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Small fiber neuropathy: A burning problem

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ABSTRACTSmall fiber neuropathy is increasingly being recognized as a major cause of painful burning sensations in the feet, especially in the elderly. Although strength remains preserved throughout the course of the disease, the pain and paresthesias are often disabling. Diabetes mellitus is the most common identifiable cause of small fiber neuropathy, and impaired oral glucose tolerance and individual components of the metabolic syndrome are often associated with it. Some cases, however, are idiopathic. Skin biopsy (with an evaluation of the density of intraepidermal nerve fibers) and tests of autonomic nerve function are useful for the diagnosis. Management involves controlling pain and identifying and aggressively treating the underlying cause.

KEY POINTS

  • Symptoms of small fiber neuropathy typically start with burning feet and numb toes.
  • Causes and associated conditions can be found in over 50% of cases. These include glucose dysmetabolism, connective tissue diseases, sarcoidosis, dysthyroidism, vitamin B12 deficiency, paraproteinemia, human immunodeficiency virus infection, celiac disease, neurotoxic drug exposure, and paraneoplastic syndrome.
  • Findings on routine nerve conduction studies and electromyography are typically normal in this disease.
  • Management includes aggressively identifying and treating the underlying cause, advising lifestyle modifications, and alleviating pain.


 

References

Figure 1. Symptoms are pain, burning, numbness, and autonomic dysfunction (lack of sweating) in the hands and feet in a stocking-glove distribution. Strength is not affected. Tendon reflexes are normal, as are nerve conduction studies.

An estimated 15 to 20 million people in the United States over age 40 have some type of peripheral neuropathy.1 In many, the impairment is purely or predominantly in small nerve fibers, and the clinical presentation consists of pain, burning, tingling, and numbness in a length-dependent or stocking-glove distribution. (“Length” refers to distance from the trunk; distal fibers are affected first.) Symptoms typically begin in the feet and slowly ascend to the distal legs, at which point the hands may also be affected (Figure 1).

In many of these patients, the findings on neurologic examination, nerve conduction studies, and electromyography are normal, although some may show signs of mild distal sensory loss on physical examination. The lack of objective findings on routine nerve conduction studies and electromyography may lead many physicians to attribute the symptoms to other disorders such as plantar fasciitis, vascular insufficiency, or degenerative lumbosacral spine disease.

The past 2 decades have seen the development of specialized tests that have greatly facilitated the diagnosis of small fiber neuropathy; these include skin biopsy to evaluate the density of nerve fibers in the epidermis and studies of autonomic nerve function. Common etiologies have been identified for small fiber neuropathy and can be specifically treated, which is critical for controlling progression of the disease. Pain management is becoming easier with more available options but is still quite challenging.

WHAT IS SMALL FIBER NEUROPATHY?

Figure 2.

Small fiber neuropathy is a disorder of the peripheral nerves that primarily or exclusively affects small somatic fibers, autonomic fibers, or both, resulting in sensory changes and autonomic dysfunction when both types are involved (Figure 2).2

Peripheral nerve fibers can be classified according to size, which correlates with the degree of myelination.

  • Large nerve fibers are heavily myelinated and include A-alpha fibers, which mediate motor strength, and A-beta fibers, which mediate vibratory and touch sensation.
  • Medium-sized fibers, known as A-gamma fibers, are also myelinated and carry information to muscle spindles.
  • Small fibers include myelinated A-delta fibers and unmyelinated C fibers, which innervate skin (somatic fibers) and involuntary muscles, including cardiac and smooth muscles (autonomic fibers). Together, they mediate pain, thermal sensation, and autonomic function.

Small fiber neuropathy results from selective impairment of small myelinated A-delta and unmyelinated C fibers.

Sensory symptoms: Pain, burning, tingling, numbness

Damage to or loss of small somatic nerve fibers results in pain, burning, tingling, or numbness that typically affects the limbs in a distal-to-proximal gradient. In rare cases, small fiber neuropathy follows a non-length-dependent distribution in which symptoms may be manifested predominantly in the arms, face, or trunk.

Symptoms may be mild initially, with some patients complaining of vague discomfort in one or both feet similar to the sensation of a sock gathering at the end of a shoe. Others report a wooden quality in their feet, numbness in their toes, or a feeling as if they are walking on pebbles, sand, or golf balls. The most bothersome and fairly typical symptom is burning pain in the feet that extends proximally in a stocking-glove distribution and is often accompanied by stabbing or aching pains, electric shock-like or pins-and-needles sensations, or cramping of the feet and calves.

Symptoms are usually worse at night and often affect sleep. Some patients say that their feet have become so exquisitely tender that they cannot bear having the bed sheets touch them, and so they sleep with their feet uncovered. A small number of patients do not have pain but report a feeling of tightness and swelling in their feet (even though the feet appear normal).

Examination often reveals allodynia (perception of nonpainful stimuli as being painful), hyperalgesia (perception of painful stimuli as being more painful than expected), or reduced pinprick and thermal sensation in the affected area. Vibratory sensation can be mildly reduced at the toes. Motor strength, tendon reflexes, and proprioception, however, are preserved because they are functions of large nerve fibers.

Autonomic symptoms

When autonomic fibers are affected, patients may experience dry eyes, dry mouth, orthostatic dizziness, constipation, bladder incontinence, sexual dysfunction, trouble sweating, or red or white skin discoloration.2 Examination may show orthostatic hypotension and skin changes. The skin over the affected area may appear atrophic, dry, shiny, discolored, or mildly edematous as the result of sudomotor and vasomotor abnormalities.

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