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Nerve fibers (axons) can be classified as either large fibers or small fibers and the distinction has a direct bearing on peripheral neuropathy in terms of symptoms and prognosis.

Large Nerve Fibers

Large fibers are long nerve fibers that are myelinated that enables very fast conduction of impulses to the brain. They carry non-nociceptive information and are not normally associated with pain. Lesions or injury to large fibers can affect many functions including:

• Motor function
• Vibration perception
• Positional sense
• Perception of temperature

Symptoms associated with large fiber neuropathy include:

• Numbness
• Tingling
• Weakness
• Pain
• Loss of deep reflexes

Small Nerve Fibers

Some small nerve fibers are myelinated and some are not and each type involves different sensations. They contain nociceptors which are highly sensitive to pain and paresthesia (abnormal sensations such as tingling, pricking, or burning). Small nerve fiber neuropathy is the most common type in people over the age of 50 and is often unrecognized by physicians. An underlying cause for small fiber neuropathy is found in less than 10% of patients, many of whom suffer from idiopathic peripheral neuropathy. Small fiber peripheral neuropathy is very painful to the point of being debilitating and responds slowly to medication, if at all.

Symptoms of small fiber neuropathy are many and include:

• Pain described as burning, stabbing, prickling, jabbing, or lancinating (piercing)
• Sensation of "broken glass", "burning sand", or "ice pick in the bone"
• Tight band-like pressure
• Insensitivity to heat and cold
• Autonomic dysfunction - related to functioning of the organs

Small fiber neuropathy may be caused by:

• Diabetes
• HIV infection
• Amyloidosis (presence of amyloid protein deposits either in specific organs or throughout the body)
• Other systemic diseases

Small fiber neuropathy accounts for most cases of idiopathic peripheral neuropathy where no cause can be found for the condition. In peripheral neuropathy involving both types of fibers, small fiber damage usually precedes large fiber dysfunction and occurs typically in the lower limbs.