Small fiber peripheral neuropathy

Small fiber peripheral neuropathy is a type of neuropathy. It is also called a small fiber neuropathy, small fiber sensory neuropathy (SFSN) and C fiber neuropathy.

Naming due to pathology
This type of neuropathy is named after the specific portion of the peripheral nervous system that is affected. Small nerve fibers are the nerve fibers near the skin's surface that carry sensation. They are unmyelinated (meaning they don't have a fatty layer of insulation), and conduct slowly. They carry mostly pain and temperature sensation. Nerves to muscles and other sense organs (like pressure touch) are usually myelinated, conducting faster.

Symptoms
The key symptoms are insensitivity to heat and/or cold. There may be other neuropathic symptoms which are common to all polyneuropathies, such as tingling, numbness, muscle weakness, cramps, pain and other symptoms. People describe the pain as an "ice pick being poked into bone", "like an electric shock", "or walking on broken glass". In some cases, the pain causes a loss in the ability to feel when injury occurs, so one may be bleeding or have a skin injury without even knowing it.

Topographic pattern
Like many polyneuropathies, the symptoms usually start in the longer nerves and progressively attack shorter nerves. This means that most often the symptoms start in the feet and progress upwards, and usually symptoms are more severe in the feet. However, patients with Fabry disease have isolated small fiber engagement, and can have a more widespread small fiber disruption.

Diagnosis
This neuropathy is considered a separate clinical entity from a regular large-fiber polyneuropathy. Small fibers are difficult to diagnose. The diagnosis of a large-fiber (common) polyneuropathy is much easier. Many large-fiber polyneuropathies have minor small-fiber engagement, and small-fiber engagements are often implied if the patient has minor small-fiber symptoms in addition to large-fiber symptoms. The clinical picture of an isolated small-fiber neuropathy is characteristic, but the diagnosis is not always easy.

It is often a disorder diagnosed by ruling out everything else. In fact, nerve conduction tests and electromyography (EMG tests), which are good in diagnosing other neuropathies, are usually bad in detecting small fiber neuropathies. Quantitative sensory testing (QST) can be used to measure more objective changes in the temperature sensation. An elevated heat-detection threshold, heat-pain threshold, a reduced cold detection threshold or cold pain threshold may indicate a small-fiber neuropathy. A conventional nerve biopsy is not useful, since in this procedure mostly large-fiber nerves are studied. A skin biopsy (with the measurement of intraepidermal nerve fiber density) can be used for a diagnosis, but is not commonly available. This test allows for direct visualization of the un-myelinated nerve fibers (the "small fibers") in the epidermal layer of the skin, and requires taking a small skin sample.

Causes
The classic example of a small fiber neuropathy is Fabry disease. Sometimes the disorder is caused by diabetes, HIV, Erythromelalgia, postherpetic neuralgia, CRPS, alcoholism, and many other nerve pain conditions; its cause is also commonly idiopathic. Since there are no known causes for most cases and most tests do not identify it, not much money is spent on curing idiopathic cases.

Importance as a marker of Fabry disease
Small fiber neuropathy is important in the early detection of Fabry disease, which is currently treatable. Fabry disease can lead to stroke, heart and kidney failure. Small fiber neuropathy is an early sign. The diagnosis of a small fiber neuropathy could lead to an early diagnosis of Fabry disease, which could make organ damage less likely.

Treatment
Treatment is based on the underlying cause, if any. Where the likely underlying condition is known, treatment of this condition is indicated treated to reduce progression of the disease and symptoms. For cases without those conditions, there is only symptomatic treatment.

There is no current treatment to cure small fiber peripheral neuropathy, but Intravenous immunoglobulin (IVIG) is often used as well as plasmapheresis.