Small Fiber Neuropathy: An Overview

Learn more about this uncommon nerve disorder, from obtaining a diagnosis to finding treatment.

 

Small fiber neuropathy (SFN), a nerve disorder, is marked by severe pain attacks. Typically, the attacks begin in the hands and feet. Most patients first describe it as a stabbing, burning, or abnormal sensation of the skin, such as tingling or itchiness. Others, however, have a more generalized pain even from the start, saying it affects the whole body. Some say the pain is more severe when they are resting or sleeping. Others report that their feet are so tender that even bed sheets touching them seem like torture.

The condition, also known as small fiber peripheral neuropathy, affects the body’s peripheral nerves that primarily or exclusively affect the body’s small (versus large) nerve fibers, such as those in the skin and those that mediate pain and thermal sensation. These nerve fibers also help to regulate the body’s largely unconscious functions, such as heart rate and digestion.

Below is an overview of SFN, from obtaining a diagnosis to finding treatment.

Nerve model. Source: 123RFSmall fiber neuropathy, or SFN, affects the small fiber peripheral nerves outside the brain and spinal cord.

Symptoms

At first, SFN symptoms may be mild and vague. Some patients say they feel like they have a sock gathered at the end of a shoe. Others say they have numbness in the toes or feel as though they are walking on sand or pebbles.

Patients report that they cannot feel pain concentrated in a small area, such as from a pinprick. But they may have higher sensitivity to pain in general and they may report pain from a stimulation that typically does not cause pain. One example would be feeling a worsening pain from a surgical incision with no cause, such as infection. In some patients, there is a reduced ability to distinguish between hot and cold. However, in others, a warm or cold trigger can provide a pain attack.

Other symptoms may include bowel problems, urinary issues, rapid heartbeat or palpitations, abnormal sweating, and dry eyes and mouth. Some people get a sharp drop in blood pressure when they stand, which can cause fainting, dizziness, or blurred vision.

 

How Common is SFN? Why Does It Occur?

Exactly how many people have SFN is unknown. Experts estimate that about 40 million individuals in the US may have peripheral neuropathy of both types—large fiber and small fiber. And some have both types of nerve fibers involved. Symptoms can begin in the teen years or in adulthood. While some researchers have found that the condition affects more men than women, other studies are unclear as to gender differences in the prevalence of the disease.

Genetic mutations are suspected to play a role. These mutations involve the SCN9A and SCN10A genes. When these genes have mutations, they affect the functioning of structures found in nerve cells that transmit pain signals to the spinal cord and brain. As a result, there may be an enhanced transmission of pain signals.

In recent years, experts have begun to believe that the diagnosis of SFN should be at least suspected in anyone who visits a doctor complaining of widespread pain. This is especially true for those patients who may have been told by their doctor they do not understand why they are experiencing pain, or for those patients suspected of simply seeking painkillers.

Getting to the Root Cause

Diabetes is the most common identifiable cause of the SFN, but many other causes and conditions are linked to the disease as well. Restless leg syndrome is often associated with small fiber neuropathy, for example, as is having pre-diabetes, rheumatoid arthritis, hepatitis C, Parkinson’s disease, chronic kidney disease, and fibromyalgia. The use of statins, chemotherapy, and alcohol has been linked with SFN. In many patients, no underlying causes are found, however.

Among similar chronic conditions, SFN is often mistaken for plantar fasciitis, an inflammatory foot condition that causes heel pain, as well as vascular insufficiency or degenerative spine disease.

Diagnostic Tests

To obtain a diagnosis, your doctor will take a careful history and ask you to describe your symptoms, when they began, and how they have progressed. Testing may include nerve conduction studies, which assess the large nerve fibers only, done to rule out the involvement of those fibers. This test measures how quickly an electrical impulse moves through your nerve and can assess nerve damage. During the test, two electrode patches are placed on the skin over the nerve. As one electrode stimulates the nerve with a very mild electrical impulse, the other records it. The speed is assessed by measuring the distance between electrodes and how much time it takes for the impulses to travel between them.

After that, specialized studies may be needed to assess the small fibers. One such test is a skin biopsy to evaluate nerve fiber density. Another is a test known as QSART or quantitative sudomotor axon reflex testing to assess sudomotor autonomic function. The skin biopsy is typically a 15-minute procedure that involves taking 3-millimeter diameter punch from the leg or thigh. Two or three samples may be needed. The lab then analyzes the number of fibers in a given area. The QSART measures sweat output, which reflects the functioning of nerve fibers. Electrodes are placed on the wrist and leg to record the volume of sweat once a mild stimulation on the skin results in triggering the sweat glands to produce. The output is then compared with normal values.

Once a diagnosis is confirmed, your doctor is likely to perform other tests to pin down the cause. These may include but are not limited to: a complete blood count, a metabolic and lipid panel, measurement of thyroid hormones, and a 2-hour oral glucose tolerance test.

 

Treatment Options

Treatment of small fiber neuropathy typically targets both the nerve pain and the underlying cause. Because problems with glucose metabolism are common with this condition, control of blood sugar and lifestyle improvements, such as weight control and regular exercise, may be integrated into your treatment plan.

Your doctor may also work with you to treat any co-existing conditions, such as celiac disease and autoimmune diseases, which, when managed, can help reduce nerve problems and improve symptoms.

Pain management is important. Among the options:

  • gabapentin (Neurontin, Horizant), a nerve pain medication and anticonvulsant
  • tramadol (Ultram, ConZip), a narcotic
  • topical anesthetics, such as skin patches

Outlook

Unfortunately, the symptoms of small fiber neuropathy are known to worsen over time, and the condition has the potential to progress to affect the body’s large nerve fibers as well. The good news is that progression is typically slow and by treating the underlying cause, pain management can be possible.

 

Updated on: 11/20/18
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