Diagnosis and Treatment of Pain in Small Fiber Neuropathy
Definition of Small Fiber Neuropathy
A small fiber neuropathy occurs when damage to the peripheral nerves predominantly or entirely affects the small myelinated (Aδ) fibers or unmyelinated C fibers. The specific fiber types involved in this process include both small somatic and autonomic fibers. The sensory functions of these fibers include thermal perception and nociception. These fibers also are involved in a number of autonomic and enteric functions.
Most small fiber neuropathies occur in a length-dependent fashion, resulting in loss of function in a stocking distribution in the lower extremities. When the condition is more advanced, a glove-like loss in the upper extremities also may occur. In rare cases, a non–length dependent neuropathy results in symptoms involving the trunk, face, proximal limbs, or other focal areas [2•, 3•].
Anatomically, the small nerve fibers may be damaged or destroyed in these conditions, resulting in a loss of small nerve fibers and/or abnormal nerve fiber morphology. However, the pathogenesis of injury to small nerve fibers is not well understood. Small nerve fiber neuropathies can occur without large nerve fiber involvement, but in some cases they occur concomitantly or progress to involve large nerve fibers.
Symptoms of Small Fiber Neuropathy
Symptoms of small fiber neuropathy can vary widely in severity. Many individuals report the gradual onset of distal symptoms that include vague disturbances of sensation in the feet. These symptoms may include the feeling of a wrinkle in a sock that cannot be removed or of small pebbles or sand in the shoe. Others may report a cold-like pain, tingling or a pins and needles sensation. More severe symptoms of small fiber neuropathy may include burning pain that often is persistent, although it may vary in intensity throughout the day. Many patients also report transient electric shock–like pain, usually lasting only seconds, but quite severe and potentially multiple times per day. Many symptoms worsen during periods of rest and at night. In addition to spontaneous pain, many individuals report allodynia and hyperesthesia. Patients with small fiber neuropathy frequently complain that the bedsheets are exquisitely painful, and therefore, wear socks or use “foot tents” to keep the sheets from making physical contact with the feet.
Small nerve fiber neuropathies also may result in autonomic and enteric dysfunction. Patients often do not identify the relationship of these symptoms to their sensory complaints; however, when asked, they may report dry eyes, dry mouth, postural lightheadedness, presyncope, syncope, abnormal sweating, erectile dysfunction, nausea, vomiting, diarrhea, constipation, early satiety, difficulty with urinary frequency, nocturia, and/or voiding [4, 5].
Examination Findings in Small Fiber Neuropathy
One of the hallmarks of a pure small fiber neuropathy is a normal or near normal physical and neurologic examination. The coordination, motor, and reflex examinations will be normal. Light touch, vibratory sensation, and proprioception also may be normal, resulting in diagnostic confusion in some situations. Patients may have decreased pinprick, decreased thermal sensation, or hyperalgesia in the affected region. There may be mildly decreased vibratory sensation in some individuals. Associated skin changes in affected areas may include dry, cracked, or shiny skin, with decreased moisture on the surface of these affected areas as well.