Small fiber neuropathy (SFN) is caused by damage to the small, unmyelinated fibers in the peripheral nerves that innervate the skin and internal organs, including the cardiovascular system, gastrointestinal tract, and bladder, among others. These fibers convey pain and temperature sensations from the skin, and mediate autonomic functions.^1,2 Dysfunction of the small nerve fibers can cause sensory or autonomic symptoms. The neurological examination and EMG and nerve conduction studies that measure the large nerve fibers are usually normal, so that the diagnosis requires examination of the small nerve fibers in the skin.^3,4

Skin samples are routinely obtained by punch skin biopsy at standard sites, and the density of the small nerve fibers is determined by morphometry after immunostaining with an antibody to the axonal marker PGP- 9.5. Two standardized tests are available; the Epidermal Nerve Fiber Density (ENFD) test measures the density of the small sensory nerve fibers in the skin, and the Sweat Gland Nerve Fiber Density (SGNFD) test measures the density of the small autonomic nerve fibers in the sweat glands.^5,6 The SGNFD test may also show a decrease in nerve fiber density in some patients with small fiber neuropathy whose ENFD test is normal.⁷

Making the diagnosis of SFN provides an explanation for the symptoms, and helps direct further testing for potentially reversible causes, whose treatment can result in amelioration of the neuropathy. Outcome studies have also found that providing an explanation of their symptoms and reassuring the patients that their evaluation is complete, is as important to patients as treatment of their neuropathic pain.^8,9 Understanding the cause of their symptoms helps the patient accept their illness, cope with their symptoms, and be more receptive to their physician’s recommendations and treatments.

Symptoms of sensory small fiber neuropathy include numbness, hypersensitivity, and spontaneous painful or annoying sensations called paresthesias. The latter can present as tingling, burning, freezing, stinging, stabbing, itching, squeezing, tearing, buzzing, aching, or electric sensations that fluctuate in severity. The skin can be hypersensitive to pressure or touch, and innocuous stimuli can provoke unpleasant sensations, called dysesthesias; clothes can feel like sandpaper against the skin, and the pressure from wearing shoes or socks can cause pain and limit walking. These sensations can occur anywhere in the body, including the feet, arms, legs, torso, scalp, face, or even the mouth.^10,11 At the same time, however, patients can experience a loss of sensation, and inability to feel painful stimuli, as when stepping on a sharp nail or burning their fingers on a hot stove, without being aware of the injury.

Symptoms of autonomic small fiber neuropathy include abnormal sweating or temperature regulation, lightheadedness or fainting when standing up from hypotension or tachycardia, gastroparesis with bloating and constipation or diarrhea, incomplete bladder emptying or difficulty initiating a stream, sexual dysfunction from hypo or hypersensitivity, dryness and thinning of the skin, hair loss in the legs, and ridged or brittle nails.^12–14

The differential diagnosis includes other conditions that can cause pain including multiple sclerosis or myelopathy, Reflex Sympathetic Dystrophy, Regional Pain Syndrome (RSD), polymyalgia rheumatica, fibromyalgia, Erythromelalgia, Raynaud syndrome, restless leg syndrome, neuroma, fasciitis, or somatic pains resulting from anxiety or depression.^15–21 Patients are sometimes diagnosed as having neuropathic pain, although that requires demonstration of the underlying disease or lesion.²² Autonomic symptoms can be misdiagnosed as being due to cardiac arrhythmia, colitis or irritable bowel syndrome, chronic bladder infections, skin allergies, or emotional stress.

The causes of small fiber neuropathy are diverse, and in some cases, the neuropathy is the first manifestation of an underlying systemic disease. The most common cause is diabetes mellitus or glucose intolerance.²⁰ Other causes include metabolic syndrome, hypothyroidism, Sjögren’s syndrome, lupus, scleroderma, mixed connective tissue disease, sarcoid, vasculitis, inflammatory bowel disease, psoriasis, unspecified inflammatory conditions, Guillain-Barre syndrome, nutritional deficiencies such as B12 deficiency, celiac disease, Lyme disease, HIV-1 infection, hepatitis C infection, Fabry disease including in female carriers, amyloid, alcohol abuse, neurotoxic drugs including statins, toxins, or vaccinations.^23–50

In cases where no cause can be found, the neuropathy is called idiopathic. In some cases, a progressive neuropathy that affects both the small and large nerve fibers can first present as small fiber neuropathies before progressing to involve the large fibers, at which point it can also cause electrodiagnostic abnormalities. A listing of the known causes of small fiber neuropathy, and the corresponding diagnostic tests, is provided in the table below.

One large study reported that in approximately 40% of patients, no cause for the neuropathy was found on initial evaluation, but in a 2 year follow-up study, a potential cause was identified in 25% of these patients. Thirteen percent of patients went on to develop large fiber neuropathy.²⁴ Progression of the underlying neuropathy can be evaluated by repeating the biopsy and monitoring the ENFD.

Therapy in patients with small fiber neuropathy is directed at both the underlying cause, if one can be identified, and at ameliorating the symptoms. Some acute onset, otherwise idiopathic small fiber neuropathies were reported to respond to prednisone, presumably because they were caused by inflammatory mechanisms.³⁹ Painful paresthesias can be treated using oral medications for neuropathic pain such as Lyrica or Cymbalta, or with topical medications such as Lidoderm.