By Todd Levine, MD
Peripheral nerves are actually composed of individual nerve fibers of varying sizes. The size of the nerve (or axon) is dependent on how much insulation (or myelin) is wrapped around the axon. The amount of myelin determines how fast the nerve will conduct electricity. The size of the nerve conveys different types of information.
Our largest nerves work about as fast as an airplane flies and transmit information that affects our balance. The medium-sized nerves help move our muscles. The smallest nerves, or nerves with no myelin or very little myelin, work about as fast as a person can walk and convey information about pain, burning, tingling, etc. So, small fiber neuropathy is when these small nerves are damaged; the result is pain, numbness and/or tingling.
While most disease processes affect all the different nerve sizes equally, there are some diseases that will preferentially affect only the small nerve fibers. The most common condition to cause a small fiber neuropathy is diabetes or pre-diabetes, but certain vitamin deficiencies, infections or immune system disorders can also preferentially affect these small fibers.
The diagnosis of a small fiber neuropathy is often difficult. The typical things that neurologists do to detect neuropathy are a physical exam and nerve conductions studies, and these are often normal if a patient has damage to only their small fibers. This is because the small fibers are too small to detect by this standard testing. A simple technique using a small three-millimeter punch biopsy can objectively prove that there is damage to the small fibers. A sample of skin is taken and then, using sophisticated staining of the nerves, the number of nerves per millimeter of skin can be detected. If a person has too few small nerves in their skin, their small fibers are damaged.
The treatment for small fiber neuropathies is primarily directed at treating the underlying cause. If a patient has diabetes, diet and exercise can be effective in controlling the pain. If a patient has vitamin deficiencies, these can be replaced and the neuropathy will hopefully improve. Certain infections can be treated as well. Other conditions such as Sjogren’s syndrome, lupus, and sarcoid can be treated and, in turn, improve the neuropathy and the pain. Such treatment can consist of corticosteroids, intravenous immune globulin (IVIG) or other immunosuppressive medications. If no treatable cause is identified, a patient may have their pain symptomatically treated with drugs like gabapentin, pregabalin, duloxetine, or nortriptyline. If the pain is severe, a patient may have to be treated with narcotic agents to control the pain.
As discussed in previous blogs, there are many causes of neuropathy and different types and sizes of nerves can be affected. A thorough exam by a neurologist who specializes in neuropathy and/or neuromuscular disease is very helpful. In some instances, the cause is obvious; in others, there is a cause but it is harder to determine.