Neuropathy Evaluation for Painful Burning Sensations
Small fiber neuropathy (SFN) is a major cause of painful burning sensations usually in the lower extremities.
Although strength usually remains preserved, the pain and paresthesias are often disabling.
- Diabetes mellitus is the most common identifiable cause of small fiber neuropathy, however multiple other condition with metabolic and or immune/inflammatory dysregulaation may also lead to SFN
- Skin biopsy (with an evaluation of the density of intraepidermal nerve fibers) are the standard for diagnosis
- Management involves controlling pain and identifying and aggressively treating the underlying cause.
An estimated 15 to 20 million people in the United States over age 40 have some type of peripheral neuropathy.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
WHAT IS SMALL FIBER NEUROPATHY?
SFN is a disorder of the peripheral nerves that primarily or exclusively affects small somatic fibers, autonomic fibers, or both, resulting in sensory changes and/or autonomic dysfunction
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) mediate pain, thermal sensation, and autonomic function.
SFN 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
- Autonomic fibers involvement
Symptoms may include:
dry eyes, dry mouth, orthostatic dizziness, constipation, bladder incontinence, sexual dysfunction, trouble sweating,
Signs may include:
skin may appear red, atrophic, dry, shiny, discolored, or mildly edematous as the result of sudomotor and vasomotor abnormalities.