if it carries motor or sensory function. So as to avoid any weakness, a sensory fascicle is removed. Removal of this fascicle may cause a patch of numbness similar to a sensory nerve biopsy.

Surgical Procedure

A nerve biopsy (excluding fascicular biopsies) takes about thirty minutes and is performed in day surgery under sterile conditions. It can either be performed with only a local injection of lidocaine, or with some intravenous sedation. About half of patients request sedation. The incision is about 1.5 inches long, so that a one-inch piece of nerve can be removed. This size is required for an adequate diagnostic yield. The skin is then closed with two layers of sutures that absorb under the skin. In addition to numbness occurring, the risks of the procedure include about a 1% chance of infection, as well as few percent chance of wound tenderness from neuroma formation. To prevent this latter complication, the cut nerve is always buried away from the skin so that tenderness rarely occurs. Simultaneous nerve and muscle biopsies can be performed via the same incision, when required.

Another alternative is to repair the biopsied nerve with either a local vein or absorbable collagen conduit to promote regeneration over time.

Sural nerve biopsy under local anesthesia in day surgery. The yellow loop is around the sural nerve and the blue loop is around the lesser saphenous vein.

Day of Surgery

If sedation is requested, then you must not eat or drink after midnight the day before the procedure. You may take your normal medication with a sip of water the morning of surgery. Furthermore, for sedation you need to have some laboratory tests performed beforehand in pre-admission testing. No aspirin or Motrin should be taken the week before or after the surgery. You arrive in day surgery an hour before the procedure where you meet the surgeon (if you haven't already). After the procedure you are observed in the recovery room for about an hour. You may go home with a family or friend taking you home.

Sciatic nerve biopsy. The sciatic nerve is exposed (above, between arrows). Next, a portion of the nerve is separated into its component fascicles using a microscope (below, each fascicle is in a white loop). Each fascicle is tested with electrical stimulation before one or two are chosen for removal.

Recovery

Overall this is not a painful procedure. It is normal to have some bruising near the wound for about a week or so. The occlusive dressing is removed after 3 days, revealing some stickers underneath. These stickers are left in place until you see your surgeon about one week later for a wound check. You may shower right after surgery with either the occlusive dressing or stickers exposed. For the first week after surgery, you may work, but avoid strenuous activity with the affected area. Also avoid unnecessary walking for about a week while the wound heals. Although rare, signs of infection include discharge, redness, and progressively worsening pain. If these occur, you should notify the surgeon. For procedures below the knee, you should elevate the leg as much as possible the first three days after surgery to prevent swelling, which can normally occur. If an ace bandage is used, this can be removed for showers, but should be re-applied for the first three days.

Results

The laboratory results take about 1-2 weeks to be completed by the pathologist. These results are faxed to your referring physician for analysis. In general, you should follow-up with him or her in regard to an explanation and further treatment.

Further Reading

National Institute of Neurological Disorders and Stroke
Hereditary Neuropathies Information Page