Small Fiber Peripheral Neuropathy

Small fiber peripheral neuropathy is a type of neuropathy that occurs from damage to the small unmyelinated peripheral nerve fibers. These fibers, categorized as C fibers, are present in skin, peripheral nerves and organs.[1] The role of these nerves is to innervate the skin (somatic fibers) and help control autonomic function (autonomic fibers).

Symptoms

Sensory symptoms of small fiber neuropathy are highly variable. Common complaints include paresthesiasdysesthesias, and insensitivity to pain. Paresthesias are abnormal sensations. They are often described as numbness, burning, cold, prickling, pins and needles along with other symptoms. Dysesthesias are altered normal sensations. A light breeze, the feeling of clothes, or even a soft touch can cause pain.[3] Insensitivity to pain can be particular problem. 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.

Causes

The classic example of a small fiber neuropathy is Fabry disease. Sometimes the disorder is caused by diabetesHIVErythromelalgiapostherpetic neuralgiaCRPSalcoholism, 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.

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

There are many possible causes of small fiber neuropathy. The most common cause is diabetes or glucose intolerance.[4] Other possible causes include hypothyroidismSjögren’s syndromeLupusvasculitissarcoidosis, nutritional deficiency, Celiac diseaseLyme diseaseHIVFabry diseaseamyloidosis and alcoholism.[5] A 2008 study reported that in approximately 40% of patients no cause could be determined after initial evaluation.[6] When no cause can be identified, the neuropathy is called idiopathic. A recent study revealed dysfunction of a particular sodiumchannel in a significant portion of the patient population with an idiopathic small fiber neuropathy.[7]

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.

References

  1. ^ Overview of Small Fiber Neuropathy. Therapath Pathology.
  2. ^ Zhou, Lan (2000). Small fiber neuropathy: A burning problem Cleveland Clinic Journal of Medicine, vol.76 5.
  3. ^ Latov, Norman. Peripheral neuropathy: when the numbness, weakness, and pain wont stop. American Academy of Neurology (AAN) quality of life guides, 2007, p.8.
  4. ^ Polydefkis, M, et al. (2003). “New insights into diabetic polyneuropathy”. JAMA 290: 1371–6.
  5. ^ Overview of Small Fiber Neuropathy. Therapath Pathology.
  6. ^ Devigili, G, et al. (2008). “The diagnostic criteria for small fiber neuropathy; from symptoms of neuropathology”. Brain 131: 1912–1925.
  7. ^ Faber, C, et al. (2011). “Gain of function Na(V) 1.7 mutations in idiopathic small fiber neuropathy”. Annals of Neurology.